Bull Electrical-jun03.qxd

(V. Poulsen), and by mechanical alterna-tors (E. Alexanderson). Semiconductorsnow play an increasing role, but valves arestill used in high-power transmitters.

As their name suggests, the waves com-prise an electric and a magnetic fieldwhich are aligned at right angles to one

another. The electric field isformed by the rapid voltage fluctu-ations (oscillations) in the aerial.Current fluctuations create themagnetic field.

Electromagnetic waves cannot,

by themselves, convey any informa-tion. They are essentially radio fre-quency carriers, and arrangementshave to be made for the audio fre-quency speech and music signals tohitch a ride. This is done by modu-lating the radio frequency carrierwith the audio frequency signals.

If the amplitude of the carrier isvaried in sympathy with the signal,the process is known as amplitudemodulation (a.m.), and typicalwaveforms are depicted in Fig.1.1.Varying the carrier frequency is, ofcourse, known as frequency modu-lation (f.m.).

Marconi's Morse signals weretransmitted by simply switchingthe carrier on and off. It was R. A.Fessenden who, in 1906, used acarbon microphone (said to bewater cooled) to directly modulatethe radio frequency (50kHz) out-put of an alternator and be the firstto transmit speech and music.

The oscillations produced by the

transmitter are fed to an aerial sys-tem in order to radiate the electro-magnetic energy. The lower thefrequency the longer the wave-length and the bigger the aerial.

Aerial designs vary, but thedipole adopted by Hertz in 1886 isstill deployed at high, very high

physicist who first demonstrated the exis-tence of electromagnetic waves in 1886.

Before the valve era, radio frequencyoscillations were generated by using anelectrical discharge to shock-excite a tunedcircuit (H. Hertz and G. Marconi), by thenegative resistance of an electric arc

T owards the end of the 19th century,sending a radio signal a few hun-dred yards was considered a major

achievement. At the close of the 20th, manwas communicating with space probes atthe outermost edge of the solar system.

No other area of science and technologyhas affected the lives of peoplemore completely. And because itis so commonplace and afford-able, it is accepted without a sec-ond thought. The millions whoenjoy it, use it, even those whoselives depend upon it, often havelittle more than a vague notion ofhow it works.

This series of articles will viewthe technology in a historicalperspective and try to dispel itsmysteries. The main purpose,however, is to present a variety ofpractical circuits for set buildersand experimenters. And, witheconomy in mind, basic compo-nents and assemblies are repeatedin different receivers.

Radio uses electromagnetic

waves to transport speech, musicand data over vast distances at thespeed of light.

The electromagnetic waves aregenerated by making an electriccurrent oscillate at frequenciesranging from 10kHz (ten thou-sand Hertz) to more than 100GHz(one-hundred thousand millionHertz).

The lowest frequencies areused for submarine communica-tions because of their ability topenetrate water to a considerabledepth: the highest mainly forsatellite communications. Mostradio listeners are served by theportion of the spectrum extendingfrom 150kHz to 110MHz.

Frequency of oscillation ismeasured in Hertz in honour ofHeinrich Rudolf Hertz, the

Part 1: Introduction, Simple Receivers and a Headphone Amp.

398 Everyday Practical Electronics, June 2003

Fig.1.1. Modulation and detection: (A) Radio frequencycarrier wave. (B) Audio frequency signal. (C) Carrier wavemodulated by audio signal. Average value of imposed audiosignal voltage is zero. (D) Diode detector, D, working intoload resistor, R, rectifies the modulated carrier wave.Reservoir capacitor, C, removes residual radio frequenciesand enables the audio frequency output voltage to approachits peak value. (E) Recovered audio frequency signal.

MODULATED CARRIERWAVE INPUT

RECOVERED AUDIO FREQUENCYSIGNAL OUTPUTC R

Da k

A)

B)

C)

D)

E)

RESIDUAL RADIO FREQUENCIES REMOVEDBY SHUNTING ACTION OF CAPACITOR, C

Everyday Practical Electronics, June 2003 399

IMPORTANT EVENTS1831In published papers and a letter deposited with the RoyalSociety (opened in 1937), Michael Faraday tentatively proposeselectromagnetic wave theory.1864James Clerk Maxwell's mathematical analysis of Faraday's workpublished in his paper: A Dynamical Theory of the Electro-mag-netic Field.1888Heinrich Rudolf Hertz uses a crude spark transmitter andreceiver to demonstrate the existence of electromagnetic waves.1889Sir Oliver Lodge lectures on the need to tune the transmitter toreceiver, a condition he called “syntony”.1893Sir Oliver Lodge uses an invention of Edouard Branley's as asensitive detector of electromagnetic waves. (The coherer).1901Guglielmo Marconi transmits radio signals across the Atlantic.1904Sir John Ambrose Fleming patents the diode valve.1906Dr Reginald Fessenden modulates a carrier wave and broad-casts speech and music. Dr Lee de Forest makes a patent appli-cation for his triode valve, the first electronic amplifying device.1913Major Edwin Howard Armstrong invents the regenerative receiver.1918Armstrong invents the superheterodyne receiver.1921Armstrong invents the super regenerative receiver and W. G.Cady uses quartz crystals to stabilize oscillators.1933Armstrong demonstrates his system of frequency modulatedradio transmission.1947John Bardeen, Walter Brattain and Dr William Shockley developthe transistor at the Bell Telephone Laboratories.

and ultra high frequencies. The elevated wire and earth arrange-ment used by Marconi in the 1890's is still used for the radiationof low and medium frequencies.

Transmitter powers range from the miserly one or two watts,radiated by amateurs who specialize in low power communication,to the two-million watts output from some medium wave broad-cast transmitters.

Radiation from the transmitter reaches the receiver by either theground wave (line of sight or diffraction around the earth's curva-ture), or the sky wave (reflected between the ionosphere and thesurface of the earth. Propagation path is frequency dependant: byground wave up to 500kHz, then gradually shifting to sky waveuntil, above 30MHz, the waves are no longer reflected back by theionosphere and escape out into space.

Solar radiation has a profound effect on the charged particles whichmake up the ionosphere, and propagation conditions vary between nightand day, seasonally, and according to the eleven-year sunspot cycle.

Reception involves three essential functions: picking up the

energy radiated by transmitters, selecting one station from all therest, and extracting the modulation from the carrier wave in orderto make the transmitted speech or music audible to the listener.

Receiving aerials respond to either the electric or the magnetic field

radiated by the transmitter. Sets that use telescopic rod or wire aerialspick up the electric field. Receivers that have loop aerials, i.e., a coilwound on a frame or a ferrite rod, respond to the magnetic field.

Portable receivers usually incorporate both: long and mediumwaves (150kHz to 1·6MHz) are covered by a loop with a ferriterod core, and the v.h.f. f.m. band (88MHz to 108MHz) and short-wave bands by a telescopic aerial.

In order to select one station from the thousands that are spread

across the radio frequency spectrum, the receiver has to be tuned tothe carrier frequency of the transmitter.

Sir Oliver Lodge was stressing the importance of tuning, a con-dition he called “syntony”, as early as 1889, and he patented hissystem in 1897. This is one of the most fundamental patents inradio, and his method is still universally adopted.

Lodge's invention exploits the way an inductor (coil) and capac-itor combination resonate at a particular frequency. If the capacitoris connected in parallel with the inductor (see Fig.1.2a) the circuitpresents a high impedance at its resonant frequency and a lowerimpedance at all others.

Connecting the capacitor in series with the inductor (Fig.1.2b)results in a low impedance at resonance and a higher impedance atother frequencies. If the inductor or the capacitor (usually thecapacitor) is made variable, it is possible to tune the circuit acrossa range of frequencies.

Sharpness of tuning depends mainly on resistive and other loss-es in the inductor or coil: the lower these losses the sharper thetuning. Resistive and dielectric losses in the capacitor also affectperformance, but, with modern tuning components, these are usu-ally so small they can be ignored.

The ability of the coil to resonate sharply, i.e., be more selective, isknown as its “Q” factor: the sharper the resonance the higher the Q.

Resonant tuned circuits magnify signal voltages, a phenomenonthat is crucial to radio reception. If a signal of the same frequencyas the resonant frequency of the tuned circuit is applied to thecoil/capacitor combination, its voltage will be increased in pro-portion to the Q of the coil. With a Q of 100, a 1mV signal will bemagnified to 100mV or 0·1V. We will be returning to this later.

With amplitude modulated signals (a.m.) the process of recov-

ering the modulation is essentially one of rectification. In Fig.1.1d,diode, D, rectifies the incoming radio frequency carrier wave andcapacitor, C, shunts residual radio frequencies to ground (earth)leaving only the audio frequency modulation. Capacitor, C, alsoexhibits a reservoir action enabling the audio frequency voltage toapproach its peak value.

The diode and, indeed, any other a.m. demodulator, is called adetector, a hang-over from the earliest days of radio when glasstubes filled with metal filings were used to simply detect the pres-ence of electromagnetic waves.

In 1889, whilst working on the protection of telegraph equip-ment from lightning, Sir Oliver Lodge noticed that metal surfaces,separated by a minute air gap, would fuse when an electrical dis-charge occurred close by. He used the phenomenon to detect elec-tromagnetic waves, and called devices of this kind coherers.

About this time, Edouard Branley discovered that a spark in thevicinity of a mass of metal particles lowered their resistance.Lodge found this arrangement to be more sensitive and, in 1893,adapted it for use as a detector.

Subsequently, J. A. Flemming's diode valve, patented in 1904,and crude semiconductor devices, were used as rectifiers in orderto demodulate signals.

A)

B)

Fig.1.2. Basic tuned circuits. Combining an inductor (coil) and acapacitor produces a circuit which resonates at one particularfrequency. The resonant frequency can be varied by changingthe amount of inductance or capacitance (usually the latter).Thetuning of all radio receivers and transmitters depends upon thisphenomenon.

PARALLEL TUNED CIRCUITPresents a high impedanceat its resonant frequency anda lower impedance at otherfrequencies.

SERIES TUNED CIRCUITPresents a low impedance atits resonant frequency and ahigher impedance at otherfrequencies.

The most popular of the semiconductor

detectors was the “crystal” or “cat'swhisker” which consisted of a short lengthof springy brass wire touching a crystal ofgalena (lead sulphide). Adjustment of thepoint of contact was critical, but these crys-tal detectors could be more sensitive thanFlemming's diode valve. They were muchless expensive.

The modern equivalent of the crystaldetector is the point contact germaniumdiode. Here, a gold-plated wire contacts awafer of germanium, the assembly beingenclosed within a glass tube. These diodesare still used to demodulate the signals inmost domestic a.m. radios.

The simplest receiver, known as a

Crystal Set, consists of nothing more thana coil, tuning capacitor, diode detector, anda pair of earphones.

A typical circuit diagram for a CrystalSet Radio is given in Fig.1.3, where induc-tor or coil L1 is tuned by variable capacitorVC1 to the transmitter frequency. DiodeD1 demodulates the signal, which is fedstraight to the earphones. There is noamplification.

A long (at least 20 metres), high (7metres or more) aerial and a good earth (aburied biscuit tin or a metre of copper pipedriven into damp ground) are required inorder to ensure audible headphone recep-tion. The earphones originally used withthese receivers had an impedance ofaround 4000 ohms and were very sensitive(and heavy and uncomfortable). They areno longer available, but a crystal earpiece,which relies on the piezoelectric effect,will give acceptable results. Low imped-ance “Walkman” type earphones are NOTsuitable.

Quite apart from the absence of amplifi-

cation, two factors seriously limit the per-formance of crystal receivers.

Germanium diodes become increasinglyreluctant to conduct as the applied voltagefalls below 0·2V, and this makes the receiv-er insensitive to weak signals. Silicondiodes have a threshold of around 0·6V,and are, therefore, unsuitable for circuits ofthis kind.

The earphone loading imposes heavydamping on the tuned circuit, reducingits Q and, hence, its selectivity, i.e. itsability to separate signals. With such lowselectivity insensitivity can be a bless-ing, and crystal sets are normally onlycapable of receiving a single, strongtransmission on the long and mediumwavebands. (They will sometimesreceive more than one if a shortwave coilis fitted).

The aerial and diode can be connected totappings on the tuning coil in order toreduce damping, but the improvement inselectivity is usually at the expense ofaudio output.

When valves cost a week's wages andhad to be powered by large dry batteriesand lead/acid accumulators, the construc-tion of simple receivers of this kind couldbe justified. With high performance tran-sistors now costing only a few pence orcents, crystal sets are now regarded as“nostalgic pieces”.

Some readers may, however, wish tobuild one out of curiosity, or for the novel-ty of having a receiver that does not requirea power supply. Moreover, the componentsrequired are all used in more complexreceivers to be described later.

Ferrite loop aerial L1 and polythene

dielectric variable capacitor VC1 form thetuned circuit. Point contact germaniumdiode D1 demodulates the signal; capacitorC1 bypasses residual r.f. (radio frequen-cies) to earth and also exhibits a reservoiraction, enabling the a.f. (audio frequency)output to approach its peak value. The

recovered audio signal is fed directly to acrystal earpiece.

Signal voltages induced in the ferrite loopaerial by the radiated magnetic field aremuch too weak to produce an output fromthe detector, and the component is used heresimply as a tuning coil. The ferrite core does,however, reduce the number of turnsrequired for the coil winding, thereby reduc-ing its resistance and increasing its Q factor.


CRYSTAL SETResistor

RX 4k7 0·25W 5% (onlyrequired if set is connected toamplifier)

CapacitorsC1 10n disc ceramic.VC1 5p to 140p (minimum)

polythene dielectric variable capacitor (see text)

SemiconductorsD1 0A47 germanium diode

MiscellaneousL1 ferrite rod, 100mm (4in.)

× 9mm/10mm (3/8in.) dia., with coil (see text)

Crystal earpiece and jack socket tosuit.; plastic control knob; plastic insulat-ed flexible cable for aerial wire, down-lead and earth connection, 30 metres(100 ft) minimum; buried biscuit tin or 1metre (3ft) of copper pipe for earth sys-tem; 50gm (2oz) reel of 26s.w.g.(25a.w.g.) enamelled copper wire, fortuning coil; card and glue for coil former;multistrand connecting wire; crocodileclips or terminals (2 off), for aerial andearth lead connection; solder etc.

Approx. CostGuidance Only ££44

excl. earpiece & wire

HEADPHONE AMPLIFIERIF SET CONNECTED TO

RX 4k7. ONLY REQUIRED

EARPIECETO CRYSTAL

ABOVE THE GROUNDLOCATED 7 METRES (20 FT)20 METRES (60 FT) OF WIRELONG WIRE AERIAL: AT LEAST

L1

(25 A.W.G.)26 S.W.G.

100t

(SEE TEXT)EFFECTIVE EARTH

D1OA47

a k

VC15p TO140p

C110n

RX

a kOA47

Fig.1.3.This simplest of radio receiversuses a germanium diode as the “cat’swhisker” crystal detector.

The author’s Crystal Set “knock-up” on a wooden baseboard. This uses two screwterminals for the Aerial and Earth wire connections instead of croc. clips.

The circuit is simple enough to be

assembled on the work bench, and a print-ed circuit board layout is not given. Thecomponents and the various interwiringconnections are illustrated in Fig.1.4.

Full construction and winding details for

the ferrite tuning/aerial coil L1 are shownin Fig.1.5. The coil is made from 26s.w.g.(25a.w.g.) enamelled copper wire, closewound on a cardboard former. This sameferrite tuning coil forms the loop aerial inthe following TRF Receiver.

The r.f. bypass capacitor C1 can, in prac-tice, be omitted with no noticeable reduction

in performance. However, if the set is to beconnected to either the headphone amplifier(Fig.1.10) or speaker amplifier describednext month, this component, together withdiode load resistor, RX, must be included.

Audio frequency amplification after

the diode detector will permit the use oflow impedance Walkman type earphonesor even loudspeaker operation. It will do

nothing, however, to overcome thediode's insensitivity to weak signals. Forthis we must have radio frequency ampli-fication of the signals picked up by theaerial before they reach the detector.(The standard circuit for a transistorportable receiver has three stages ofradio frequency amplification ahead ofthe diode).


100 TURNS OF 26SWG (25AWG)ENAMELLED COPPER WIRE,CLOSE WOUND

9mm ( / in)DIAMETER x100mm (4in)FERRITE ROD

3 8ENDS OF WINDINGSECURED BYNARROW STRIPS OFMASKING TAPE

THIN CARD ROLLED AROUND ROD TOMAKE A FORMER 60mm (2 / IN) LONG BY13mm ( / in) DIAMETER. APPLY GLUE TOCARD WHILST ROLLING.

3 81 2

Fig.1.5. Construction and windingdetails for the ferrite rod tuning/aerialcoil L1. This loop aerial is also used inthe TRF Receiver.

CRYSTALEARPIECE

JACK PLUGAND

SOCKET

L1

a k

L1

(25 A.W.G.)26 S.W.G.

100t

D1

RXC1

DIODE LOAD RESISTOR, RX, IS ONLYREQUIRED IF CRYSTAL RECEIVER ISCONNECTED TO HEADPHONE AMPLIFIER.

TO EARTH, BISCUIT TIN OR 1M COPPERPIPE IN DAMP GROUND. CONNECTION TOCENTRAL HEATING PIPEWORK WILL OFTENSUFFICE.

TO AERIAL, AT LEAST 20 METRES(60FT) OF WIRE MOUNTED 7 METRES(20FT) MINIMUM ABOVE THE GROUND

VC1A

G

O

Fig.1.4. The components and various interwiring connections for the simple crystalset. This circuit is easily “lashed-up” on the workbench and no circuit board layoutis given. The author’s demonstration set is shown in the heading photograph.

Receivers with tuned circuits and ampli-fication, at signal frequency, ahead of thedetector stage were known, during thevalve era, as tuned radio frequency, ort.r.f., receivers.

This arrangement was adopted byFerranti when they designed their popularZN414 radio i.c. Introduced in 1972, thechip relied upon a then new manufacturingtechnique developed byBell Laboratories andknown as collector diffu-sion isolation.

No bigger than a singletransistor, and requiring apower supply of only 1·5V,the device enabled trulyminiature receivers to bebuilt, one of which wasfeatured on the BBCTV science programme,Tomorrow's World. Thechip is still produced, but ina plastic package instead ofthe original metal case andwith the type numberMK484.

The chip's internal architecture, in block

form, is depicted in Fig.1.6. The very highimpedance input stage minimizes dampingon the tuned circuit, enabling it to maintain ahigh Q factor and, consequently, reasonableselectivity. This is followed by three stagesof radio frequency amplification ahead of atwo transistor detector or demodulator.

Chip characteristics and pinout detailsare also listed with the block diagramFig.1.6. Internal capacitors impose the lowfrequency operating limit, and the perfor-mance of the transistors determines thetail-off at high frequencies.

MK484 Specification . . .Supply Voltage 1·1V to 1·8V

(via external load resistor)

Current Drain 0·3mA to 0·5mA(affected by signal level)

Frequency range 150kHz to 3MHz(peaks at 1MHz)

Input Impedance 1·5 megohms

Output Impedance 500 ohms

Sensitivity better than 100V

Power Gain 70dB

Internal Component Count:10 Transistors15 Resistors

4 Capacitors

R.F.AMP

R.F.AMP

R.F.AMP

R.F.AMP DET.

INPUT2

1OUTPUT

3GROUND

123

OUTPUTINPUT

GROUND

Fig.1.6. Block diagram showing the internal arrangement ofthe MK484 radio i.c.



excl. case, earpiece, wire & batt.

TRF RECEIVERResistors

R1 100kR2 3k9 see Table 1.1R3 470R4 2M2 see Table 1.1R5 4k7R6, R7 120 (2 off)

All 0·25W 5% carbon film

PotentiometersVR1 4k7 rotary carbon, log.

CapacitorsC1 1n polystyrene (see text)C2 100p polystyrene or “low k”

ceramic (see text)C3, C6, 10n disc ceramic (2 off)C4 100n disc ceramicC5 220n disc ceramicC7 47µ radial elect. 16VC8 100µ radial elect. 16VC9 1 radial elect. 16VVC1 5p to 140p (minimum)

polythene dielectric variable capacitor

Table 1.1: MK484 TRF Receiver(Values of resistors R2 and R4 for

different supply voltages)

Voltage R2 R4

1·5V 100 180k3V 1k 1M4·5V 1k8 2M26V 2k2 2M29V 3k9 2M2

The circuit diagram for a simple TRF

Receiver using the MK484 i.c. is given inFig.1.7. Inductor or coil L1 is “tuned” byvariable capacitor VC1, from roughly550kHz to 1·7MHz, i.e., over the mediumwave band.

Provision is made for toggle switch S1ato connect an additional capacitor acrossL1 to tune it to a lower frequency longwave station. For BBC Radio 4 on 198kHz,a non-standard component is required, andthis is made up from capacitors C1 and C2.

Tuning coil L1 is wound on a ferrite rodin order to form a loop aerial which, as wehave seen, responds to the magnetic fieldsradiated by transmitters. The high perme-ability ferrite material concentrates thelines of magnetic force, and the signaldeveloped across the coil is equal to thatpicked up by an air-cored loop of around200mm (8in.) diameter. The threshold sen-sitivity of IC1 is better than 100µV, andthis is sufficient for the reception of strongsignals via the loop.

Current drawn by IC1 increases as sig-

nal strength increases, and the gain of theMK484 is supply voltage dependant.Connecting all of the stages to the supplyvia the audio load resistor R3 produces ameasure of automatic gain control (a.g.c).(Increased current demand at high signallevels increases the voltage drop across theload resistor thereby reducing the gain ofthe chip.) IC1 input pin 2 is biased viaresistor R1, and r.f. bypass capacitors C3and C4 ensure stability.

The value of the audio load resistor(sometimes called the a.g.c. resistor) R3can range from 470 ohms to 1000 ohms.Selectivity is better when the value is keptlow: gain is greater when the value is high.Most readers will need all the selectivitythey can get and a 470 ohm resistor is usedin this circuit.

Optimum supply voltage with a 470ohm load is around 1·2V: more than thiscan cause instability problems, less willreduce gain. The voltage delivered by afresh dry cell can be as high as 1·7V andthe chain of silicon diodes, D1 and D2,

SemiconductorsD1, D2 1N914 silicon signal diodes

(2 off)TR1 BC549C npn small signal

transistorIC1 MK484 radio i.c.

MiscelllaneousS1 d.p.d.t. centre-off toggle

switchL1 ferrite loop aerial: 100mm

(4in.) × 9mm/10mm (3/8in.) dia. ferrite rod withcoil (see text)

Printed circuit board available from theEPE PCB Service, code 392; plastic case,size and type to choice; plastic control knob(2 off); 50gm (2oz) reel of 26s.w.g.(25a.w.g.) enamelled copper wire, for tuningcoil; card and glue for coil former; crystalearpiece and jack socket to suit; multistrandconnecting wire; 9V battery, clips and hold-er; p.c.b. stand-off pillars; mounting nutsand bolts; solder pins; solder etc.

bc

e

B19V

123OUTPUT

INPUT

GROUND

ak1N914

D11N914

D21N914 AUDIO

OUTPUT

TUNING

VOLUME

a

a

k

k

VC15p TO140p

C11n

C2100p

LONGWAVE

MEDIUMWAVE

R1100k

C310n

C4100n

IC1MK484

2 1

3

R23k9

R3470Ω

*

*

*

SEE TEXT

C5220n

C610n

R42M2

R54k7

C747µ

R6120Ω

+

+

+

C8100µ

R7120Ω

C91µ

VR14k7

S1b

ON/OFFPOLE

S1a

POLE

TR1BC549C

BC549C

ebc

C1 AND C2 COMBINE TO GIVE THENON-STANDARD CAPACITOR VALUEREQUIRED TO TUNE RECEIVER TOBBC RADIO 4 ON LONG WAVE BAND(198kHz). SEE TEXT.

SEE TABLE 1 FOR VALUES OFR2 AND R4 FOR ALTERNATIVESUPPLY VOLTAGES.

MK484

+9V

0V

IN

GND

OUT

L1

Fig.1.7. Complete circuit diagram for the MK484 TRF Receiver.

each of which begins to conduct at a 0·6Vthreshold, holds the supply at the correctpotential.

Some readers will no doubt wish to usethe circuit with different supply voltages,and the value of dropping resistor R2should be altered to avoid excessive currentdrain. Table 1.1 gives suitable values forthis resistor for various battery voltages.

The output from IC1 pin1 is low, so the

audio amplifier stage, TR1, is included toincrease it to a useable level. The signalfrom IC1 pin 1 is applied to TR1 base (b)via d.c. blocking capacitor C5, and the out-put is developed across collector (c) loadresistor R5.

Emitter (e) bias is provided by resistorR6 which is bypassed by capacitor C7. Basebias is derived via resistor R4. Connecting

this resistor to the collector rather than thesupply rail provides a measure of negativefeedback, stabilizing the stage against tem-perature and transistor gain variations. Thevalue of this resistor has to be optimized fordifferent supply voltages, and appropriatevalues are given in Table 1.1.

Headphone and loudspeaker amplifierswill, in most cases, be connected across thesame battery power supply, and resistor R7and capacitor C8 decouple the tuner circuitin order to prevent instability. Bypass capac-itor C6 connected across collector load R5is also included to avoid instability. Audiooutput is taken from the collector of transis-tor TR1 and connected to the Volume con-trol VR1, via d.c. blocking capacitor C9.

A crystal earpiece can be connected inplace of the volume control but the output,especially when weaker signals are beingreceived, will be barely adequate. With thisarrangement there is no need for resistorR7, and provision is made, on the printedcircuit board, for it to be shorted out (seeFig.1.9).

If you have not already made up the

“tuning coil” for the Crystal Set, construc-tion of the TRF Receiver should com-mence with the winding of the ferrite loopaerial as detailed earlier in Fig.5 and asfollows. Wind thin card (a postcard isideal) around a 100mm (4in.) length of9mm (3/8in.) diameter ferrite rod until an

overall diameterof 13mm (1/2in.)is achieved. Applyadhesive as thecard is wound on.

Coil L1 consists of 100 turns of 26s.w.g.(25a.w.g.) enamelled copper wire closewound, i.e. with turns touching. Secure thestart and finish of the winding with thinstrips of masking tape wound tightlyaround the former. The task of producing aneat coil can be eased by slightly spacingthe turns as they are wound on and repeat-edly pushing them together with the thumbas the winding proceeds.

The inductance of this loop winding ishigher than normal in order to ensure fullMedium Wave coverage with the specifiedtuning capacitor (medium wave loops usu-ally comprise about sixty turns on a 9mm(3/8in.) dia. rod). Longer rods, which willincrease signal pick-up, may be used if alarger receiver can be tolerated.

Use rubber bands, strips of card or woodor plastic blocks to secure ferrite loopaerials. Do not use metal mounts as thesecan form a shorted turn and dramaticallyreduce efficiency.

Most of the TRF Receiver components

are assembled on a small printed circuitboard (p.c.b.). The topside component lay-out, off-board interwiring and a full-sizeunderside copper foil master are shown inFig.1.8. This board is available from theEPE PCB Service, code 392. How to con-nect an earpiece directly to the Receiverp.c.b. is illustrated in Fig.1.9.

Insert and solder in position the resistorsand capacitors first and the semiconductorslast. The 3-pin MK484 radio i.c. must bemounted close to the board to preventinstability: leave just sufficient lead lengthfor the application of a miniature crocodileclip as a heat shunt whilst soldering.

Take care to remove all traces of theenamel from the ends of the coil winding inorder to ensure a good connection. Solderpins, inserted at the lead-out points, willsimplify the task of off-board wiring.

392

1.65in (41.9mm)1.

12in

(28

. 4m

m)

R1

R3

R6

R2

R7

R5

R4

C4

C5

D2

D1C3

C9

C8

C7

C6

IC1 TR1

INGND OUT

e b c+ +

+a

a

k

k VOLUME

TUNING

0V

L1

C2

C1TO

BATTERY

AUDIO TOAMPLIFIER

VC1 S1a S1b

VR1

+

A G O

V

V

Fig.1.8. Printed circuit board component layout, interwiringto off-board components and full-size underside board cop-per track master.

Fig.1.9. Connecting a crystal earpiece, via socket, directly tothe p.c.b. in place of the Volume control. Note the “shorting”link wire.

R3 R2

R7

R5

R4

C5

D2

D1

C9

C8

C6

TR1e b c +

+a

a

k

AUDIOOUTPUT

TO BATTERYVIA S1b

SOCKET TOSUIT CRYSTAL

EARPIECE

LINK SHORTSOUT R7

(Polythene dielectric type)

Some retailers supply extenders for thestubby spindle. If an extender has to befabricated, use a 6mm diameter tubularstand-off bolted to the central hole in thespindle (bolt is usually 2mm). Grip thecapacitor spindle when tightening bolt. Donot tighten against internal end stop.

Secure capacitors to bracket or frontpanel with two bolts driven into threadedholes in its front plate. Bolts (usually 2mm)must not extend through the thicknessof the front plate.

Twin-gang capacitor lead-outs are usual-ly thin metal strips. Moving vanes (the stripoften marked ‘G’) should be connected toground or the 0V rail. Strips for the fixedvanes, normally used to tune the aerial andoscillator circuits in a superhet receiver, areoften marked “A” and “O”. One or bothshould be connected to the “hot” end of thesimple receiver’s tuning coil.

Internal trimmers should be set to mini-mum capacitance when a variable is usedwith simple receivers. Twin-gang polyvari-cons have two trimmers. Four-gang unitshave four trimmers.

For complete coverage of the mediumwave band with this ferrite loop aerial, thevariable capacitor should have a 5pF mini-mum capacitance and a maximum capaci-tance of at least 140pF. The aerial tuningsection of most polythene dielectric vari-ables, as used in transistor portables,should be suitable.

If necessary, both sections (aerial and oscil-lator) can be connected in parallel to ensurethe necessary maximum capacitance. Thiswill, however, double the minimum capaci-tance and slightly curtail coverage at the highfrequency end of the band.


A G O

The single transistor HeadphoneAmplifier circuit illustrated in Fig.1.10, willensure an acceptable output via Walkmantype ’phones. The audio input signal is cou-pled to the base of transistor TR1 via d.c.blocking capacitor C1. Base bias resistorsR1 and R2, fix the standing collector currentat around 4mA. Emitter bias is provided byR3, which is bypassed by C3.

Walkman type ’phones form transistorTR1’s collector load, both earpieces beingwired in series to produce an impedance of64 ohms. Bypass capacitor C2 acts as ahigh frequency shunt across the ’phoneleads. This measure avoids instability prob-lems and is particularly necessary when theamplifier is used with some of the moresensitive receivers to be described later inthe series.

Bypass capacitor C4 ensures stabilitywhen tuner and amplifier stages are pow-ered by the same battery, particularly whenbattery impedance rises as it becomesexhausted. On/off control, S1b, is one halfof a two-pole, centre-off, toggle switch.

All parts, except the ’phone socket and

battery, are assembled on a small p.c.b. andthe component layout, off-board wiringand full-size copper foil master are given inFig.1.11. This board is available from theEPE PCB Service, code 393.

Follow the assembly sequence suggestedfor the TRF radio board. Again, solder pinsat the lead-out points will ease the task ofoff-board wiring.

Wiring the output to the tip and centrering on the jack socket will result in theseries connection of the earpieces and pro-duce a nominal 64 ohm load for transistorTR1.

Check the two p.c.b.s for poor soldered

joints and bridged tracks. Check theorientation of electrolytic capacitors andsemiconductors.

Make sure the off-board wiring has beencorrectly routed and, if all is in order, con-

nect the battery powersupply. Current con-sumption of thetuner/amp with a 9Vsupply and resistorsR2 and R4 as speci-fied in Table 1.1should be approxi-mately 2·5mA.

Readers who arekeen to minimize bat-tery drain couldreduce bias resistor,R2, in Fig.1.10. to4·7k or less. This

HEADPHONE AMPLIFIERResistors

R1 39kR2 5k8

(see text)R3 150

CapacitorsC1 1µ radial elect. 16VC2 100n disc ceramicC3 47µ radial elect. 16VC4 100µ radial elect. 16V

SemiconductorsTR1 BC549C npn small

signal transistor

MiscellaneousS1 d.p.d.t. centre-off toggle

switch (see text)

Printed circuit board available from theEPE PCB Service, code 393; jack sock-et to suit ’phones; multistrand connect-ing wire; 9V battery, clips and holder;p.c.b. stand-off pillars; solder pins; solderetc.

SeeSSHHOOPPTTAALLKKppaaggee

1.3in (33 mm)

1.05

in (

26. 7

mm

)

393

'GROUND' SIDEOF AUDIO INPUT

AUDIO INPUTFROM RECEIVER

VR1 SLIDER

TO BATTERYVE VIA S1b+

TO BATTERY VE

SOCKET FORHEADPHONES

R2

R1

R3

C2

C1

C3

C4

TR1e

b

c

++

+

Fig.1.11. Headphone Amplifier printed circuit board compo-nent layout, wiring details and full-size copper foil master.

bc

eB19V

SEE TEXT

S1b

ON/OFFPOLETR1

BC549CBC549C

AUDIOINPUT

PHONES

9V VE TO+RADIO P.C.B.

C4100µ

C347µ

+

+

+

R139k

C2100n

C11µ

R25k8

R3150Ω

0V

ebc

Fig.1.10. Circuit diagram for a single transistor HeadphoneAmplifier.

will lower the standing current drawn by the Headphone Amplifier atthe expense of maximum undistorted output.

The add-on amplifier stage will permit low-impedance Walkmantype ’phones to be used with the Crystal Set (Fig.3). A Volume con-trol is unnecessary: simply connect a 4700 ohm resistor (RX) to act asa diode load in place of the ’phones, and link receiver to amplifier viad.c. blocking capacitor, C1.

Performance of the MK484 TRF Receiver and Headphone

Amplifier combination is far superior to the simple Crystal Set.The MK484 is, moreover, sufficiently sensitive to operate from aferrite-cored loop and an external wire aerial and earth are notrequired.

Selectivity is barely adequate, and very powerful signals tend tospread across the dial. Rotating the ferrite loop aerial to null out theoffending station will, however, usually effect a cure. (Loop aerialsare directional and signal pick-up falls to a minimum when the axis ofthe coil is pointing towards the transmitter).

Although the circuit will permit the clear reception of a number ofstations, sensitivity is not sufficient for the reception of weak signals.A simple add-on circuit, which will transform the performance of thereceiver and make it as selective and sensitive as a commercial super-het, will be described next month.

Next month’s article will also include an amplifier for readers whowant loudspeaker operation, and a design for another simple, but highperformance, medium and long wave receiver using individualtransistors.


excl. ’phones


circuit formed by combining an inductor(coil) and a capacitor. It will be recalledthat magnification is limited mainly byresistive and other losses in the coil.

If losses are kept low, the coil is said tohave a high Q factor. With careful designand construction, Q factors in excess of100 are not difficult to achieve. However,when the coil is connected into circuit, theloading or damping effect of valves, tran-sistors and other components reduces its Qsignificantly.

If a signal at the resonant frequency isapplied to the coil and capacitor combina-tion, its voltage will be increased in pro-portion to the Q factor. Thus, with a Q of100, a 1mV signal will be magnified to100mV or 0·1V. Off-resonance signalsare not magnified in this way, and thegreater the Q the more selective the tunedcircuit.

however, that the credit, at least for theregenerative receiver, is Armstrong's.

Armstrong's discoveries made reliable,

long distance radio communication a pos-sibility. The valve oscillator eventuallyformed the basis of powerful transmitterscapable of operating at higher frequenciesthan those attained hitherto.

His regenerative circuit provided a sim-ple and inexpensive means of greatlyincreasing receiver sensitivity and selectiv-ity. It remained the most popular receivingsystem until the superhet (another inven-tion of Armstrong's) achieved dominanceduring the 1930s.

In Part One we touched on the signal

magnifying effect of a resonant tuned

IN Part One, last month, we looked at thefundamental principles and earlyhistory of radio, and detailed the con-

struction of a TRF (tuned radio frequency)receiver. Its selectivity and sensitivity arebarely adequate and, this month, a simpleadd-on unit that will transform its perfor-mance is described.

Also included is an alternative circuitfor a simple medium wave portable radio.But first, some historical background.

The invention of the triode valve, by Lee

de Forest, in 1906, made available, for thefirst time, a means of amplifying radio fre-quency signals.

These early valves were only partiallyevacuated (soft), hand made, and extreme-ly expensive. Their operation was not com-pletely understood, and they were used,initially, as detectors and audio frequencyamplifiers.

Lee de Forest called his invention theAudion. It was only later that Eccles, of flip-flop fame, described the device as a triode.

In 1913, whilst experimenting with a tri-

ode detector, Edwin Howard Armstrongrealized that radio frequencies were pre-sent at the anode (plate in the USA) or out-put port of the valve. He connected a sec-ond tuning coil in the output circuit andnoticed a dramatic rise in receiver sensitiv-ity when he brought it closer to the gridtuning coil. When the coils were veryclose, the circuit began to oscillate.

He had, almost by accident, discoveredthe regenerative detector and valve oscilla-tor. He patented the circuit in October1913, two months before his twenty-thirdbirthday.

Lee de Forest, Alexander Meissner,Irving Langmuir, C. S. Franklin and othersmade similar discoveries, and there wasmuch patent litigation. In 1921, theColumbia Court of Appeals gave judge-ment to Lee de Forest on the basis of alegal technicality. It is widely accepted,

474 Everyday Practical Electronics, July 2003

Part 2: Regeneration, Q-Multiplier, Reflex Radio and a Speaker Amp.

Completed circuit boards for the following (left to right, clockwise): MK484 TRFReceiver (Pt 1 – June ’03), Headphone Amplifier (Pt 1 – June ’03), SpeakerAmplifier (this issue) and Q-Multiplier (this issue).

Everyday Practical Electronics, July 2003 475

By progressively feeding back energy tothe tuned circuit in phase with the incom-ing signal, i.e. positive feedback, the resis-tive and other losses in the coil are gradu-ally overcome and very high Q factors canbe achieved. As the positive feedback isincreased beyond the point where the loss-es are eliminated, the circuit begins tooscillate.

Armstrong found that feedback, orregeneration as it came to be known,increases the strength of weak signals by afactor of 1000 or more and, because theincrease is due wholly to the dramatic risein tuned circuit Q, there is also a bigimprovement in selectivity.

Regeneration enhances weak signalsmore than strong ones, and the systemexhibits an a.g.c. (automatic gain control)action. In practice, however, the effect isnot pronounced, and there is a muchgreater range of output levels than is thecase with a superhet receiver with conven-tional a.g.c. circuitry.

Regenerative receivers are easily

swamped by powerful signals (the tuningtends to lock onto strong carriers), andtheir sharply peaked selectivity curveattenuates the higher audio frequencies.Overloading by strong signals can, howev-er, be avoided by fitting a simple inputattenuator, and some top cutting is not too

high a price to pay for a big improvementin selectivity.

Skill is required of the operator if highperformance is to be achieved. The regen-eration control has to be carefully set tobring up weak signals, and the input leveladjusted to prevent overload. This, morethan anything else, brought about the grad-ual demise of the system after Armstronginvented the superhet, with its more user-friendly controls, in 1918.

Like the author, readers will no doubt bekeen to get high performance at the lowestpossible cost, and a regenerative receiver is asclose as it comes in the field of radio and elec-tronics to getting something for nothing. Anumber of modern examples of the techniquewill, therefore, be included in the series.

By applying regeneration, or Q multipli-cation, to its ferrite loop aerial, the perfor-mance of the MK484 TRF Receiverdescribed last month can be dramaticallyimproved. Levels of sensitivity and selec-tivity approaching those of a domesticsuperhet can be achieved. Because of thesmall voltages developed across the loopaerial, swamping by strong signals is notnormally a problem, especially if the fer-rite rod is rotated for minimum pick-up, asdescribed last month.

The circuit diagram of the simpleadd-on Q-Multiplier unit is given inFig.2.1 where L1 is the receiver's ferriteloop aerial, and field effect transistor,TR1, provides the radio frequencyamplification needed for the multiplyingprocess.

Amplification, and hence the level offeedback, is controlled by potentiometerVR1, which varies the voltage on the drain(d) of transistor TR1. Bypass capacitor C1eliminates potentiometer noise.

The received signal is taken from thetuned circuit via d.c. blocking capacitor C3(biasing puts a positive voltage on theinput pin of the MK484 radio i.c.), and R2is TR1’s gate (g) bias resistor. Source (s)bias is developed across resistor R1, whichis bypassed, at radio frequencies, bycapacitor C2.

The circuit is configured as a Hartleyoscillator with feedback from the source ofTR1 being coupled to the tuned circuit bycoil L2. This is two turns of plastic insu-lated hook-up wire wound over the“earthy” end of tuning coil L1. If the feed-back coil L2 is wound in the same direc-tion as the tuning coil, the correct connec-tions for Q-enhancing positive feedbackare as shown in Fig.2.2.

With the exception of the Regeneration

control, VR1, all of the components aremounted on the smallprinted circuit boardas illustrated inFig.2.2. A full-size

copper track master is also included. Thisboard is available from the EPE PCBService, code 397.

Commence construction by mountingthe resistors and capacitors on the boardfirst and the transistor TR1 last. It is goodpractice to clip a miniature crocodile cliponto the leads of field effect devices to actas a heat shunt whilst they are being sol-dered into position.

Terminal pins, inserted at the boardlead-out points, will simplify the task ofoff-board wiring, details of which are alsogiven in Fig.2.2. Locate the multiplierp.c.b. close to the receiver's ferrite loopaerial in order to keep the leads to coil L2reasonably short; no more than two orthree inches (50mm to 75mm).

g

d

s

R7 ON RADIO P.C.B.TO 9V VIA+

FERRITE ROD

L1

L2

VR110k

C11µ

C2100n

R11k

R21M

C322p

+

TR12N3819

2N3819

ds g

C1 C2

VC1S1a

C3

THESE COMPONENTSON MK484 RADIO P.C.B.(PART 1)

Fig.2.1 (above). Circuit diagram for the simple add-onQ-Multiplier for the MK484 TRF Receiver. (Tuning coil L1was covered last month.)Fig.2.2 (right). Printed circuit board component layout, wiringand full-size copper foil master for the Q-Multiplier.

+ TUNING

A

G

'Q' MULTIPLIER

1.0in (25.4mm)

1.2i

n (3

0.5m

m)

TO 9VBATTERY+V

V

VR1

L2, TWO TURNS OF PLASTIC INSULATEDHOOKUP WIRE. ALL TURNS WOUND INTHE SAME DIRECTION.

TO PIN 2OF MK484RECEIVER I.C.

TO MK484RECEIVER(PART1)

L1

VC1

O

C1

C2R1

R2

C3

TR1

g

d

s

L2

397

Check the p.c.b. for poor soldered joints

and bridged copper tracks, and check theorientation of TR1 and electrolytic capaci-tor C1. If all is in order, connect the “hot”end of VR1 to the receiver's power supply.

Tune in a weak station on the receiverand advance VR1. Perceived signalstrength will greatly increase, and receivertuning may have to be adjusted slightlybecause of the improved selectivity.

Weak signals, formerly below the sensi-tivity threshold of the MK484 i.c., can nowbe made clearly audible. Maximum sensi-tivity and selectivity are obtained with VR1set close to the onset of oscillation, whencurrent consumption of the Q-Multiplierwill be approximately 1mA.

The unit should go into oscillation whenVR1 is approaching its maximum setting.If difficulty is encountered with receiversthat have a lower supply voltage, increasethe number of turns on L2 and/or reducethe value of resistor R1. Conversely, if theaction is too vigorous, connect a fixedresistor of, say, 10 kilohms in series withVR1 and/or increase the value of R1.

Other than the need to connect L2 in thecorrect sense for positive feedback, the cir-cuit is not critical and very easy to set up.

Problems of alignment will be encoun-

tered if this Q-Multiplier is used withsuperhet receivers. The ganged tuningstages (aerial and oscillator) of a superhetcannot be maintained in perfect alignmentover the full swing of the tuning capacitor.(When correctly adjusted, alignment is per-fect at three points on the dial).

This inherent defect is unnoticeable withbroadly tuned ferrite loop aerials of normalQ. However, loop tuning becomesextremely sharp when Q is increased, andthe misalignment is then very apparent.

Experienced constructors who have adomestic portable they are not afraid ofmodifying can overcome this by disconnect-ing the set's aerial trimmer capacitor (orturning an integral trimmer to minimum

capacitance) andsubstituting a 25pFvariable capacitoras a front panelcontrol. By thismeans the align-ment or tracking ofthe receiver can bec o n t i n u o u s l yadjusted. Somerepositioning of the coil on the receiver'sferrite rod may also be necessary.

The boost in performance given by the

add-on Q-Multiplier eliminates the needfor the single transistor headphone amplifi-er described last month. A circuit for con-necting Walkman type ’phones directly tothe radio p.c.b. is given in Fig.2.3, whereTR1 is the audio amplifier following theMK484 radio i.c. used in last month’s TRFReceiver, and VR1 is a potentiometer con-nected in place of the original collectorload resistor R5.

Details of the wiring between the receiv-er p.c.b., the potentiometer and the jacksocket are shown in Fig.2.4.


Q-MULTIPLIERResistors

R1 1kR2 1M

All 0·25 5% carbon film

PotentiometersVR1 10k rotary carbon, lin.

CapacitorsC1 1 radial elec. 16VC2 100n disc ceramicC3 22p disc ceramic

SemiconductorsTR1 2N3819 field effect transistor

MiscellaneousL1/L2 ferrite loop aerial (last month), with the addition of

two turns of plastic-covered wire – see text

Printed circuit board available from the EPE PCB Service,code 397; plastic case, size and type to choice; control knob;connecting wire; solder pins; solder etc.


excl. case & ’phones


Fig.2.3. Circuit modifications for connecting headphones to lastmonth’s TRF Receiver. The front-end Q-Multiplier must beincluded to ensure sufficient output – see text.

CONNECTED)HEADPHONES. (SHANK NOTIMPEDANCE WITH 32 OHMSOCKET TO GIVE 64 OHMSAND RING OF HEADPHONECONNECT OUTPUT TO TIP

OFF

M.W. L.W.

bc

e

B19V

TR1BC549C

BC549C

PHONES

C8100µ

C747µ

++R6

120Ω

ebc

VR14k7 SK1

C610n

C5220n

R42M2

VOLUME

POLE

S1b

INPUT

+9V

0V

R3 R2

R7

R4

C5

D2

D1C8

C6

TR1e b c

+a

a

k

VOLUME

VR1

LINK IN PLACE OF C9

+

JACK SOCKET TOSUIT HEADPHONESEARPIECES IN SERIESFOR 64Ω

TO BATTERYVIA S1BTO BATTERY VE

R5 AND C9 CAN BEDELETED IN THISAPPLICATION. (SEEPART 1 FOR CIRCUIT)

LINKR6

V

Fig.2.4. Interwiring details to enableheadphone use.

Completed prototype of the speaker version of the MK484 TRF Receiver, withQ-Multiplier and Speaker Amplifier p.c.b.s.

EPE Online

Note that the circuit boards used in EPE Online projects are available from the EPE Online Store at www.epemag.com (also note that the codes for the boards in the online store are prefixed with 7000, so a board with a code of say 256 will appear as 7000256 in the online store).

The high cost of valves in the early daysof radio led designers to contrive ways ofusing them twice, first as radio frequencyamplifiers and then, after the signal hadbeen demodulated, as amplifiers of therecovered audio frequency signal.

This technique, known as reflexing, wasadopted again when transistors were firstintroduced in the 1950s. At that time,devices with a modest specification costthe present day equivalent of £10 ($15) ormore, and there was the same incentive touse them twice.

Although the cost of transistors hasplummeted, some of the reflex designsdeveloped during the 1960s combined sim-plicity with good performance, and theyare well worth a second look.

In 1964, Sir Douglas Hall published thefirst transistor reflex circuit in whichimpedances along the signal path wereroughly matched. It was subsequentlyreworked by G. W. Short and others,including the author, and the version givenhere uses current production transistorsand components. It also incorporates asmooth and effective regeneration controland an optional circuit for preventingstrong-signal overload.

The circuit diagram for the MW Reflex

Radio is given in Fig.2.5. Coil L1 is a fer-rite loop aerial tuned by variable capacitorVC1. If desired, an additional capacitor,

made up of C1 andC2, can be switchedacross the coil by S1a totune it to Radio 4 on theLong Wave band. Thisarrangement works well inareas where the transmissioncan be received at reasonablestrength.

Coupling winding L2 matches the lowimpedance presented by the base (b) oftransistor TR1 to the tuned circuit.Potentiometer VR1 controls the gain of thecircuit at radio frequencies and acts as theQ-Multiplier or regeneration control.

Transistor TR1 functions as a groundedemitter stage at both radio and audio fre-quencies, with the amplified signals beingdeveloped across collector (c) load resistorR1.

Transistor TR2 is configured as an emit-ter follower buffer at radio frequencies, theoutput being developed across radio fre-quency (r.f.) choke L5. The relatively highimpedance at the base of TR2, in thismode, optimizes signal transfer from thecollector of TR1, whilst the low emitterimpedance is a reasonable match for detec-tor diode D1.

The demodulated signal is fed back tothe base of TR1, via coupling coil L2, forfurther amplification at audio frequency.Residual radio frequencies are removed bycapacitor C4.

Output from the collector (c) of TR1is directly coupled to the base (b) ofTR2, which functions as a common

emitteramplifier at

audio frequen-cies. Emitter bias is

provided by resistor R3which is bypassed by capacitor

C8.Audio output is developed across col-

lector load resistor R2 and is coupled to theVolume control by d.c. blocking capacitorC10. Radio frequencies are removed fromthe audio signal path by the shunting actionof capacitor C9.

Power is connected to the circuit viaswitch S1b, and a low current l.e.d., D2,with its dropping resistor R5, act as anoptional On indicator. Readers may wish toconnect the tuner and a power amplifier tothe same battery, and the necessary supplyline decoupling is provided by resistor R4and capacitor C11.

Connecting the collector of TR1 to the

“hot” end of the tuning coil L1, via capac-itors C5 and C6, provides the positive feed-back needed for the Q multiplying process.In order to keep the feedback at the correctlevel, the capacitors must have a very lowvalue, and C5 comprises about 6mm(1/4in.) lengths of plastic covered hook-upwire twisted together.

Low value ceramic capacitor C6 isplaced in series with C5 so that the twistedwires can have a reasonable length. Shuntcapacitor C7 improves the action of VR1,the Regeneration or Q-Multiplier control.


bc

e

bc

e

B19V

AUDIOOUTPUT

L51mH

R.F. CHOKEREGEN.

VOL.

M.W.L.W.

OFF

C1 AND C2 COMBINE TO GIVEA NON-STANDARD CAPACITORVALUE (1000p + 100p = 1100p)REQUIRED TO TUNE L1 TO BBCRADIO 4 ON LONG WAVE BAND(198KHz). SEE TEXT

'TWISTED WIRE'CAPACITOR, C5TUNING

M.W. L.W.

OFF

C3

L3

L4

L3/L4RWR 331208NO

OPTIONAL WAVE TRAP. ONLYONLY REQUIRED IF RECEIVERIS SWAMPED BY A POWERFULLOCAL TRANSMITTER. (SEE TEXT)

L1

L2

C11000p

C2100p

C62p2

R14k7

R24k7

TR1BC549C

BC549CTR2

C72p2

C410n

D1OA47

VR14k7

ak+

+

C847µ

R3680Ω

C910n

+C101µ

VR24k7

C11100µ

R53k9

D2

a

k

R4100Ω

akOA47

BC549C

ebc

2mAL.E.D.

ka

FLAT

S1a

POLE

S1b

POLE

VC15p TO140p

(SEE TEXT)

Fig.2.5. Circuit diagram for the MW Reflex Radio. Provision is provided for one preset longwave station – BBC Radio 4.

Since the 1960s, often powerful local

stations have proliferated on the mediumwave band and they tend to swamp thesesimple receivers. (The author lives almostin the shadow of a transmitter mast, and theradiated energy is strong enough to badlyoverload domestic superhets.)

Other readers may be affected in thisway, and the simple tuned circuit formedby L3 and C3 is included to attenuate theoffending signal. The device is known as aWave Trap.

The tuned circuit takes more than oneform and the principles are discussed later.The value of trap tuning capacitor C3 mustbe selected so that the adjustable core ofthe coil can be set to the frequency of theoffending station. Table 2.1 gives a rangeof values for the medium wave band.

Coupling winding L4 matches the highimpedance trap to the low impedancecircuit formed by L2, VR1 and thebase/emitter junction of TR1.

Most of the components for the MW

Reflex Radio are mounted on a smallprinted circuit board. The component sideof the board and the off-board wiringtogether with a full-size copper track mas-ter are illustrated in Fig.2.6. The board isavailable from the EPE PCB Service,code 398.

Solder the resistors and r.f. choke L5 inplace first, then the capacitors and, finally,the semiconductors. Germanium diodescan be damaged by excessive heat duringsoldering, and the leads of diode D1should, therefore, be long enough topermit the attachment of a miniaturecrocodile clip to act as a heat shunt. Thedropping resistor for the optional on/offindicator l.e.d., D2, is not mounted on thep.c.b.

With very compact receivers, it is possi-ble for radio frequency choke L5 to inter-act with the ferrite loop, L1, and produceunwanted feedback (some designs use thisas a means of providing preset regenera-tion). Assuming that the p.c.b. will bemounted in the same plane as the loop aer-ial (it invariably is), the choke should bemounted vertically on the board to avoidthis problem. Miniature radio frequencychokes look like, and are colour coded inthe same way as, resistors, the code givingthe value in microhenries (1000H =1mH).


Table 2.1: Wave Wrap Tuning Ranges with StandardValue Capacitors

Capacitor Frequency (kHz) Frequency (kHz)Value pF (core fully In) (core fully Out)

33 1300 170047 1100 140068 900 120082 820 1100

100 740 1000120 680 900150 600 800180 550 700220 500 650

Notes:(1) The above tuning ranges are obtained with a Toko type

RWR331208 tuning coil(2) Use polystyrene or “low k” ceramic capacitors

Interior of the headphone version of the MW Reflex Radioshowing the wave trap, receiver and headhone amp p.c.b.s.

S1a

S1b

L3/L4

C1

TR1

TR2

e

e

b

b

c

c

+

+

+C8

C10

C11

C1

C2

C5

L5

SCREEN THESE LEADS IFLONGER THAN 75mm (3in)

AUDIO OUTPUT'LIVE' LEAD

AUDIO OUTPUT'EARTHY' LEAD

L2 L1

VC1

TUNING

A

G

O

VR1

'Q' MULTIPLIER

VOLUME

R5

VR2

SECURE ENDS OFWINDINGS WITHSTRIPS OF MASKINGTAPE

WAVE TRAPP.C.B. (SEETEXT)

D2

L.E.D.

FLAT

R1

R3

R2

R4

C4

D1

C9

C7

C6a

a

k

k

2.0in (50.8mm)

1.6i

n (4

0.6m

m)

TOBATTERY

+V

V

398

Fig.2.6. Printed circuit board component layout, interwiring and full-size copper foilmaster for the MW Reflex Radio. Ferrite loop aerial construction was covered inPart 1. The Wave Trap is only needed where a local transmitter swamps the receiv-er. Connect VR1 directly across L2 when the trap is not wanted.

FERRITE LOOP AERIALL1 – 100 turns 26s.w.g. (25a.w.g.) on 13mm

(½in.) dia. former around 9mm (3/8in.) x100mm (4in.) long ferrite rod. Formerthin card wound around rod and glued.

L2 – 10 turns 26s.w.g. on strip of cardwrapped around “earthy” end of L1. Allturns wound in same direction.


Again, solder pins inserted at the lead-out points will simplify off-board wiring.Readers who do not require the Wave Trapshould, of course, connect coupling coil L2directly to the “hot” end of VR1.

The loop aerial is the same as the one

used in the MK484 TRF Receiverdescribed last month, but with the additionof a ten turn base coupling winding, L2.

A strip of card or masking tape, woundover the “earthy” or start end of L1, willmake it easier to add the coupling coil. Thebase winding of L2 must be connected inthe correct sense to ensure positive feed-back, and full details are given in Fig.2.6.

Check the completed p.c.b. for poor sol-

dered joints and bridged tracks, check theorientation of the semiconductors and elec-trolytic capacitors, and make sure the off-board wiring has been correctly routed.

If all is in order, connect the board to a9V battery. Current consumption, withoutl.e.d. D2, should be around 3mA.

Connect the Reflex Radio p.c.b. to theHeadphone Amplifier described last monthor the Speaker Amplifier described later.Switch on and tune in a weak signal at thelow frequency end of the medium waveband and advance Regen. control VR1.

Signal strength should increase dramati-cally. If it does not, reverse the connectionsto L2. The circuit should begin to oscillate (ahiss in the speaker) when VR1 is close to itsmaximum setting. If the Q-Multiplier actionis too fierce, untwist capacitor C5 a little.

If the Wave Trap (L3/L4 and C3) optionhas been fitted, tune the receiver to theoffending station, then, with a plastic trim-ming tool, adjust the core of L3/L4 until itsperceived strength has been reduced asmuch as possible.

If provision has been made for receptionof BBC Radio 4 on long waves, switch incapacitors C1 and C2 and peak the signalwith tuning capacitor VC1.

Simple receivers are swamped by pow-

erful signals, which can also degrade theperformance of complex communicationsequipment. An old, but effective, solutionis to attenuate the offending signal before itgets into the receiver. If this is not practica-ble (e.g., sets with loop aerials), then itshould be attenuated before it reaches theamplifier stages.

This single-frequency attenuator is calleda Wave Trap. It is no more than a tuned cir-cuit resonant at the frequency of the signalto be blocked out or shunted to ground.

RejectionThe most common arrangement is

shown in Fig.2.7a. Here the parallel tunedcircuit, connected in the aerial lead, pre-sents a high impedance to the unwantedsignal and heavily attenuates it. Thisarrangement is very effective when theinput impedance of the receiver is less than1000 ohms (most are either 50 ohms oraround 600 ohms).

A parallel tuned trap can be inserted in aloop aerial coupling circuit if an imped-ance matching winding is provided. This

REFLEX RADIOResistors

R1, R2, 4k7 (2 off)R3 680R4 100R5 3k9


PotentiometersVR1 4k7 rotary carbon, lin.VR2 4k7 rotary carbon, log.

CapacitorsC1 1000p polystyrene

(see text)C2 100p polystyrene

(see text)C3 see Table 2.1, only

required if using Wave Trap

C4, C9 10n disc ceramic(2 off)C5 twisted wires (see text)C6, C7 2p2 disc ceramic (2 off)C8 47 radial elect. 16VC10 1 radial elect. 16VC11 100 radial elect 16VVC1 5p to 140p (minimum),

polythene dielectricvariable capacitor

SemiconductorsD1 OA47 germanium diodeD2 low current (2mA) l.e.d.TR1, TR2 BC549C npn small

signal transistors(2 off)

MiscellaneousL1/L2 ferrite loop aerial: 100mm

(4in.) x 9mm/10mm3/8in.) dia. ferrite rodwith coil (see text)

L3/L4 tuning coil, Toko RWR331208 (only requiredif Wave Trap fitted)

S1 d.p.d.t. centre-off toggleswitch (see text)

Printed circuit board available from theEPE PCB Service, code 398; plasticcase, size and type to choice plastic con-trol knob (3 off); 50g (2oz) reel of26s.w.g. (25a.w.g.) enamelled copperwire, for tuning coil; card and glue for coilformer; l.e.d. holder; connecting wire; 9Vbattery and clip; stand-off pillars; solderpins; solder etc.


excl. case, wire & batt.


Completed speaker version of the MW Reflex Radio.

Author’s head-phone version ofthe MW ReflexRadio.

Interior layout of the Reflex Radio built by a friend (Dr. P. O’Horan) of the author.Note the air-spaced tuning capacitor, trimmer capacitor in place of twisted wires,and mods. to amplifier p.c.b. This set worked “first time” and is claimed to be asgood as many small superhet sets.

EPE Online


has already been described in connectionwith the MW Reflex Receiver project, andthe basic circuit is repeated in Fig.2.7b.Without the matching winding, L2, the lowimpedance presented by the loop couplingcircuitry would appear directly across L1and reduce its Q to a useless level.

AcceptanceIt will be recalled (see Part 1 – June ’03)

that a capacitor and inductor in series pre-sent a low impedance at resonance and amuch higher impedance at other frequen-cies. The circuit is shown in Fig.2.7c.

In the past, this type of trap was oftenconnected between the aerial and earth ter-minals of receivers with a high inputimpedance where it exhibited a shuntingeffect at the unwanted frequency. (Somevalve era receivers had aerial couplingwindings that were self-resonant within themedium wave band, and this resulted in thehigh input impedance). It is occasionallyencountered in superhet receivers designedaround integrated circuits, where it is usedto shunt the intermediate frequency.

The trap tuning inductor should, prefer-

ably, be screened to prevent re-radiation ofthe unwanted signal within the receiver. Amodern Toko coil is suitable, and details ofsuppliers are given in the Shoptalk column.

Capacitors should, preferably, have apolystyrene dielectric but “low k” ceramicplate components can be used. Differentfrequency ranges, within the medium waveband, for various standard capacitor values,are given in Table 2.1.

A universal printed circuit board, which

can accommodate any of the trap configu-rations described here, is illustrated along-side Figs.2.7a to 2.7c. The copper trackside master of the board is also includedThis board is available from the EPE PCBService, code 399.

Mount the board as close as possible tothe “trapping point” to keep connectingleads as short as possible. Aerial trapsshould be located inside any metal chassis,alongside the aerial terminal.

Tune in the offending station and, using

a plastic trimming tool, adjust the “trap”tuning coil core until maximum attenuationis achieved.

Traps of this kind work well on the longand medium wave bands where swampingproblems are usually encountered. As fre-quency increases, trap bandwidth widens,and the circuit is no longer capable ofsingle-signal attenuation.

Many readers will require a loudspeakeroutput from the various receivers describedin the series. A suitable power amplifiercan be built using the Philips TDA7052integrated circuit.

The designers of the i.c. adopted aninternal bridge configuration for the outputstage, and the device will deliver reason-able power at low supply voltages (450mWwith a 4·5V, and 1W with a 9V, supply intoan 8 ohm speaker). It also eliminates theneed for a speaker coupling capacitor. Theexternal component count is minimal; justtwo capacitors.

The circuit diagram for a simpleTDA7052 Speaker Amplifier is shown inFig.2.8, where bypass capacitors C1 and

C2 ensure stability at radio and audio fre-quencies. The input pin (IC1 pin 2) isconnected to the signal source via a poten-tiometer (Volume control) and a d.c. block-ing capacitor. These components areincluded with the radio receiver circuits,but are shown again here in the interests ofclarity. This input arrangement must alsobe adopted if the amplifier circuit is to beused with other equipment.

The printed circuit board component

layout, wiring and full-size copper trackmaster for the Speaker Amplifier is illus-trated in Fig.2.9. This board is also avail-able from the EPE PCB Service, code 400.


1.0in (25.4mm)

0.9i

n (2

2.9m

m)

L1

L2

C1

L1L2

C1

COIL SCREENING CANTO RECEIVER EARTHOR 0V RAIL.



TO AERIALINPUT ONRECEIVER

TO AERIAL

L1/L2

L1/L2

L1/L2

TO LOW IMPEDANCECIRCUIT IN RECEIVER

COUPLING WINDING L2PREVENTS THE L1/C1TUNED CIRCUIT FROMBEING DAMPED BYTHE LOW IMPEDANCECIRCUIT IN THERECEIVERCOUPLING

WINDINGCONNECTED IN SERIES WITHLOW IMPEDANCE CIRCUIT INTHE RECEIVER

CONNECT ACROSS HIGHIMPEDANCE CIRCUIT IN

RECEIVER

ACCEPTOR CIRCUIT

LOW IMPEDANCE ATRESONANT FREQUENCYOF L1/C1 SHUNTSUNWANTED SIGNALTO GROUND TO HIGH IMPEDANCE

CIRCUIT IN RECEIVER

A)

B)

C)

C1

C1

C1

L1L2 C1

NOTUSED

NOTUSED

TO AERIAL INPUTON RECEIVER

AERIAL

REJECTOR CIRCUIT

REJECTOR CIRCUIT

HIGH IMPEDANCE ATRESONANT FREQUENCYOF L1/CI BLOCKS ENTRYOF UNWANTED SIGNALINTO RECEIVER

399

OptionalWave Trap

circuit board.

FOR MEDIUM WAVES, L1/L2 IS A TOKO RWR 331208 TYPE TUNINGCOIL. SEE TABLE 2.1 FOR CAPACITOR C1 VALUES.

Fig.2.7. Various Wave Trap circuit arrangements to prevent “swamping” of receiversby powerful, usually local, signals. Wiring details and a full-size printed circuit boardare also illustrated.

An 8-pin d.i.l. socket is recommendedfor IC1 as this makes substitution checkingeasy. Again, solder pins at the lead outpoints are a help when carrying out the off-board wiring.

On completion, check the board forbridged tracks and poor soldered joints,and check the orientation of the i.c. and theelectrolytic capacitor.

If all is in order, connect an 8 ohmspeaker, the volume control potentiometer,and a 9V battery. With the volume controlturned to minimum, current consumptionshould be approximately 5mA. With agood output from the speaker, current drainwill increase to approximately 100mA.

The noise level of the amplifier is

extremely low and its output is free fromaudible distortion up to the power levelsquoted earlier. With a sensitivity of 40mVr.m.s., it can easily be driven to full outputby any of the receivers in this series (otherthan last month’s Crystal Set).

Output short circuit protection is builtinto the chip, which shuts down whendissipation becomes excessive. It will not,however, withstand prolonged abuse, espe-cially with higher supply voltages.

Current consumption for a given poweroutput, although acceptable, is almosttwice that of other i.c.s which have morecomplex external circuitry; e.g., the

LM386N-1 and the TBA820M. (The May2002 edition of EPE contained full circuitdetails of these and other amplifiers).

Most of the components for the circuits

in Part Two are widely available and no dif-ficulty should be encountered in obtainingthem. The semiconductors are not particu-larly critical. Most n-channel junction f.e.t.sshould work in the add-on Q-Multiplier.

Similarly, most high gain (hfe at least400) small signal npn transistors should besuitable for the Reflex Radio. The BC239C,

BC547, 2N2926 and 2N3711 have beentried and found acceptable. A germaniumpoint contact signal diode must be used(small signal silicon diodes are not suit-able), but the actual type is not critical andan OA90 or OA91 could be substituted.

Semiconductor base connections varyand must be checked.

Next month's article will describe highperformance regenerative receivers for bothgeneral coverage (150kHz to 30MHz) andthe amateur bands. We also hope, spacepermitting, to take a close look at coils,variable capacitors and tuning systems.


SPEAKER AMPLIFIERCapacitors

C1 100n disc ceramicC2 220 radial elect. 16V

SemiconductorsIC1 TDA7052 (Philips) power amp

MiscellaneousLS1 8 ohm miniature loudspeaker (see text)

Printed circuit board available from the EPE PCB Service,code 400; 8-pin d.i.l. socket; multistrand connecting wire; twin-core screened audio cable, if leads to Volume control will belonger than 75mm (3in.); solder pins; solder etc.


excl. speaker


INSIGNAL

1µ

220µ

C10

C2

4k7VR2

CLARITYREPEATED HERE FORCIRCUITS. THEY ARE ONLYINCLUDED WITH THE RADIOAND VOLUME CONTROL ARETHE BLOCKING CAPACITOR 8Ω

LS1TDA7052

IC1

1

6

3

2 5

8

+

+

C1100n

+ +3V TO 9V

0V

1

2

3

4 5

6

7

8SUPPLY VOLTAGE VE+

INPUT

INPUT GROUND

N.C.

SIGNAL OUTPUT

SIGNAL OUTPUT

N.C.

OUTPUT GROUND

TOP VIEW OF TDA7052

1.2in (30.5mm)

1.6i

n (4

0.6m

m)

IC1

C1

C2

++

SIGNAL INPUTVIA BLOCKINGCAPACITOR

VOLUMECONTROL

SCREEN LEADS TOVOLUME CONTROLIF MORE THAN 3in(75mm) LONG

TO 8LOUDSPEAKER

Ω

TO BATTERY400

Fig.2.8. Circuit diagram for the Speaker Amplifier.

Fig.2.9. Speaker Amplifier printed circuit board component layout, wiring and full-size copper foil master. Note the Volume control pot. is included in the radio circuits.

TO SUPPLY

TAKE A LOOK,A FREE ISSUEIS AVAILABLE

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costs just $10.99 (US)

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The amplified audio signal is devel-oped across drain load resistor R2 andcoupled to the audio amplifier stages viad.c. blocking capacitor C3. Residualradio frequencies are bypassed bycapacitor C4.

In some versions of the circuit, a transis-tor type audio transformer (e.g., an LT44)is substituted for the resistive load in orderto maximize signal transfer. However, evenwith this arrangement, the audio output isvery low, and at least one additional stageof pre-amplification will be required ahead

Further, because tuned circuit Q is raised,there is also a dramatic improvement inselectivity.

Source bias is developed across resis-tor R3, which is bypassed by capacitorC2, and signal detection occurs by wayof rectification at the gate/source junc-tion of transistor TR1. (In the originalvalve versions, diode rectificationbetween grid and cathode resulted indetection or demodulation). Low valueblocking capacitor C1 prevents theaudio voltage developed across gateresistor, R1, being shorted by the tuningcoil.

IN Part Two we reviewed the history andtheory of regeneration and used thetechnique to improve the performance

of two, portable, radio receivers. Thismonth a regenerative receiver designed forserious listening on the long, medium andshort wave bands will be described.

For a regenerative receiver to perform

well, three basic requirements have to bemet. (1) Its regeneration control must besmooth, completely free from backlash,and have a minimal effect on tuning.

(2) The tuned circuit to which the Qenhancing positive feedback is appliedmust be isolated from the aerial: failure todo this will result in reaction dead spots onfrequencies (or harmonics) at which theaerial is resonant.

(3) Provision must be made, at thereceiver input, for the attenuation of pow-erful signals, otherwise the regenerativecircuit will lock onto strong carriers and itwill not be possible to receive weak trans-missions on adjacent channels.

A modern, transistor version of

Armstrong’s circuit, patented in 1913, isshown in Fig.3.1. In the original, the feed-back coil (tickler coil in the USA), L3, wasconnected in place of the radio frequencychoke, L4, and regeneration controlled byadjusting its proximity to tuning coil L2.This was eventually superceded by thevariable capacitor system shown here,where increasing the capacitance of VC1increases feedback through L3.

Choke L4, in the drain (d) circuit ofTR1, acts as a load for the radio frequencycomponent of the amplified signal, con-trolled amounts of which are fed backthrough L3 to overcome losses in the tunedcircuit (L2/VC2) and increase its efficien-cy or Q factor.

Armstrong discovered that the tech-nique permits the amplification of weaksignals by a factor of more than 1000.

568 Everyday Practical Electronics, August 2003

Part 3: Regenerative Receivers – A modern-day version ofArmstrong’s 1913 circuit.

g

d

s

AERIAL

EARTH

TUNING

REGEN. OUTPUTAUDIO

+

+

VC150p

VC2200p

L1

L2

L3

C122p

R11M

C2100n

R32k7

2N3819TR1

C34 7µ

C41n

R422k

C547µ

L41mHR.F.

CHOKE

R24k7

VR147k

+9V

0V

Fig.3.1. Circuit diagram for a modern transistor version of Armstrong’sRegenerative Detector. In Armstrong’s circuit, patented in 1913, feedback coil L3 isconnected in place of r.f. choke L4 and its proximity to L2 is adjusted to controlregeneration. VR1 presets regeneration control range.

of the headphone or speaker amplifiersdescribed in Parts 1 and 2.

The gain of TR1, and hence its willing-ness to regenerate, is determined by presetpotentiometer VR1, which adjusts thedrain voltage. By this means the circuit canbe optimized for different transistors andcoil feedback winding ratios.

Using gradually improving versions of

Lee de Forest’s triode valve as the amplify-ing device, Armstrong’s circuit, followedby a one or two valve audio amplifier,remained popular as a domestic receiveruntil the end of the 1920s in the USA andwell into the 1930s in the UK.Regenerative receivers were still beingmanufactured by Ever-Ready (their ModelH) as late as the 1950s, and they were con-structed by amateurs up to the close of thevalve era.

It is not easy to use low impedance, cur-rent amplifying bi-polar transistors in thiscircuit. However, it saw something of arevival in amateur circles following theintroduction of the field effect transistor(f.e.t.), with its more valve-like characteris-tics, in the late 1960s.

Smooth regeneration can be obtained

more readily, and with simpler coils, byconfiguring the Q multiplier, or regenera-tive detector, as a Hartley oscillator. Atypical circuit is given in Fig.3.2, wherefeedback from the source(s) of the dual-gate MOSFET, TR1, is connected to atapping on the tuning coil L2.

The level of feedback is controlled byVR1, which varies the gain of the transistorby adjusting the voltage on its gate g2.Preset potentiometer VR2 (wired as a vari-able resistor) determines the source biasand optimizes the action of the regenera-tion control for individual tuning coils andtransistors.

Audio output is developed across drainload resistor R3. The stage is decoupledfrom the supply rail by resistor R2 andcapacitor C5, and the filter network formedby C4, R4 and C6 removes radio frequen-cies from the output.

During the valve era, the functions of

signal detection and Q multiplication orregeneration were invariably carried outby a single device. This combining offunctions can make it more difficult toobtain the smooth, backlash-free controlof regeneration which is crucial to theefficient operation of a receiver of thiskind. Best modern practice uses separatetransistors.

The dual-gate MOSFET circuit illus-trated in Fig.3.2 can be used just as a Q-multiplier by increasing the value ofcapacitor C4 to 100nF. Filter components,R4, C6 and coupling capacitor C7, can beomitted when the stage is configured inthis way.

The “hot” end of the tuned circuit must,of course, be connected to gate g1 of thetransistor, and resistor R1 is best retainedto hold gate g1 at 0V during coil changing.

A field effect transistor (f.e.t.), biased

into the non-linear region of its

characteristic curve, forms an excellentdetector stage. The “drain bend” version(the transistor equivalent of the valve“anode bend” detector) is included asTR3 in the Regenerative Receiver designillustrated in Fig.3.4. This arrangementis discussed later.

Alternatively, the audio output can betaken from the source of the f.e.t. We thenhave the transistor equivalent of the valve“infinite impedance” detector. The modi-fied circuit, using the component number-ing of Fig.3.4 for ease of comparison, isshown in Fig.3.3.

High-value source bias resistor R9 isbypassed only at radio frequencies bycapacitor C10 (C9 is omitted), and C13 isincreased to 47F to decouple the stagewhich is now in thecommon drain mode.The r.f. filter compo-nents, R8 and C15, andthe original decouplers,R6 and C11, are notrequired.

There is little tochoose between the twodetectors: both workwell, imposing very lit-tle damping on the tunedcircuit. In theory there issome gain with the drainbend version whilst thegain of the source fol-lower is slightly lessthan unity.

In practice, the needto ensure non-linearityover a range of f.e.t.characteristics results inthe drain bend circuitproviding very littlegain. If the value of r.f.bypass capacitor C10 inFig.3.3 is reduced, thesource-follower detectormay become unstablewhen the regenerationcontrol is critically set.

Everyday Practical Electronics, August 2003 569

Fig.3.2. Circuit diagram for an improved Regenerative Detector based on a Hartleyoscillator. VR2 presets regeneration control range. This circuit forms an excellentQ-Multiplier for use with a separate detector, in which case increase capacitor C4value to 100nF and delete C6, C7 and R4.

g

d

s

AUDIOOUTPUT

TO 'HOT' ENDOF TUNING

COIL

R710k

R922k

C1347µ

C10100n

C144 7µ

TR32N3819

+

+

+9V

0V

Fig.3.3. Source-follower detector cir-cuit. Transistor equivalent of the valve“anode-bend” detector.

The full circuit diagram for a HighPerformance Regenerative Radio incorpo-rating the essential features described hereis given in Fig.3.4. It is easy to set up andperforms well.

Grounded base stage, TR1, isolates thetuned circuit L2/VC1 from the aerial andTR2 functions as the Q-Multiplier. Fieldeffect transistor TR3 is a drain bend detectorand transistor TR4 an audio preamplifier.

Although excellent Q multipliers canbe designed around dual-gate MOSFETS(metal-oxide semiconductor field effecttransistors), devices of this kind arebecoming more difficult to obtain. Forthis reason a j.f.e.t. (junction field effecttransistor) is used in the Q multiplierstage.

Performance is not compromised andthese simpler devices are widely available.The circuit in Fig.3.2 should assist anyreaders who might wish to experiment withdual-gate MOSFETS as an alterative.

Some readers may be plagued by a

medium wave transmission which is pow-erful enough to swamp the receiver, and L1and C1 act as a Wave Trap, blocking outthe offending signal. Wave trap circuitswere discussed in Part 2 last month, andcomponent values and a printed circuitboard design were also given.

Potentiometer VR1 connected as theemitter resistor for transistor TR1, controlssignal input, and resistors R2 and R3 fixthe base bias. The base of TR1 is “ground-ed” at radio frequencies by capacitor C5;and R1, C3 and C4 decouple the stage fromthe supply. Blocking capacitor, C2, pre-vents the grounding of TR1 emitter whenaerials are connected to the receiver via abalun transformer and coaxial cable.

The grounded base configuration resultsin a low input impedance and a high outputimpedance, and the stage can be coupleddirectly to the tuned circuit without impos-ing excessive damping. Because TR1 is apnp transistor, its collector (c) can be con-nected to the 0V rail via coil L2, eliminat-ing the need for a coupling winding.

Stability is ensured by stopper resistor

R4 and by maintaining a low level of basebias on TR1. Constructors may wish to tryreducing the value of resistor R3 (not lessthan 47 kilohms) to improve performancewhen low gain transistors are used in theaerial input circuit.

However, if this is overdone the stagewill no longer be unconditionally stable,

and control of regenerationwill become erratic, especiallywhen tuning capacitor VC1 is set ata low value.

It could be argued that using a fieldeffect transistor, with its near square lawcharacteristics, in the TR1 position, wouldreduce the receiver’s susceptibility tocross-modulation.

Cross-modulation occurs when a powerfulsignal drives the input stage into non-lineari-ty. It then begins to function as a modulator,imposing the strong signal on adjacent weak-er signals and spreading it across the dial.

The regenerative circuit of Fig.3.4 willlock onto a powerful signal long before it isstrong enough to make TR1 non-linear.The measures taken to avoid this (wavetrap and input attenuator) will, therefore,also prevent cross-modulation. Moreover,p-channel field effect transistors are not sowidely available, and this militates againsttheir use.

The simplest possible tuning arrange-

ment is depicted in Fig.3.4. and the hand-

wound, short wave coil L2 is illustrated inFig.3.6.

The tuning capacitor VC1 is a 10pF to260pF unit formed by connecting both a.m.gangs of a polyvaricon (polythene dielec-tric) capacitor in parallel. Typical connec-tion details are shown in Fig.3.7, and thecopper track side of a printed circuit boardsuitable for mounting most screw or tagfixed variable capacitors of this kind is alsoshown.

Regeneration, or Q multiplication, is

provided by TR2, a field effect transistorconfigured as a Hartley oscillator.Feedback is taken from TR2 source (s) to atapping on coil L2 via preset potentiometerVR2 and its bypass capacitor C6. Thisarrangement enables the control of regen-eration to be optimized for different coiland transistor combinations.


b

ce

g

d

s

TUNING

AERIAL

EARTH

C210n

L1 C1

WAVE TRAPSEE TEXT

VR11k

C3100µ

C4100n

C5100n

R447Ω

+

++

R3100k

R28k2

R1150Ω

TR1BC557

L2

VC110p TO 260p

R51M

VR247k

C61n

2N3819TR2

VR44k7

VR54k7

C8100µ

C710µ

VR34k7

SEE TEXT AND FIG. 3.6FOR DETAILS OF L2

ebc dgs

BC557BC549C

2N3819

INPUTATTEN.

REGEN

Feedback is adjusted by regenerationcontrol VR4, which varies the voltage onthe drain (d) of TR2 thereby altering itsgain. The range of adjustment is fixed bypresets VR3 and VR5, and the action of theregeneration control can be made very gen-tle and smooth when the swing of VC1 isnot too great. Potentiometer noise is elimi-nated by capacitor C7.

Drain bend detector, TR3, is biased intonon-linearity by resistor R9, which isbypassed at audio and radio frequencies bycapacitors C9 and C10. Audio output isdeveloped across drain load resistor R7,and R6 and C11 decouple the stage from

the supply rail. Residual radio frequenciesare filtered out by R8, C13 and C15, andthe signal is coupled to transistor TR4through d.c. blocking capacitor C14.

Audio preamplifier stage, TR4, is neces-sary in order to boost the weakest signals.Emitter bias is provided by resistor R13,which is bypassed by C16; R10 is the basebias resistor, and R12 is TR4’s collectorload.

Supply rail decoupling is effected byR11 and C12. The output signal is takenfrom TR4 collector, via blocking capacitorC17, and the audio output level is set byVolume control VR6.

Speech signals, especially when they are

overlaid by noise, can be greatly clarifiedby reducing the response of the system tolow and high audio frequencies. Telephonecompanies throughout the world operateon this principle, and heavily attenuate fre-quencies below 300Hz and above 3000Hz(3kHz). Narrowing the response leavesspeech intelligible while removing parts ofthe spectrum that carry a good deal of thenoise.

The values of the capacitors in the receiv-er’s audio signal path, i.e. from the collectorof TR3 onwards, can be chosen to tailor the


REGENERATIVE RADIOResistors

R1, R6, R11 150 (3 off)R2 8k2R3 100kR4 47

R5, R10 1M (2 off)R7 10kR8, R13 470 (2 off)R9 22kR12 4k7


PotentiometersVR1 1k rotary carbon, lin.VR2 47k carbon preset, horizontalVR3, VR5 4k7 carbon preset, horizontal (2 off)VR4 4k7 rotary carbon, lin.VR6 4k7 rotary carbon, log.

CapacitorsC1 only required if Wave Trap fitted

(see Part 1)C2 10n disc ceramicC3, C8,

C11, C12 100 radial elect. 25V (4 off)C4, C5,

C10 100n disc ceramic (3 off)


excl. case & batt. & wire


C6 1n polyesterC7, C9 10 radial elect. (2 off)C13, C15 1n polyester or ceramic – see text and

Table 3.1 (2 off)C14, C17 47 radial elect. – see text and Table 3.1 (2 off)C16 47 radial elect. – see text and Table 3.1C18 10n polyester or ceramic – see text and Table 3.1VC1 10p to 260p polythene dielectric variable capacitor

SemiconductorsTR1 BC557 pnp small signal transistorTR2, TR3 2N3819 n-channel field effect transistor (2 off)TR4 BC549C npn silicon transistor

MiscellaneousL1 only required if Wave Trap fitted (see Part 1)L2 tuning coil, hand-wound (see Fig.3.6)S1 d.p.s.t. toggle switch

Printed circuit boards available from the EPE PCB Service, codes405 (Regen) and 406 (T/Cap); diecast or aluminium box for chassis,at least 200mm x 150mm x 75mm (8in. x 6in. × 3in.), or aluminiumsheet to fabricate a base and front panel; aerial and earth screw ter-minals; one large and three small plastic control knobs; audio typescreened leads; 50g (2oz) reel of 24s.w.g. (23a.w.g.) enamelledcopper wire for tuning coil; plastic tube, 20mm (3/4in.) outside diam-eter (o/d) for coil former; 9V battery (PP3) and clip; connecting wire;nuts, bolts and washers; solder pins; solder etc.

bc

e

g

d

s B19V

ON/OFF

OUTPUTAUDIO

+

+

+

+

+

+

2N3819TR3

C910µ

C10100n

R922k

C151n

C144 7µ

R6150Ω

R11150Ω

R710k

R8470Ω

R101M

C11100µ

C12100µ

C131n R12

4k7

C1647µ

R13470Ω

VR64k7

LOG.

C1810n

C174 7µBC549C

TR4*

* *

*

* *

*

SEE TABLE 3.1FOR ALTERNATIVE

CAPACITOR VALUES

S1a

(0V)

VOLUME

SCREEN

Fig.3.4. Complete circuit diagram for the High PerformanceRegenerative Radio. See Table 3.1 for alternative capacitor values.Note the Wave Trap is optional – see text and Part 2 (July ’03).

Table 3.1. Capacitor Values for a Wideor Narrow Audio Frequency Response

Part Wide NarrowNo. Response Response

C13 1n 10n

C14 47 100n

C15 1n 10n

C16 47 47

C17 47 100n

C18 10n 330n

audio response. Increasing the value ofshunt capacitors C13, C15 and C18, willreduce response to high frequencies.

Reducing the value of coupling capaci-tors, C14 and C17, will attenuate low fre-quencies. Reducing the value of bypasscapacitor C16 introduces selective negativefeedback which also inhibits response atthe lower audio frequencies.

Suggested alternative values for thesecapacitors are given in Table 3.1. Readerswill no doubt wish to experiment until theaudio response meets their needs.

Even small power amplifiers induce

large voltage variations in the supply rail,and the four transistors in this circuit musthave their own battery supply (or a supplyisolated by an electronic regulator andample smoothing).

Voltage fluctuations on a common supplywill cause erratic regeneration, problemswith electronic tuning systems (describednext month) and low frequency oscillationor “motor boating”. The receiver battery isswitched by S1a. The other half of the tog-gle switch, S1b, can be used to control thesupply to the Speaker Amplifier (describedlast month) or other audio amplifiers.

The pnp, bipolar transistor used as the

r.f. amplifier, TR1, is not particularly criti-cal. Any small signal device with an fT ofat least 100MHz and an Hfe of 200 or moreshould perform well. The audio preamplifi-er, TR4, can be almost any small signal npnsilicon transistor, but low-noise, high gain(Hfe at least 400) devices are to bepreferred.

Most n-channel field effect transistorsshould function in the detector (TR2) andQ multiplier (TR3) stages. In addition tothe specified 2N3819’s, the BF244A,BF245B, J304, J310, TIS14, K168D andMPF102 have all been “in circuit” testedand found to be satisfactory.

Note that base connections for all ofthese devices vary and should be checked.

Most of the receiver components are

assembled on a compact printed circuitboard (p.c.b.). The topside component lay-out, together with the full-size undersidefoil master pattern and off-board wiring areillustrated in Fig.3.5. This board is avail-able from the EPE PCB Service, code 405(Regen).

The tuning coil L2 and variable capaci-tor VC1 are mounted separately. This givesgreater freedom in the choice of tuningarrangements. A small p.c.b. which willtake most miniature screw or tag fixingpoythene dielectric variable capacitors isshown in Fig.3.7. This board is also obtain-able from the EPE PCB Service, code 406(T/Cap).

Solder pins inserted at the lead-outpoints ease the task of off-board wiring.They should be inserted into the printedcircuit board first. Follow these with theresistors, then the capacitors, smallest first;and, finally, the semiconductors. It is goodpractice to use a miniature crocodile clip asa heat shunt whilst soldering the fieldeffect transistors in place.

On completion, the p.c.b. should beexamined for poor soldered joints and

bridged tracks, and the orientation of semi-conductors and electrolytic capacitorsshould also be checked.

It is a good idea to wire the printed cir-cuit board to the controls and tuning com-ponents on the work bench, and test itbefore mounting it in an enclosure. Currentconsumption of the receiver should be inthe region of 4mA.

Details of the tuning coil L2 are given

in Fig.3.6. It is wound on an off-cut of20mm (3/4in.) outside diameter plasticelectrical conduit and preset VR2 and itsbypass capacitor C6 are located at oneend of the former. Solder tags are used toanchor the windings and the presetpotentiometer.

Plastic electrical conduit for the hand-wound coil is retailed at most DIY outlets.Suppliers of enamelled copper wire andtuning capacitors are mentioned in theShoptalk column. The remaining compo-nents are widely available.

The specified variable capacitor (VC1)will tune coil L2 from 4·8MHz to14·6MHz. This covers the 20, 30 and 40metre amateur bands, and the 25, 31, 41,and 49 metre broadcast bands.

Details of hand-wound coils covering150kHz to 30MHz will be described nextmonth, together with switched coil packs,incorporating commercial coils, for gener-al coverage and amateur bands receivers.

Constructors who like to experimentwith their own coils should tap longwaveinductors at 5 per cent of the total turns,and all other coils at 10 per cent. The short-wave coil covering up to 30MHz mayrequire a 15 per cent tapping point tosecure regeneration when the tuning capac-itor is set at maximum. Commercial coilsin spares boxes canbe pressed into ser-vice by adding turnsto form the sourcetapping point.

Construction of

the set must be rigidand robust or thereceiver will notperform well, espe-cially on the short-wave bands. Diecastboxes are best forchassis or enclo-sures, but receiversassembled on or inaluminium boxesare acceptable. Ametal front panel isessential for screen-ing purposes.

Layout is not par-ticularly critical, butlocate the tuningcomponents close tothe relevant solderpins on the receiverprinted circuitboard. Keep signalinput leads awayfrom output leads.The regenerationpotentiometer VR4

can be located in any convenient position(it is decoupled from the signal circuits).

The general interwiring from the p.c.b.to the off-board components is shown inFig.3.5. Some variable capacitors aresecured by screws driven into their frontplates. Check the length of the screws toensure that they do not project too far andfoul the capacitor vanes.

Metal potentiometer cases should beconnected to the 0V rail. Leads betweenthe aerial terminal, the input attenuator,VR1, and the receiver printed circuit board,should be screened, as should the leadsbetween the volume control, VR6, thepower amplifier and the receiver board.The screening must, of course, be connect-ed to the “ground” or 0V rail.

Most polyvaricon capacitors designedfor a.m. or a.m./f.m. portable receivers willbe suitable. The calibrated dial reproduced(half-size) in Fig.3.8 should be reasonablyaccurate if the tuning capacitor mentionedin the Components List is used and the coilis wound in accordance with Fig.3.6.


OptionalWave Trap

circuit board,not shown in the

author’s model below.

General layout inside the metal case. The speaker amplifiermodule is in the foreground. The wave trap is mounted on theunderside boxed chassis.

COIL L2, 20 TURNS OF 24S.W.G. (23 A.W.G.)ON A 20mm (0.75in) DIAMETER PLASTIC

FORMER (ELECTRICAL CONDUIT)

9mm (0.375in) 15mm (0.625in)

32mm (1.25in)

FRONT PANEL

C/SUNK BOLT

COIL FORMER PUSH FITOVER WOODEN PLUG

1) START, TO GATE OF TR22) TAP, TO SOURCE OF TR2 VIA VR23) FINISH, TO GROUND (0V RAIL)

PRESET VR2ATTACHED TOSOLDER TAGS.

C6 INSIDE FORMER

SOLDER TAGS 18 TURNS

56mm (2.25in)

2 TURNS

(3)

TAP (2)(1)

Fig.3.6. Coil winding details for the tuning coil L2. The coil former also carriespreset VR2 and capacitor C6 – see photo opposite.

Fig.3.5. Printed circuit board component layout, interwiring details and full-size copper foil master for the RegenerativeReceiver. Note you will need an additional VR2 preset and capacitor C6 for each waveband coil.


80mm (3.15in)

46m

m (

1.81

in)

C4C3

R9

C10

C9 C13 R7

R8

C11R13

C16

R3

R2 R1

R6

VR3C15

C14

R10

C18

C5

C2

R5

R4

C8

C7

VR5 R12

R11

C17

C12

e

e

b

b

c

c

s

s

d

d

g

g

TR1

TR2

TR3 TR4

++

+

+

+

+

+

+

+

L1 C1

AUDIO OUTPUTTO

AMPLIFIER

S1a

ON/OFF

0V

+9V

VR1 VR4 VR6

TO BATTERY

VC1

VR2

C6

WAVE TRAP(SEE TEXT)

EARTH AERIAL

TO 0V (GROUND)

TO TR2 SOURCE

TO TR2 GATEL2

S1b SWITCHES SEPARATE POWERSUPPLY TO AUDIO POWER AMPLIFIER (SEE TEXT)

S1

SCREEN

405

INPUT ATTEN. REGEN. VOLUME

TUNING

Polyvaricons intended for inex-pensive ‘hi-fi’ systems often have300pF or larger a.m. gangs. Thesecapacitors have a deeper case,around 20mm (3/4in.) compared tothe 10mm (3/8in.) or so for the lowervalue units. Only one gang shouldbe connected if a capacitor of thiskind is fitted.

The accompanying photographsshow this simple version of thereceiver assembled on the metalchassis used to test and evaluate thecircuits. A 6:1 reduction drive is fit-ted to the spindle of the tuning capac-itor but this is not adequate for easytuning over the shortwave bands.

Further, the value of the tuningcapacitor, whilst it gives good cov-erage with the single coil, is toohigh and regeneration becomes dif-ficult to adjust at the high frequency end ofthe range. These questions are addressednext month when more refined tuning sys-tems are discussed.

Readers may wish to try connecting onlyone of the capacitor gangs into circuit. Thisgives a swing of 5pF to 130pF and cover-age with the specified coil is around

6·5MHz to 15·5MHz.Coverage is reducedbut control of regener-ation, at the higherfrequencies, is easier.

Connect the receiv-

er to the Speaker

Amplifier described in Part 2. Connect anaerial comprising at least 30 feet (10metres) of wire located as high as possibleand well clear of any telephone or powerlines and earthed objects.

Set preset VR2 to maximum, and presetsVR3 and VR5 to minimum resistance.Rotate the slider of Regen. control VR4 toput the maximum voltage on the drain (d)of TR2. Set the other potentiometers tohalf-travel.

Now connect a fresh 9V battery and tunein a weak signal with variable capacitorVC1, set close to its maximum value.

Reduce the resistance of preset VR2until the Q multiplier begins to oscillate(indicated by a rushing sound or faint whis-tle). Turn down Regeneration control VR4.The receiver should slide gently out ofoscillation.

When regeneration is set close to oscil-lation, the perceived strength of signalswill be greatly increased and tuning muchsharper.

Turning down VR1 to attenuate inputsignals, as necessary, gradually open thevanes of VC1 to tune the receiver up in fre-quency. Less regeneration will be requiredto maintain sensitivity as the tuning capac-itance is reduced, and VR4 will have to beprogressively turned down. When VC1 isfully open, increase the value of presetVR5 until regeneration can be set justbelow the threshold of oscillation whenVR4 is at minimum.

Refine the adjustment of preset poten-tiometers VR2, VR3 and VR5 until theaction of the regeneration control VR4 is asgentle as possible across the entire tuningrange. Because of the wide swing of thetuning capacitor, VC1, in this simpleversion of the receiver, presets VR3 and


5

8

MHz5. 5 5. 7

6

7 910

11

1213

1414 .514 .6

THIS P.C.B. WILL ACCOMODATE MOST SCREW ORSOLDER TAG MOUNTED VARICON CAPACITORS

51mm (2.0in)

26m

m (

1.02

in)

5p TO 25p

5p TO 25p

MOVING VANESGROUNDED(TAKEN TO

0V RAIL)

MOVING VANESGROUNDED(TAKEN TO

0V RAIL)

5p TO 130p

5p TO 130p

406

Theloudspeaker mounted on acase side struttand boxed chassis.

Fig.3.8. Half-size calibrated dial (MHz). Calibration with thespecified tuning capacitor and coil L2. Receivers will vary, butit is a good guide to coverage.

Front panel fascia of single band version showinggeneral layout .

Fig.3.7. Tuning capacitor mounting p.c.b. details.Connections to a typical a.m./f.m. four-gang polythenedielectric variable capacitor. Capacitance values and con-nections may vary and should be checked.

(Above) The tuning capacitormounted on the p.c.b.

(Right) Completed tuning coilshowing preset soldered to tagsbolted on one end of former.

VR5 will have to be set close to minimumresistance to give VR4 sufficient control.

Best results will be obtained if the

Regenerative Radio is operated with VR1set to attenuate the input as much as possi-ble and the audio frequency gain (Volume)control turned up to ensure adequate soundoutput. This is good practice with complexsets and essential with this simple receiver.If this procedure is not followed it will beimpossible to hear weak signals close infrequency to powerful ones.

For best reception of a.m. (amplitudemodulated) signals the Regeneration con-trol VR4 must be adjusted, as the receiveris tuned across the band, to keep the Qmultiplier circuit close to oscillation. Whenregeneration is correctly set, rocking thetuning control should produce a faint whis-tle on either side of the station.

A useful technique when searching forvery weak signals is to advance the

regeneration controluntil the stage is justoscillating in theabsence of a carriersignal. When a stationis tuned in its carrierwill suppress theoscillation and theweak signal willbecome audible.

This practice was

widely adopted whenregenerative receiverswere popular fordomestic listening.Unfortunately therewas usually no radiofrequency stage toisolate the detectorfrom the aerial, and

an oscillating valve with up to 100V on itsanode forms a good transmitter.

Reception was, therefore, marred bywhistles and howls propagated by neigh-bouring receivers. The problem became soacute that, in 1928, the BBC issued ahandbook guiding the public on the cor-rect operation of the regeneration control.During that year so many complaints werereceived that listeners were warned thattheir licenses would be withdrawn if theydidn’t exercise more restraint.

With this modern transistor design, thegrounded base radio frequency stage iso-lates the very low powered oscillator andinterference problems do not arise.

For the reception of amateur s.s.b.

(single-side-band) transmissions, theregeneration control must be advanceduntil the Q multiplier is oscillating. Thelocally generated oscillation replaces the

carrier suppressed at the transmitter so thatthe signals can be demodulated in the usualway (more about this later).

Very precise tuning is required to clarifythese signals, and the simple slow motiondrive fitted on this version of the receiver iscompletely inadequate. Fortunately, the oper-ation of the regeneration control produces avery slight shift in the Q multiplier’s frequen-cy of oscillation, and this can be used to finetune and clarify these transmissions (increas-ing the drain voltage produces a very slightreduction in the gate to source capacitance).

The signals will still be difficult toresolve, however, and next month a tuningsystem dedicated to the three most popularamateur bands will be described.

An “earth” connection may improvereception. Guidance on constructing anearth system was given in Part 1.

The Regenerative Radio described here

is a modern evocation of the 1913 circuitthat made man’s dream of long distanceradio reception a reality. However, itshould not be regarded as a historical nov-elty. Correctly built, connected to a decentaerial and skillfully operated, it will permitthe reception of at least 90 per cent of thesignals receivable on a modern, high per-formance communications receiver.

Skilful operation is the key to unlockingits performance. The need for this is, per-haps, the main reason why it was replacedby the more easily controlled superhetreceiver.

It lacks automatic gain control, automat-ic input attenuators and pushbutton tuning.It does, however, offer a standard of per-formance out of all proportion to the mini-mal outlay of money and effort involved inits construction.

Next month’s article, dealing with morerefined tuning systems and general cover-age and amateur bands coil packs, will helpreaders to get the best out of the receiver.

Close-up view of the mounting arrangement for the tuningcoil and variable capacitor. Note the reduction drive fitted tothe tuning capacitor spindle.


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simple circuits should substitute air-spacedvariables. Remember that moving vanesare connected to the “ground” or 0V rail:fixed vanes are connected to the “hot” endof the tuning coil.

The Q of a tuned circuit reduces as tun-

ing capacitance increases. With regenera-tive receivers, positive feedback must,therefore, be gradually increased in orderto keep the receiver in a sensitive condi-tion. Setting up the circuit to ensure suffi-cient feedback at maximum capacitancecan make regeneration fierce when the tun-ing capacitor is turned low, and smoothcontrol can only be secured over a limitedcapacitor swing.

For long and medium wave coils, themaximum capacitance should be nogreater than 400pF. On the highest short-wave range (up to 30MHz), smoothregeneration will be difficult to achieve ifthe maximum capacitance exceeds100pF, and a 50pF component is to bepreferred.

A reasonable compromise for generalcoverage regenerative receivers spanning150kHz to 30MHz is a 200pF tuningcapacitor, and provision for reducing theswing to around 100pF on the highest fre-quency shortwave range is most desirable.Receivers covering the narrow amateurbands are best tuned with a 25pF variablecapacitor.

one tuned circuit, and Franklin introducedcapacitor ganging in 1907.

Air-spaced variables with ceramic insu-lation are the components of choice fortraditional tuning systems. They are morestable and have a higher Q factor (thegreater the Q the more selective the tuningcircuit) than solid dielectric capacitors.Unfortunately, the increasing use of elec-tronic tuning is making them something ofan expensive rarity.

A selection of the kind of air-spacedunits still manufactured in the UK and theUSA is shown in the photographs. Valuesrange from 5pF to 365pF or more; and one,two, and three-gang versions can beobtained – at a price!.

Variable capacitors with a polythenedielectric are the standard tuning compo-nent in inexpensive domestic receivers.They were discussed and illustrated in PartOne. The solid dielectric results in smallersize and makes them less prone tomicrophony (electro-mechanical feedbackvia the capacitor vanes). Minimum capaci-tance is lower (5pF instead of 10pF pergang).

Because of their low cost, ready avail-ability and versatility, polyvaricons havebeen chosen for the receivers described inthis series of articles. Constructors wishingto get the most out of these comparatively

IN Part Three the basic requirements of aregenerative receiver for serious listen-ing on the long, medium and shortwave

bands were listed and a simple but effec-tive practical circuit described.

This month, we explore the merits oftuning systems and give details of add-oncoil packs to extend the coverage of lastmonth’s High Performance RegenerativeRadio into the amateur bands.

Ease of tuning is crucial in a receiver to

be used for searching for weak signals.Large movements of the control knobshould produce only a small change in fre-quency and the drive must be free frombacklash. For these requirements to be metcomponents must be of good quality andset construction rigid and strong.

The Regenerative Radio designdescribed last month incorporated the sim-plest possible tuning arrangements.Alternative and more refined systems willnow be described.

Charles S. Franklin, an engineer who

spent most of his career in the service ofMarconi, invented the variable capacitor in1902. Receivers began to have more than

640 Everyday Practical Electronics, September 2003

Part 4: Tuning systems, coils and coil packs for general coverageand the amateur bands.

Group of air-spaced variable capacitors. Spindle couplers.

Slow-motion drives.

Tuning “drums”, spindle, springs and cord.

Fixed capacitors can be connected in

series with tuning capacitors in order toreduce their swing. This technique, adopt-ed in the circuits illustrated in Fig.4.3 andFig.4.4, eases regeneration problems andreduces the tuning rate.

A 1000pF fixed capacitor will reduce theswing of a 260pF variable to 206pF. A470pF fixed capacitor will reduce theswing of a 365pF variable to 205pF.

Always use polystyrene or low K ceramiccomponents for this purpose. Medium andhigh K ceramic capacitors (usually valuesabove 200pF or so) have a lower Q factor andthis will impair the efficiency of the circuit.

The sought after standard when commu-

nications receivers were tuned by variablecapacitors was a frequency change of 5kHzfor each full turn of the tuning control. Thiswas seldom achieved with basic superhetdesigns, especially on the high frequencybands.

Tuning has to be set to within 25Hz orso of the signal frequency in order to clar-ify a single-sideband transmission. Doingthis at 28MHz calls for a very slow tuningrate and a receiver of robust and rigidconstruction.

The tuning rate can be reduced bymechanical or electrical means, or acombination of both. The first methodinvolves gears, pulleys or epicyclic balldrives: the second a low value fine tuningor vernier capacitor wired in parallel with

the main tuningcomponent.

Both methods havethe same drawback: anarrangement whichproduces an acceptabletuning rate at 2MHz isstill much too fast at20MHz. (Constant tun-ing rates can only beachieved with super-hets of complexdesign; e.g., sets withtunable intermediatefrequency (i.f.) ampli-fiers or synthesizedoscillators).

The only mechani-cal reduction systemcurrently available tohome constructors isthe epicyclic balldrive. A pointer mounting flange is some-times fitted and the drives usually offer a6:1 reduction. They can be coupled in tan-dem to give ratios of 36:1 and, if three areused, almost 220:1. Standard andminiature versions are depicted in thephotographs.

A drum or large pulley driven by a cordwrapped around a 6mm (¼in.) spindleforms an effective slow motion drive, andthe component parts are shown in thephotographs. The cord is secured to thedrum and tensioned by a spring. Althoughstill fitted in most capacitor-tunedportable radios, the parts are no longerretailed.

Some constructors will, however, beable to salvage them from discardedreceivers, and the drums will usually fitdirectly onto the stubby spindles of poly-varicon capacitors. Combining a large (say100mm or 4in.) drum with an epicyclicdrive gives a worthwhile reduction ofaround 120:1.

A fine tuning, or vernier, capacitorvalue of 10pF is a good compromise for ageneral coverage receiver. The main tun-ing component is called the Bandset, andthe fine tuner the Bandspread, control. Ifboth are fitted with 6:1 reduction drivesthe arrangement represents a very accept-able tuning system for simple receivers.

Everyday Practical Electronics, September 2003 641

General Coverage and Amateur Bands coil pack p.c.b.stogether with the Regen. Radio board (last month).

Electronic tuning systemsexploit the way the capacitanceacross a semiconductor diodejunction can be varied by apply-ing a reverse bias. Special diodes,known as “varicaps” or “varac-tors”, with swings of up to 500pF,are produced for this purpose.

Capacitance change is reason-ably linear over the mid-range ofreverse bias. At low bias levels,when capacitance is approachingmaximum, the tuning rate is high-er. At high bias levels the rate ofchange is much lower.

Varicaps are comparativelyinexpensive and very convenientto use. The potentiometer Tuningcontrol can be mounted remotelyfrom the diode and this greatlysimplifies receiver layout.Bandspreading involves no morethan the addition of a secondpotentiometer.

There are disadvantages. They

have a comparatively low Q,especially when the capacitanceis approaching maximum.

Moreover, high value typeshave a high minimum capaci-tance, and thermal drift can begreater than with conventionalvariable capacitors. The draw-backs become more evidentwhen high capacitance diodes areused above 10MHz or so, but lowcapacitance types are satisfactoryfor fine tuning throughout theshortwave spectrum.

A typical Varicap Tuner circuit

is given in Fig.4.1, where D1 isthe varicap diode and VR1 thepotentiometer (Bandset) that setsthe reverse bias. Signal frequen-cies are isolated from the biasnetwork by resistor R1 (the diodepasses no current so the resistordoes not reduce the voltage).Capacitor C2 eliminates poten-tiometer noise and C1 preventsthe bias being shorted to the 0Vrail through the tuning coil.

Potentiometer VR2 produces asmall change in the bias and actsas a fine tuning or Bandspreadcontrol. Potentiometer VR3

TO GROUNDEDEND OF

TUNING COIL L2

D1KV1236

ORBB105

TO ‘HOT’ ENDOF TUNING COIL L2

C11n

SEE TEXT

BANDSET

BANDSPREAD

SET TUNINGRANGE

R2

470

VR1100k

VR24k7

VR3220k

0V

C2100n

C3

100

TO 9V+

*

*

*

*

**

R1100k

+a

k

a k

D1BB105

a k a k

XXX XXX

TYPE No

D1KV1236

(TWO DIODES INSNAP-APART PACK)

FIg.4.1. Circuit diagram for a simple Varicap Tuner. ForBandset and Brandspread tuning, use half of a twin KV1236varicap. For Bandspread tuning only, use a BB105 varicapdiode and a 22pF capacitor for C1. Also, omit VR2 and con-nect VR1 to VR3.

determines the minimum bias voltagethereby fixing the varicap’s swing.

In mains powered equipment, the biassupply must be well smoothed and regulat-ed. With battery equipment, regulation isstill essential if the tuning potentiometerhas a calibrated dial. If the varicap actsonly as a low value fine tuning capacitor,regulation, although still desirable, can bedispensed with, but supply-line fluctua-tions must not be imposed by other circuits(e.g., audio power amplifiers).

The “electronic tuning” Varicap Tuner is

assembled on a small printed circuit boardthat must be mounted close (within 50mmor 2in.) to the coil or coils and wavechange

switch. Tuning potentiometers can belocated in any convenient position.

Details of the printed circuit board(p.c.b.) topside component layout, full-sizecopper foil master and the off-board wiringdetails are shown in Fig.4.2. This board isavailable from the EPE PCB Service, code412. Full tuning and just vernier tuningalternatives were given earlier. Generalguidance on construction is given later.


a

k

VR3

D1

C1

R1

R2

C2

C3+

BANDSPREAD

BANDSET

VR1

VR2

TO +9V TO +12V

TO GATE OR TR2(’HOT’ END OF COIL L2)

TO RECEIVER0V RAIL1.1in (27.9mm)

1. 5

in(3

8. 1

mm

)

412

412

Fig.4.2. Printed circuit board component layout, wiring details and full-size under-side copper foil master for the Varicap Tuner. Clockwise rotation of the pots. reducesthe capacitance and increases frequency. The lead-off wires go to last month’sRegenerative Radio.

VARICAP TUNERResistors

R1 100kR2 470


PotentiometersVR1 100k rotary carbon, lin.VR2 4k7 rotary carbon,

lin. (see text)VR3 220k enclosed

carbon preset

CapacitorsC1 1n polyester or

polystyrene (full tuning) or

22p ceramic “low k”(bandspread only) – see text

SemiconductorsD1 KV1236 varicap diode

(full tuning) orBB105 varicap diode

(bandspread only) –see text

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 412(Varicap); multistrand connecting wire;insulated p.c.b. mounting stand-off pillar(4 off); solder pins; solder etc.


excl.


It is our understanding that the onlycommercial coils available to home con-structors in the UK are those produced bythe Japanese manufacturer Toko. Theseminiature coils, with their ferrite cup orslug tuned cores and bright plated brasscans, are ubiquitous.

Any reader who has removed the backof a transistor radio will have seen them oran imitation. The adjustable cores permitwide variation of the inductance.

The circuit diagram for a switched coil-

pack, General Coverage Receiver incor-porating Toko inductors is shown inFig.4.3. Coils L2a to L2f are tuned by theBandset or Tune capacitor VC1, formedby connecting both a.m. gangs of a poly-varicon (polythene dielectric capacitor) in

parallel. The tuning sweep being reducedto around 200pF by series capacitor C19.

Fine or Bandspread tuning is carriedout with VC2, one of the f.m. gangs of asecond polyvaricon. The swing of thiscomponent is reduced to around 10pF bycapacitor C20.

Readers using the receiver primarily forlong and medium wave listening shoulddelete capacitor C20 and leave VC2 at itsfull value. Where shortwave coverage isthe area of interest, readers may prefer toreduce capacitor C20 to, say, 6·8pF inorder to produce a slower tuning rate.

The inductors (L2a to L2f) are switchedby S2a, and the TR2 source bias presets(VR2a to VR2f) by S2b. The circuitdetails around TR2 were given last month,see Fig.3.4.

Connections to the coil windings varyin order to increase inductance and/or to

improve the feedback tapping ratio.Details of the base connections are alsogiven so that constructors wishing to usethe coils individually, without the switch-ing, can easily do so.

Amateur transmissions occupy narrow

segments of the high frequency spectrumand the actual band allocations are listedin Table 4.1. Speech signals are in a modeknown as single-sideband (s.s.b.), and tun-

General Coverage Receiver.

ing has to be very precise to make themintelligible. Because of the narrow bandsand need for critical tuning, improvedresults will be obtained with the coil andcapacitor combinations illustrated inFig.4.4.

The circuit arrangement shown coversthe three most popular allocations: 80, 40,and 20 metre bands. Coils L2a, L2b andL2c are switched by S2a, and brought toresonance within the band by fixed capaci-tors C19, C20 and C21.

Tuning is by one of the 5pF to 25pF f.m.gangs of a polyvaricon. Even this swing isexcessive for the 40m and 20m bands andswitch S2c connects series capacitors C22and C23 into circuit in order to reduce it.

Source bias presets VR2a to VR2c,together with their bypass capacitors C6ato C6c, are switched by S2b. Again, coilbase connections are given for readers whowish to use crocodile clips to connect coilsinto circuit. (The coil can/screen must besecurely held in place or vibrations willaffect tuning).

Coils, presets and capacitors are assem-

bled on printed circuit boards. These boards

are


TUNE

FINE TUNE

L2aCAN1A350EK

L2bRWO6A

7752

BASEB

BASEA

BASEA

L2cYMO56A356EK

TO COLLECTOROF TR1

VIA R4 (PART 3 FIG.3.4.)

L2dTKANS32696A

BASEA

BASEB

BASEA

L2eKXNK3767

L2fKXNK3766

S2a1

2

3 4

5

6

S2b

7

89 10

11

12

VR2a470k

VR2b220k

VR2c22k

VR2d10k

VR2e10k

VR2f10k

C6a C6b C6c C6d C6e C6f

TO SOURCE OFQ-MULTIPLIER (TR2)

(PART 3 FIG.3.4.)

TO GROUND(0V RAIL)

TO GATE OFQ-MULTIPLIER (TR2)

(PART 3 FIG.3.4.)

*C19

1n

*C20

22p

VC25p - 25p

VC110p - 260p

( ) ( ) ( ) ( ) ( ) ( )

BAND

C6a TO C6f1n (1000p)

*SEE TEXT

Fig.4.3. Coil pack circuit diagram for providing a switched6-band General Coverage version of last month’s Regen.Radio (see Fig.3.4). Maximum capacitance for VC1 (Tune) isset to 200pF approx. by series capacitor C19 and for VC2(Fine) to 10pF approx. by C20.

TO GROUND (0V RAIL)

ALTERNATIVE COIL BASE CONNECTIONS(VIEWED FROM UNDERSIDE)

TO SOURCE OFQ-MULTIPLIER VIA VR2


A

TO GROUND (0V RAIL)

TO SOURCE OFQ-MULTIPLIER VIA VR2

TO GATE OFQ-MULTIPLIER (TR2)(PART 3 FIG. 3.4/5)

B

TO GATE

TO SOURCEVIA VR2

TO GROUND (0V)(PART 3 FIG.3.4/5)

TO GATE

TO SOURCEVIA VR2

TO GROUND (0V)(PART 3 FIG.3.4/5)

TKANS32696A 154FN8A

6439KXNK3767

AND

COIL BASE CONNECTIONSVIEWED FROM UNDERSIDE

Fig.4.3a (above) and Fig.4.4a (below). Base connections forthose wishing to “plug-in” individual coils, without any switch-ing, into the General Coverage and Amateur Bands receivers.

TKANS32696A

154FN8A6439

KXNK3767

C1915p

C2082p

L2a L2b L2c

C2182p

S2a

12

3

VR2a220k

VR2b100k

VR2c22k

C6a1n

C6b1n

C6c1n

S2b

4 5 6

TO COLLECTOR OF TR1VIA R4 (PART 3 FIG.3.4.)


(PART 3 FIG.3.4.)

C2318p

C2227p

S2c7

8 9

TO GROUNDOR 0V RAIL

TO SOURCE OFQ-MULTIPLIER (TR2)

(PART 3 FIG.3.4.)

BAND

TUNE

VC25p - 25p

Fig.4.4. Circuit diagram for a 3-band Amateur Band version of last month’s Regen. Radio(see Fig.3.4). Bands shown with an asterisk (*) are wider in the USA: 3·5MHz-4MHz and7MHz-7·3MHz. The coil and capacitor combinations will give full USA coverage.

Table 4.1: Amateur Band Allocationsand Marker Frequencies

Band Frequency Marker CrystalMetres Allocation Frequency Harmonic

MHz MHz80 3·5 to 4 3·58 Fundamental

(3·5 to 3·8 in UK)40 7 to 7·3 7·16 Second

(7 to 7·1 in UK)30 10·1 to 10·15 10·74 Third20 14 to 14·35 14·32 Fourth15 21 to 21·45 21·48 Sixth12 24·89 to 24·99 25·06 Seventh10 28 to 29·7 28·64 Eighth

The crystal frequency has been rounded up to 3·58MHz.

COVERAGE (S2)1 150kHz-335kHz2 525kHz-1·3MHz3 900kHz-2·2MHz4 2·8MHz-7·1MHz5 6·5MHz-15MHz6 12MHz-31MHz

COVERAGE (S2)1 3·5MHz-3·8MHz*

(80 metres)2 7MHz-7·1MHz*

(40 metres)3 14MHz-14·35MHz

(20 metres)


12

3

4

5

6

78

9

10

11

12

A

B

S2

TO SOURCE OF TR2(PART 3 FIG.3.5.)

TO GATE OF TR2(PART 3 FIG.3.5.)

TO 0V (GROUND)(PART 3 FIG.3.5.)

VC1 VC2

TUNE FINE TUNE

C19

C20

L2f

L2d

L2b

L2e

L2c

L2a

VR2e

VR2c

VR2a

VR2f

VR2d

VR2b

C6a

C6b

C6d

C6c

C6e

C6f

3.0in (76.2mm)

2. 0

in(5

0. 8

mm

)

413

CANTAG


excl. slow-motion drives & case

GENERAL COVERAGE REC.(Coil Pack)

PotentiometersVR2a 470k enclosed carbon presetVR2b 220k enclosed carbon presetVR2c 22k enclosed carbon presetVR2d to 10k enclosed carbon preset (3 off)

VR2f

CapacitorsC6a to C6f 1n polycarbonate (6 off)C19 1n polystyreneC20 22p polystyrene or ceramic “low k”VC1 10p to 260p polythene dielectric

variable capacitor (see text)VC2 5p to 25p polythene dielectric

variable capacitor (see text)

MiscellaneousL2a CAN1A350EK Toko screened (metal can) coilL2b RWO6A7752 Toko screened (metal can) coilL2c YMOS6A356EK Toko screened coilL2d TKANS32696A Toko screened coilL2e KXNK3767 Toko screened coilL2f KXNK3766 Toko screened coilS2 2-pole 6-way rotary switch

Printed circuit boards available from the EPE PCB Service,code 413 (Coil Gen. Cover) and code 406 (T/Cap – tworequired); slow-motion drive (2 off); spindle extenders and/orcouplers (see text); front panel card and protective 2mm thickPerspex sheet; p.c.b. stand-off pillars; connecting wire; solderpins; solder etc.

Note: Case is the Regenerative Radio in Part 3.

HAND-WOUND COILS(General Coverage Rec.)

PotentiometersVR2 100k enclosed carbon preset (4 off)

(Ranges 1 to 4)VR2 (Range 5) 22k enclosed carbon presetVR2 (Range 6) 10k enclosed carbon preset

CapacitorsC6 (Ranges 1 to 6) 1n polyester (6 off)

Coils (see Table 4.2 and Fig.4.7)Enamelled copper wire for coils, 50g (2oz) reels, sizes: 36s.w.g.

(32a.w.g.), 32s.w.g. (30a.w.g.), 24s.w.g. (23a.w.g.), 18s.w.g.(16a.w.g.); plastic tube, 20mm (¾in.) outside diameter (o/d) for coilformer; thin card for coil bobbins; adhesive; clear cellulose; nuts;bolts, washers and solder tags.


Fig.4.5 General Coverage Receiver coil pack printed circuit board compo-nent layout, full-size underside copper foil master and interwiring details toband selection switch S2, two varicon (polythene) variable capacitorp.c.b.s and lead-off wires to last month’s Regen. Radio (see Fig.3.5).Wiring to actual unit must be kept as short as possible and direct.

available from the EPE PCB Service, codes413 (Gen.) and 414 (Amateur).

Details of the component side of thegeneral coverage coil pack board, full-sizecopper foil track master and the wiring tothe wavechange switch and tuning capaci-tors are given in Fig.4.5.

The component side of the amateurbands board, p.c.b. foil master togetherwith the wiring to the wavechange switchand tuning capacitor are shown in Fig.4.6.Swing reducing capacitors, C22 and C23,are mounted on the tags of switch S2c: pro-vision is not made for these components onthe printed circuit board.

Note that the wiring to the variablecapacitor is correct for the component sug-gested in the Components List.Alternatives should have their values andconnections checked.

Coil pack, capacitors and wavechangeswitch S2 must be mounted very close toone another and all wiring kept as short aspossible and direct. These componentsmust also be very close to the terminal pinside of the Regen. Radio printed circuitboard. Lead lengths should certainly be nomore than 75mm (3in.). Guidance notes onconstruction are given later.

With a little care and patience, efficient

coils can be wound by hand on the plastictubing manufactured for plumbing servicesand electrical conduits.

With the Regen. Radio circuit given inPart Three, the “feedback” tapping shouldbe about 5% of the total number of turnsfor the longwave coil, and 10% of the totalon all other ranges. The highest shortwaverange; i.e., up to 30MHz, may require a


AMATEUR BANDS(Coil Pack)

PotentiometersVR2a 220k enclosed carbon presetVR2b 100k enclosed carbon presetVR2c 22k enclosed carbon preset

CapacitorsC6a to C6c 1n polyester (3 off)C19 15p polystyrene or ceramic “low k”C20, C21 82p polystyrene or ceramic “low k” (2 off)C22 27p polystyrene or ceramic “low k”C23 18p polystyrene or ceramic “low k”VC1 5p to 25p polythene dielectric variable

capacitor (see text)

MiscellaneousL2a TKANS32696A Toko screened (metal can) coilL2b 154FN8A6439 Toko screened (metal can) coilL2c KXNK3767 Toko screened (metal can) coilS2 4-pole 3-way rotary switch

Printed circuit boards available from the EPE PCB Service,code 414 (Amateur) and code 406 (T/Cap); slow-motion drive(see text); spindle extender/coupler (see text); front panel cardand protective 2mm thick Perspex sheet; p.c.b. stand-off pillars;connecting wire; solder pins; solder etc.

Note: Case is the Regenerative Radio in Part 3.


excl. slow-motion drive & case


TO GATE OF TR2(PART 3 FIG.3.5.)

TO SOURCE OF TR2(PART 3 FIG.3.5.)

2.45in (62.2mm)

1. 5

in(3

8.1

mm

)

414

12

3

4

5

6

78

9

10

11

12

A

B

C

TO 0V (GROUND)(PART 3 FIG.3.5.)

C22

C23

S2

C6a

C6b

C6c

C19

C20

C21

VR2a VR2b VR2c

L2a L2b L2c

VC1

SINGLE F.M. GANG OFPOLYVARICON CAPACITORPROVIDES 5p - 25p TUNINGCAPACITOR

TUNE

BAND SELECT

CANTAG

Fig.4.6. Amateur Bands coil pack printed circuit board com-ponent layout, full-size copper foil master and interwiring torange switch, tuning capacitor p.c.b. and lead-off wires tolast month’s Regen. Radio (see Fig.3.5). All wiring should bekept short and direct.

Completed Amateur Bands coil pack p.c.b. wired to thewavechange switch.

15% tapping if the value of the tuningcapacitor exceeds 100pF.

Winding details for a range of coils for ageneral coverage receiver are given inTable 4.2. The specified coverage is basedon a tuning capacitor with minimumcapacitance of 10pF and a swing of 200pF;i.e. both gangs of a polyvaricon with theswing limited by C19. (On the highestshortwave range, just one gang should beswitched into circuit to give a swing ofaround 115pF.)

The higher inductance long and mediumwave coils must be sectionalized to reduceself-capacitance and maintain an accept-able tuning range. This is done by windingthe coils in a series of “pies” (the tradition-al term) or piles, held in place by card bob-bins. Full details are given in Fig.4.7.

Thin card (postcard) glued with Durofix,or a similar quick setting adhesive, is idealfor the coil bobbins. It is a good idea to dipthe bobbins in cellulose paint, in orderstiffen them, allowing the paint to hardenbefore sliding them onto the plastic tubing.

When producing close-wound coils

(turns touching), wind the turns on tightlyand slightly spaced, and keep pushing themtogether with the thumb of the hand hold-ing and rotating the former, as the windingproceeds. For space-wound coils, just con-centrate on winding on the correct numberof turns as tightly as possible, then careful-ly even out the spacing with the tip of ascrewdriver (avoid damaging the enamelcoating) when the ends of the coil havebeen anchored.

A coat of clear cellulose can be appliedto hold the turns in place. Coils wound onbobbins can be protected by strips of mask-ing or insulating tape but they must not beimpregnated.

Solder tags are a convenient means ofanchoring the ends of the windings. Wiregauges are not especially critical, but thick-er material may not be accommodated inthe bobbins or on the formers.

The coils can be connected into circuit

by short (no more than 75mm or 3in.) fly-ing leads terminated with miniature croco-dile clips. Source bias preset VR2 andcapacitor C6 can be mounted on soldertags at the end of the coil former (seeFig.3.6, last month) when this connectionmethod is adopted.

If hand-wound coils are arranged in acoil pack with wavechange switching, pro-vision must be made to short out the coilnext higher in inductance to the one in use.If this is not done it will be tuned by itsself-capacitance to resonate within the tun-ing range of the coil in circuit and drawenergy from it. This will cause a regenera-tion dead-spot.

Switches can be obtained which short allunused windings. They are much to be pre-ferred for packs of unscreened coils.

Screening afforded by the metal cansand cup cores makes shorting arrange-ments unnecessary with Toko coils.

An accurately calibrated dial adds

greatly to the enjoyment of using a receiv-er and the following guidance is offered toreaders who do not have access to a signal

generator, crystal calibrator or frequencycounter.

On medium waves, careful listening,during daylight hours, aided by a copy ofthe local and regional transmitter schedules(e.g., the Radio Times), should enable sta-tions and frequencies to be identified andthe receiver dial calibrated.

This procedure would be impossiblytedious on shortwaves. Here the simplestcalibration method is to keep the Regen.receiver’s tuning in step with the tuning ofanother receiver with an accurate, prefer-ably digital, dial.

If the calibrating receiver has a beat fre-quency oscillator (b.f.o.), switch it on and

place its aerial lead close to the regenera-tive receiver’s p.c.b. Advance the regenera-tion control until the Q-multiplier isoscillating. When both sets are tuned to thesame frequency the calibrating receiverwill pick up the signal radiated by the Q-multiplier circuitry and reproduce it as atone.

Adjust the regenerative receiver’s tuningto make the tone lower in pitch until it is analmost inaudible fluttering. This is the zerobeat position and the tuning of the tworeceivers is then very precisely matched.By setting the calibrating receiver to pre-cise spot frequencies, the regenerativereceiver’s dial can be marked out.


Table 4.2: Details of Hand-Wound Coils

No. Wave Turns Turns S.W.G. A.W.G. Type of Winding Range VR2(S2) Band 1-2 2-31 LW 600 20 36 32 5 bobbins of 120 141kHz- 100k

plus pile of 20 345kHz2 MW1 160 16 32 30 4 bobbins of 40 520kHz- 100k

plus bobbin of 16 1·28MHz3 MW2 100 10 32 30 Close wound 1MHz- 100k

2·6MHz4 SW1 35 3 24 23 Close wound 2·6MHz- 100k

6·7MHz5 SW2 12 2 24 23 Spaced 6·4MHz- 22k

16·8MHz6 SW3 6 1 18 16 Spaced 13·5MHz- 10k

33·8MHz

NOTES:(1) Enamelled copper wire used throughout.(2) Tuning capacitor for Ranges 1 to 5: 10pF min. to 210pF max.(3) Tuning capacitor for Range 6: 5pF min. to 120pF max.(4) The Range 6 shortwave coil (SW3) has a separate 24s.w.g. feedback winding located at

the “earthy” end of the tuned winding· Finish of both windings connected to the 0V rail andall turns wound in the same direction.

COIL FORMERS SHORT LENGTHS OF20mm DIAMETER PLASTIC ELECTRICALCONDUIT

PIE (PILE)WOUND COIL

2M OR 8BA NUTS, BOLTSAND SOLDER TAGS

WINDINGS INCARD BOBBINS

(1) START

(2) TAP

(3) FINISH

(2)

LAYER WOUNDCOIL

(1)

(3)

‘EARTHY’ OR REGENERATIONEND OF COIL NEXT TO CHASSIS WOODEN PLUG

BOLTED TO CHASSIS

45mm(1.75in)

FORMER

CARDRINGS

3MM (1/8in) WIDE CARD STRIPWOUND AROUND FORMER

3mm (1/8in)

SLIT FOR WIRE ENTRY

THIN CARD RINGSAS BOBBIN CHEEKS

NOTCH FORWIRE EXIT

20mm(3/4in)

CARD BOBBINS FOR PIEOR PILE WOUND COILS

3mm (1/8in)

Fig.4.7. Construction details for producing hand-wound coils for the GeneralCoverage Receiver. see Table 4.2 for winding details and bands covered. A collec-tion of completed hand-wound coils is shown below.

1 – START of winding (to TR2 gate of Q-multiplier, see Fig.3.5 Part 3)2 – TAPPING (to TR2 source of Q-multiplier, see Fig.3.5 Part 3)3 – FINISH of winding (to 0V rail or ground, see Fig.3.5 Part 3)

Solder pins at the lead-out points ease

the task of off-board wiring. They shouldbe located in the printed circuit boardsfirst. Follow these with the resistors; thenthe capacitors, smallest first; and, finally,the semiconductors.

Take care to insert the coils in the correctpositions on the printed circuit board.Identification lettering on the cans is quick-ly erased by handling. Readers wishing topreserve it should apply a piece of clearsticky tape. Stressing the coil pins canresult in open-circuit windings, and theyshould be treated with great care.

Some variable capacitors are secured byscrews driven into their front plates. Checkthe length of the screws to ensure that theydo not project too far and foul the capacitorvanes.

Wiring between the printed circuitboards making up the receivers should beshort and direct. The ground plane on thecoil pack p.c.b.s must be connected direct-ly to the ground or 0V pin on theRegenerative Radio board. TheRegeneration control may function errat-ically if there is a separate return via thecase metal chassis.

On completion the printed circuit boards

should be checked for poor soldered jointsand bridged tracks. Check the orientationof semiconductors and electrolytic capaci-tors, the positioning of Toko coils, and theinter-board wiring.

The electronic tuning printed circuit boardshould be wired to the potentiometers, on theworkbench, and tested before being mounted

in the receiver. Coil packs can be tested byapplying flying leads, terminated with croc-odile clips, to the receiver solder pins.

Receiver printed circuit boards,

wavechange switch and tuning capacitormust be assembled on or in an aluminiumchassis or box. An aluminium front panel ismost desirable.

The arrangement used for assessing theprototype circuits is shown in the variousphotographs and itemized in the ComponentsList. Whilst it proved to be sufficiently rigidit should be regarded as no more than ade-quate: diecast boxes and air-spaced tuningcapacitors are much to be preferred.

Detailed guidance on setting up was

given last month. Because of the GeneralCoverage Receiver’s tuning capacitorswing, presets VR3 and VR5 have to beturned close to minimum resistance inorder to give Regeneration control VR4 awide enough range of control.

On the highest shortwave range, biaspreset, VR2, should be optimized at thefrequency overlap point rather than at tun-ing capacitor maximum. Coil cores shouldbe adjusted to give continuous coverage,but note the gaps between 335kHz and525kHz, and 2·2MHz and 2·8MHz. A half-size (approx.) calibrated dial is shown inFig.4.8.

Coil core adjustments with the AmateurBands Receiver are very critical. The dial ofthe prototype receiver is reproduced, half-size (approx.), in Fig.4.9, and this shouldgive some idea of the tuning capacitor posi-tion at the various marker frequencies.

With this version of the Regen. receiver,presets VR2a to VR2c should be adjustedso that the Q-multiplier begins to oscillatewhen Regeneration control VR4 justmoves away from the zero position.Presets, VR3 and VR5, should be set asclose to maximum resistance as possible.

Correctly operated, the General

Coverage Receiver is sensitive and selec-tive. It is capable of receiving transmis-sions from all over the world.

The Regeneration control is completelyfree from backlash but, on the shortwavebands, when tuning capacitance is low, thetransition into oscillation is very abrupt.(For the reasons given earlier, this is a com-mon problem with regenerative receivers).Fitting a ten-turn potentiometer in the VR4

position will overcome this difficulty andmake the control of regeneration extremelysmooth.

Adjusting the input attenuator, VR1,changes, very slightly, the damping on thetuned circuit (L2/VC1). This potentiometercan, therefore, be used as an alternativemeans of gently controlling regeneration atthe threshold on the higher shortwavebands.

The Amateur Bands receiver’s lowvalue tuning capacitor makes tuning mucheasier. Increasing the Q-multiplier’s(TR2) drain voltage causes a slight reduc-tion in its gate to source capacitance,thereby increasing the frequency of oscil-lation. Regeneration potentiometer, VR4,can, therefore, be used as a very fine tun-ing control, and single-sideband (s.s.b.)signals are easy to clarify. Breakthroughfrom powerful broadcast transmitters canusually be eliminated by turning down theinput attenuator, VR1.

Amateur signals are very closely spacedand the selectivity of the receiver isinevitably inferior to that of a superhet withnarrow i.f. filters. However, only the tunedsignal is clarified and interfering stationson adjacent channels are high-pitched,unintelligible and not too distracting.Speech reproduction is clear and distinct.

Activity on the amateur bands, especial-ly 20 metres, varies, and 80 metres can benoisy. Begin by listening on 80 and 40metres on Sunday mornings.

Next month we embark on the construc-tion of a Super Regeneration Receiver,claimed to be the most sensitive of single-device receiving systems, and a simplecrystal calibration aid.

12

13

14

15

1618

20

22

24

26

28

30

9

11

13

15

8

7

6. 5

3

5

6

7

4

1.5

2

1

0.7

1

0.8

0. 6

0. 5

5

1 .1

1.2

0.2 0 .

25

0.3

0. 1

5

MHz

14 .3

14 .

4

14. 1

14

3.8

14. 3

27. 1

6

3.58

7. 1

7

7 .2

7.3

3 .9

4

3.7

3. 5

3. 4

14.2

MHz 1 2 3

Fig.4.8. General Coverage Receiver dial, approx. half-size.Receivers will vary, but it gives a good idea of coverage tobe expected.

Fig.4.9. Calibrated dial (half-size approx.) for the AmateurBands Receiver. Typical frequency “markers” are indicatedby the arrowhead pointers.

General Coverage coil pack p.c.b.wired to the wavechange switch.


follow the signal modulation. Because ofthis, the circuit combines detection withsignal amplification, delivering an audiooutput that has a logarithmic relationshipto the applied signal.

This condition introduces noticeabledistortion on a.m. (amplitude modulation),but the drawback is not so pronouncedwith f.m. (frequency modulation).Logarithmic operation imposes a limitingaction on noise spikes and strong signals,and the inherent a.g.c. (automatic gaincontrol) is very apparent.

In the case of separately quenchedreceivers, they can be made to operate ineither mode. Self-quenching receivers,which are, in effect, squegging oscillators,operate logarithmically.

Whatever the mode, the enormousincrease in selectivity afforded by regener-ation or Q multiplication is lost when thecircuit is re-configured as a super-regener-ator. Moreover, the quenched oscillatorradiates hash at signal frequencies and,unless precautions are taken, interfereswith other receivers.

1000 times the quench frequency, and thisconfines the circuit to the radio spectrumabove 20MHz.

Oscillations in the super-regenerator

build up from a signal voltage developedacross the tuned circuit. In the absence ofan external signal, the random movementof electrons, or “noise”, triggers the action.

During the build-up, the amplitude ofthe oscillations can exceed that of the sig-nal by as much as a million times. Thesephenomena give rise to the enormous sen-sitivity of the circuit and the loud hissemitted by the speaker under no-signalconditions.

It is important that Q-multiplieroscillations die away during quenchcycles or they will build up againfrom fading oscillations instead of thesignal. Some means of controllingquench amplitude must, therefore, beprovided.

Signal frequency oscil-

lations can be“quenched” before theyreach their maximumamplitude. The peakamplitude of the oscilla-tions is then proportionalto signal voltage and thereceiver is said to operatein the linear mode.

If the quenching actionis such that the oscilla-tions reach, or evenmomentarily rest at, theirmaximum value, the cir-cuit is said to be in loga-rithmic mode. Signalsacross the tuned circuitthen speed up the rate atwhich the oscillationsrise to, and decay from,their maximum value.

Operation in logarith-mic mode causes Q-mul-tiplier current drain to

THIS month we consider the techniqueknown as super-regeneration. Noother radio circuit, it is claimed, pro-

duces more gain from a single valve ortransistor.

Readers who have access to a FrequencyCounter may wish to use it as a digitalreadout dial for the RegenerativeReceivers covered in Parts Three and Four,and the Direct Conversion receiver to bedescribed later in the series. Details of asimple buffer amplifier for linking receiverto counter are given this month.

Before we tune-in to the subject ofsuper-regeneration, readers may care tobuild the simple low-cost Crystal Markercalibration aid shown opposite. This isaimed particularly at last month’s amateurbands receiver.

Regeneration, whereby positive feed-back from an amplifier is used to cancelout losses in a tuned circuit and increase itsQ factor, was covered at length in PartThree. For the greatest increase in sensitiv-ity and selectivity, the feedback has to besufficient to almost completely overcomethe losses. A little more and the circuitoscillates and becomes unsuitable for pro-cessing signals (other than single-sidebandtransmissions).

In practice it is impossible to set andhold a Q-multiplier on the very thresholdof oscillation when a signal is beingreceived, and the ultimate sensitivitywhich regeneration can offer is never fullyrealised.

The super-regenerative receiver over-comes this by imposing an oscillating volt-age on the Q-multiplier to repeatedlysweep it across the critical threshold. This“quench oscillation”, as it is called, can beprovided by a separate stage or the multi-plier itself can be made to perform a dualfunction.

Quenching must be at a supersonic fre-quency or it will be heard as a tone in theheadphones or loudspeaker. In practice,the signal frequency is usually more than

700 Everyday Practical Electronics, October 2003

Part 5: Super-regeneration: A highly sensitive receiving system

The amateur bands receiver describedlast month presents particular calibrationproblems. Signals can be weak and diffi-cult to resolve; sometimes the bands aredead.

Constructors who do not have access toa calibrated receiver with a b.f.o. needsome means of locating them. This cantake the form of a simple and inexpensivecrystal marker and, as the bands have a har-monic relationship, one crystal will pin-point them all.

A circuit diagram for a simple amateurbands Crystal Marker is given in Fig.5.1,where transistor TR1 and crystal X1 areconfigured in Clapp’s version of a Colpitt’soscillator. The crystal acts as a tuned circuitwith high inductance, very low capacitance,extremely high Q, and exceptional stability.

Feedback from TR1 emitter is appliedto the capacitance tap provided by C2 andC3. Crystal loading capacitor C1 is usual-ly a 5pF to 60pF variable used to set thecrystal frequency to its stated value againsta known standard. The simpler circuitgiven here is accurate enough for our pur-poses.

Base bias is fixed by resistors R1 andR2, and TR1 emitter bias is developedacross R4. Collector load resistor R3 mustnot be greater than R4 or oscillation will beinhibited. The signal output is taken fromTR1 collector, via capacitor C4. CapacitorC5 avoids the possibility of erratic opera-tion with ageing batteries.

The marker crystal used in this circuit isa 3·579545MHz component used in thecolour sub-carrier circuitry of American

TV receivers. It is widely available at lowcost. Its fundamental lies within the 80metre band, the second harmonic withinthe 40 metre band (just outside the UKallocation), and the third harmonic withinthe 20 metre band.

If the marker unit is placed very close to

the receiver, the coil cores can be adjusteduntil the relevant harmonic beats with theoscillating Q-multiplier to produce anaudible tone. Band allocations, markerfrequencies and harmonic numbers werelisted last month in Table 4.1.

This particular circuit will oscillatewith 1MHz to 15MHz crystals, and unitscut to convenient round-figure frequen-cies can be used to calibrate the generalcoverage receiver’s shortwave ranges. A1MHz crystal will inject signals at rea-sonably close intervals. It is easy to losetrack of the higher harmonics, and an8MHz or 10MHz crystal is needed toprovide unambiguous markers at higherfrequencies.

The Crystal Marker unit is assembled on

the printed circuit board illustrated inFig.5.2, together with the p.c.b. foil masterand wiring. This board is also available fromthe EPE PCB Service, code 415 (Marker). Ifit is to be used for general calibration pur-poses, a socketshould be fitted sothat the crystal canbe changed easily.

The lead spacing on common HC-49/Uand the lower profile U4 crystals is4·88mm. This spans three pins on an i.c.holder, from which a socket can be cut.

X1

R2

R1

C1 C3

C5

R4

R3

C4

C2

TR1

eb

c

S1ON/OFF

TO 9V+

TO 0V

OUTPUTTERMINALS

1.3in (33.0mm)

1. 2

in(3

0. 5

mm

)

415

B1

b

c

e

B19V

X13.579545

MHz

C127p

R1100k

R2120k

C2220p

C3220p

R41k2

R3

680

C410p

MARKERSIGNALOUTPUT

C5100n

ON/OFF S1

+9V

0V

TR1BC549C

ebc

BC549C

Fig.5.1. Circuit diagram for an amateurbands Crystal Marker.

Fig.5.2 (right). Crystal Marker printedcircuit board component layout, inter-wiring and full-size copper foil master.Also shown is the completed prototype;note the spare crystal compartment.

1.3in (33.0mm)

1. 2

in(3

0. 5

mm

)

415

CRYSTAL MARKERResistors

R1 100kR2 120kR3 680R4 1k2


CapacitorsC1 27p polystyrene or

ceramic “low k”C2, C3 220p polystyrene or

ceramic “low k” (2 off)C4 10p disc ceramicC5 100n disc ceramic

SemiconductorsTR1 BC549C npn silicon

transistorX1 3·579545MHz crystal

(American TV coloursub-carrier)

MiscellaneousS1 s.p.s.t. toggle switch

Printed circuit board available fromthe EPE PCB Service, code 415(Crystal); plastic case, size 111mm x57mm x 22mm approx.; 4mmterminal/socket (2 off); 2-pin crystalholder, cut from i.c. socket (3-pins, seetext); 9V battery and holder; connectingwire; solder pins; solder etc.


excl. case & batt.


Everyday Practical Electronics, October 2003 701

A practical, semiconductor interpreta-tion of a Super-Regenerative V.H.F.Receiver is shown in the circuit diagram ofFig.5.3. Originality is not claimed: thedesign is typical of many produced duringthe late sixties and seventies.

Grounded base signal amplifier TR1 iso-lates the regenerative detector TR2 fromthe aerial circuit. The audio preamplifierstage TR3 boosts the demodulated signalso that a decent loudspeaker output can bedelivered by the simple audio power ampli-fier described in Part Two.

Emitter resistor R1 and bias resistors R2and R3 fix the operating conditions of tran-siistor TR1. Incoming signals from the aer-ial are applied to TR1 emitter via capacitorC1, and coupling coil L1 acts as the collec-tor load. The stage is decoupled from thesupply by R4 and C3, and C2 grounds TR1base (b) at radio frequencies.

Input impedance is of the order of 50ohms: a reasonable match to coaxial aerialcables. Quite short whip aerials are a quar-ter wavelength long at v.h.f., and they, too,can be adjusted to present a decent match.

The output from the r.f. stage is lightlycoupled by L1 to the tuned circuit formedby coil L2 and tuning capacitor VC1.Increasing coupling to maximize signaltransfer is likely to result in the erraticoperation of the detector.

Super-regenerative detector, TR2, is

configured as a Colpitts oscillator. Colpitt’scapacitor tapping is a little obscure withthis v.h.f. version of his circuit. The inter-nal gate-source capacitance of TR2 formsone element and trimmer capacitor VC2the other. The trimmer presets the feedbackthat makes the transistor oscillate at thesignal frequency.

The source (s) of TR2 is held at r.f.potential by r.f. choke L3 and C5 is a d.c.blocking capacitor. Quenching action isadjusted by potentiometer VR1, the rangeof control being confined to the criticalregion by resistor R6. Supply line decou-pling is provided by R5 and C4, and thiscapacitor also eliminates potentiometernoise.

The tuning capacitor VC1 is one of the5pF to 25pF f.m. gangs of a polyvaricon.

Stray capacitances with this simple receiv-er are comparatively low, and the swing isconsequently too great for the required tun-ing range (88MHz to 108MHz). Moreover,if the tuning capacitance is too high, thesuper-regenerator will behave erratically ornot function at all. Accordingly, fixedcapacitor C6 reduces the maximum capac-itance to suit the tuning range, and alterna-tive values for different bands are given inFig.5.5. Winding and construction detailsof the coils, including r.f. choke L3, arealso depicted in Fig.5.5.

An audio signal is developed across TR2

source resistor R7; and C7, R8 and C10remove residual radio and quench frequen-cies. The signal is then applied to the base(b) of audio preamplifier TR3 by d.c.blocking capacitor C9.

The preamplifier stage is biased by R9and emitter resistor R12. Resistor R11forms TR3’s collector (c) load which isshunted by capacitor C8 in order to attenu-ate the higher audio frequencies.Decoupling is effected by R10 and C14.Blocking capacitor C15 couples the outputto Volume control potentiometer VR2.

Audio and radio frequency bypasscapacitors, C12 and C13, ensure the


Fig.5.3. Full circuit diagram for a Super-Regenerative V.H.F. Receiver. Successful alternative semiconductors are shown inset.The other half of switch S1(b) controls the power supply to the simple Power Amp module from Part 2.

stability of the circuit, and resistor R13 iso-lates the battery and its supply leads atradio frequencies (signal pick-up by off-board wiring can make simple v.h.f.receivers behave erratically). The otherhalf of the On/Off switch S1 (S1b) controlsthe supply to the audio power amplifiermodule, see Part Two.

As with the Regenerative Radio (Part 3),separate battery supplies for the receiverand audio power amplifier are strongly rec-ommended. Even low power audio ampli-fiers can cause significant voltage swingson the supply rail and this will disturb theoperation of receivers of this kind, evenwhen decoupling is generous.

Transistor types are not critical, and base

connections for a number of alternativedevices are included in Fig.5.3. The r.f.stage transistor, TR1, should have a highfT, preferably not less than 500MHz.

Of all the devices tested in the TR2position, only the 2N3819 would oscillateup to 150MHz, and available samples ofJ310 did not work well in this circuit. Thesuggested alternative transistors will,

however, function on the v.h.f. f.m. band.Any small-signal npn transistor shouldwork in the TR3 position, but a low-noisedevice with an hfe of 500 or more is to bepreferred.

All of the components, with the excep-

tion of tuning capacitor VC1, swing limit-ing capacitor C6 and potentiometers VR1and VR2, are mounted on a single-sidedprinted circuit board. The topside compo-nent layout, full-size copper foil master

and the off-board wiring details are illus-trated in Fig.5.4. This board is availablefrom the EPE PCB Service, code 419,together with the small variable tuningcapacitor p.c.b., code 406.

Begin construction by inserting solderpins at the lead-out and coil mountingpoints, then solder the resistors and capac-itors in position. Mount the semiconduc-tors last. The leads of TR1 and TR2 shouldbe kept quite short: just leave sufficient toattach a miniature crocodile clip to act as aheat shunt during soldering.


Fig.5.4. Printed circuit board component layout,interwiring to off-board components and full-sizeunderside copper foil master for the Super-Regenerative Receiver. The small tuning capaci-tor p.c.b. first appeared in Part 3, code 406.

Completed Super-Regen. circuit board.

The simple coils are hand-wound and

full details are given in Fig.5.5. Couplingand tuning coils, L1 and L2, are formed bywinding them around the shank of a 10mmdrill bit. Wind the turns on tightly, andbend the ends as shown in the diagram,before withdrawing the drill from the coil.

Radio frequency choke L3 is wound ona short length of 6mm diameter plasticpotentiometer spindle (you could use apiece of wood dowelling). Holes, drilledclose to the ends, secure the turns of wire.

Scrape the enamel from the wire untilbright metal is exposed, then thoroughly“solder tin” the ends of the windings.Failure to make a perfect connection willprevent the receiver functioning at thesefrequencies.

Coupling and tuning coils are mountedon solder pins. The short, horizontal exten-sion to the tuned winding permits the coilto be squeezed or extended to adjust itsinductance and frequency coverage.

Quite small changes in coil dimen-sions, wiring and components have asignificant effect on coverage at these fre-quencies. If, however, the receiver isconstructed as described, the coils depict-ed in Fig.5.5. should be within gentle“squeezing and pulling” range of thespecified bands.

The Range 1 coils, which span the v.h.f.f.m. band, should be soldered in place first.Broadcast signals on these frequencies arestrong and reliable, and this is of greatassistance during the setting-up process.The Range 2 coils cover the v.h.f. AircraftBand, and Range 3 the Two Metre Amateurband.

Check the printed

circuit board for poorsoldered joints andbridged tracks. Checkthe semiconductorsand electrolytic capa-citors are correctly orientated. If all is inorder, the board can be tested on the work-bench before being mounted on a chassisor in an enclosure.

Connect variable capacitor VC1 to thereceiver p.c.b. using the leads of capacitorC6 as the “hot” connection, and wire upcontrols VR1 and VR2, see Fig.5.4. Usescreened audio/coaxial leads to connect thereceiver to the audio power amplifierdescribed in Part Two. Set the vanes oftrimmer capacitor VC2 to quarter-meshand connect the batteries, via S1. Currentconsumption should be in the region of3mA.

Advance Quench control VR1 until aloud hiss is heard in the speaker, indicat-ing that TR2 is oscillating and quenchingor squegging. If the set seems dead, or ifthe hiss dies away at the maximum orminimum setting of the tuning capacitor,adjust trimmer VC2. The setting of VC2is fairly critical and varies from transistorto transistor. It should, however, liebetween 10 percent and 50 percent of fullmesh.

Now connect a short length of flex(about 600mm or 24in.) to act as an aerial,and rotate tuning capacitor VC1 very slow-ly. Broadcast transmissions should beheard. When they have been identified, coilL2 can be compressed or expanded untilthe entire band is covered.

Despite the fairly broad selectivity of thesuper-regenerative detector, tuning at thesefrequencies is quite critical. Demodulationof the f.m. signal is achieved by tuning thereceiver onto the carrier’s side skirts. Thereare thus two, closely spaced, points on thedial where each station can be heard com-paratively free from distortion.

After tuning the receiver, refine theadjustment of Quench control VR1: bestresults will usually be obtained with it setas low as possible.

The printed circuit board and tuning

capacitor must be rigidly mounted andlocated so that the wiring between the twois as short as possible. Indeed, the back ofthe capacitor should almost touch thereceiver p.c.b. Tuning will be considerablyeased if tuning capacitor VC1 is fitted withsome form of slow-motion drive.

The accompanying photographs showthe board and VC1 mounted on the metalchassis and front panel used to evaluateother receivers in the series. The arrange-ment works well and the printed circuitboard is reasonably accessible for coilchanging.

If an aluminium box is used as an enclo-sure, make sure it is big enough for thecoupling and tuning coils to be spaced atleast 25mm (1in.) from its metal sides.


Fig.5.5. Individual coils and r.f. choke details for the threetuning ranges. Tuning coils L1 and L2 both have a 10mminternal diameter. Wind around the shank of a 10mm drill

Completed receiver board with alternative bands coils.

Fig.5.6. Circuit diagram for a simple front-end modificationfor tuning the input to the receiver. Note that L4 and L5 areidentical to L1 and L2.

The electronic tuning (Varicap Tuner)system, described last month, can be usedwith this receiver; at least on the v.h.f. f.m.band. The fine-tuning version, incorporat-ing a BB105 varicap diode, should be built,and the connections between the boardsmust be as short as possible.

The varicap diode bias supply can betaken from the battery used to power thereceiver. When this arrangement is adopt-ed, it is imperative that the audio poweramplifier be connected to a separatebattery.

Readers who wish to carry out seriousexperiments with super-regenerativereceivers should fit an air-spaced variable

capacitor with ceramic insulation. AJackson C804 with a 3pF to 10pF swingwould be suitable, and series capacitor C6would only be required on Range 3. Theslightly higher Q of the air-spaced compo-nent should help to maximize the operatingfrequency of the circuit.

The grounded-base r.f. stage (TR1) iso-

lates the super-regenerative detector fromthe aerial, helps to reduce the radiation ofoscillator “hash”, and makes the perfor-mance of the receiver more predictable.However, the simple arrangement adoptedhere provides little or no signal gain, andreaders may wish to try an additional tunedcircuit at the input in an attempt to improveperformance.

The circuit diagram for a simple front-end modification is given in Fig.5.6. Aerialcoupling coil L4 and the additional tunedwinding L5 are duplicates of L1 and L2. A2pF to 22pF trimmer capacitor, VC3, tunesL5 to the centre of the band.

Emitter resistor R1 is connected to the0V rail via a tapping on L5. One turn abovethe 0V rail is a good starting point, but theposition which gives the best signal-to-noise ratio should be found by trial anderror. The emitter resistor R1 must bebypassed by additional capacitor C16.

Increasing TR1’s collector current mayimprove performance. To do this, connectresistors ranging in value from 100kilohms to 22 kilohms in parallel withR2 (the lower the value the greater thecurrent).

The additional coils and trimmer capac-itor can be mounted on a short soldertagstrip, and L4 and L5 must be orientatedat right angles to L1 and L2. If the


SUPER-REGEN. RECEIVERResistors

R1, R10, R12 470 (3 off)R2 47kR3, R6, R7, R11 10k (4 off)R4 180

R5 1kR8 15kR9 1M8R13 47


PotentiometersVR1 10k rotary carbon, lin.VR2 4k7 rotary carbon, log.

CapacitorsC1 82p disc ceramicC2, C3, C12 100n disc ceramic (3 off)C4 22 radial elect. 16VC5 1n disc ceramicC6 (Range 1) 15p ceramic “low k”

(Range 2) 10p ceramic “low k”(Range 3) 6p8 ceramic “low k”

C7 4n7 polyesterC8, C10 10n disc ceramic (2 off)C9, C15 47 radial elect. 16V (2 off)C11 47 radial elect. 16VC13, C14 100 radial elect. 16V (2 off)C16 10n disc ceramic (optional – see text)VC1 5p to 25p polythene dielectric variable

capacitor (see text)

Approx. CostGuidance Only ££1144excl. case, batt, wire, slow-motion drive & whip aerial


VC2 2p to 10p min. film dielectric trimmercapacitor

VC3 2p to 25p min. film dielectric trimmer(optional – see text)

SemiconductorsTR1 2N2369 npn small signal, high frequency

transistorTR2 2N3819 n-channel field effect transistorTR3 BC549C npn low power general purpose

transistor

MiscellaneousL1, L2,

(L4, L5) hand-wound with 18s.w.g. (16a.w.g.)enamelled copper wire – see text andFig.5.5)

L3 r.f. choke, hand-wound with 36s.w.g.(32a.w.g.) – see Fig.5.5

S1 d.p.s.t. toggle switchSK1 coaxial aerial socket

Printed circuit board available from the EPE PCB Service,code 419 (Super-Regen.) and optional 406 (T/Cap); 50g (2oz)reel 18s.w.g. (16a.w.g.) enamelled copper wire for coils; 50g(2oz) 36s.w.g. (32a.w.g.) enamelled copper wire for r.f. choke;20mm approx. length, 6mm dia., plastic rod for r.f. choke former;one large and two small control knobs; telescopic whip aerial(optional); tagstrip (see text); batteryholder, with clips; slow-motion drive, spindle extender and/or coupler (optional – seetext); multistrand connecting wire; solder pins; solder etc.

Super-Regen. Receiver board mounted on the metal chassis, via stand-off pillars,and wired to the tuning capacitor.

modification causes instability, fix a metalscreen between the tagstrip and the printedcircuit board.

This simple Super Regen. circuit effec-

tively demonstrates the extremely highsensitivity of Armstrong’s super-regenera-tive system. Frequency stability is remark-ably good, hand-capacitance effects are nottoo pronounced and the super-regeneration

control is smooth and effective. Automaticgain control action (a.g.c.) is very evident,and the loud hissing which characterizescircuits of this kind is completely sup-pressed by a strong signal.

On the v.h.f. f.m. band, careful adjust-ment of the tuning and super-regenerationcontrols enables an acceptable compromiseto be struck between audio output and dis-tortion. However, as constructors of theHazeltine Fremodyne (including the

author) found, performance falls short of aconventional superheterodyne receiverwith a ratio detector or Foster-Seeley dis-criminator as the signal demodulator.

Domestic f.m. superhets have seven ormore tuned circuits and perhaps five tran-sistors amplifying at radio frequencies. Abasic super-regenerator has only one tunedcircuit and one valve or transistor provid-ing gain at radio frequencies. Armstrong’sgenius continues to inspire.


HISTORYThe discovery of super-regeneration is attributed to that great

American radio pioneer, Howard H. Armstrong. In presenting hispaper on the technique to the American Institute of RadioEngineers in 1922, he paid tribute to the earlier work of Turnerand Bolitho. L. B. Turner had used critical biasing to hold a valveon the threshold of oscillation and make a signal-activated relaymore sensitive. Bolitho patented a refinement of Turner’s designin which the mechanical relay was replaced by a valve.

RCA purchased Armstrong’s patents and various experi-menters made attempts to apply the technique to medium wavereception which, by that time, had growing commercial impor-tance in the USA. Designs usually incorporated a small frameaerial to overcome shortcomings in selectivity, and crude audiofiltering to reduce the whistle from quenching oscillations.

For reasons already outlined in the main text, the circuit isunsuited to amplitude modulated broadcast reception, particu-larly on medium waves. Not surprisingly, therefore, the conceptwas not taken up by domestic receiver manufacturers, and own-ership of the patent rights failed to produce much income forRCA (Radio Corporation of America).

During the twenties and thirties the circuit was used exten-sively by the police and emergency services. The Second WorldWar saw the technique adopted for I.F.F (identification-friend-or-foe) responders. These devices received a radar pulse andresponded, with a minimum of delay, by transmitting an identify-ing signal.

German air interception radar systems also relied upon thecircuit. Perhaps the best known of these war-time applicationswas the American “walky-talky”, in which the super-regenerativedetector doubled as the transmitter valve.

The post-war years saw the establishment of high-quality fre-quency modulation broadcasting systems in Europe andAmerica. In 1947, B. D. Loughlin developed a design for a dou-ble-triode (12AT7) super-regenerative f.m. tuner, one valve beingused as a frequency changer, the other as the detector. Knownas the “Fremodyne”, the circuit was licensed by the AmericanHezeltine Electronics Corporation. Whilst the Fremodyne suf-fered all of the defects super-regenerators are prone to, it didhave the advantages of simplicity and low cost.

At the present time, super-regeneration is still used for simpletoy and model control receivers, car central locking and garagedoor opening systems.

Some readers who have built one of theversions of the regenerative radiosdescribed in Parts Three and Four willhave access to a Digital FrequencyCounter, and they may wish to use it togive a digital readout of receiver tuning.

Damping of the receiver’s tuned circuitmust be kept to an absolute minimum,and the signal levels across it are usuallytoo low to reliably trigger most frequen-cy counters. Direct connection is, there-fore, out of the question, and a bufferamplifier must be placed between the twounits.

The circuit diagram for a suitable Buffer

Amplifier is given in Fig.5.7.Field-effect transistor TR1 is arranged

as a common drain or source followerstage. Its high input impedance, togetherwith the low value of coupling capacitorC1 minimises disturbance and damping tothe receiver’s tuned circuit. The d.c. poten-tial on TR1 gate is held at 0V by resistorR1 and TR1’s output is developed acrosssource load resistor R3. Supply rail decou-pling is provided by R2 and C2.

The circuit output stage consists of an r.f.transistor, TR2, configured as a common-emitter amplifier. Bias resistors R4 and R5

fix the operating conditions, and the signalis applied to TR2 base via capacitor C3.The output signal is taken from TR2 col-lector to the Frequency Counter, via capac-itor C6. The collector load resistor is R7,and R6 and C5 decouple the output stage.

Fig.5.7. Circuit diagram for a Buffer Amplifier for a Digital Frequency Counter. A fre-quency counter, connected to the tuned circuit via this buffer, will give a digital dis-play of the receiver tuning. This arrangement is NOT suitable for the Super-Regen.Receiver – see text.

Most n-channel field-effect transistorswill prove suitable for TR1. The BF199,BF494 and 2N222A were in-circuit testedin the TR2 position and they all workedwell. Base connections vary and should bechecked.

All of the Buffer Amp. components are

mounted on a small printed circuit boardand the component layout, copper foil mas-ter and wiring detils are shown in Fig.5.8.This board is available from the EPE PCBService, code 420.

Solder pins at the lead-out points simpli-fy the off-board wiring, and they should beinserted into the p.c.b. first. Follow thesewith the resistors, then the capacitors and,finally, the transistors. As before, keep thetransistor leads just long enough to permitthe use of a miniature crocodile clip as aheat shunt during soldering.

Check the printed circuit board for poor

soldered joints and bridged tracks, and checkthe positioning of the transistors. Connectthe unit to a 9V supply. Current consumptionshould be in the region of 7mA.

Make a very short connection, certainlyno more than 75mm (3in.), between thebuffer amplifier’s input and the “hot” endof the receiver’s tuned circuit. Connect theamplifier’s 0V rail to the ground or 0V tagon the tuning capacitor. Power for theamplifier can be taken from the receiverbattery.

Connect the output from the bufferamplifier to the Frequency Counter via ashort (no more than 600mm or 24in.)length of screened cable.

Set the counter’s input controls, tune in a

station and advance the regeneration con-trol, turning down the receiver’s inputattenuator, as necessary. When tuning iscorrect and the regeneration setting hasbeen optimized, the counter should displaythe “tuned” frequency.

The counter will normally only give afrequency reading when a station is tunedin. To check tuning on a quiet part of thedial, advance the regeneration control untilthe Q-multiplier is oscillating. The counterwill then display the operating frequency.

When receiving single-sideband signalsthe regeneration control has to be advanced

until the Q-multiplieris oscillating in orderto replace the carriersuppressed at thetransmitter. Thecounter will, there-fore, give a continu-ous frequency readoutas the receiver istuned across the ama-teur bands.

The Buffer

Amplifier andFrequency Countercombination cannotbe used with theS u p e r - R e g e n .Receiver. Here theoscillating stage is“squegging” and the

counter cannot distinguish between signaland quenching frequencies. Moreover,buffer amplifier loading, although extreme-ly light, makes detector operation erratic.

The arrangement is also unsuitable forsuperhet receivers (to be offered in a laterissue) in which the oscillator runs at a high-er frequency than the reception frequency.Additional circuitry is required to accom-modate the difference.

Direct conversion receivers incorporatean oscillator that operates at signal fre-quency. The Buffer Amplifier and counterset-up will, therefore, form an accuratedigital tuning display. Widely used by ama-teur radio enthusiasts for the reception ofsingle-sideband transmissions, this receiv-ing system will be covered next month.


Fig.5.8. Buffer Amplifier printed circuit board component layout, wiring details andfull-size copper foil master.

BUFFER AMP.Resistors

R1 1MR2, R6 150(2 off)R3, R7 470(2 off)R4 33kR5 10kR8 120


CapacitorsC1 2p2 disc ceramicC2, C4

to C6 100n disc ceramic (4 off)C3 1n disc ceramic

SemiconductorsTR1 2N3819 n-channel field

effect transistorTR2 BF241 npn small signal,

high frequencytransistor

MiscellaneousPrinted circuit board available from the

EPE PCB Service, code 420; input con-nector, to suit frequency counter; single-core screened audio cable; multistrandconnecting wire; solder pins; solder etc.



The Buffer Amplifier wired to the Regenerative Radio(Part 3) and the tuning capacitor p.c.b.s.

A frequency counter wired, via theBuffer Amplifier, to the RegenerativeRadio and showing an amateur bandfrequency readout.

A simple technique adopted by radioamateurs involves feeding signals pickedup by the aerial straight into a productdetector and amplifying the audio frequen-cy output. Sometimes the signals areamplified before being passed to the detec-tor, but the crucial feature of the techniqueis the direct conversion of the radiofrequency transmissions to an audiofrequency signal, hence the term directconversion.

Passive product detectors use an

arrangement of between one and fourdiodes to combine the two radio frequencyinputs and produce an audio output. A typ-ical circuit diagram is given in Fig.6.2. Itsoperation will be described later, but theimportant features are simplicity, low cost,and immunity to overloading by strongsignals. On the down side, the circuitattenuates the signal by about 6dB.

The simplest receivers place a passivedetector of this kind immediately after theaerial tuned circuit. There is no amplifica-tion at radio frequencies, and a very highgain audio amplifier is needed to overcomedetector losses and make the signalsaudible.

effect, a mirror image of reverse polarityremoved by the rectifying action of thedetector in the receiver. With conventionalamplitude modulation, around 75% of thetransmitter power is, therefore, wasted. Bysuppressing the carrier and one of its sidebands, transmitter efficiency is greatlyincreased and bandwidth halved.

Equipment for the reception of single-

sideband transmissions must include anoscillator to replace the missing carrier.When the carrier has been restored, a diodeor some other non-linear detector canmake the signal intelligible in the usualway.

Simple regenerative receivers arecapable of resolving single-sideband trans-missions if the Q-Multiplier is made tooscillate and restore the missing carrier.An Amateur Bands Regenerative Receiverwas described in Parts Three and Four.

Better performance can be obtainedfrom a mixing circuit that combines thesignal and local oscillation and, at the sametime, recovers the wanted audio. Circuits ofthis kind are known as product detectors.They function in the same way as mix-ers in superhet receivers, butthe output is at audiorather than at radiofrequencies.

SUPPRESSED carrier single-sideband(s.s.b.), a highly efficient method oftransmitting speech by radio, will be

considered this month. A popular and sim-ple technique for receiving these signals isknown as direct conversion, and a circuit isincluded.

Radio frequency transmissions cannot,

by themselves, convey information. Theyare no more than carriers, and the speechor music has to be impressed upon them bya process known as modulation.

The amplitude of a carrier can be variedin sympathy with a speech signal, and theprocess is known as amplitude modula-tion. This is the oldest and still the widestused method of transmitting speech andmusic by radio. It was described in PartOne of the series.

If a 1000kHz carrier is amplitude modu-lated by a 3kHz signal, two sidebands,each 3kHz wide, are produced. The radiotransmission then occupies a bandwidth of6kHz, extending from 997kHz to1003kHz.

Assuming a reasonable depth of modula-tion, some 50% of the total power suppliedby the transmitter is expended on the carri-er and 25% on each of the sidebands. Justone sideband is carrying all of the informa-tion in the signal. The other sideband is, in

Part 6: Single-sideband and direct conversion.

HISTORYSingle-sideband transmission was invented by John R.

Carson, an American engineer. Initially, the technique was usedto conserve channel space in carrier-current telephonesystems, but, by the late 1920s, it was being deployed at low(60kHz) radio frequencies for the transatlantic telephoneservice.

During the Second World War, single-sideband transmitterswere used, by the American forces, for long distance radio com-munication. The British avoided the technique, claiming it wastechnically too demanding for a battlefield environment, but theyadopted it after the war.

American amateurs began to test the system in the 1940s,and it was taken up, by amateurs, world-wide, during the sixties.At the present time it is the standard mode of speech transmis-sion on all of the high-frequency amateur bands.

Everyday Practical Electronics, November 2003 793

794 Everyday Practical Electronics, November 2003

Even when an efficient aerial is avail-able to deliver the strongest possible sig-nals to the receiver, results are likely to bedisappointing. Moreover, the high levels ofaudio frequency amplification are usuallyaccompanied by hum pick-up andmicrophony problems.

The microphony arises because thereceiver components and wiring act as amicrophone, picking up vibrations. Thismanifests itself as clanging and ringingsounds when the receiver is touched oradjusted, the sounds growing into howls asfeedback from the loudspeaker takes over.

The situation can be improved by pre-ceding the passive detector with a stage ofradio frequency amplification. Theimproved gain distribution does a greatdeal to overcome the problems outlinedabove, but receivers of this kind stillrequire high levels of audio amplification.

An active product detector, which gives

signal amplification rather than attenua-tion, will usually transform the perfor-mance of these simple receivers. Adrawback with this arrangement is the

possibility of cross modulation beingcaused by overloading.

When this occurs, weak signals aremodulated by strong ones within the prod-uct detector and the strong signals seem tospread across the entire band. Careful useof the receiver’s input attenuator will domuch to prevent this.

Unwanted envelope detection, or rectifica-tion, of powerful, amplitude modulated trans-missions is a more prevalent cause of break-through with simple active or passive productdetectors. Again, the strong signals spreadacross the band and cannot be tuned out.

The full circuit diagram for an amateurbands Direct Conversion Receiver isshown in Fig.6.1. A radio frequency ampli-fier, consisting of TR1 and TR2, precedesthe product detector, TR3. The carrierreplacement oscillator is TR5, its outputbeing buffered and amplified by TR4.Audio amplification is provided by TR6and TR7.

Field-effect transistors, TR1 and TR2

form a cascode where TR1 is configured inthe common source and TR2 in the ground-ed gate mode. This combination gives about20dB of gain. Input and output impedancesare high and damping on the tuned circuits,L1/C4 and L2/C5, is minimal.

Source bias to TR1 is provided by resis-tor R3 which is bypassed by capacitor C6.The gate (g) of TR2 is held at half the sup-ply voltage by resistors R1 and R2, andgrounded at radio frequencies by capacitorC3. The stage is decoupled from the supplyrail by resistor R4 and capacitor C2.

All of the windings of coil L1 are con-nected in series to provide an appropriatetapping ratio for Input Attenuator poten-tiometer VR1. This helps to maintain the Qfactor and selectivity of the tuned circuit.The aerial is connected to the circuit viacapacitor C1, which is included to protectany preamplifiers or converters againstshorting by VR1.

The tuned circuit formed by coil L2 andcapacitor C5 acts as the drain (d) load forTR2, and the output is coupled to the gate(g) of TR3, the product detector, by capac-itor C7.

Despite the isolation between the inputand output ports afforded by groundingthe base of TR2, the cascode r.f. stage isnot unconditionally stable. Indeed,because of the light loading on both tunedcircuits, the stage will come close to oscil-lation when they are precisely aligned tothe same frequency.

Accordingly, resistor R5 is connectedacross the coupling winding L3 to provideadditional damping. This increases thestability margin and the problems areavoided. When the alternative passivedetector circuit (described later), given inFig.6.2, is used, the damping imposed bythe diodes and balance potentiometerserves the same purpose.

Field-effect transistor TR3 is configured

as a product detector. Signal input is to thegate, oscillator input to the source throughcapacitor C11, and the audio frequencyoutput is taken from the drain.

The drain load resistor is R8 and R7, C8and C9 decouple the stage from the supplyrail at audio and radio frequencies. Sourcebias for TR3 is provided by resistor R9,and the d.c. potential on the gate is held at0V by R6.

A Colpitts oscillator, TR5, replaces the

carrier suppressed at the transmitter. Thecapacitance tap across tuning coil L4 isformed by capacitors C20 and C21, andfeedback is developed across emitter resis-tor R17. The stage is biased by resistorsR18 and R19, and heavily decoupled byR15, R16, C14 and C15.

All sections and windings of oscillatorcoil L4 are connected in series to obtain therequired inductance, and it is brought toresonance within the 7MHz amateur bandby capacitor C22. Main tuning is by VC1,one of the 5pF to 25pF f.m. gangs of apolyvaricon tuning capacitor, and capacitorC23 reduces its swing to restrict coverageto the 7MHz amateur band.

Without some means of fine tuning, thesingle-sideband signals will be difficult toclarify. A shift of one or two picofarads isall that is required, and the varactor prop-erties of an ordinary power rectifier diode,D1, can provide this.

Reverse bias (Fine tuning) is controlledby potentiometer VR2. Resistor R20 iso-lates the signal circuits and capacitor C24prevents the bias being shorted by tuningcoil L4. Increasing the bias reduces thecapacitance across the semiconductorjunction of D1.

The output from oscillator TR5 has to be

amplified to ensure the correct operation ofeither the transistor or the alternative diodemixer. The oscillator also needs isolating inthe interests of frequency stability.

Buffer transistor TR4 performs thesefunctions. Oscillations are applied to itsbase by capacitor C16, and the stage isbiased by resistors R13, R14 and R12. Theoutput is developed across collector loadresistor R11; and R10 and C10 decouplethe stage from the supply rail.

The directly coupled preamplifier

formed by transistors TR6 and TR7 is anadaptation of the front-end circuitry foundin most high-fidelity amplifiers.

Current through TR6 is kept below100A by high-value collector load resis-tor R22 and d.c. feedback resistor R21. Thelow collector current reduces the noiseintroduced by the stage.

Preset VR3 sets the gain of the two tran-sistor combination. The calculated valuewith VR3 at maximum is approximately200, and 2000 with it turned to zeroresistance.

The biasing of transistor TR7 is deter-mined by resistors R21 and R26. Theamplifier’s output, developed across col-lector load resistor R25, is coupled to thesignal feedback loop by capacitor C29, andto the Volume control potentiometer VR5by C30. Supply line decoupling is provid-ed by R24 and C31 and, in view of the highlevel of gain, is generous.

Signal-to-noise ratios are improved, and

clarity increased, if the frequency responseof a speech communication system isrolled off below 300Hz and above 3000Hz.

The low values of capacitors C17 andC30, and bypass capacitor C27, reduceresponse to the lower audio frequencies(gain reducing negative feedback, whichincreases as frequency lowers, is intro-duced because of the low value of C27).Bypass capacitors, C12, C18, and C28 cur-tail the high frequency response.

Signal feedback preset VR4 can be pro-gressively shunted by capacitor C26. Thisincreases negative feedback, and reducesgain, at the upper audio frequencies andenables the response of the amplifier to betailored to suit individual preferences.

The active product detector depicted in

the circuit of Fig.6.1 is not balanced andthis makes the receiver more vulnerable tobreakthrough from powerful broadcasttransmissions.

An alternative, balanced arrangement isshown in Fig.6.2, where signal diodes D2and D3 are switched in and out of conduc-tion, by the oscillator voltage, in order toproduce the desired mixing action.

To avoid confusion, it should be stressedthat the silicon diodes are used here asswitches and their poor performance asrectifiers of weak signals is not relevant.The oscillator must, however, be vigorousenough drive the diodes into conduction,and buffer stage TR4 ensures the deliveryof sufficient power.

Balancing potentiometer VR6 optimizesimmunity to the envelope detection ofstrong, amplitude modulated signals. Some

advocates of the circuit bring the poten-tiometer out as a panel control so that it canbe adjusted to reduce break-through undervarying reception conditions.

The balanced diode arrangement shownin Fig.6.2 attenuates signals by about 6dB.The active product detector included inFig.6.1 gives about 6dB of gain. The 12dBdifference is very noticeable, but readersmay wish to try the balanced alternativeand the increased protection against break-through that it offers.


Fig.6.2. Alternative passive balancedproduct detector. This circuit can besubstituted for TR3, the active detec-tor, see text.

Fig.6.1. Circuit diagram for the Direct Conversion Receiver for the 7MHz (40m)amateur bands.

L5R

.F.C

HO

KE

1mH L5

R.F

.CH

OK

E1m

H

*S1a

*SE

E T

EX

T

*D1

1N40

02

Even small audio power amplifiersimpose voltage swings on the supply rails,especially when batteries are ageing. Thesesignal induced fluctuations can cause lowfrequency instability, and the receiver andpower amplifier must have separate batter-ies. This is particularly important whenelectronic tuning bias is derived directlyfrom the receiver battery.

Switch S1a controls the supply to theReceiver; S1b (the other half of the switch)is used to switch the amplifier battery.

Transistor types are not critical and base

connections for alternative devices areshown inset in Fig.6.1. In the interests ofstability, the r.f. stage field-effect transistors

should have the same lead out sequence asthe specified 2N3819. Buffer stage transis-tor TR4 must be a small-signal r.f. type withan fT of at least 200MHz.

Almost any small-signal npn transistorin the BC107, BC108 and BC109 families,or their plastic-cased variants, should workin the oscillator and audio preamplifierstages. For best performance, TR6 shouldbe a high-gain, low-noise device such asthe BC549C.

The forward resistance of the diodes usedin the alternative product detector (Fig.6.2)should, preferably, be matched with a testmeter, and it will be easier and cheaper toproduce a matched pair if silicon types areused. Most small power rectifier diodes canbe pressed into service as a “varactor” diodefor D1. Readers who prefer to use a true var-actor diode should fit a BB105.

Any miniature 1mH r.f. choke will besuitable for L5. If a wire-ended componentis substituted for the bobbin wound choke,mount it vertically and as close as possibleto the printed circuit board.

Most of the components for the Direct

Conversion Receiver are assembled on asingle printed circuit board (p.c.b.). Thetuning capacitor (VC1) has its own smallp.c.b. for ease of assembly and mounting.These boards are available from the EPEPCB Service, codes 423 (Dir. Conv. Rec.)and 406 (T/Cap).

The Receiver topside p.c.b. componentlayout, together with the underside copperfoil master and interwiring to off-boardcomponents, is illustrated in Fig.6.3.Solder pins, located at the lead-out points,


DIRECT CONVERSION RECEIVERResistors

R1, R2 47k (2 off)R3 270R4, R5, R7, 220 (4 off)

R15R6 470kR8, R9, 4k7 (4 off)

R13, R17R10, R23 47 (2 off)R11, R16 470 (2 off)R12 100R14 10kR18, R19, 220k (4 off)

R21, R22R20 100kR24 150R25 680R26 560R27 1k8 (optional – see Fig.6.2 and text)

All 0·25W 5% carbon film or better

PotentiometersVR1 1k rotary carbon, lin.VR2 100k rotary carbon, lin.VR3 470 enclosed carbon presetVR4 100k enclosed carbon presetVR5 4k7 rotary carbon, log.VR6 1k enclosed carbon preset

(optional – see Fig.6.2 and text)Capacitors

C1, C11, C19 1n ceramic (3 off)C2, C3, C6, C9, 100n ceramic (8 off)

C13 to C15,C25

C4 68p ceramic “low k” or polystyreneC5 100p ceramic “low k” or polystyreneC7 56p ceramic “low k”C8 100 radial elect. 25VC10 47 tantalum bead 35VC12, C18 47n ceramic (2 off)C16 10p ceramicC17 470n ceramicC20 180p polystyrene or ceramic “low K”C21 470p polystyrene or ceramic “low k”C22 39p polystyrene or ceramic “low k”C23 27p polystyrene or ceramic “low k”C24 4p7 ceramic “low k”C26, C28 10n ceramic (2 off)C27, C29 47 radial elect. 25V (2 off)C30 1 radial elect. 25VC31 470 radial elect. 25VVC1 5p to 25p one f.m. gang of a polythene

dielectric variable capacitor (see text)Semiconductors

D1 1N4002 rectifier diode (see text)


excl. case, slow-motion drive, batt. & optional items


D2, D3 1N4148 signal diode (2 off)(optional – see Fig.6.2 and text)

TR1, TR2, 2N3819 n-channel field effect transistorTR3 (3 off)

TR4 BF199 npn high frequency transistor orsimilar (see text)

TR5 2N3904 npn low power, small signal,transistor

TR6, TR7 BC549C npn small signal transistor

MiscellaneousL1, L2/L3 154FN8A6439 Toko screened (metal can)

coil (2 off)L4 KXNK3767EK Toko screened (metal can)

coilL5 1mH r.f. choke (see text)S1 d.p.s.t. toggle switch

Printed circuit boards available from the EPE PCB Service,codes 423 (Dir. Con. Rec.) and 406 (T/Cap); screw terminal foraerial and earth (2 off); slow-motion drive (optional); one largeand three small plastic control knobs; audio type screened cable;multistrand connecting wire; p.c.b. stand-off pillars; front panelcard and protective 2mm thick Perspex sheet; battery holder andconnector; solder pins; solder etc.

Note: Case is Regenerative Radio from Part 3.

SWITCHED 3-BAND VERSION(Component changes and additions – see Fig.6.4 and

Table 6.1)Capacitors

C20 (14MHz Band) 82p polystyrene or ceramic “low k”C21 (14MHz Band) 180p polystyrene or ceramic “low k”C22 (3·5MHz Band) 82p polystyrene or ceramic “low k”

(14MHz Band) not required – see textC23 (3·5MHz Band) 200p polystyrene or ceramic “low k”

(14MHz Band) 15p polystyrene or ceramic “low k”VC1 (3·5MHz Band) 5p to 130p polythene dielectric

variable capacitor (one a.m. gang ofspecified cap.)

(14MHz Band) 5p to 25p polythene dielectric variablecapacitor (one f.m. gang of specifiedcap.)

MiscellaneousL1, L2/3

(3·5MHz Band) 154AN7A6440 (2 off)(14MHz Band KXNK376EK (2 off)

L4 (3·5MHz Band) 154FN8A6439(14MHz Band) KXNK37677EK

S2 4-pole 3-way rotary switch

Note: The main circuit (Fig.6.1) and components list coversthe 7MHz Band.

Also, you will need two additional main p.c.b.s (code 423) ifyou go for the switched-band option; but you only build the frontend up to, and including, r.f. choke L5 and resistor R20.

will simplify off-board wiring, and theyshould be inserted first. Follow these withthe tuning coils.

Next, solder in place the resistors, thenthe capacitors, smallest first, and, finally,the diodes and all the transistors.Semiconductor leads should be just longenough to permit the use of a miniaturecrocodile clip as a heatsink duringsoldering.

If the alternative product detector(Fig.6.2) is used, mount preset VR6 anddiodes D2 and D3 on the p.c.b. solder pinsconnected to coil L3. Resistors R5 to R9,together with capacitors C7, C8, C9 andC12, and transistor TR3, must be removedfrom the board.

Failure to remove capacitor C12 willresult in the oscillator output being shortedto the 0V rail. Use wire links to connect thebuffer stage, TR4, to the diodes via capac-itor C11, and the output from the diodes tor.f. choke L5.

The printed circuit board can be testedbefore being mounted on a chassis or in anenclosure. First, check the placement of allof the components and examine the printedcircuit board for poor soldered joints andbridged tracks.

Use the leads of capacitor C23 to makethe “hot” connection to tuning capacitor

VC1 and wire up the three off-board poten-tiometers. Connect the audio output to theSpeaker Amplifier described in Part Twowith screened cable. Receiver current con-sumption should be approximately 13mAwith a 9V supply.

Set the cores of all of the coils about twoturns down from the tops of the cans. The coreof L4 is particularly brittle and a plastic trim-ming tool should be used for the adjustments.


Completed circuit board for the Direct Conversion Receiver.

Fig.6.3. Direct Conversion Receiver (7MHz)printed circuit board component layout,interwiring and full-size copper foil master.The small tuning capacitor p.c.b. firstappeared in Part 3.

L5

Connect an aerial (at least 10 metres or30ft of wire located as high as possible).Set input Attenuation control VR1 for max-imum input and turn up Volume controlVR5. There should be a faint hissing in theloudspeaker and signals should be heard ifTuning capacitor VC1 is slowly rotatedand/or the core of L4 is adjusted.

If you have access to a Communications

Receiver, connect a short aerial wire to itand lay the wire near to the DirectConversion set. Switch on the communica-tions receiver’s b.f.o. (beat frequency oscil-lator) and tune it to 7MHz.

Turn Tuning capacitor VC1 to maximumcapacitance, and slowly adjust the core ofL4 until a tone is heard in the speaker of thecommunications receiver. This receiver is,of course, picking up the signal radiated bythe oscillator in the direct conversion set.

With input and volume controls turnedup, rotate VC1 towards minimum capaci-tance. Even if tuning coils L1 and L2 arebadly out of alignment, the direct conver-sion set should pick up some signals.

When a single-sideband transmissionhas been tuned in, adjust the cores of L1and L2/L3 for maximum output. It will benecessary to turn back the Input Attenuatorand the Volume control as the circuits arebrought into alignment.

If a communications receiver is not

available, use the simple Crystal Markerdescribed in Part Five. Placing this unitclose to the input of the direct conversionreceiver will inject a 7·16MHz signal. Thisis just above the UK upper band limit of7·1MHz (at the centre of the USA 7MHz to7·3MHz allocation).

Readers in the UK should set VC1 closeto minimum capacitance before adjustingthe core of oscillator coil L4 until a tone isheard in the speaker. If necessary, switchthe marker on and off to make sure that thecorrect signal has been identified. Adjustthe cores of L1 and L2/3 to peak theresponse.

Connect the long aerial wire tothe receiver, slowly turn VC1towards maximum capacitance andtune in a single-sideband signal.Adjust the cores of L1 and L2/L3for maximum output.

The printed circuit board and

tuning capacitor must be rigidlymounted, preferably in a diecast oraluminium box. At the very least analuminium chassis and front panelshould be provided.

The accompanying photographsshow the board mounted on thechassis and front panel assemblyused for the evaluation of most ofthe receivers in the series. Thisarrangement works well andmicrophony (feedback via the speaker) isnot a problem at normal volume levels.

Keep the tuning capacitor, VC1, veryclose to the connecting pins on the receiv-er printed circuit board. Because of theprovision of a fine tuning system, it is notabsolutely essential to fit a slow motiondrive to VC1. It will, however, make thereceiver more pleasant to operate.

Some readers may wish to extend cover-

age to the 3·5MHz and 14MHz amateurbands. Attempts to switch the tuning andoscillator coils are likely to result in unsta-ble and erratic operation. A better solutionis to construct three separate front-endboards. (Components up to, and including,r.f. choke L5 and resistor R20 are required.)

A block diagram of the arrangement isgiven in Fig.6.4, where a four-pole, three-way, rotary switch S2 connects the threeboards into circuit. The aerial is switchedfrom board-to-board by section S2a, thebattery power supply by S2b, the audiooutput from the product detector by S2c,and the fine tuning voltage from the sliderof VR2, by S2d.

The polythene dielectric tuning capaci-tors used in a.m./f.m. receivers have two

25pF and two 130pF (or thereabouts)gangs. One of the 25pF f.m. gangs is con-nected to the 7MHz board, the other to the14MHz board. The 3·5MHz front-end istuned by one of the 130pF a.m. gangs.

Providing each board with its own vari-able capacitor avoids the problems associ-ated with switching high impedance r.f.circuits. Only one r.f. lead, the aerial, isswitched. This is at low impedance and canbe screened without causing problems.

Connections between the tuning capaci-tor gangs and the three front-ends must beas short as possible. This calls for a com-pact arrangement of the boards around thetuning capacitor.

Aerial and audio links between theboards and switch S2 must be screened.Earth the screen at one end only: do not useit to carry the negative supply rail to theboard.

Align the additional boards in one of theways described earlier. Details of amateur


Receiver p.c.b. mounted on the metal chassis and wired to thetuning capacitor board.

Fig.6.4 (right). Block diagram showing switching of three p.c.b.“front-ends” to give coverage of the 80 (3·5MHz), 40 (7MHz)and 20 (14MHz) metre amateur bands. VC1 is a 4-ganga.m./f.m. variable capacitor, one gang permanently connectedto each board. One a.m. gang is not connected.

Completed 7MHz version of the DirectConversion Receiver. If a 3-Band ver-sion is to be built, the band changerotary switch is mounted in theblanked-out hole just below the tuningknob.

band allocations were given in Part Four.Toko coil numbers and tuning and swing-reducing capacitor values for the 3·5MHzand 14MHz bands are given in Table 6.1.

Tuning has to be very carefully adjusted

to transform the garbled sounds into clearspeech. Fine tuning potentiometer VR2shifts the tuning across just one or twoamateur transmissions and is very usefulfor clarifying signals. Reception may alsobe improved by an earth connection.

Activity on the amateur bands varies.The 7MHz allocation was chosen for thereceiver described here because plenty ofsignals can usually be heard during the day.When setting up the tuning on 3·5MHz or7MHz, listen in around 10a.m., when thesebands are busy. Activity on 14MHz is morevariable, but the early afternoon will usual-ly produce some signals.

Readers who have access to a frequencycounter can use it to obtain a digital displayof operating frequency. A buffer circuitwas described last month. It should be con-nected, by very short leads, to the emitterof oscillator transistor TR5.

The version with an active product detec-

tor is sensitive. Using ten yards of flex as anaerial, quite weak signals can brought up toa good loudspeaker volume when the sim-

ple power amplifier described in Part Twois used. Gain preset VR3 will probably haveto be turned well down. If the passive prod-uct detector is used, sensitivity should beadequate if VR3 is set for maximum gain.

Signals are reproduced with great clari-ty, and the receiver is not fatiguing to listento. After a five-minute warm-up period,frequency drift is minimal.

Unlike diodes and other devices thatdemodulate by a rectifying action, the het-erodyning product detector will continue tofunction down to the lowest signal levels.In theory, therefore, the ultimate sensitivityof receivers with product detectors shouldbe greater. In practice, received and inter-nally generated noise are limiting factors.

The receiver cannotmatch the selectivity ofa superhet. Its perfor-mance in this respectis similar to theRegenerative Receiverdescribed in Part Four.When the bands arebusy, the severe crowd-ing of transmissionsresults in a faintbackground of unintel-ligible chattering. How-ever, only the wantedsignal is clarified, andthis makes the interfer-ence less distracting.

There are no discernible overloading orcross-modulation problems with eitherproduct detector. Indeed, the feature whichseparates this receiver from the regenera-tive set is its ability to cope with strong sig-nals without the need for adjustment of theinput attenuator.

Breakthrough from powerful broadcasttransmitters can be a problem after dark,particularly on the 7MHz band. Carefuladjustment of the cores of L1 and L2/L3will usually clear up the problem, andrefinements of this kind should be carriedout during the setting up process. Keepingthe signal input low will also minimizeinterference of this kind.

Next Month: The Superhet


Table 6.1: Toko Coils and Tuning Capacitor Values for threepopular Amateur Bands (80, 40 and 20 metre)

Band L1 C4 L2/L3 C5 L4 C22 C23 VC1 C20 C21(pF) (pF) (pF) (pF) (pF) (pF) (pF)

3·5MHz 154AN7 33 154AN7 47 154FN8 82 200 130 180 470(80) A6440 A6440 A6439

7MHz 154FN8 68 154FN8 100 KXNK 39 27 25 180 470(40) A6439 A6439 3767EK

14MHz KXNK 47 KXNK 82 KXNK – 15 25 82 180(20) 3767EK 3767EK 3767EK

Notes.(1) Fixed capacitors must be polystyrene or low-k ceramic types.(2) 3·5MHz band: the values quoted here will give coverage of the wider USA allocation.(3) 7MHz band: the C23 value given here restricts coverage to the UK’s 7.1MHz band limit.

For the wider 7·1MHz to 7·3MHz allocation, connect VC1 directly across L4.(4) The quoted tuning capacitor values are nominal. Measured values for the component

used in the prototype receivers are: f.m. gangs, 4pF to 22·5pF; a.m. gangs, 4pF to127pF.

A small signal r.f. transistor must be used for TR5 on the 14MHz board to maintain oscilla-tor output at the higher frequency. Any of the r.f. types listed in Fig.6.1, together with the2N2369, should prove suitable.

The Receiver board wired to the Speaker Amplifier (Part 2)using screened cable. These boards must have separatebattery supplies.

The two separate power supply battery“packs”, one for the receiver and theother for the amplifier, located on therear panel.

Calibrated tuning dial (half-size approx.) for the 7MHz ver-sion of the Direct Conversion Receiver.

Germanium diodes are still widely usedfor the detection or demodulation of the sig-nal (see Part One). In addition to recoveringthe audio modulation, the diode produces ad.c. voltage which varies relative to signalstrength. This is used to change the basebias on the transistors in the i.f amplifierand reduce their gain when strong signalsare being received. This facility is known asautomatic gain control, or a.g.c. for short.

Audio amplifier stages are similar tothose discussed earlier in the series (seePart Two).

Superhet receivers are prone to spurious

responses. At their simplest, they take theform of a whistle that changes in pitch asthe set is tuned through a station. The prin-ciple cause is second channel or “image”interference.

Second channel interference arisesbecause the mixing process can produce anoutput, at the intermediate frequency, fromtwo, different, incoming signals. These areat the oscillator frequency minus the inter-mediate frequency, and at the oscillatorfrequency plus the intermediate frequency(the “second channel”). The image signalis, therefore, spaced from the wanted sig-nal by twice the intermediate frequency.

Whistles are caused when the interfer-ing signal is not quite twice the i.f. dis-tance from the wanted station, say one ortwo kilohertz out, the resulting beat pro-ducing the whistle as the receiver is tunedthrough the station.

Strong signals spaced twice the i.f.above the frequency reading on a silentpart of the dial can be heard and mistaken

The inclusion of frequency changingcircuitry makes the superhet different fromall other types of receiver. Unfortunately, itbrings with it some unique problems, andthe evolution of the circuit has been large-ly shaped by attempts to overcome them.

A block diagram for a basic superhetreceiver is given in Fig.7.1. The receiver istuned to the desired station by a Pre-selector stage comprising one or moretuned circuits.

The Local Oscillator operates at a high-er frequency than the incoming signal, thedifference between the two being equal tothe intermediate frequency (i.f.). This dif-ference has to be maintained over theswing of the tuning capacitor. How this isdone is described later.

The input signal and local oscillation arecombined in the Mixer stage. This resultsin the production of a number of frequen-cies, and a tuned circuit in the mixer’s out-put selects oscillator frequency minussignal frequency; i.e., the desired interme-diate frequency.

Signals, at this new and fixed frequency,are amplified in the I.F. Amplifier. In basicreceivers, intermediate frequency ampli-fiers usually comprise two transistors cou-pled by radio frequency transformers(known as i.f. transformers) tuned to thei.f. Higher performance receivers incorpo-rate a ceramic, mechanical or crystal filterto improve selectivity.

THIS month we begin to consider asimple to operate receiving systemcapable of the highest levels of selec-

tivity and sensitivity. Known as the super-sonic heterodyne, or superhet for short, ithas been the dominant receiver technologyfor seventy years. Its application is nowuniversal, from the simplest domesticportable to state of the art equipment usedin government listening stations.

With the superhet system, all incoming

signals, whatever their wavelength, arechanged to a fixed frequency at whichalmost all of the amplification takes place.

Frequency changing is achieved by“beating” or “heterodyning” a locally gen-erated oscillation with the incomingsignal. The frequency difference betweensignal and oscillation is kept constant, andthe resulting beat frequency is, therefore,also fixed and constant.

By converting all signals to a fixed fre-quency, known as the intermediate fre-quency (i.f.), the problems of instabilityand inconsistent performance that canafflict multi-stage tuned radio frequency(t.r.f.) receivers are avoided. The maincause of this instability is feedback via theganged tuning capacitor and wavechangeswitch wiring. The inconsistent perfor-mance arises from the change in theQ-factor and impedance as the tuningcapacitor is rotated.

In the superhet, ganged variable capaci-tors and coil switching are eliminated fromthe fixed frequency i.f. stages, and veryhigh levels of gain can be achieved withoutinstability. Moreover, performance is notaffected by changes in tuning capacitorvalues.

Fixed frequency amplification makespossible the introduction of crystal,mechanical or ceramic filters capable oftailoring selectivity to suit all types of sig-nal and reception conditions (more aboutthis later). It is also easier to provideeffective automatic gain control (a.g.c.)systems.

Part 7: The Superhet: Preselectors, mixers and oscillators

Everyday Practical Electronics, December 2003 873

Fig.7.1. Block schematic diagram of a basic superhet receiver.

for transmitters operating at the actual dialreading. These “phantom” stations becomemore intrusive as the receiver is tuned upthrough the shortwave bands.

This is because simple preselector stagesbecome less and less able to reject signalsspaced by only 910kHz (i.e twice a stan-dard i.f. of 455kHz) from the wanted sta-tion as the reception frequency rises above5MHz or so. It is for this reason that inex-pensive receivers sometimes appear so“lively” on the shortwave bands.

Reception of signals in close proximityto the receiver’s i.f. can also be a cause ofwhistles. More difficult to trace are whis-tles caused by harmonics produced by thenon-linear action of the receiver’s owndetector.

The problem of second channel interfer-

ence can be greatly reduced by eitherincreasing the selectivity of the preselectorstage, or raising the intermediate frequen-cy, or both.

High performance receivers of the valveera with i.f.s of 460kHz had three tunedcircuits in the preselector stage. When thei.f. was increased to 1600kHz, two tunedcircuits were considered adequate.Domestic valve receivers often adopted abandpass arrangement to improve prese-lector action on medium and shortwaves. Atypical circuit diagram, featuring moderncoils, is given in Fig.7.2.

The lower the intermediate frequency,the greater the selectivity of the receiver(more about this later). This imposes limi-tations on raising the i.f. to reduce secondchannel interference, and designers beganto include two or more frequency chang-ers. This permitted the use of a high firsti.f. to minimize image interference fol-lowed by a low second to ensure adequateselectivity.

Modern, high performance communica-tions receivers often have a first i.f. of45MHz, followed by a transitional stage of10·7MHz and a final of 455kHz. Each ofthe three frequency changers tends to causespurious responses, and these receivershave to be designed with the utmost care orthe attempted cure only aggravates theproblem.

Whistles caused by the reception of sig-nals close to the intermediate frequencycan usually be cured by the insertion of awave trap, tuned to the intermediate fre-quency, at the aerial input. Wave traps werediscussed in Part Two.

Problems caused by the radiation of har-monics produced by the detector can beavoided by adequate post-detector filtering,sensible receiver layout, and screening.

The term frequency changer was coinedearly in the development of the superhet. Itis something of a misnomer. Combiner,mixer, or even modulator would betterdescribe the process which takes placewhen the local oscillation heterodynes orbeats with the incoming signal to producethe intermediate frequency.

Mixer is currently the most favoured term,but audio engineers and radio engineers havea completely different understanding of theword. Audio engineers demand a high degreeof linearity in a mixing stage to ensure thatthe signals being merged onto the same pathremain unchanged. They must not interactwith one another to produce new frequencies.

Fig.7.2. Circuit diagram for a bandpass tuned circuit for improving the selectivity ofthe r.f. preselector stage. The coil type shown is for medium wave reception.

The Superhet had its origins during the earliest years of radio.Indeed, seminal patents predate the valve.

In 1901, Dr. R. A. Fessenden patented a system whereby twounmodulated signals, radiated by a transmitter, were combined atthe receiver. The signals differed in frequency by about 1kHz andthe resulting beat note produced an audible tone from adiaphragm activated directly by the fine iron wire core of thereceiver’s tuning coil. Fessenden’s receiver could have beendescribed as a sonic heterodyne.

He later improved the arrangement by substituting a locallygenerated oscillation for one of the transmitted signals. In thisway the frequency difference and, hence, the tone of the beat,were brought under the control of the operator.

These experiments predated the valve, and high frequencyalternators were used to generate oscillations in the 50kHz to100kHz range at the transmitting and receiving stations.

American Navy researchers increased the sensitivity ofFessenden’s system in 1913, when they used a rectifier to mix thesignals and produce an audible beat note in conventional earphones.

Lee de Forrest patented his triode valve in 1903 and, by 1913,researchers in Europe and America had discovered its usefulness asan oscillator. During that same year, Captain H. J. Round used a sin-gle valve as oscillator and mixer. He called his circuit the Autodyne.

The 1914-18 war gave an enormous impetus to the develop-ment of the superhet. There was a need to operate at frequenciesbetween 500kHz and 3000kHz which, at the time, were consid-ered very high. The triode was still the only amplifying deviceavailable to designers.

Unfortunately, because of the capacitance between its grid andanode (input and output pins), it became unstable at frequencies

much above 100kHz. By adopting the superhet principle andconverting all incoming signals to a frequency below 100kHz,these primitive valves could be made to provide the necessaryamplification.

Captain H. J. Round, M. Latour and Major E. H. Armstrongfor the Allies, and W. Schottky for the Germans, researched theproblem. Schottky filed a patent application in June 1918, but hewas unable to develop his circuit. Armstrong’s patent was filedin December, six months later. Like Schottky’s, it made refer-ence to all of the essential features of the superheterodyne.

Unlike Schottky’s, it was based on the construction of a work-ing receiver. Credit for the invention of the circuit is, therefore,usually ceded to Armstrong.

In 1918, L. A. Hazeltine patented a feedback circuit known asthe Neutrodyne, which neutralized the triode’s grid-anode capac-itance and made stable amplification at higher radio frequenciespossible. This, together with the development of the screen-gridvalve (which greatly reduced the capacitance between the inputand output), prolonged the life of the t.r.f. receiver through the1920s in America and well into the 30s in Europe. The superhetthen began to dominate the domestic receiver market.

Subsequent developments have been directed towards elimi-nating spurious responses, increasing selectivity, and overcom-ing the effects of oscillator drift. The weight and mechanicalcomplexity of some of the best receivers of the valve era havebeen replaced by electronic sophistication in the high perfor-mance sets manufactured today.

In terms of the crucial ability to resolve the weakest signalsunder difficult reception conditions, there have been few, if any,significant improvements since the end of the valve era.

874 Everyday Practical Electronics, December 2003

Conversely, radio mixers (other thanmixers that function purely as modulators)must posses a degree of non-linearity toensure that the local oscillation combinesor heterodynes with the signal to producethe intermediate frequency at the output.

Diodes, which display the mostextreme form of non-linearity, were usedas mixers in primitive superhets (they stillare in some communications receiversand receivers operating at frequenciesapproaching 1GHz). This is why mixerswere referred to as the first detector in theearly days of radio. Mixers which func-tion by passing two signals through anon-linear device are known as additivemixers.

The development of multi-grid valves inthe late 1920s and special frequencychanger valves in the 30s permitted a dif-ferent mode of mixing. Known as multi-plicative mixing, the locally generatedoscillation was applied to a subsidiary gridplaced in the electron stream so that itmodulated the incoming signal frequency.The valve operated in a linear fashion, andthe intermediate frequency was selected atits output (anode), by a tuned circuit, in theusual way.

This form of mixing was the norm untilthe end of the valve era. The introductionof the transistor in the 1960s broughtabout a return to non-linear, additivemixers.

As we have seen, the oscillator must

operate at signal frequency plus intermedi-ate frequency over the full swing of thetuning capacitor. Maintaining this constantdifference is known as tracking.

Taking the medium waveband as anexample, the signal or preselector circuitswill tune from 550kHz to 1650kHz.Assuming an i.f. of 450kHz, the oscillatorwill have to run from 550 + 450 = 1000kHzto 1650 + 450 = 2100kHz.

The difference in coverage is achievedby using a lower inductance tuning coil inthe oscillator circuit. Its inductance is cal-culated so that the frequency of oscillationis 2100kHz when the tuning capacitor isset to its minimum value.

This measure, by itself, is not sufficientto ensure accurate tracking. The maximum

value of the tuning capacitor must also bereduced to ensure that the frequency ofoscillation is 1000kHz when its vanes arefully meshed. Sometimes a lower valueoscillator section is ganged with the signaltuning capacitor. More commonly, bothsections of the ganged capacitor are identi-cal and a fixed capacitor is placed in serieswith the oscillator tuner to reduce its max-imum value. This fixed capacitor is knownas a padder.

With the correct choice of inductor andpadder values, tracking is perfect at threepoints on the dial: close to either end andaround the centre point. At all other pointsthe error is minimal.

The oscillator in

some of the first gener-al coverage, solid statereceivers ran below thesignal frequency on thehighest shortwaverange (12MHz to30MHz) because of thelimited performance ofearly transistors.

Operating the oscil-lator above signal fre-quency is, however,standard procedure.Indeed, simple arith-metic will reveal that,with workable tuningswings, it becomes dif-ficult and then impos-sible, as frequency islowered, to run theoscillator below signalfrequency.

It’s now time for us to take a look atsome circuits.

Increasing the efficiency of the preselec-

tor circuits will do much to overcome theinherent defects of the superhet system. InFig.7.2, two tuned circuits L2/VC2 andL3/VC3, have been coupled to increase

selectivity. Known as a bandpass filter, itsperformance is compared with a singletuned circuit in the graph of Fig.7.3.

Correctly set up, this arrangement passesa narrow band of frequencies beyond whichthe response falls off comparatively steeply.In this way, good selectivity is achievedwithout attenuating the higher audio fre-quencies on the boundaries of the modula-tion envelope (see Part One). For thisreason, two tuned circuits, coupled induc-tively, formed the basis of all intermediatefrequency transformers (i.f.t.s) during thevalve era. (The miniature i.f.t.s used in mosttransistor radios are now singly tuned.)

Returning to Fig.7.2, signal transfer iseffected by C1 and C2, known as the seriesand shunt (or top and bottom) capacitorsrespectively. As frequency increases, cou-pling increases via C1 and reduces via C2.With falling frequency the reverse is thecase. This ensures that the response of thefilter is reasonably constant over the swingof ganged tuning capacitors, VC2 andVC3.

Coupling windings, L1 and L4, matchthe high impedance tuned circuits to theaerial and base of a bipolar transistormixer. The high impedance gate of a field-effect transistor can be connected directlyto winding L3. Preset trimmer capacitors,VC1 and VC4, and adjustable inductorcores, permit the two circuits to be exactlymatched to one another in a process knownas alignment.

The values quoted for capacitors C1 andC2 will produce a passband, on mediumwaves, around 10kHz wide. Reducing C1and increasing C2 will narrow theresponse, ultimately to a single peak.

When the preselector has two tuned

circuits the receiver requires a three-gangtuning capacitor. Three-gang air-spacedvariables are still manufactured but areexpensive. An alternative is to link two,polythene dielectric tuning capacitors toproduce a four-gang arrangement at mod-est cost. Details of this solution are given inthe mechanical diagram Fig.7.4.


Fig.7.3. Comparing the selectivity of a single tuned circuitwith two tuned circuits in a bandpass filter.

Author’s method of providing a metal brace between two solid dielectric tuningcapacitors to form a four-gang unit.

Contact faces on the linkage strip andbrass spindle couplers should be tinned andthe parts assembled on a length of 6mm(1/4in.) spindle before being “sweated”together. This ensures perfect alignment.

Lack of screening precludes the inser-tion of a signal frequency amplifying stagebetween VC1 and VC2. These capacitorscan, therefore, only be used for the band-pass circuit already described. Such a stagecould, however, be provided between VC2and VC3 if adequate screening is installed.Instability does not normally arise throughinteraction between signal and oscillatorcircuits because they operate at differentfrequencies. Receiver component layoutshould, however, separate them as much aspossible.

The self-oscillating mixer circuit used in

almost all of the transistor portablereceivers manufactured since the 1960s isshown in Fig.7.5.

The tuned circuit formed by tuningcapacitor VC1 and the ferrite loop aerial,L1, selects the station. To minimize damp-ing, signal transfer to the low impedancebase of transistor TR1 is via couplingwinding L2.

This transistor version of AlexanderMeissner’s oscillator is tuned by L5 andVC4, with capacitor C5 acting as theswing-reducing padder. Feedback fromTR1 collector (c) to emitter (e) is via coilsL3 and L4, the latter winding providingoscillator injection at the emitter. Oscil-lation amplitude is sufficient to drive thedevice into the non-linear region of itscharacteristic curve and this produces thedesired mixing action. (This is an additivemixer).

Base bias for TR1 is fixed by resistorsR1 and R2, and capacitor C1 is a d.c.blocking capacitor. Emitter bias is provid-ed by resistor R3 which is bypassed by C3.Transformer IFT1, with its tuned primary,selects the 455kHz intermediate frequency(i.f.).

Supply rail decoupling is achieved byR4 and C4. Trimmer capacitors VC2 andVC3, together with the adjustable cores inL1, L5, and IFT1, facilitate the accuratesetting up of the circuit.

Anyone who has used a correctlyaligned transistor portable (can there beanyone who hasn’t?) will know that thissimple and economical circuit works sur-prisingly well on medium and long waves.This is due, in no small measure, to the

high Q (and hence selectivity) of the ferriterod aerial and its modest signal pick-up.Injecting stronger signals via a long wireaerial results in spurious responses, crossmodulation problems and oscillator“pulling” (the oscillator tends to lock ontostrong signals).

Performance of the simple mixer/oscil-lator deteriorates on the shortwave bands.Again, the modest signal pick-up of theset’s whip aerial does much to hide thedefects of the circuit, but spurious respons-es increase as frequency rises.

Moreover, a reluctance to oscillatebecomes evident, above 10MHz or so, ifthe amount of tuning capacitance in cir-cuit is much above 200pF. Emitter biasresistor R3 may have to be selectedto ensure reliable operation at higherfrequencies.

As far as the author is aware, three wind-ing oscillator coils are not available, tohome constructors, for the long and short-wave bands. On these ranges, TR1 emitter(e) should be connected, via resistor R3and its bypass capacitor C3, to a tap closeto the “earthy” end of coil L5.

Separating the mixer and oscillatorstages and the use of field-effect transistorsimproves performance. Field-effect tran-sistors have a square law relationshipbetween their drain current and gate/sourcevoltage, and overloading produces only thesecond harmonic.

Bipolar transistors, on the other hand,exhibit an exponential base/collector cur-rent relationship, and overloading producesa range of odd and even harmonics.


Fig.7.5. Circuit diagram for a bipolar transistor mixer/oscillator stage used in mostdomestic portable receivers. Oscillator coil (L3 to L5) and “padder” capacitor C5 arefor medium wave reception.

Fig.7.4. Mechanical details for linking solid dielectric tuning capacitors to producea four-gang unit.

A circuit, popular since the 1960s, forsimple general coverage communicationsreceivers, is given in Fig.7.6.

Dual-gate MOSFET TR1 acts as themixer and TR2, a junction f.e.t., is used asthe maintaining device in an Armstrongoscillator. The high impedance gates ofTR1 minimize damping on the signal fre-quency tuned circuits and permit a directlink to the oscillator. Gate two (g2) of TR1is held at about 1V by connecting it, viasignal isolating resistor R3, to the source(most dual-gate MOSFETs work well asmixers with this arrangement).

This modern version of Armstrong’soscillator performs reliably up to about30MHz. It is, however, prone to squegging(going in and out of oscillation at a super-sonic frequency) and stopper resistor R5and the decoupling resistor, R4, preventthis. Source bias on TR2 is optimized bypreset VR2 to ensure reliable oscillation,on the highest shortwave range, when thetuning capacitor is set at maximum.

The MOSFET mixer requires an oscilla-tor injection of between 1Vr.m.s. and2Vr.m.s. A higher voltage is developedacross coil L4 and low value couplingcapacitor C4 reduces it to within thisrange. This is a multiplicative mixer. Theoscillator voltage modulates the signal. Itdoes not drive theMOSFET into non-linearity.

Padder capacitorC5 can be connectedin series with oscilla-tor coil L4 in order tosimplify wavechangeswitching. Oscillatoroutput is, however,more constant whenC5 is connecteddirectly in series withvariable capacitorVC4.

Power supply to theoscillator should bewell smoothed andregulated to minimizedrift.

Any receiver intended for serious listen-

ing must have some means of attenuatingstrong signals at the aerial input (see PartThree). Rotary potentiometer VR1 is ade-quate, but the switched resistor arrange-ment shown in Fig.7.7 is more reliable andless noisy at high frequencies.

The levels quoted take no account ofaerial impedance and are approximate.

Padder values and Toko coil numbers for

a five-band general coverage receiver aregiven in Table 7.1. The circuit diagram ofFig.7.6 is more sensitive, less vulnerable tocross modulation, less noisy than themixer/oscillator arrangement depicted inFig.7.5, and more immune to oscillatorpulling. With its 455kHz i.f. it is still affect-ed by spurious responses, and at least onemore tuned circuit is needed ahead of themixer to make its performance acceptablein this respect, even at medium frequencies.


FIg.7.6. Circuit diagram for a 5-Band Tuner/Mixer using a dual-gate mixer with an f.e.t. Armstrong oscillator (455kHz i.f.). SeeTable 7.1 for coil and padder capacitor details. (No constructional details are given for this circuit.)

Table 7.1: Toko Coil Numbers and Padder CapacitorValues for a Superhet Receiver with a 455kHz I.F.

Band Preselector Oscillator Padder Coverage(L1/L2) (L3/L4) (C5 pF) (MHz)

LW CAN1A350EK RWR331208N2 150 0·14 to 0·3MW RWR331208N2 YMRS80046N 330 0·53 to 1·6SW1 154FN8A6438EK 154AN7A6440EK 680 1·5 to 4SW2 154FN8A6439EK 154AN7A6441EK 1500 3·5 to 12SW3 KXNK3767EK KXNK3766EK 2000 10 to 30

Notes:(1) See Fig.7.6 for circuit diagram.(2) The quoted tuning ranges are approximate. The tuning capaci-

tors, VC2/VC4, should have a minimum value of not much morethan 10pF and a maximum of at least 300pF.

(3) Alignment on LW will be easier, and tracking improved, if a fixedcapacitor of 56pF is connected across L2 and 150pF isconnected across L4.

Providing additional signal frequency

tuned circuits involves considerable com-plication, especially with multibandreceivers. Another way of reducing spuri-ous responses is to increase the intermedi-ate frequency, and this solution is adoptedin the circuit diagram for a 3-BandShortwave Superhet Tuner/Mixer depictedin Fig.7.8.

Some readers have reported difficultiesin obtaining dual-gate MOSFETs, so twojunction f.e.t.s, TR1 and TR2, have beencascaded (or cascoded!) to form the mixer.Oscillator injection is via the gate of TR1,which is biased at half the supply voltageby resistors R3 and R4. The signal path isisolated by R2 and C5 bypasses the biassupply at radio frequencies.

Field-effect transistor TR3 is the main-taining device for the Hartley oscillator,feedback from its source being applied to atapping on tuning coil L2b, via resistor R8and wavechange switch S1c. Padder capac-itor C6b is placed in series with coil L2b inorder to simplify wavechange switching,and capacitor C8 couples the oscillator tothe mixer stage.

Hartley’s oscillator may be marginallymore drift free than Armstrong’s at higherfrequencies, but it was chosen because thesimple tapped coil permits the series con-nection of commercial windings to pro-duce the inductance values needed for the1·6MHz i.f. Voltage regulator IC1 stabi-lizes the supply to the oscillator to reducedrift.

The coils shown in Fig.7.8 cover the

4·5MHz to 11·8MHz range with a 5pF to130pF tuning capacitor swing.Experimenters are urged to try this busyrange first.

Details of the coils and padders for twoother ranges, affording continuous cover-age from 1·7MHz to 30MHz, are given inFig.7.9. The differing connections to theToko coil windings should be noted.

Intermediate frequency transformers for1·6MHz are no longer retailed, and IFT1comprises a standard Toko coil tuned bycapacitor C2.

The mixer/oscillator stage and its six

tuning coils are assembled on a singleprinted circuit board. The component sideand wiring to the 3-way 4-pole rotarywavechange switch are illustrated inFig.7.10, together with a full-size coppertrack master. This board is available fromthe EPE PCB Service, code 426.


Table 7.2: Toko Coil Numbers and Padder Capacitor Values fora Shortwave Superhet Receiver with a 1·6MHz I.F.

Range Preselector Core Oscillator Core Padder Coverage(L1a to L1c) Depth (L2a to L2c) Depth (pF) (MHz)

(mm) (mm)

SW1 154FN8A6438EK 2·5 TKANS32696 2·5 147 1·7 to 4·7

SW2 154FN8A6439EK 1·5 BKANK3334 2·5 330 4·5 to 11·8

SW3 BKXNK3766 2 BKXNK3766 1·5* 1500 10 to 30

Notes:(1) See Fig.7.8 for circuit diagram.

(2) The specified coverage is obtained with a 5pF to 130pF tuning capacitor.

(3) Core depths are measured down from the top of the can.

(4) *The measurement quoted for Range 3 oscillator coil (L2c) is the height of the coreabove the top of the can.

(5) The core of intermediate frequency transformer IFT1 is set 2mm down from the top ofthe can.

Fig.7.7. Receiver input signal attenuator circuits.(A) Simple variable potentiometer arrangement.(B) Switched resistor network. Note: S1 is a 1-pole12-way rotary switch, make-before-break.


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Fig.7.8. Circuit diagram for the mixer and oscillator stages for the 3-Band SWSuperhet Tuner/Mixer. Coils for Range Two (4·5MHz to 11·8MHz) are shown. Forranges One and Three, see Fig.7.9.

Solder pins at the lead-out points simpli-fy off-board wiring. They should be insert-ed first, followed by the Toko coils. Typenumbers on the coil cans are easilyremoved by the smallest amount of han-dling: if they are to be retained they shouldbe protected by clear sticky tape.

Follow the coils with the resistors, thenthe capacitors. Mount the transistors andthe i.c. last. It is a good idea to use aminiature crocodile clip as a heatshuntwhen soldering the field effect transistorsin position.

The coils have been arranged on the

board so that the highest frequency induc-tors are closest to the wavechange switch.This minimizes lead-lengths where strayinductance would have the greatest impact.The wavechange switch and tuning capaci-tor must, however, be located as close aspossible to the printed circuit board, andthe photographs show the prototypearrangement.

Source resistor R8 and blocking capac-itor C7 affect the willingness of TR3 tooscillate. Reducing the value of R8 andincreasing C7 makes oscillation more vig-orous. These components are used to linkthe p.c.b. to the wavechange switch S1.This makes them accessible to readers whowish to experiment, or who need to com-pensate for a low gain transistor.

Coil winding ratios make more sourceresistance necessary on Range One, andthis takes the form of resistor R9 on theprinted circuit board. See also Fig.7.9.

Provision is made on the board for a pairof trimmers, VC1/VC3, for each waverange. This will be necessary if an air-

spaced tuning capacitor, without integraltrimmers, is fitted. The suggested polyvari-con has its own trimmers, which will servefor a single range version, or for rangethree in a multirange receiver.

Check the board for poor soldered joints

and bridged tracks. Check the orientationof IC1, the semiconductors and tantalum

capacitor C10. Check the wiring to thewavechange switch S1.

If all is in order, connect a fresh 9V bat-tery. Current consumption should be in theregion of 10mA and the voltage at the out-put of IC1 should be precisely 5V.

Almost any medium wave receiver will

act as the second mixer, i.f. strip, detector


Fig.7.9. Circuit diagrams for the mixer/oscillator coil switching and padder arrangements for Shortwave Range One (1·7MHz to4·7MHz) and Range Three (10MHz to 30MHz). VC2 and VC4 are ganged tuning capacitors and VC1 and VC3 are 2p to 22ptrimming capacitors. See Fig.7.8 for Range Two details and full circuit.

Printed circuit board mounted on chassis (underside) and wired-up to the rotarywavechange switch.

and audio amplifier. A car radio, which haseffective screening, is particularly suitable.

The Regenerative Radio described inPart 3 and Part 4 works well as an i.f. strip(the Q-multiplier control can be used toresolve single-side-band signals).Combinations of this kind are known assupergainers.

Connect the mixer/oscillator to thereceiver via not more than 600mm (2ft) ofscreened cable. If a transistor portable isbeing used, wind about six turns of hook-up wire around the ferrite rod aerial andconnect the cable to this, as shown inFig.7.11.

The i.f. strip in the receiver is already

aligned, and the simple proceduredescribed below should enable readerswho do not have access to a signal genera-tor to get the shortwave mixer/oscillatortuner working.

Commence the tuning of the i.f. by set-

ting all trimmers, including the tuningcapacitor’s integral trimmers, to minimumcapacitance and set the coil and IFT1 coresto the depths quoted in Table 7.2.

Connect the tuner board to a mediumwave receiver. Now switch on the receiverand tune it to a silent part of the dial at the


Fig.7.11. Linking the Mixer/Osc. to aportable radio. The radio provides thesecond mixer, i.f. amplifier, detectorand audio amplifier functions.

Above: Chassis view of tuning capaci-tor mounting, which is mounted directlyabove the p.c.b.

Fig.7.10. The 3-Band SW SuperhetTuner/Mixer printed circuit board com-ponent layout, interwiring and full-sizecopper foil master. VR1, S1 andVC2/VC4 all viewed from the rear.

extreme high frequency end of the mediumwave band. Mark the dial setting on thereceiver (the tuning of the shortwavemixer/oscillator unit will be altered if thesetting is changed).

Connect a battery and an aerial to the

shortwave mixer/oscillator unit, switch it toRange 2 and rotate the tuning capacitor.Signals should be picked up around thedial.

Tune in a station with the tuning capaci-tor (VC2/VC4) set close to maximum andadjust the core of L1b to peak the output.Now tune in a station with VC2/VC4 set

close to minimum and adjust trimmercapacitors, VC1b/VC3b, to peak theoutput.

Repeat these adjustments until no fur-ther improvement can be obtained.

Switch to Range 3 and carry out the

same procedure. The difficulty here will befinding a station at the high frequency endof the dial: try listening in the early after-noon. If integral trimmers are being usedfor the alignment of this range, set them tothe lowest possible value, then switch backto Range Two and refine the adjustment ofVC1b/VC3b to allow for the additionalstanding capacitance.

Switch to Range 1 and repeat theprocess. As frequency lowers the settingsof cores and trimmers becomes more criti-cal and no signals will be heard if the unit

is significantly out ofalignment. Settingthe cores to the stat-ed depths and a littlepatience shouldensure success.

The calibrateddial of the prototypereceiver is repro-duced, half-size, inFig.7.12. Oscillatorcoil core and trim-mer settings have avery significanteffect on coverage,and it is only usefulas a general guide.

The setting of thecore of IFT1 is not

critical. After carrying out the above proce-dure it should be adjusted to peak the out-put from the unit.

The precise frequency limits of each

range cannot be set or defined without anaccurately calibrated signal generator. It is,however, possible to align the unit wellenough for it to give a very acceptable levelof performance.

The use of field effect transistors bringsall of the advantages mentioned earlier.Adopting a 1·6MHz i.f. greatly reduces theproblem of spurious responses.

Tuning rate, with a single epicyclicreduction drive, is much too fast. Ways ofmaking tuning less critical were discussedin Part Four, and next month a constant ratefine tuning system will be described.

882 Everyday Practical Electronics December 2003

Fig.7.12. Calibrated tuning dial (half-size approx.) for the3-Band SW Superhet Tuner.

PIC Virus ZapperMost of our components advertisers should be able to suggest a suitable

display module for the PIC Virus Zapper, but it would be advisable to checkout the pin-out arrangement of the one being offered before purchasing; youcan, if you wish, hardwire it to the p.c.b. The alphanumeric display moduleused in the prototype was one that originally came from MagentaElectronics ( 01283 565435 or www.magenta2000.co.uk).

For those readers unable to program their own PICs, a ready-programmedPIC microcontroller can be purchased direct from the author for the sum of £5(add £1 for overseas). He will be supplying either the PIC16F84 or thePIC16F627, both devices are pin-for-pin compatible. Orders (mail only)should be sent to Andy Flind, 22 Holway Hill,Taunton, Somerset,TA1 2HB.Payments should be made out to A. Flind.

The software is available on a 3·5in. PC-compatible disk (Disk 6) from theEPE Editorial Office for the sum of £3 each (UK), to cover admin costs (foroverseas charges see page 891). It is also available Free via the Downloadsclick-link option on the home page when you enter our main web site atwww.epemag.wimborne.co.uk, then enter the PIC Microcontroller sourcecodes folder and select PIC Virus Zapper.

Christmas CheeksAfter browsing through a few components catalogues, it would appear that

finding an l.e.d. with an identical specification to the one used in theChristmas Cheeks prototype is likely to cause some problems. Most of the redultrabright and superbright l.e.d.s that we looked at seem to have “viewingangles” ranging from 50° to 60° and not the 30° listed by the author.

The types that appear to meet the author’s specification or better are oneswith “water clear” packages. You can, of course, use the standard ultrabrightor superbright and accept a more diffused light intensity.

Practical Radio Circuits–7As with previous Practical Radio Circuits projects, the inductors are really

the only “specials” in this series. The author quotes two sources but hasinformed us that there may be a small delay as some coils have had to be re-ordered from Japan. However, by the time you read this issue they shouldhave arrived.

All the Toko coils are available from JAB Electronic Components( 0121 682 7045 or www.jabdog.com), mail order only, or from Sycom,Dept EPE, PO Box 148, Leatherhead, Surrey, KT33 9YW ( 01372 372587or www.sycomcomp.co.uk). You will need to order by quoting their typenumbers as listed in the parts box and on the circuit diagrams.

The polyvaricon (polythene dielectric) variable capacitor will normally befound listed as a “transistor radio” type and consist of an antenna and oscilla-tor section, plus trimmers. They are currently stocked by ESR Components( 0191 251 4363 or www.esr.co.uk), code 896-110 and SherwoodElectronics (see page 896), code CT9. The one in the prototype wasobtained from Maplin ( 0870 264 6000 or www.maplin.co.uk), codeAB11M.

PIC Nim MachineWe do not expect any buying problems to be encountered when gathering

together parts for the PIC Nim Machine project.A pre-programmed PIC16F877-20 (20MHz) microcontroller can be pur-

chased from Magenta Electronics ( 01283 565435 or www.magen-ta2000.co.uk) for the inclusive price of £10 each (overseas add £1 p&p). Thesoftware is available on a 3·5in. PC-compatible disk (Disk 6) from the EPEEditorial Office for the sum of £3 each (UK), to cover admin costs (for over-seas charges see page 891). It is also available Free via the Downloads click-link option on the home page when you enter our main web site atwww.epemag.wimborne.co.uk, then enter the PIC Microcontroller sourcecodes folder and select PIC Nim.

Teach-In 2004 Part 2No problems to report on this month’s instalment of our new Teach-In ’04

series. All the various transistor types mentioned should be readily availablefrom our components advertisers, particularly Cricklewood Electronics( 0208 452 0161).

Good News! We have just been informed that Magenta Electronics( 01283 565435 or www.magenta2000.co.uk) have produced two Teach-In components kits, one for the main components, including a plug-in bread-board, and the other containing most of the additional parts listed under“miscellaneous” in Part 1. See their ad. on pages 816/817.

Details of all the prices and ordering information for this month’s printedcircuit boards can be found on page 891.

PLEASE TAKE NOTECardboard Clock (Nov ’03)

Use normal thin card for the clock wheels as thick (art supplies) cardmay be too heavy and stop it. Use a rectangular file to make the groovesthat support the wheels axles. Depth is correct when a screw laid in thegroove is half buried. Point “D” is the 15mm horizontal section ofsuspension hook W2. Cement end B of the pendulum post (A1) to thebackboard.

For the pendulum rod start with a 1 metre length of 24s.w.g. wire, anddraw it slightly, to straighten it. Before bending it, mark the bend pointswith a pencil. The letter p shows on which side to position the pliers,when making bends Q and R.When readjusting the coil table rely on therubber band, and not on the bolt through hole E.

455kHz i.f. transformers. When greaterselectivity is required, a crystal, ceramic ormechanical filter is built into the i.f. ampli-fier. Low frequency final i.f.s have fallenout of use.

Crystal

The first filters relied upon the piezo-electric nature of quartz, and the wayslices of this crystalline material can bemade to resonate, very sharply, at radiofrequencies.

Professor W. G. Cady exploited thisphenomenon, for the first time, in 1921,when he used a slice of quartz to controlthe frequency of an oscillator.

During the 1930s, circuits incorporat-ing one or two crystals (Robinson’scrystal gate and the Simmonds-Robinson two crystal filter) were form-ing part of high performance receivers.Passbands as narrow as 300Hz (forMorse reception) can easily be achievedwith this technique.

the screened-grid valve, measures that per-mitted amplification at high frequencies,i.f.s of around 100kHz persisted foranother decade.

Second channel interference problems(see Part 7) intensify as the i.f. is loweredand/or reception frequency is raised, andmanufacturers began to adopt the nowstandard i.f. of between 450kHz and470kHz. The growing demand for domes-tic sets with shortwave coverage no doubtcontributed to this development.

Despite the higher i.f., valve superhetswith two, double-tuned i.f. transformerswere selective enough for domestic listen-ing. High performance communicationsreceivers either incorporated a crystal filter(see below) or had a second mixer/oscilla-tor stage that produced a final i.f. between50kHz and 100kHz.

Current practice with simple transistorreceivers is to provide three, single-tuned,

LAST month’s article covered theadvantages and drawbacks of thesuperheterodyne (or superhet for

short) receiver. A design for a mixer/oscil-lator unit with a 1·6MHz intermediate fre-quency (i.f.) output was described. Raisingthe i.f. to 1·6MHz, instead of the moreusual 455kHz, reduces susceptibility tosecond channel interference, the majordefect of the superhet.

This final part deals with the intermedi-ate frequency amplifier, which providesmost of the selectivity and gain. Teamedwith last month’s 3-Band SW SuperhetTuner/Mixer, it forms a high performancereceiver for the shortwave listener.

Selectivity increases as the intermediate

frequency is lowered.Let us assume a receiver with the low

i.f. of 100kHz tuned to a station on900kHz with an adjacent transmitter oper-ating on 910kHz. Signal spacing is 10kHz(910kHz minus 900kHz), and the adjacentstation is, therefore, off tune from thewanted one by a mere 1·1 percent. Normalpreselector, or signal frequency, tuned cir-cuits will be unable to prevent the unwant-ed station reaching the mixer.

After mixing, the wanted signal willemerge at the i.f. frequency (1000kHzlocal oscillation minus 900kHz signal,equals 100kHz i.f.). The adjacent stationwill, therefore, emerge at a frequency of90kHz (1000kHz minus 910kHz).Heterodyning has increased the differencebetween the signals to 10 percent, and thetuned circuits in the i.f. amplifier are betterable to remove the unwanted station.

Performing this calculation with an i.f.of 455kHz reveals a difference, after het-erodyning, of around two percent. Theimprovement with the higher i.f. is, there-fore, very much less.

The first superhets had i.f.s below

100kHz in order to ensure stable operationwith triode valves. Despite the introduc-tion, during the 1920s, of neutralizing and

Part 8: A double-conversion superhet receiver for the SW bands

Everyday Practical Electronics, January 2004 47

Group photograph of this month’s printed circuit boards: Second Mixer/OscillatorI.F. Amplifier (top); Signal-Strength Meter (left) and B.F.O./Product Detector (right).

MechanicalThe Collins Radio Corporation of

America subsequently developed mechani-cal filters in which the resonant elementsare vibrating metallic discs linked by a rod.Filters of this kind can produce passbandsless than 400Hz wide.

They are expensive but capable of veryhigh performance. The signal is transferredto, and extracted from, the disc resonatorsby a coil wound on the link rod and mount-ed in the field of a permanent magnet.

Miniature mechanical filters are current-ly manufacture by Toko, see Table 8.1.Signal transfer involves the piezoelectriceffect and the resonant mechanical elementis a phosphor bronze strip. Performance isinferior to the Collins filters, but they arevery inexpensive and widely used for lessdemanding applications. Passbands can beas narrow as 3kHz.

CeramicDevelopment of the ceramic filter by

Murata and Vernitron coincided with theintroduction of transistors and the resultingdrive towards miniaturization. Ceramic fil-ters also exploit the piezoelectric effect. Inthis respect they are similar to crystal fil-ters, but they have a lower Q-factor (i.e.,they resonate less sharply) and insertionlosses are greater.

Manufactured from lead zirconatetitanate and other compounds, they aresmaller and usually cheaper. They arewidely used in radio equipment, fromdomestic portables to high performancecommunications receivers.

Details of a number of low and mediumpriced ceramic filters, together with detailsof Toko’s mechanical unit, are given inTable 8.1. Bandwidths (passbands) arequoted at the 6dB down (i.e., half-voltage)points. The figures represent the fullpassband, not the deviation from centrefrequency.

The i.f. amplifier printed circuit boardused here will accommodate any of thefilters listed.

The perfect filter would cause zero

attenuation, have a flat response within thepassband, sharp cut-off at the passbandboundaries, and completely attenuate allother frequencies. In practice, filters havean uneven response (ripple) within thepassband, spurious responses beyond, andan insertion loss ranging from around threeto ten decibels (dB).

Ceramic filters are prone to spuriousresponses, which are much reduced whenthey are combined with tuned matchingand i.f. transformers.

The specified filter performance will

only be realized if its input and outputimpedances are correctly matched to theamplifying devices in the i.f. strip. This isnot easy to achieve with bipolar transistors(automatic-gain-control alters bias levelsand changes input impedance).

Serious mismatching causes an asym-metrical passband, increased insertion

losses and ripple. In practice, the filtersdetailed here will perform well despiteslight imperfections in matching. They area very convenient way of achieving highselectivity with an intermediate frequencyof 455kHz.

During the valve era, gain within the i.f.

amplifier was invariably provided by a sin-gle variable mu valve. Variable mu valvesdisplay a large change in amplificationwhen the bias voltage is shifted by thea.g.c. circuitry. To achieve the same levelof gain, transistor superhets require two i.f.amplifier stages.

Increasing use is being made, in domes-tic portables, of integrated circuits thatcontain all but the audio stages on a singlechip. When these devices are employed, aminiature filter and its matching transform-ers provide selectivity.

Modern communications receivers usu-ally have two, specialized, integrated cir-cuit “gain blocks” in the final i.f. amplifier.Excellent a.g.c. performance, predictableimpedances (for filter matching) and highlevels of gain are achievable with thesedevices.

Unfortunately, they are not widelyavailable to home constructors and, con-tinuing the practice adopted throughoutthe series, the i.f. amplifier “strip”described here uses easy-to-obtain, non-critical semiconductors.

48 Everyday Practical Electronics, January 2004

To secure acceptable selectivity, the1·6MHz output from our first mixer(Part 7) must be lowered to 455kHz.Constructors seeking higher selectivitywill then be able to use one of the 455kHzfilters already described. The i.f. amplifiermust, therefore, be preceded by a secondmixer/oscillator.

The full circuit diagram for theSecond Mixer/Oscillator is given inFig.8.1. Here IFT1 is tuned to the1·6MHz incoming frequency, by capaci-tor C2, its low impedance couplingwinding ensuring a correct match to thefirst mixer (See Part 7).

Signals developed across IFT1 areapplied to TR1 gate (g), the second mixer,the local oscillation being injected at itssource (s). Biasing of this stage is by resis-tor R2, which is bypassed by capacitor C4.

Tuned transformer IFT2 selects the455kHz heterodyne and acts as TR1’sdrain (d) load. The stage is decoupled byR1 and C1.

The second oscillator must operate at2055kHz (1600kHz plus 455kHz). Tunedcircuit L2/C6 determines the frequency ofoscillation and the maintaining device isTR2, an n-channel field effect transistor.Low impedance mixer coupling windingL1 minimizes damping on the tunedcircuit.

Feedback for this Hartley oscillator istaken from the source of TR2 and appliedto a tapping on L2 via bias resistor R5 andbypass capacitor C9. Supply line decou-pling is by R4 and C10.

The second oscillator frequency can be

altered, over narrow limits, by potentiome-ter VR1 which varies the reverse bias onvaricap diode D1. This tunes the second

i.f. amplifier across the broad passband ofthe first, in effect shifting the first i.f. Thisresults in a bandspread or fine tuning sys-tem that has a constant rate, irrespective ofreception frequency.

The fine-tuning rate is preset by VR2.Potentiometer “noise” is eliminated bycapacitor C11 and resistor R6 isolates thesignal circuits from the bias network.Blocking capacitor C7 prevents the short-ing of the bias through coil L2.

Intermediate frequency transformers,

IFT2 and IFT3, ensure an acceptablematch to any of the Toko or Murata filterslisted in Table 8.1. A filter, X1, with a totalbandwidth of 4kHz is recommended forgeneral listening on the shortwave bands.Wider filters can be used to avoid side-band cutting, but selectivity will be exces-sively compromised if the passband isgreater than 6kHz.


Fig.8.1. Full circuit diagram for the Second Mixer/Oscillator I.F. Amplifier for the Dual-Conversion SW Superhet Receiver.(See Part 7 for the first Mixer/Oscillator stages.)

Bipolar transistors TR3 and TR4 provide

amplification at the 455kHz second i.f.This circuit is used, with minor modifica-tions, in all domestic superhets fabricatedfrom discrete components.

Transformers IFT4 and IFT5 couple thestages and tune the amplifier to 455kHz.Emitter bias to TR3 is developed across R8and Gain preset VR3. This stage is decou-pled by R7 and C16, and the emitter resis-tors are bypassed by C14. Emitter bias toTR4 is provided by R11 with C18 as thebypass capacitor.

Point contact germaniumdiode D2 demodulates the a.m.signals, and residual radio fre-quencies are filtered by C20, R13and C21. (Diode detectors were dis-cussed in Part 1.) The audio output isdeveloped across resistor R14 and cou-pled to the a.f. gain (Volume) control,VR4, by capacitor C24.

Provision is made for the r.f. output ofthe i.f. strip to be connected, via capaci-tor C19, to a product detector so thatsingle side-band (s.s.b.) signals can beclarified. Toggle switch S2a selectseither the a.m. output or theclarified s.s.b. signals.The other half ofthis switch,

S2b, connects the B.F.O. and ProductDetector to the power supply (seeFig.8.6).

Supply line decoupling of the entirei.f. amplifier at r.f. and a.f. is accom-

plished by capacitors C22 and C23.The other half (S1b) of the On/Off

switch S1 is used to connect a sepa-rate battery to the simple Speaker

Amplifier (Part 2). Separate bat-teries prevent the audio

amplifier’s current swingsdisturbing the electronic

tuning systems. Power“on” is indicated by low

current l.e.d. D4.

The d.c. voltage produced by the detec-

tor diode D2 is used to control the gain ofTR3 and TR4 by varying their base bias.As signal strength increases, the voltagebecomes more negative. This reduces thebias and, hence, the current through thetransistors, and gain falls.


Fig.8.2. Printed circuit board topside component layout, interwiring to off-board components and full-size copper foil master forthe Second/Mixer/Oscillator I.F. Amplifier.

The a.g.c. (automatic gain control)circuit is a little obscure. A chain of com-ponents, R9, R10, D3, R12 and D2 form ashared bias network. Including detectordiode D2 introduces the signal-relatedvoltage variation. Stabilization againsttemperature changes is provided by diodeD3 (which must be a silicon component to

provide compensation for the silicontransistors).

The a.g.c and bias network is connectedto TR3 and TR4 via the IFT’s couplingwindings. The full a.g.c. voltage is appliedto the base of TR3. Final stage TR4 is con-nected via resistor R10 to limit the a.g.c.action so that, even under strong signal

conditions, sufficient output is available todrive the detector. The a.g.c. line isbypassed by C12.

Details of the printed circuit board(p.c.b.) topside component layout, full-sizecopper foil master and the off-board wiringdetails are shown in Fig.8.2. General guid-ance on construction is given later.


I.F. AMPResistors

R1, R7, R8 100(3 off)R2, R15 2k2 (2 off)R3 220kR4, R5,

R14 4k7 (3 off)R6 100kR9 82kR10, R12 8k2 (2 off)R11 1kR13 470


PotentiometersVR1 100k rotary carbon,

linearVR2 220k enclosed carbon

presetVR3 1k enclosed carbon

presetVR4 4k7 rotary carbon, log.

CapacitorsC1, C4, C10,

C14 to C16,C18, C22 100n ceramic (8 off)

C2 220p polystyrene orceramic “low k”



excl. case & batts

C3, C5,C13, C17 included in i.f. can


C7 27p ceramic “low k”C8 47p ceramic “low k”C9, C20,

C21 10n ceramic (3 off)C11 47 radial elect. 16VC12 10 radial elect. 16VC19 22p ceramic “low k”C23 470 radial elect. 16VC24 470n ceramic or polyester

SemiconductorsD1 BB105 varicap diodeD2 OA90 germanium diodeD3 1N4148 signal diodeD4 5mm low current (2mA)

red l.e.d.TR1, TR2 2N3819 n-channel field

effect transistor (2 off)TR3, TR4 MPSH10 high frequency

transistor or similar (2 off)

InductorsIFT1, L1/L2 154FN8A6438 Toko

screened (metal can)coil (2 off)

IFT2 7MCS4718N Tokoscreened (metal can)coil

IFT3 7MCS4786N Tokoscreened coil

IFT4 YHCS1A590R Tokoscreened coil

IFT5 YHCS11100AC2 Tokoscreened coil

MiscellaneousX1 CFU455IT Murata 4kHz

bandwidth ceramic filter(see text)

S1, S2 d.p.d.t. toggle switch (2 off)

Printed circuit board available from theEPE PCB Service, code 428 (I.F. Amp);case, aluminium box 203mm x 152mm x76mm (8in. x 6in. x 3in.) – 2 off (see text);white card and rubdown lettering for frontpanel; protective 2mm thick Perspex sheetfor front panel; two large and four smallplastic control knobs; aluminium sheet fortuning capacitor (Part 7) screen; l.e.d. hold-er; audio type screened cable; multistrandconnecting wire; p.c.b. nylon stand-offpillars; battery holder and connectors; nuts,bolts and washers; solder pins; solder etc.

Table 8.1: Mechanical and Ceramic I.F. Filters

Ceramic Filter: Murata Type CFUType Input/output 6dBNo. impedance bandwidth

CFU455IT 2k 4kHzCFU455H2 2k 6kHzCFU455G2 2k 9kHz

(Case size: L8mm x W7mm x H8mm)

Ceramic Filter: Murata Type CFGType Input/output 6dBNo. impedance bandwidth

CFG455J 2k 3kHzCFG455I 2k 4kHzCFG455H 1k5 6kHz

(Case size: L11mm x W7mm x H9·5mm)

Ceramic Filter: Murata Type CFMType Input/output 6dBNo. impedance bandwidth

CFM455J1 2k 2·6kHzCFM455I 2k 4kHzCFM455H 2k 6kHz

(Case size: L20mm x W7·3mm x H10·3mm)

Insertion loss for all of the filters, except the CFM455I and the CFG455J, is 6dB.Insertion loss for the CFM455I is 7dB; for the CFG455J, 8dB.

Mechanical Filter: Toko Type HCFMType Input/output 6dBNo. impedance bandwidth

HCFM2455A 1k/1k5 4kHzHCFM2455B 1k5/2k 6kHzHCFM2455C 2k 8kHz

(Case size: L8·2mm x W3·5mm x H9·5mm)

A Signal-Strength Meter is useful formonitoring reception conditions andassessing different aerial systems. It canalso be of assistance during the setting upprocess.

The Signal-Strength Meter bridge circuitgiven in Fig.8.3 enables the standing volt-age on the a.g.c. line to be nulled out sothat the meter pointer can rest at zero underno-signal conditions.

Transistor TR1 and collector load resis-tor R2 form two arms of the bridge: preset

potentiometer VR2 provides the other two.Setting the slider (moving contact) of VR2to the same potential as the collector ofTR1, in the absence of a signal, brings themeter pointer to zero.

The base of TR1 is connected to thea.g.c line via current limiting resistor R1.The a.g.c voltage becomes more negativeas signal level increases, current throughTR1 decreases, and the voltage at its col-lector becomes more positive, driving overthe meter pointer.

Panel meter full-scale deflection (f.s.d.)is set by preset VR1, and the bridge is sta-bilized against thermal drift by diode D1,which mirrors the base/emitter junction inTR1.

Meters of lower sensitivity can be usedif the value of R1 is reduced. It ought not,however, to be taken below 100 kilohms.With this value it should be just possible touse a 1mA meter. Meters of higher sensi-tivity should be shunted to around 100A.

Details of the printed circuit board top-side component layout, full-size copperfoil master and interwiring are given inFig.8.4. General guidance on constructionis given later.


Fig.8.3. Circuit diagram for the Signal-Strength Meter.

SIGNAL-STRENGTH METERResistors

R1 220k (seetext)

R2 4k7All 0·25W 5% carbon film

PotentiometersVR1 100k enclosed carbon

presetVR2 4k7 enclosed carbon

preset

SemiconductorsD1 1N4148 signal diodeTR1 BC549C npn small

signal transistor

MiscellaneousME1 100A moving coil

panel meter

Printed circuit board available fromthe EPE PCB Service, code 429 (Sig.Meter); multistrand connecting wire;solder pins; solder etc.


Approx. CostGuidance Only ££55..5500

excl. meter Fig.8.4. Signal-Strength Meter printed circuit board component layout, wiring andfull-size copper foil master.

Panel Meterand circuit

board mountedin one corner ofthe receiver top

chassis.

EPE Online


Readers interested in Morse and singleside-band (s.s.b.) speech transmissions willrequire a beat frequency (b.f.o.) and carrierreplacement oscillator.

Morse is transmitted by interrupting thecarrier, and the receiver must contain anoscillator to beat with these signals andmake them audible. Single side-band trans-missions have their carrier suppressed atthe transmitter and it must be restored, byan oscillator in the receiver, so that theycan be demodulated (see Part 6).

The simplest arrangement, widelyadopted during the valve era, is to injectthe oscillation directly into the i.f. strip,just ahead of the detector. Unfortunately,this activates the a.g.c system anddepresses receiver sensitivity. Accord-ingly, a simple product detector has beenincluded to ensure the complete isolationof the oscillator.

The full circuit diagram for a B.F.O. and

Product Detector is shown in Fig.8.5.Transistor TR2, coil L2 and capacitor C6form a 455kHz Hartley oscillator. Its fre-quency can be altered slightly by varicap

diode D1 in order to change the beat noteor to accommodate the upper and lowerside-band transmission modes. This oscil-lator circuit is almost identical to the sec-ond mixer/oscillator, and the function ofthe various components has already beendescribed.

The oscillator’s output is coupled, viacoil L1, to the source pin of productdetector TR1. Signals are applied to TR1

gate, and the audio output is developedacross drain load resistor R3. Residualradio frequencies are filtered out by C2, R5and C4. C5 is the d.c. blocking capacitor.Choke RFC1 prevents the leakage of oscil-lations into the supply rail.


Fig.8.5. Complete circuit diagram for the B.F.O. and Product Detector. For clarity, switch S2a is also shown on the I.F. Ampcircuit. This circuit is required for the reception of s.s.b. transmissions.

The B.F.O. boardhoused in analuminium screeningbox.

Product detectors were discussed inPart 6. They function in the same way asmixers but combine close radio frequenciesto deliver an audio signal at the output port.

Details of the printed circuit board top-side component layout, full-size copperfoil master and interwiring are shown inFig.8.6. General guidance on constructionis given later.

Transistor types for the three circuits are

not critical and alternatives, with base con-nections, are included in Fig.8.1. Otherf.e.t.s, including the BF245B, J304 andJ310, have been in-circuit tested and foundto be satisfactory.

Detector diode D2 must be a point con-tact germanium type. The specified OA90is often used in this circuit, but the OA47and others should perform equally well.

The Murata CFU455IT filter is recom-mended, but the i.f. printed circuit boardwill accept any of the units scheduled inTable 8.1. Readers whose requirements areless demanding can dispense with the filterand link the coupling windings of IFT2 andIFT3 with a 100pF low-k ceramic capaci-tor. The performance implications of thisare discussed later.

Full details of the preselector and firstmixer/oscillator stages, and the switchedcoil pack, were given in Part 7. Details of asimple Speaker Amplifier were given inPart 2.

The three circuits are assembled on sep-

arate printed circuit boards (p.c.b.s). Thispermits the complete screening of the b.f.o.unit and makes it easier for constructors toomit items they do not require. Theseboards are available from the EPE PCBService, codes 428 (I.F. Amp), 429 (Sig.Meter) and 430 (B.F.O./Prod. Det.).

The component side of the I.F. Amplifi-er printed circuit board, together with afull-size copper foil master, is given inFig.8.2. Component and foil sides of theSignal-Strength Meter p.c.b. are shown inFig.8.4 and the two sides of the B.F.O. andProduct Detector board are depicted inFig.8.6.

Off-board wiring is made easier by theuse of solder pins at the p.c.b. lead-outpoints. They should be inserted first. Followthese with the resistors, then the coils, andthen the capacitors, smallest first.

The semiconductors should be solderedin place last, their lead lengths just longenough to attach a miniature crocodile clipto act as a heatshunt. (Heat shunting is goodpractice with f.e.t.s and germanium diodes).

Filter matching transformers IFT2 andIFT3 are only 7mm square (the 10mm ver-sions are no longer retailed), and particularcare should be taken to avoid bridgingtracks when they are soldered onto thep.c.b. Avoid stressing the pins of the coils:this can result in open-circuit windings.

On completion, double-check eachboard for bridged tracks and poor solderedjoints. Check the orientation of the semi-conductors and polarized capacitors, andcheck coil and i.f.t. placements.

Connect a fresh 9V battery and checkcurrent drain. The I.F. Amplifier shouldconsume around 4mA, the B.F.O. andProduct Detector around 6mA.


B.F.O. and PROD. DETECTResistors

R1, R6 220k(2 off)

R2, R5 470(2 off)

R3 10kR4 4k7R7 47R8 100R9 100k

All 0·25 5% carbon film

PotentiometersVR1 100k rotary carbon, lin.VR2 220k enclosed carbon

preset

CapacitorsC1 100 radial elect. 16VC2, C3,

C9 10n ceramic (3 off)C4 47n ceramicC5, C12 47 radial elect. 16V

(2 off)



excl. case


C7 27p ceramic “low k”C8 47p ceramic “low k”C10 100n ceramicC11 47 tantalum bead, 16V

SemiconductorsD1 BB105 varicap diodeTR1, TR2 2N3819 n-channel field

effect transistor (2 off)

MiscellaneousL1/L2 RWO6A7752EK Toko

screened (metal can) coilRFC1 1mH r.f. chokeS2 see I.F. Amp listing

Printed circuit board available from theEPE PCB Service, code 430 (B.F.O./Prod.Detect.); screening box for B.F.O., size76mm x 51mm x 25mm (3in. x 2in. x 1in.) –see text; audio type screened cable; multi-strand connecting wire; p.c.b. nylon stand-off pillars; solder pins; solder etc.

Fig.8.6. Printed circuit board topside component layout, interwiring to off-boardcomponents together with the underside copper foil master.

The chassis and front panel described inPart Three could be used for this receiver.Construction of a double-conversion super-het does, however, represent a considerableeffort, and many readers will want a more“finished” look. The arrangement adoptedfor the prototype receiver is shown in thevarious photographs.

Case and chassis are formed from twostandard aluminium boxes, with theirU-shaped covers fixed back-to-back. Thecoil pack and first mixer/oscillator p.c.b.,from last month, is mounted, along withthe various controls, below the chassis.The tuning capacitor, i.f. amplifier, b.f.ounit, signal-strength meter and audioamplifier are located above the chassis.Receiver and audio amplifier batteries aresecured, by strong rubber bands, to a hard-board platform fixed alongside the coilpack.

A small screening box encloses theb.f.o. unit, and a metal screen is fixedaround the tuning capacitor. These mea-sures are not absolutely essential, but theydo help to prevent spurious responses.

The front panel is marked out on whitecard and annotated with rub-down letter-ing, see photograph. Acrylic sheet, 2mmthick (the kind used for DIY double glaz-ing), protects the panel. The speaker ismounted beneath the top of the case, whichis finished with car spray paint. Rubberfeet protect the underside.

Details of the interwiring connections

between the printed circuit boards are alsoshown in Fig.8.2, Fig.8.4 and Fig.8.6.Screened cable must be used where indi-cated, but it can be of the ordinary audiovariety.

Note the use of the 0V pin on the i.f.amplifier board as the common negativeconnection for all of the receiver p.c.b.s.Do not rely on the outer braiding ofscreened leads to act as negative returns forthe power supply.

The negative supply lead for the SpeakerAmplifier is connected directly to theappropriate pin on its p.c.b. (see Part 2).Connections to the battery positive termi-nals are, of course, via switch S1a for thereceiver boards, and S1b for the SpeakerAmplifier.

Provision is not made on any p.c.b. forthe l.e.d dropper resistor R15. This compo-nent is included in the wiring to the PowerOn l.e.d. D4.

Mount the p.c.b.s on insu-lated stand-offs, and connectthe “earthy” side of the i.f.board input to the metal chas-sis, close to the tuning capaci-tor. This point should also beused as the chassis connectionfor the first mixer/oscillatorp.c.b. (see Part 7). Connect theaudio amplifier board to chas-sis at the a.f. gain or volumecontrol.

Alignment of the com-

pleted Double-Conversion SWReceiver cannot be undertaken

without the aid of a signal generator cover-ing 1·6MHz to 30MHz. The procedure is asfollows:

(1) Switch receiver and signal generator onand allow a thirty-minute warm-uptime. Always keep the generator outputas low as possible. Sensitivity willincrease as alignment progresses, andeventually a direct connection betweenreceiver and generator will no longer berequired: just place the generator’s out-put lead close to the injection point.

(2) Coil and i.f. transformer cores, espe-cially the small slug type, are brittle.Use a plastic trimming tool for theadjustments.

(3) Potentiometers, VR1, VR2 and VR3,associated with the I.F. Amplifier,should be set at mid-travel. Switchreception mode to a.m. and turn upVolume control VR4.

(4) Connect a high impedance voltmeter (adigital testmeter is suitable) across theSignal-Strength Meter output points onthe i.f. board and switch it to a low (say2V) range.

(5) Using the signal generator, inject amodulated 455kHz signal at the drainof the second mixer, TR1. Adjust thecores of IFT2, IFT3, IFT4 and IFT5 formaximum reading on the testmeter.

(6) Filter X1 determines the i.f. frequency,and this may not be precisely 455kHz.Rock the generator tuning controlknob, very gently, in order to set its out-put exactly at the filter resonance. Ifthis changes the generator setting,refine the alignment of IFT2 to IFT5.

(7) Remove the voltmeter and connect theSignal-Strength Meter p.c.b. in itsplace. Switch off the generator andadjust preset VR2 on the meter board tobring the pointer to zero. Reconnect thegenerator and inject a low-level signal.Adjust preset VR1 on the meter boardto set the pointer at mid-travel. Thetestmeter is no longer required.

(8) The first i.f. can range from around1600kHz to 1650kHz. It should be cho-sen to avoid interference from anymedium wave stations that are strong inthe reception area. For the purpose ofthis guidance, it will be taken as1600kHz.

(9) Set the signal generator to 1600kHzand inject a weak signal at the input tothe i.f. amplifier board. Adjust the coreof L1/L2 (the second oscillator coil),then the core of IFT1, for maximumreading on the meter. Refine the adjust-ment of IFT2.

This completes the alignment of the sec-ond mixer/oscillator and the i.f. amplifier.

Readers should now refer back to Part 7

for the circuit details and ComponentNumbers to be quoted in connection withthe alignment of the preselector and firstmixer/oscillator stages.


General layout of p.c.b.s within the topside chassis. Note the screening bracketaround the tuning capacitor. The small p.c.b., to the side of the main board, is theSpeaker Amp from Part 2.

Padder capacitors, C6a to C6c,have been calculated on the basis of a5pF to 130pF tuning capacitor,VC2/VC4. Fixed capacitors shouldbe wired in series with higher valuepolyvaricons (tuning capacitors) toreduce their swing to not much morethan 130pF. Trimmers inside the caseof the tuning capacitor are best notused for Range Three in this applica-tion. Set them at zero and mount sep-arate trimmers on the p.c.b.

Remember that the receiver willfunction with the oscillator runningat 1·6MHz above or below the signalfrequency. The oscillator core settingthat produces the lowest inductance(giving the highest frequency) is thecorrect one.

Screw large cores down to increaseinductance. With the small cores, induc-tance is at maximum when they are flushwith the top of the can.

Frequency coverage is determined whol-ly by the setting of the oscillator coil cores,L2, and trimmers, VC3. Signal frequencycores, L1, and trimmers, VC1, are adjustedto optimize alignment, thereby peakingsensitivity and minimizing second channelinterference.

(10) Inject a 1600kHz signal at the gate offirst mixer, TR1. Adjust the core ofIFT1 for maximum meter reading.

(11) Switch to Range Three. Set trimmers,VC1c and VC3c, at about 10 percentmesh and turn tuning capacitor,VC2/VC4, to maximum value. Injecta 10MHz signal at the aerial terminaland set the core of oscillator coil, L2c,for maximum response. Adjust thecore of signal frequency coil, L1c, tofurther peak the response.

(12) Set tuning capacitor, VC2/VC4, at itsminimum value. Inject a 30MHz sig-nal at the aerial terminal and adjustoscillator trimmer, VC3c, for maxi-mum response. Adjust signal circuittrimmer, VC1c, to further peak theresponse.

(13) Swing the tuning capacitor back tomaximum and refine the adjustmentof the coil cores. Then back to mini-mum and peak the trimmers again.

(14) Repeat this process on Ranges Oneand Two, injecting the appropriate fre-quencies (see Table 7.2) at the tuninglimits. Carry out refining adjustmentson all ranges to ensure continuouscoverage.

(15) When the three ranges have beenaligned, finally peak the responsewith the tuning capacitor set ataround 15 percent, and then 85 per-cent, mesh. This will help to optimizetracking over the tuning capacitor’sswing.

This completes the alignment of thereceiver.

The beat frequency oscillator can now

be tuned, i.f. gain adjusted, and the signal-strength meter settings refined.

(16) Switch to single-side-band mode, setb.f.o. tuning potentiometers, VR1 andVR2, to mid position, and preciselytune the receiver to a steady signal.Adjust the core of b.f.o. coil, L1/L2,for zero beat in the loudspeaker.

(17) Adjust i.f. gain preset, VR3. Mid-posi-tion should suit most users, but if thereception area is difficult and/or theaerial very inefficient, maximum gainmay be required. This will, however,increase inter-station receiver noise.

Readers blessed with a good aerialin an ideal location may choose a min-imum setting. Altering VR3 affects theadjustment of the signal-strengthmeter.

The receiver is sensitive and internally

generated noise is comparatively low: thefield-effect transistor mixers are probablycontributing to this.

Adopting a 1·6MHz first i.f. results in asignificant reduction in second channelinterference, but the problem is not com-pletely eliminated. Two tuned circuits inthe preselector would make performancevery acceptable in this respect (see Part 7).

The specified filter provides good selec-tivity but there is a noticeable attenuation ofthe upper audio frequencies. With the filterin circuit, weak signals sandwiched

between powerful ones can bereceived free from side-band-splatter.

Without the filter, the weak sta-tions cannot be heard. Some top notecutting is not too high a price to payfor the clear reception of weak and“difficult” stations.

High selectivity makes tuning criti-cal. The simple reduction drive on themain tuning control is not adequate. Areduction of 36-to-one or 100-to-onewould be more appropriate. The con-stant-rate fine tuning or bandspreadcontrol does, however, ensure that thearrangement is workable.

If the superhet’s oscillator is not

crystal controlled it will drift. Highselectivity only makes the problem moreapparent.

With this receiver, after a ten-minutewarm-up, drift is unnoticeable up to12MHz or so and small enough not to be aproblem at higher frequencies. Specialmeasures were not taken to minimize it,and performance in this respect could nodoubt be improved.

Mounting the speaker under the top ofthe cabinet caused microphony when weaksignals were reproduce at good volume.This was cured by loosely packing the topcompartment with plastic sponge.

The aluminium box chassis and case areno more than adequate. To get the best outof the receiver, its various sections shouldbe enclosed in diecast boxes and an (expen-sive) air-spaced tuning capacitor fitted.

In a simple comparison test with a mod-

ern, high-performance communicationsreceiver (price tag in excess of £1000), tun-ing was swept from 5MHz to 12MHz.Every station picked up by the communica-tions receiver could be heard equally wellon this radio. The test did, however, exposefour, second-channel responses. These tookthe form of “phantom” stations (see Part 7).

A sweep across the 3·5MHz amateur bandrevealed the superiority of the commercialreceiver, but very little would have beenmissed by a user of the set described here.

56 Everyday Practical Electronics January 2004

Receiver front panel layout and lettering.

Underside chassis showing wiring to rear of front panel controls, coil pack p.c.b.(Part 7) and arrangement of the battery packs on a hardboard platform.

Bull Electrical-jun03.qxd

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