BASIC ELECTRONICS

Part 5

A Course of Training Developed for

THE UNITED STATES NAVYby the New York firm of

gement Consultants and Graphiological Engineers

S^ALKENBURGH, NOOGER & NEVILLE, INC.

(CENTRAL!1

T

'1RAHXJ

pted to J3r» and Commonwealth Usage

Special F i Training Investigation Team of

the Roy al & Mechanical EngigSj$£ 3?J

^ mm

LONDON

iHE TECHNICAL PRESS, LTDNEW YORK

THE BROLET PRESS

British and Commonwealth Edition first published 1959

©Copyright 1959 by

VAN VALKENBURGH, NOOGER & NEVILLE, INC.New York, U.S.A.

All rights reserved

American Edition first published 1955

©Copyright 1955 by

VAN VALKENBURGH, NOOGER & NEVILLE, INC.

New York, U.S.A.

U.S. Library of Congress Catalog Card No. 55-6984

All rights reserved

.'.'JAN

&rar;es

76/32. t

Made and printed by Offset in Great Britain by

William Clowes and Sons, Limited, London and Beccles

PREFACE

IN THESE six Manuals on BASIC ELECTRONICS and the five which have pre-

ceded them on BASIC ELECTRICITY, there lies the core of an illustrated

Course of Technician Training—carefully planned, brilliantly simplified, and radi-

cally new—which was developed some years ago at the request of the United States

Navy by a distinguished New York firm of management consultants and graphio-logical engineers, Messrs. VAN VALKENBURGH, NOOGER & NEVILLE, INC.The Course has since become standard in U.S. Navy Training Schools. More than50,000 men have taken it as an essential part of their training to technician level in

14 different Navy trades; their average training time has been cut by half; andsupplies of Course materials are now held as part of the Navy's official War Mobiliza-tion Stores.

The text of the Course was subsequently released in a condensed form to the

general public in the United States, where it has proved an outstanding success. In

addition to large sales to individuals, to schools and to technical institutions of all

kinds, more than a score of world-famous companies have taken the published

Manuals for use in their Apprentice Training Schemes, and have found that they

enable them to turn out qualified technicians both faster and at less cost than did

the old methods of text-book and lecture. Several American trade unions (who take

a keen interest in the "up-grading" of their members to more skilled and better-paid

jobs) have chosen the Manuals as the best available training materials for their

purpose.

This notable Series is now being made available, in a revised, reset, and suitably

re-worded edition, to users in Britain and the Commonwealth.

While negotiations with the American authors were still in progress, word reached

the British publishers that there had recently been set up, under command of Train-

ing Headquarters, Royal Electrical and Mechanical Engineers, at Arborfield in

Berkshire, a special "Electronics Training Investigation Team" whose task was to

devise solutions for some of the training problems which would face the British

Army when National Service ended, and when the Army's increasingly elaborate

electrical and electronics gear would have to be manned and serviced by recruits

entering the Army with none of the technical knowledge which many National

Servicemen had hitherto brought with them into the Forces.

It seemed possible that most of the REME requirements for a new-style, yet

technically sound, instructional approach could be met by a suitably edited British

version of the VVN&N Manuals. A visit to Arborfield was accordingly arranged,

where the reception given to the Manuals, with their attractive appearance andproved record of success, was enthusiastic; and after a careful evaluation of their

merits and potential suitability had been made, War Office consent was secured to

a proposal that the work of adapting text and illustrations to British notation andterminology should be undertaken by the Electronics Team at Arborfield.

Later, while this work was still proceeding, a decision was reached to adopt the

revised Manuals as basic texts for the training of future REME technicians, and anorder for large numbers of complete sets of the Manuals was placed. Early interest

was also shown by several other branches of the Armed Forces, notably the Royal

Corps of Signals and the Royal Air Force. Military Advisers to the High Com-missioners of at least six leading Member Nations of the Commonwealth submitted

early proofs of the English edition to their respective Ministries of Defence.

The original U.S. Navy Course was based on a novel technique of teaching

developed by the Authors after extensive research and practical experience with

thousands of students. Immense pains were taken to identify and present only the

essential facts about each new concept or piece of equipment. These facts were

then explained in the simplest possible language, one at a time; and each was illus-

trated by a cartoon-type drawing. Nearly every page in every one of the Manuals

carries one or more of these brilliantly simple "visualizations" of the concept

described.

The approach throughout is non-mathematical. Only the simplest equations

needed for working with the fundamental laws of electricity are employed. Yet

there has been no shirking of essentials, even when they are difficult; and students

with higher qualifications and educational background find nothing in the Manuals

to irritate or slow them down. They merely pass on to the next subject quicker

than the rest.

Despite their Services background, the Manuals have been proved suitable for

civilian use. Their purpose, however, is limited to the training of technicians, not

of engineers. They aim to turn out men capable of operating, maintaining, and

carrying out routine repairs to the equipment described—not men capable of invent-

ing or improving it.

They present a unique simplification of an ordinarily complex set of subjects—so

planned, written and illustrated as to become the best and quickest way to teach or

learn BASIC ELECTRICITY and BASIC ELECTRONICS that has ever been

devised.

In these Manuals, first things come first—and only the essentials come anywhere.

Their accuracy and thoroughness, combined with their extreme lucidity, will maketheir publication a landmark in technical education in Britain and the Common-wealth.

TABLE OF CONTENTS

Sectionpage

1 Introduction to Receivers 5 j

2 Receiver Aerials5 13

3 TRF Receivers—R.F. Amplifier Stage 5.21

4 TRF Receivers—Detector Stage 5 29

5 TRF Receivers—Audio Amplifier Stage 5.38

6 The Superheterodyne Receiver 5 43

7 Fault-finding5 j^

8 General Review of Receivers 5 92

9 Miscellaneous Electronic Circuits 5 95

10 Frequency Modulation : Transistors 5. 100

Index5J01

This Course in

BASIC ELECTRONICS

comprises 6 Parts

This is PART 5

It is preceded by a Course in

BASIC ELECTRICITY

comprising 5 Parts

all uniform with this volume.

Part 1 explained the General Principles of Electricity.

Part 2 described and discussed D.C. and D.C. Circuits.

Parts 3 and 4 described and discussed A.C. and A.C. Circuits.

Part 5 described and discussed A.C. and D.C. Machines.

BASIC ELECTRONICS

will be followed by a further Course in

BASIC SYNCHROS & SERYOMECHANISMS

in two Parts

also uniform with this volume

§ I: INTRODUCTION TO RECEIVERS 5.1

The History of Communications

Since the earliest days, Man has always tried to increase the distance over whichhe could send messages. The modern radio transmitter and receiver are merely

the latest and most efficient of a long series of devices which he has invented anddeveloped to the same end.

5.2 [§|

The History of Communications {continued)

Some of the more primitive methods of communication—human messengers andhoming pigeons, for instance—today have only limited application.

But we still use semaphore signals and interrupted flashes of light to conveymessages. Coloured lights, rockets and flares are only more up-to-date versions of

those warning hilltop fires which flashed news of the coming of the Spanish Armadaacross the length and breadth of England—from Plymouth to "the burghers of

Carlisle." Whistles and sirens are still used in ways differing only in degree fromthe uses to which they were put in the days of the Roman Empire.

SOME JJJDUilJj/] VERSIONS OF

rJiJJJJ J J J J£ METHODS ARE

Signal lamps

Sirens

§'1 5.3

The History of Communications (continued)

These simple signalling systems, however, are at best slow and unreliable. If the

wind is blowing from the wrong direction, sound signals may not be received. In

thick fog or heavy rain, visual signals fail to deliver their message. Runners and

pigeons are slightly more reliable; but their rate of travel is relatively slow.

The problem of rapid and reliable communication was only finally solved by

harnessing electricity to the task.

Improvements on the inventions of Morse, Bell and Marconi have led to the

development of modern telegraph, telephone and wireless communication systems

which are capable of transmitting messages almost instantaneously over thousands

of miles.

tfl*^DE COMMUTATIONSR^^^

5.4

The History of Communications (continued)

B

tSSCOMMUN.c^,^

Transmission by telephone and telegraph is, of course, still limited to placeswhich can be physically reached by a wire or cable; but with the advent of "wire-less" communication (or, as it is more commonly called, radio communication) theuse of electricity for transmitting messages has reached its most advanced stage.

This remarkable electronic system, radio, consists of two parts—the transmitterand the receiver. The transmitter, as you learnt in Part 4 of Basic Electronics,sends out the message in the form of radio waves. The radio receiver picks upthese radio waves, and converts them into the message which was originally put intothe transmitter.

This Part of Basic Electronics deals with the receiving end of radio communica-tion—the receiver.

§1]

The Jobs a Receiver Performs

5.5

The jobs which a receiver must perform are very much the same in radio, in

television, in radar and in echo-sounding equipment. Both the type of signal goinginto the receiver and the type of signal coming out of the receiver are different for

each type of equipment; but the stages which an incoming signal must go throughbefore it emerges as a useful output are almost identical whether the receiver is used

for radio, television, radar or echo-sounding.

The function of any receiver can be broken down into five separate steps, as

follows:

1 . Picking up incoming signals. In radio, television and radar, the incoming signals

are electro-magnetic carrier waves sent out by a transmitter. When these wavescut across the receiving aerial, a very weak current is caused to flow in the aerial.

This current varies in frequency and in amplitude in such a way as to duplicate

the signal radiated from the transmitter aerial.

In echo-sounding equipment, the "aerial" is an underwater microphone which^converts the incoming signal to a weak current flow, and so serves the samepurpose as the radio and radar aerials.

RADIO

MODULATEDCARRIER WAVE

UNDERWATER <2%t+*MICROPHONE A# ""-

m,^mm RADAR ECHO —

Selecting the desired signal. Manytransmitters are sending out sig-

nals which reach a receiver aerial,

however; and of these manysignals, the receiver must be able

to select the one desired. Every

transmitter uses a different fre-

quency, while the receiver contains

circuits which can be tuned to

any frequency the operator desires

to receive.

By tuning these circuits to the

frequency of the signal of one of

the transmitters, you can select that

desired signal and reject all others.

The more tuned-circuits used in a

receiver, the sharper is the tuning

of that receiver.

5.6 [§l

The Jobs a Receiver Performs (continued)

3. Amplifying the desired r.f. signal. The currents generated by the incoming

signals in the aerial are extremely weak. R.F. amplifiers similar to those you

have already studied are used to amplify these weak signals before they reach

the detector.

4. Detecting, or demodulating, the amplified signal. A detector stage follows the

last r.f. amplifier in a receiver. The detector does the important job of separat-

ing the "envelope" of the signal from the r.f. carrier. Because the envelope is

the modulation of the signal, a detector is sometimes called a "demodulator."

The signal, after demodulation, may be either a voice or code signal, as in

communications radio receivers; or else a rapid change of voltage, as in radar

or television receivers.

5. Amplifying the audio or video signals. In radio receivers, the audio signal

which comes from the detector undergoes further amplification. Audio voltage

amplifiers and power amplifiers similar to those you have already studied build

up the audio signal to a sufficient strength to operate a pair of earphones or a

loudspeaker, so that the signal may be heard.

In radar receivers, the signal will show up as a "pip" on an oscilloscope. In

these receivers, video amplifiers similar to those you have already learnt about

are then used to amplify the voltage "pips." The video amplifiers take the

signal from the detector, and build it up so that it can be seen on the radar screen.

AA

f1

2

TunedCircuit

COMMUNICATIONTYPE

RECEIVER

4 5

DetectorAudioAmp.

4—

¥

-H-Ajv JU RADAR

TYPERECEIVER

Y2 3 4 5

TunedCircuit

RFAmp. Detector

VideoAmp.

§ I]5 -7

Receiver Sensitivity

There are several characteristics of a receiver which can be determined simply

by comparing the receiver output with the input signal. These characteristics will

tell you how well your receiver is working.

The first of the characteristics—there are three in all—is sensitivity.

Sensitivity can be defined as the ability of a receiver to pick up weak signals, to

amplify them, and to deliver a useful output. No matter what type of equipment

the receiver is in, sensitivity is important because many input signals which the

receiver must amplify are extremely weak. And only a sensitive receiver can

develop a sizable output from a weak input.

Receiver Not Sensitive Enough

HI-YO

Very Sensitive Receiver

[§l5.8

Receiver Selectivity

Sensitivity by itself, however, will not make a receiver good enough for use. It

must also be selective.

Selectivity may be defined as the ability of a receiver to select a desired signal,

and to discriminate against all undesired signals.

Even if every signal which reached the aerial was amplified, the output—althoughstrong enough—would still be useless because of all the interference caused by thepresence of the unwanted signals.

M,#*fr$ny*R

y£& moon *„

Selective Receiver

Si] 5 -9

Receiver Fidelity

If the receiver can pick out one signal from the many which reach the aerial

(selectivity), and can amplify it so as to produce a useful output even though the

signal may be weak (sensitivity), the receiver is good enough to be used in quite

a number of applications.

For other applications, however, one more thing is important—the receiver must

be able to reproduce the original signal without distortion. A receiver which can

do this is said to have good fidelity; a receiver which cannot has "poor fidelity."

Home radio receivers usually have good fidelity, because they are made for the

enjoyment of the listener. Communications receivers are made to reproduce speech,

but only so that it shall be intelligible; they are therefore not usually designed with

good fidelity in mind.

Radar receivers, on the other hand, must have good fidelity because the operator

can get a great deal of information from the exact appearance of the received

signal as it is displayed on his oscilloscope.

5.10[§ |

The Crystal Receiver

The first receivers used in the early 1900s were called "crystal sets." In their

simplest form they consisted of an aerial, a tuned-circuit, a crystal detector, and a

pair of earphones.

The aerial picked up any signals there might be about—in those days there werevery few of them!—and the tuned-circuit selected the wanted one. The crystal

—

usually a piece of galena or carborundum with a "cat's whisker" contact device

—

"detected" the signal in a manner you will learn about later. The resulting audio-frequency signals were then used to energise the earphones.

Simple though these crystal sets were, a fair degree of skill was needed in the

adjustment of the "cat's whisker"; and, since their sensitivity was poor, consistently

good results could only be obtained in the neighbourhood of the transmitter.

Today these wireless sets are curiosities—but the crystal detector is used in manymodern equipments.

IN THE BEGINNING...

§1]

The TRF Receiver

5.11

RF AMPLIFIER DETECTOR AUDIO AMPLIFIER

By 1920 crystal sets were on their way out, and were being replaced by tuned

radio frequency (TRF) receivers which made use of valves.

The first one or two valves, and their tuned-circuits, make up the r.f. amplifier

which gives the TRF receiver better selectivity and sensitivity than had the old

crystal sets. The detector does the same thing as did the crystal detector, but

sometimes amplifies the signal as well.

After the detector, the audio signal is amplified in the audio amplifier. The

output of this audio amplifier is a fairly powerful signal, which can be used to

drive a loudspeaker or a pair of earphones.

TRF receivers are not very often used today, but some receivers are still of

this type.

[§l5.12

The Superheterodyne Receiver

The most common type of receiver used in home radios and in other equipmenttoday is the superheterodyne receiver.

In this type of receiver, not all the r.f. amplification takes place at the incomingsignal frequency. Most of it is achieved after the incoming signal has been con-verted to an intermediate frequency (i.f.), which is always the same no matter whatthe frequency of the desired signal may be.

LocalOscillator

Detector AFAmplifier

SUPERHET

The only parts in a superhet which are additional to those in a TRF are thevariable-frequency local oscillator, the mixer and the i.f. amplifier. >

The variable-frequency local oscillator is similar to the oscillators which youhave already studied. The oscillator produces an r.f. signal which is "mixed" inthe mixer stage with the signal from the r.f. amplifier. The resulting i.f. frequencyis the difference between the input signal frequency and the local oscillatorfrequency.

The i.f. is a fixed frequency, and the i.f. amplifiers are therefore fixed-tuned.This allows them to be very accurately tuned, so that high gain and selectivity canbe obtained at the chosen frequency.

You will find out exactly how a superhet receiver works later in this Part. Forthe time being, it is enough for you to know that the advantage of the superhetover the TRF receiver is that the superhet has higher gain and greater selectivity.

§2: RECEIVER AERIALS 5.13

The Function of Receiver Aerials

The purpose of the receiver aerial is to intercept the electro-magnetic wavesradiated from the transmitter. When these waves cut across the aerial, they gener-ate a small voltage in it. This voltage causes a weak current to flow in theaerial-earth system.

This feeble current has the same frequency as the current in the transmitter. If

the original current in the transmitter is amplitude-modulated, the aerial currentwill vary in exactly the same manner.

This weak aerial current, flowing through the aerial coil, induces a correspond-ing signal voltage in the grid circuit of the first r.f. amplifier stage of the receiver.

Electromagnetic Waves

nWHCurrent

TRANSMITTER

VCurrent

RECEIVER

RECEIVER AERIALS INTERCEPT THE RADIO WAVES SENT OUT BYTHE TRANSMITTER

A receiving aerial should feed as much signal, and as little undesired interference,

to the receiver as possible. It should be constructed so that the signal is not lost

or dissipated before reaching the receiver. It should give maximum response atthe frequency or band of frequencies to which the receiver is tuned.

An aerial can also be "directional," which means that it will give best responsein the direction from which the operator wishes to receive.

The receiver aerial problem is easily solved when the receiver is operated inconjunction with a transmitter. Since the transmitting aerial is usually designedto incorporate the desirable features which have just been listed, the same aerialcan be used alternately for both transmitter and receiver.

A switch or relay is used to connect the aerial to whichever piece of equipmentis operating at any particular moment.However, wireless stations are frequently receiving-stations only; and it is then

necessary to erect a separate receiving aerial, paying attention to the four considera-tions of noise, signal loss, frequency response and directivity.

Before discussing these considerations of aerial design, you should get to knowsomething about a few of the more common types of receiving aerials.

5.14 [§2

Types of Receiver Aerials

One of the simplest and most commonly used aerials is the "inverted L." It

consists of a wire, known as a "flat-top," which is suspended horizontally between

two insulators.

The length of the wire should be from 50 to 75 feet for medium broadcast-band

reception, and from 20 to 40 feet for high frequency reception. The flat-top should

be suspended from 30 to 50 feet above ground.

A wire known as the "lead-in" is used as a transmission line from the aerial to

the receiver. It is connected near one end of the flat-top, and brought down to the

primary winding of the receiver aerial coil.

Flat-top

INVERTED L AERIAL

Another common type of aerial is the "doublet," or dipole aerial. It consists

of a horizontal wire divided into two equal sections by an insulator.

Each half of the aerial should ideally be a quarter-wavelength long, for the

frequency most commonly used.

The transmission line from the aerial is connected to the two ends of the primary

of the aerial coil.

This type of aerial will give excellent high-frequency reception, and will also give

comparatively noise-free reception on the broadcast band.

It may be of interest to note that most of the television receiver aerials with

whose appearance you are so familiar are little more than modifications of the

dipole aerial, with metal bars replacing the less rigid wires.

1/4 wavelength

i—eCUO-

1/4 wavelength

!~«JA

^

Transmissiony^\Line

DIPOLE AERIAL

§2]

Types of Receiver Aerials (continued)

5.15

Where lack of space makes horizontal

aerials impracticable, a vertical aerial is

used instead.

Vertical aerials, consisting of tele-

scoping metal masts from 3 to 14 feet in

length, are commonly used for cars and

portable receivers, and sometimes for

home broadcast receivers.

An ordinary lead-in wire is run from

the bottom of the aerial to the primary

of the aerial coil of the receiver. The

other end of the primary should be

earthed.

VERTICALAERIAL

Another type of aerial used for port-

able and home receivers is the "frame

aerial."

The aerial consists of a coil of wire

which is connected to the two ends of

the primary of the aerial coil. Manyportable broadcast-band receivers con-

tain a frame aerial within the cabinet.

The frame aerial is highly directional.

When it is pointed edgeways towards

a transmitter, the signal pick-up is

maximum; when its flat side is towards

the transmitter, the signal pick-up is

minimum. This property makes it ex-

tremely useful for radio-beacon and

direction-finding equipment.

When used in conjunction with

direction-finding equipment, the frame

aerial takes the form of a loop, and is

therefore called a "loop aerial."

5.16 .[§2

Selecting and Installing an Aerial—Noise

An important consideration in aerial installation is that of undesired radio noise(commonly called simply "noise").

Noise consists of radio waves of many frequencies, and is produced by bothman-made and natural electrical disturbances. Among the more important man-made noise producers are lifts, fans, refrigerators, vehicle ignition systems, vacuumcleaners, X-ray and diathermy equipment, and mains power lines.

No aerial can differentiate between desired signals and undesired radio noise,though steps can be taken to minimize the latter.

It is customary to compare the signal pick-up of the aerial with its noisepick-up. This relationship is known as the "signal-to-noise ratio."

A high signal-to-noise ratio is necessary for relatively noise-free reception.

DesiredSignal

WavesNOISE WAVES

AERIALS CANNOT DIFFERENTIATE BETWEEN SIGNALAND NOISE RECEIVER

NoisyReception

§2]*•»

Selecting and Installing an Aerial—Noise (continued)

There are various ways in which a high signal-to-noise ratio may be obtained.

The first is by locating the aerial as far as possible from power lines, and any

other electrical devices likely to produce noise.

Placing the aerial at right angles to the mains power lines will also reduce the

amount of noise.

HIGH SIGNAL-TO-NOISERATIO (Low Noise Pickup

LOW SIGNAL-TO-NOISE RATIO(Hi£h Noise Pickup)

The second method is by increasing the height above ground of the aerial as much

as practical considerations will allow. This tends to increase the signal strength,

and to reduce the amount of noise.

The third method involves using a good earth connection to the receiver when

provision is made for one. A poor earth lead may pick up noise; it should there-

fore be kept as short as possible, and away from noise-producing devices.

A good eartrrlead should use rubber-insulated wire, S.W.G. No. 16, or copper

braid. It should make good contact through a clamp to an earthed object, such as a

radiator or water pipe. Gas pipes should never be used for earthing purposes.

A good deal of noise may be picked up by the lead-in. If the lead-in uses two

wires, as in the case of the transmission line used with a dipole aerial, noise can be

reduced by using twisted wires, or by reversing the positions of the wires every

few feet.

Noise can also be reduced by using screened lead-in wires.

518 B 2

Selecting and Installing an Aerial—Signal Losses

The second factor to be considered in selecting and installing an aerial is thatof signal losses.

The aerial should be placed as far as possible from metal objects, chimneys,walls, and tree branches. These objects absorb radio waves, and thus reduce thestrength of the signal reaching the aerial.

A loose or swinging aerial may cause the signal to fade.

FACTORS THAT CAUSEAERIAL SIGNAL LOSSES. .

.

^r-

Aerial touchingtree branches

Aerial sur-rounded by tall buildings

Aerial swaying in the breeze

Signal losses will also be increased if a high resistance is present in the aerialcircuit. To reduce resistance, all joints and connections should be carefullysoldered; and, where possible, the aerial and lead-in should consist of a single pieceof wire with no joints.

Signal losses may be further increased by leakage of current through poor sup-porting insulators. Insulators should be made of materials such as glazed porce-lain or pyrex glass, which do not readily absorb moisture and thus provide a leakagepath for current.

Aerial Wire

Insulator

Lead-in

sant=

Tie-Wire

REDUCING AERIAL RESIST-ANCE BY ELIMINATING JOINTS

§2]5.19

Selecting and Installing an Aerial—Frequency Response and Directivity

The third consideration is that of frequency response, which is related to the aerial

length. A maximum signal, at a given frequency, will be induced in the aerial if

its length is either one-quarter or one-half the wavelength of the signal to be

received.

It is possible to change the effective length of an aerial by placing a coil or a

capacitor in series with it. Adding inductance increases the electrical length of the

aerial; while adding capacitance shortens it.

The front panel of certain receivers contains a control marked ae. tune (aerial

tuning). This control varies the size of a small capacitor, and is used to compen-

sate for variations in aerial length.

In general, however, adjustment of the aerial to the correct length is not nearly

as important for receiving equipment as it is for transmitters.

The final consideration is that of directivity. All aerials, except the vertical

type consisting of a single perpendicular wire, have directional properties, and

receive signals from certain directions better than they do from others.

A horizontal or inverted L aerial will receive best when the signal cuts the aerial

wire at right angles. For any one station, of course, the aerial may be turned so

that it produces the maximum signal pick-up. But since it is extremely unlikely

that all transmitters will be broadcasting from the same direction, the position of

the aerial will usually have to be a compromise for all stations.

Dipole aerials may be made highly directional by arranging them into systems

called "arrays," similar to those employed with television systems.

FACTORS TO BE CONSIDERED IN

SELECTING AND INSTALLINGAERIALS . .

5.20

REVIEW—Receiver Aerials

Aerial Function. The receiver aerial

picks up signals radiated by a transmitter,

and transmits these signals—via the lead-in

or transmission line—to the primary of the

receiver aerial coil. The electro-magnetic

waves cutting the aerial induce signal

voltages, which are amplified by the

receiver.

[§2

Electromagnetic Waves

Hill Illli

Inverted L Aerial. This is one of the

simplest and most commonly-used types of

aerials, consisting of a horizontally sup-

ported wire, with the lead-in attached near

one end. RECEIVER

Dipole Aerial. This type of aerial is the

same as that used in transmitters, and con-

sists of two quarter-wavelength sections

supported horizontally. It gives excellent

high-frequency response.

Frame Aerial. The frame aerial is used

in many portable and home medium broad-

cast-band receivers. Because it is highly

directional, it is also used in direction-finding

equipment.

Selection and Installation. Noise, signal

loss, frequency response, and directivity are

the four factors which must be considered

when selecting and installing an aerial.

-•-<ca>p-

RECEIVER

TT*

\

§3: TRF RECEIVERS-R.F. AMPLIFIER STAGE 5.21

Introduction to the TRF Receiver

The TRF receiver is the type of receiver you will study first.

You will recall from the first Section of this Part of Basic Electronics, "Introduc-

tion to Receivers," that the TRF consists of an r.f. amplifier, a detector, and anaudio amplifier.

So that you may have in mind the goal towards which you are working, there are

shown below the circuit diagrams of the two types of TRF receiver you will learn

about in this and the three following Sections.

TRF RECEIVER WITH A DIODE DETECTOR

5.22' B 3

The R.F. Amplifier Stage

Every TRF receiver contains one or more stages of r.f. amplification preceding

the detector. The main purpose of these amplifiers is to provide additional selec-

tivity and sensitivity.

You will recall that selectivity indicates how well a receiver receives a desired

signal and rejects unwanted signals; and that sensitivity is a measure of the re-

ceiver's ability to pick up a weak signal. Up to a point, the more r.f. amplifier

stages there are in an equipment, the greater will be its selectivity and sensitivity.

Let us now briefly review some of the principal points you have learnt about r.f.

amplifiers.

VAerial

GREATER SELECTIVITY AND SENSITIVITYOBTAINED BY USING MORE TUNED

RF STAGES

1STRF

AMPLIFIER

2NDRF

AMPLIFIER

3RDRF

AMPLIFIER

* Since the r.f. amplifier stage is designed primarily for voltage amplification, any

valve suitable for voltage amplification may theoretically be used. But in practice

triodes are not considered satisfactory, because they have a strong tendency to

produce undesirable oscillations when employed in r.f. amplifier stages. Unless

the triodes are carefully neutralized to prevent feedback, these oscillations are likely

to cause trouble.

Valves containing a screen grid do not suffer from this disadvantage; and for

this reason most r.f. amplifiers used in receivers employ either tetrodes or pentodes.

The valve which is generally preferred as an r.f. amplifier is a variable-mu pen-

tode. This type of valve not only provides considerable voltage gain, but also

minimizes certain types of interference from powerful undesired signals. Since

varying the grid bias of a variable-mu pentode changes the amount of amplification,

this type of valve is very suitable for use in circuits involving either manual or

automatic gain (volume) control

Only screen grid

valves are usedin receiver RFamplifiers

Yet even when pentodes are used in r.f. amplifiers, the number of stages of

amplification is limited because of a tendency towards instability caused by inter-

action between the stages, which can cause oscillations. You will therefore rarely

meet r.f. amplifiers containing more than two stages.

§3]

R.F. Transformers

5.23

In the schematic of an r.f. amplifier stage shown below, you will note that the

amplifier has two r.f. transformers.

The first, the aerial coil, is designed to couple the aerial circuit to the grid circuit

of the amplifier. The second, often referred to as the r.f. coil, couples the anode

circuit of the r.f. amplifier with the grid circuit of the next stage.

The coils are usually wound on a former made of cardboard or bakelite. They

are generally of the air-core type, though powdered-iron cores may also be used.

5.24 [§3

Band Switching

Note that while the primaries of transformers used in r.f. stages are untuned,

variable capacitors are connected across the secondary coils so as to form resonant

or tuned circuits. These resonant circuits are responsible for the high selectivity

and sensitivity of the TRF receiver.

If a receiver is to cover a frequency range greater than one coil and one tuning

capacitor will allow, it will be necessary to change the tuning-circuits. This is

usually accomplished by substituting- a different coil.

One system uses removable plug-in coils; while another system uses several

mounted coils whose leads run to a multi-contact rotary switch known as a "selec-

tor" or "band switch." By turning the switch, any coil may be connected to the

tuning capacitor, thus making tuning possible over the desired band of frequencies.

A receiver employing band switching is illustrated below. In this receiver the

selection of the frequency band is accomplished by rotating a four-position switch.

Both switch sections can connect any one of four r.f. coils to a variable capacitor.

§3]5.25

Ganged Capacitors

Every TRF receiver has a minimum of two tuned-circuits, one associated with

the r.f. amplifier and one with the detector.

In the early days of the TRF, every variable capacitor was connected to its own

individual tuning knob. In order to tune your radio to a station, you had to turn

each knob individually until each tuned-circuit was resonant at the frequency of

the desired station.

The later TRF receiver eliminated the need for individual tuning knobs by having

the variable capacitors of all the tuned-circuits mounted on one shaft. This allowed

the receiver to be tuned with a single control, which varied all the tuned-circuits

together and at the same time.

This is called "ganged" tuning. In a receiver having two r.f. amplifier stages,

plus a detector, a three-gang capacitor would be used.

Since all the tuned-circuits are varied together, all the variable capacitors should

have exactly equal capacitances at every setting of the gang spindle. All the tuned-

circuits would then be resonant at the same frequency at the same time—resulting

in maximum sensitivity and selectivity.

Unfortunately, no two capacitors can be manufactured exactly alike; so the

individual capacitor sections on a ganged unit will have slightly different capaci-

tances at every setting. If nothing were done to compensate for these differences

in capacitance, the tuned-circuits in a receiver would be resonant at slightly differ-

ent frequencies for every setting of the tuning knob—causing poor receiver selectivity

and sensitivity.

A receiver with such characteristics is said to be "out of alignment."

A = 200 pF

B= 195 pF

C=204pF

RESONANTTO 600 Kc/S

RESONANTTO 603 KQ/s

I3RD

RF

RESONANTTO 598 Kc/s

RECEIVER OUT OF ALIGNMENT

[§35.26

Trimmer Capacitors and Coils

The problem of misalignment can be solved by adding small variable capacitors,

called "trimmer capacitors," in parallel with the main variable tuning capacitors.

Sometimes the adjustment is made in the coil of a tuned-circuit, rather than onthe capacitors. In this case, an iron-cored slug is moved in and out of the coil,

causing the inductance to vary. This is called "permeability," or "slug tuning."In receivers covering only one band, the trimmers are usually located on the

ganged capacitors, one for each section. In receivers using band switching, thetrimmers for each range are usually mounted on, and in parallel with, the individualcoils.

These trimmer capacitors are adjusted after the main capacitors have been set

at minimum capacitance at the high end of the dial. They are adjusted to makethe total capacitance of the individual tuned-circuits the same at every setting ofthe tuning control.

The tuned-circuits will therefore be tuned to the same frequency, simultaneously,over the whole width of the band—the result being high receiver sensitivity andselectivity.

VARIABLECAPACITORS

RESONANTTO 600 Ke/s

RESONANTTO 600 Kc/s

RESONANTTO 600 Kc/S

RECEIVER IN ALIGNMENT

It sometimes happens that, although the circuits are properly adjusted at the highend of the dial, they do not tune to identical frequencies at the other end of thedial. A correction may be made for this, in some sets, if the end rotor plates areof the slotted type. Adjustments can be made by bending a portion of the slotted

plates either towards, or away from, the stator plates.

When all of the ganged circuits of a TRF receiver tune to the same frequency at

any particular dial setting, they are said to be "tracking," and the receiver is in

alignment.

§3]

Grid Bias Manual Volume Control

5.27

Since the signals arriving from different transmitters will vary in amplitude, it is

necessary to provide on the receiver a volume control so that the gain of the r.f.

amplifier and the amplitude of the output signal can both be varied.

One of the most common methods of controlling the gain of a TRF is by chang-

ing the bias voltage of the r.f. amplifier stage. This is done by placing a variable

resistor in the cathode circuit.

You will recall that the r.f. amplifier stage usually employs a variable-mu pen-

tode. Varying the bias of this variable-mu valve causes the amplification

factor of the valve to vary, and therefore the gain of the stage to vary.

If there are several r.f. amplifiers, the variable resistor may be connected in such

a manner as to vary the bias of all the r.f. amplifiers.

The fixed resistor in the cathode circuit is placed there in order to provide the

proper bias when the variable resistor is set for maximum gain at^the zero resist-

ance position.

Variation of the grid bias volume con-

trol is achieved by means of a potentiometer,

which also acts as a variable shunt across

the primary of the aerial coil.

When the moving arm of the potentio-

meter is moved to the left, the resistance

across the primary coil is reduced, while the

cathode resistance is increased. This results

in a weaker signal on the grid, and reduced

voltage amplification.

When the sliding arm is moved to the

extreme right, the resistance across the pri-

mary is increased, while the cathode resist-

ance is reduced. This produces a stronger

signal on the grid, and increased voltage

amplification.

5.28

REVIEW—R.F. Amplifier Circuit

[§3

Now pause for a moment to examine the r.f. amplifier shown above, and to review

the purpose of each of its components.

The aerial coil couples the aerial to the control grid of the r.f. amplifier. Thevariable capacitor enables the operator to tune the amplifier to the frequency of the

desired signal, and thus provides selectivity. The 25-K variable resistor acts as a volumecontrol; while the 330-ohm resistor sets the lower limit of cathode bias. Thecapacitor between the cathode and earth is the bypass capacitor.

The 100-K resistor in the screen grid circuit is the screen grid voltage dropping resistor,

which serves to keep the screen grid at a lower positive potential than the anode. The0*01-(xF capacitor in the screen grid circuit is the screen grid decoupling capacitor,

which acts as a bypass for r.f. signals and enables the screen to act as a shield betweenthe anode and the control grid.

The r.f. coil in the anode circuit acts as the anode load. The secondary of this r.f.

coil is connected into the next stage.

§4: TRF RECEIVERS-DETECTOR STAGE 5.29

What the Detector Does

The primary purpose of the detector circuit is to change the r.f. signal into a

signal which can be reproduced as sound by the headphones or loudspeaker. With-

out the detector, radio reception is not possible.

The simplest radio receiver, reduced to its bare essentials, would consist of a

detector, an aerial, and a pair of headphones. All other stages which are found in

front of the detector in more complex receivers, such as the TRF or the "superhet,"

have been put there for the primary purpose of enabling the detector to do a

better job.

In order to understand the purpose of the detector, it is necessary to review briefly

the theory of radio-telephony transmission.

In Part 4 of Basic Electronics, which dealt with radio transmitters, you learnt

that radio-telephony transmission requires the generation of a radio-frequency

carrier wave. Intelligence is impressed on this wave, one method of doing so being

to vary the amplitude of the carrier wave.

A combination of audio-frequency waves superimposed on a carrier wave is

known as an amplitude-modulated signal; and it is this combination of waves whichis picked up by the aerial of the radio receiver.

When transmitted signals reach a receiver, the desired signal is selected by tuned-

circuits. The selected signal is rectified by a crystal or valve rectifier in the detector

stage. The r.f. component is filtered out of the rectified signal, and the audio

component is changed into sound waves by earphones or a loudspeaker.

The process of detection thus includes both rectification and filtering.

THE PROCESS OF... pfcTECTlO*

Reproduction

5.30 [§4

The Crystal Detector

The simplest of all detectors is the crystal type. If you understand how it works,

you will have very little trouble understanding the operation of the somewhat more

complicated valve detectors.

A CRYSTAL DETECTOR

The modulated radio waves which are radiated from the aerials of transmitters

induce corresponding signal voltages and currents in the aerial system of the radio

receiver. These signals are then transferred to the detector circuit by means of a

radio-frequency transformer, the secondary of which is a tuned-circuit. It is this

tuned-circuit which gives the detector some degree of selectivity.

The selected signal is rectified by the detector; and the result is a pulsating d.c.

signal containing two components, one of which is radio frequency and the other

audio frequency.

The a.f. component passes through the headphones, and produces sound waves

similar to those originally used to modulate the radio wave.

The r.f. component is bypassed round the headphones by the filtering action of a

small capacitor placed across the headphones.

HOW A CRYSTALDETECTOR WORKS

*- Audio Frequencies Path

• Radio Frequencies Path

§4] 5.31

The Crystal Detector (continued)

The crystal detector possesses the advantages of simplicity and economy. It

needs no batteries, nor other local sources of power. There are no filaments to

burn out, or to produce hum and noise. In applications requiring the detection of

ultra-high-frequency signals, moreover, the crystal possesses certain decided advan-

tages over the valve detector.

The ordinary crystal detector provides no amplification (though the recently

developed transistors are—as you will discover in Part 6—crystals which are capable

of amplifying signals).

The crystal detector is therefore characterized by low sensitivity, and is usually

preceded by one or more r.f. amplifier stages.

The crystals used in the earliest radio receivers had another disadvantage. Cer-

tain portions of the face of the crystal had better rectifying properties than had

others. This made it necessary to explore the face of the crystal with a wire probe

called a "cat's whisker," until a sensitive rectifying point was found. The wire

could easily be dislodged from this sensitive point, and reception was for that

reason likely to be erratic.

In addition, dirt, grease or air-borne dust could spoil the sensitive spot, and makeit necessary to search for another spot.

These difficulties have been overcome in the more modern germanium and silicon

crystal rectifiers. These consist of small sealed cartridges containing contact wires

which cannot be dislodged. They have an extremely long life, and resist shock

and vibration better than most conventional valves.

OPEN TYPE CRYSTAL DETECTOR

SEALED GERMANIUM CRYSTAL

5.32

The Diode Detector

[§4

The fundamental circuit of the diode detector closely resembles that of the crystal

detector, and the operating principles and characteristics of these two detectors are

very similar.

CRYSTAL DETECTOR

Joo

I y

T

i

>h|—.—

,

t -i

You will see from the diagram above that the only difference between the diode

and crystal detectors. is the replacement of the crystal by a diode valve. The pro-

cesses of selection, rectification and filtering are carried out in the same way as with

the crystal detector.

When the detector is operating, current flows through the tuned-circuit during

the positive half of each signal cycle. This current flow produces what is knownas "damping," which has the effect of reducing both the voltage gain and the

selectivity of the tuned-circuit.

Because of these factors, and because it is capable of handling large signal

voltages without distortion, the diode detector is generally preceded by one or more

tuned r.f. amplifiers which provide increased sensitivity and selectivity.

The detector is usually followed by one or more stages of a.f . amplification, to

provide sufficient power to operate a loudspeaker.

§ 4] 5.33

The Grid-leak Detector

You have seen that since the diode detector cannot itself amplify, it is generally

used in a receiver which contains several separate stages of amplification. If,

however, you need a receiver in which the number of valves used has to be kept

low, you will have to use a more sensitive detector—one which amplifies as well

as detects.

In order to amplify, the detector must of necessity use a valve containing a control

grid, such as a triode, a tetrode or a pentode.

Of the triode detectors, the one which is easiest to understand is the grid-leak

detector. This is because the grid-leak detector is basically only a diode detector

followed by a stage of audio-frequency amplification.

Suppose that, to begin with, you examine the grid and cathode circuits of this

detector, and temporarily forget about the anode circuit. The result will be the

circuit shown below.

Note that this is basically the circuit of the diode detector. The control grid

of the triode is taking the place of the diode anode, the grid-leak resistor has

replaced the diode load or earphones, and the grid capacitor is acting as an r.f.

filter capacitor across the load.

When a modulated signal voltage is applied to this circuit, the grid will attract

electrons from the cathode during the positive half-cycles. The flow of current

through the grid-leak resistor to earth will produce a voltage drop across the grid-

leak resistor.

Because of the fact that current can flow in only one direction in the grid circuit,

this voltage will remain constant in polarity. The grid is thus biased, or kept at a

negative voltage with respect to the cathode.

The amount of bias will vary in accordance with the amplitude or the modulation

of the signal. In other words, the bias will vary at an audio-frequency rate.

[§45.34

The Grid-leak Detector (continued)

Now consider the complete grid-leak detector circuit.

Schematic ofagrid-leak detector

You will recall that the anode current of a triode is dependent on the grid voltage.

Consequently, the audio-frequency variations in bias should produce a correspond-

ing varying anode current.

Any radio-frequency component which there may be in the anode current is

filtered out by capacitors or by r.f. chokes placed in the anode circuit. As a result,

the voltage developed across the anode load is an amplified reproduction of the

audio-frequency voltage developed across the grid-leak resistor.

When there is no incoming signal, no bias is produced. Consequently, the anodecurrent is high when no signal is being detected. When a signal is received, the

grid becomes biased negatively, and the average amount of anode current decreases.

The amount of grief bias developed is equal numerically to the amount of grid

Current multiplied by the value of the resistance of the grid-leak. The larger the

grid-leak resistor, therefore, the greater will be the amplitude of the signal

developed.

For this reason, grid-leak detectors which were extremely sensitive would have to

use grid-leak resistors whose values were between one and five megohms.If, however, a strong signal comes in, it is quite possible that enough bias will

be created to cut off the flow of anode current during part of the cycle, thus pro-

ducing distortion.

In practice, therefore, the value of the grid-leak resistor has to be chosen so as

to be a compromise between the requirements of sensitivity and of minimumdistortion.

§ 4] 5.35

The Anode-bend Detector

The anode-bend detector employs a triode or pentode biased at, or near, cut-off.

The bias is usually provided by means of a cathode bias resistor; or, less frequently,

by means of a bias battery placed between grid and cathode. The anode current

will be at, or near, zero when no signal is being received.

*AnodeCurrent

Grid VoltageAverage value of anode current

^Signal value

applied to grid-cathode circuit

Action in anode bend detector|

When a modulated r.f. signal is impressed on the grid, there will be a pulse of

anode current during the positive half-cycle, and little or no anode current during

the negative half-cycle. The anode current will contain an amplified and rectified

version of the input signal.

The filtering of the r.f. component is accomplished by connecting a small capa-

citor between the anode and earth, and an r.f. choke in series with the anode load.

It is important that a small capacitor be used, since a capacitor which is too large

will tend to filter out the higher audio frequencies as well as the radio frequencies.

5.36 [§4

The Anode-bend Detector {continued)

In contrast to the action of the grid-leak detector, anode current in the anode-bend detector is at a minimum with no incoming signal.

Up to a certain point thereafter, the average anode current increases in direct

proportion to the amplitude or strength of the signal impressed on the grid.

Another important characteristic is that if care be taken not to drive the grid

positive, the anode-bend detector will consume no input power, and there will beno damping effect on the tuned-circuit. Consequently, the selectivity and fidelity

of the anode-bend detector is better than is that of the grid-leak detector.

On the other hand, one of the disadvantages of the anode-bend detector is thefact that its sensitivity to weak signals is much less than is that of the grid-leak

detector.

It also produces more distortion than does the diode detector; and it cannotdirectly provide a voltage to be used for automatic gain control. A typical anode-bend detector circuit is illustrated below.

Components Functions

R.F. coil and variable capacitor Provide selectivity, and couple detector topreceding r.f. amplifier stage

22-K resistor Provides cathode bias

0-5-(xF capacitor Bypasses signal round cathode bias resistor

001-jjt.F capacitor and r.f. choke Filter r.f. component of signal

270-K resistor Acts as anode load of detector

0-01-[aF capacitor Couples detector to following a.f. amplifierstage

§4]

REVIEW-

5.37

-Detectors

You have now learnt the basic principles of operation of four important types of

detectors. Let us review the basic circuits and operating characteristics of each type.

CIRCUITS

CRYSTAL DETECTOR

CHARACTERISTICSModerate sensitivity

Poor selectivity

Good fidelity

Capable of handling strong signals

Simple and economical to operate

High reliability (with modern

crystals)

s&T ..

O

DIODE DETECTORrWWA *-HT+

Low sensitivity

Poor selectivity

Good fidelity

High reliability

Capable of handling strong signals

Capable of supplying AGC voltage

GRID-LEAK DETECTOR

High sensitivity

Poor selectivity

Low fidelity

Moderate reliability

Easily overloaded by strong signals

Anode current decreases when a

signal is received

ANODE DETECTOR

Insensitive to weak signals

Good selectivity

Fair fidelity

Moderate reliability

Anode current increases when a

signal is received

ANODE-BEND DETECTOR

538 §5: TRF RECEIVERS-AUDIO AMPLIFIER

STAGEThe Audio Power Amplifier

Your next step in the study of radio receivers is to review what you learnt about

the audio power amplifier; for you will need an audio power amplifier in your

receiver to enable you to hear in your loudspeaker the signals you have picked up.

You will remember that loudspeakers produce sounds by pushing the air andmaking it move. They are themselves actuated by electrical power; and it is their

job to convert this electrical power into sound.

To enable them to do this, the first necessity is that the power supplied to themshall be sufficient for the job. It is for this reason that an audio power amplifier

is put in as the last stage of a receiver.

You will find an audio power amplifier in almost every receiver you will ever

have to operate or repair. It is as common in this type of equipment as is the r.f

.

amplifier.

§5] 5.39

A.F. Amplifier Tone Control Circuits

Now, unless something is done to correct the matter, it will frequently happen

that the sound emitted by a radio receiver will differ considerably from the original

sound applied to the transmitter.

The main reasons are that audio amplifiers do not amplify all frequencies by the

same amount, and that loudspeakers do not respond equally well tofall frequencies.

Other causes of distortion to the signal in transit are static and valve noise, both

of which are generally reproduced as high audio frequencies of a random nature.

Now, the tone or pitch of any sound depends on whether it contains a greater

proportion of high-frequency or of low-frequency waves. A high-pitched sound

has more high-frequency sound-waves; while a low-pitched sound consists mainly

of low-frequency sound-waves.

In order, therefore, to reduce the annoyance of interference by static and noise,

and to provide the deeper bass effect which most radio listeners prefer, many radio

receivers employ some means of tone control.

They accomplish this by eliminating from the signal some of the higher fre-

quencies which it contains—either shunting them to earth or bypassing them round

the output transformer.

The capacitor in the anode circuit shown above has a value such that it offers

a relatively easy path for the higher audio-frequencies; while the lower audio-

frequencies encounter a path of less opposition by travelling through the primary

coil of the transformer. In this way, the amount of high-frequency sound reaching

the loudspeaker is considerably reduced.

The variable resistor acts as a means of tone control. If the resistance is madevery high, the path through the capacitor to earth becomes one which offers high

opposition to the passage of high-frequency as well as to low-frequency signals.

Less high-frequency current will therefore flow through the bypass capacitor, and

there will be a rise in the pitch of the sound coming from the loudspeaker.

5.40 [§5

A.F. Amplifier Volume Control

You have already learnt one method of controlling the volume of a receiver.

This method involved varying the bias of the r.f. amplifier stage.

Now you will discover another commonly-used method of volume control, whichinvolves instead the detector and a.f. amplifier stages.

AFv., AMPLIFIER

DiodeDetector

ToLoudspeaker

7S»*

DETECTOR-OUTPUT VOLUME CONTROL

Notice that the detector in the circuit above is coupled to the a.f. amplifier by

means of an RC coupling circuit. The volume control is basically a voltage divider,

the moving arm tapping off the desired amount of signal voltage, which is then

applied—through the coupling capacitor—to the grid of the a.f. amplifier.

This type of volume control is also frequently employed in superhet receivers.

Some receivers employ a dual type of volume control. This type of control

regulates the gain in the first and second r.f. amplifier stages by varying the cathode

bias; and further controls the gain by varying the amplitude of the input signal

applied to the first a.f. amplifier.

GRID CONTROL OF RECEIVER

§5] 5.41

Comparison of R.F. and A.F. Amplifiers

Since most radio receivers you will encounter contain both r.f. and a.f . amplifiers,

you must possess a clear understanding of the differences between them, and of

the advantages and disadvantages of each.

Look carefully at the comparative table set out below.

R.F. Amplifiers

1. Designed to amplify frequencies

above 20,000 cycles.

2. Usually have tuned-circuits, thereby

adding selectivity.

3. Usually coupled to other stages by

r.f. transformers.

4. Precede the detector stage.

5. Designed for voltage amplification.

6. Triodes are rarely used, since they

lack stability and have to be neutral-

ised.

7. Generally employ variable-mu pen-

todes.

A.F. Amplifiers

1. Designed to amplify frequencies of

between 15 cycles and 20,000 cycles.

2. Untuned, and so do not add to selec-

tivity of set.

3. Coupled to other stages by a.f. iron-

core transformers, or by resistance-

capacitance coupling.

4. Follow the detector stage.

5. Usually designed for power ampli-

fication.

6. Very stable and not likely to oscillate.

If triodes are used, no neutralization

is required.

7. Generally employ triodes, beam-power tetrodes, or power pentodes.

5.42

REVIEW—A.F. Amplifier Circuit

Now review the functions of

the various component parts of

the a.f. power amplifier circuit

shown opposite. Notice that no

provision is made in this stage

for volume or tone control.

[§5

The 0-01-fxF coupling capacitor and the 470-K grid resistor in the control grid circuit

couple the control grid of the amplifier to the preceding detector stage. The capacitor

also eliminates the possibility of any d.c. voltages from the detector stage being im-

pressed on the control grid of the amplifier.

The 330-ohm resistor acts as a cathode bias resistor ; while the 20-jaF capacitor bypasses

the varying component of the anode current round the cathode resistor—thus preventing

the production of a varying bias and the accompanying reduction in amplification.

The primary of the output transformer acts as the anode load, and couples the amplifier

to the loudspeaker. The 0*001-jaF capacitor across the primary bypasses high-frequency

audio signals round the primary, and so reduces the amount of high-frequency sounds

emitted by the loudspeaker.

Components

0*01-(iF capacitor and 470-K resistor

330-ohm resistor

20-fi.F capacitor

0*001-|aF capacitor

Output transformer

Functions

Couples the a.f. amplifier to preceding

detector stage

Provides cathode bias

Bypasses signal round cathode bias

resistor

Prevents high-frequency audio signals

from entering loudspeaker

Acts as anode load, and couples amplifier

to loudspeaker

§6: THE SUPERHETERODYNE RECEIVER 5.43

Introduction

The superheterodyne receiver is the most popular type of receiver in use today.

Practically all commercial home radios are of this type.

You will find a superheterodyne circuit in practically every piece of electronic

equipment which contains a receiver. This includes radar, echo-sounding and

communications equipment—any device, in short, which picks up and receives a

signal.

Your knowledge of the TRF receiver gives you a good start towards learning the

superheterodyne, because it uses all the basic components of a TRF—with three

additional units.

The block diagram of a superheterodyne below shows the three additional units

—a mixer, a local oscillator, and an intermediate frequency (i.f.) amplifier.

YV

fit)AA -

RFAmplifier

Detector AFAmplifier

THE TRF RECEIVER

RFAmplifier

LocalOscillator

IF Detector AFAmplifier Amplifier

THE SUPERHET RECEIVER

5.44 [§6

The Superhet at High Frequencies

At high frequencies, the TRF receiver does not work as well as it does at lower

radio frequencies. Above 20 Mc/s, the TRF circuit does not have the necessary

sensitivity and selectivity.

The superheterodyne receiver avoids the difficulties encountered with the TRFat high frequencies by converting the selected signal frequency to a lower (inter-

mediate) frequency (i.f.) which can be amplified more easily.

/HQSPIT-OAH OIT. **H

T(/fto-

GOOD• SENSITIVITY• SELECTIVITY• STABILITY

SUPERHETRECEIVER

TENDAYSLEAVEFORAll

§6] 5.45

How the Superhet Works

If you know why the superheterodyne was developed, you will easily learn

how it works.

TRF receivers use r.f. amplifiers with variable tuned-circuits to select and amplify

the received signal. If the receiver has three r.f. stages before the detector, it will

contain four tuned-circuits. You know that, if the best selectivity and sensitivity

are to be obtained, each of these four tuned-circuits must be tuned to the samefrequency.

But it is extremely difficult to make a multi-ganged tuning capacitor each section

of which will tune its circuit to exactly the same frequency as will the other

sections. Therefore, the gain and the selectivity of the TRF receiver are both

limited; for more r.f. stages cannot conveniently be added.

The superheterodyne receiver overcomes this problem by taking the incoming

signal and converting the carrier frequency to another frequency. This new fre-

quency is called the "intermediate frequency" (i.f.); and it is constant regardless

of the frequency to which the receiver is tuned.

The i.f. signal is amplified in a series of high-gain amplifiers which are pre-

tuned to this fixed i.f. frequency. Because it eliminates the many-ganged tuning

capacitor, the superhet with its fixed frequency i.f. amplifiers can be used to give

very large gains and very fine selectivity.

Here is how the signal frequency is changed in the superhet. The incoming

signal and the output of the local oscillator are fed into the mixer valve. Theanode current varies with both of these signals, which are of different frequencies.

A beat (or difference) frequency appears in the resulting signal.

This signal is then passed through the i.f. amplifiers, which are tuned to this

difference frequency.

The i.f. signal has exactly the same modulation as the r.f. carrier. The only

change has been the substitution of the i.f. frequency for the r.f.

THE SUPERHET RECEIVERS MAKE USE

5.46 [§ 6

Selectivity of the Superhet

When you tune a superheterodyne receiver to a station of 880 kc/s, you are

setting the tuned r.f. circuit to 880 kc/s, and at the same time you are automatically

tuning the local oscillator to 1345 kc/s.

Two signals—one of 880 kc/s, the other of 1345 kc/s—are thus fed into the

mixer stage. The output of this mixer stage contains a frequency of 465 kc/s,

which is the difference between the values of its two inputs.

If at the same time the aerial picks up another (unwanted) station at a frequency

of 1100 kc/s, the signal (if it were strong enough to get by the first tuned-circuit)

would be mixed with the local oscillator output in the mixer stage. This undesired

signal of 1100 kc/s would there produce a beat-frequency of 1345 minus 1100,

equals 245 kc/s.

The i.f. amplifier tuning is fixed at 465 kc/s, and only the "beat" signal at this

frequency will be amplified—i.e. only the required (880 kc/s) station will be heard.

Thus the superhet has selected the proper input signal on the basis of the frequency

of the beat signal produced in the mixer stage.

THE yaM4ped

KEEPS THE LOCAL OSCILLATOR

"TRACKING" THE TUNED RF

If you wanted to hear the 1100-kc/s station, the receiver would have to be re-

tuned. Turning the knob changes the frequency to which the r.f. amplifier is tuned,

and at the same time changes the local oscillator frequency. A two-section ganged

tuning capacitor is used for the purpose.

Tuning the receiver does not affect the i.f. stages. When the r.f. tuned-circuit is

set at 1100 kc/s, the oscillator will be generating a signal of 1565 kc/s. The i.f.

amplifier remains tuned to 465 kc/s.

Now it is the 1100-kc/s signal which produces the 465-kc/s beat-frequency. Thebeat produced by the 880-kc/s signal is the difference between its frequency and the

1565-kc/s local oscillator frequency—namely, 685 kc/s—and this frequency will

not be amplified by the i.f. stages.

For the superhet to work properly, the local oscillator must be adjusted so that

it will always tune to a frequency which is a fixed number of kilocycles different

from the desired r.f. frequency. Thus, as the receiver—that is, the r.f. tuned-

circuit—is tuned from 550 to 1600 kc/s, the local oscillator should tune from1015 to 2065 kc/s. Then any signal picked up at the frequency to which the

receiver is tuned will produce an i.f. frequency of 465 kc/s.

The designer's choice of i.f. depends on the nature of the equipment. In most

domestic radio receivers intended for medium- and long-waveband reception, the

i.f. lies between 450 and 480 kc/s.

§ 6] 5.47

R.F. Amplifier Stage

It is not essential for a superhet receiver to contain an r.f. amplifier stage. The

signal from the aerial would then be fed through an r.f. transformer to the signal

grid of the mixer or converter stage.

But you will encounter many receivers which do contain stages of r.f. amplifica-

tion preceding the mixer; so you will have a better understanding of the operation

of superhet receivers if you know the reasons which count in favour of including

an r.f. amplifier stage.

The primary reason for having an r.f. amplifier is to improve the signal-to-noise

ratio. The mixer stage usually produces more valve noise than does an r.f. stage

of amplification. The signal, plus this valve noise, is then amplified by the follow-

ing i.f. amplifier stage.

But if the signal strength is increased by placing an r.f. amplifier stage before the

mixer,, less amplification is required in the i.f. amplifier stage. And since valve

noise produced by the mixer is not amplified as much as it was when no r.f. stage

was present, a better signal-to-noise ratio is obtained.

A further advantage of having an r.f. amplifier stage is related to radiation from

the oscillator stage.

Do not forget that this oscillator is really a low-powered transmitter. If there

is no r.f. amplifier stage, the oscillator is connected through the mixer stage to the

aerial, which will radiate some energy from the oscillator.

This radiated signal may cause interference with reception in nearby receivers;

it may also divulge the location of the receiver.

This radiation may be reduced or prevented by using one or more stages of

r.f. amplification, and by carefully screening the oscillator stage.

RADIATION FROM A SUPERHETRECEIVER MAY REVEAL THELOCATION OF A SHIP

5.48 [§ 6

R.F. Amplifier Stage (continued)

The third advantage of having an r.f. amplifier stage is concerned with selectivity.

You will recall that in the TRF receiver the r.f. amplifier stages enabled the

operator to select the desired signal from a group of signals whose frequencies

were very close to one another. The r.f. amplifier in a superhet, on the other hand,

serves to prevent interference from a signal whose frequency may be several hundred

kilocycles above that of the desired signal.

This type of interference is called "image-frequency," or "second-channel," inter-

ference.

Let us assume that you have a superhet receiver without an r.f. amplifier stage,

and that the receiver is tuned to a station operating at a frequency of 600 kc/s.

The oscillator in the receiver will be tuned to 1065 kc/s, and the resulting i.f. signal

will have a frequency of 1065 kc/s minus 600 kc/s, or 465 kc/s.

If, however, there is a powerful station nearby broadcasting at a frequency of

1530 kc/s, some of the signal from this station will enter the mixer stage, where

it will beat against the signal from the oscillator. The resulting signal will be

1530 kc/s minus 1065 kc/s, or 465 kc/s—the same intermediate frequency as that

produced by the desired station.

The i.f. amplifier stage will amplify both signals equally well, since they are

both at the correct frequency of 465 kc/s.

This type of interference produces whistles, and a confusing mixture of sounds

coming out of the loudspeaker.

So, when the intermediate frequency is 465 kc/s, second-channel interference is

produced when there is a second station broadcasting at a frequency twice the

intermediate frequency, or 930 kc/s, above that of the desired signal.

, Second-channel interference can be reduced by the use of an r.f. amplifier stage

feefore the mixer.

In any receiver in which second-channel interference might present a problem,

however, one tuned-circuit is not enough to guarantee the elimination of this inter-

ference. There may be as many as two or three stages of r.f. amplification at the

signal frequency before the signal is fed into the mixer.

These stages are not made as selective as are those in a TRF receiver; but they are

selective enough to discriminate between the desired signal and the image frequency.

These stages do not present the alignment problems of the TRF, since none of

them needs to be sharply tuned to the signal frequency.

§6]5.49

The Local Oscillator

In a superhet receiver circuit, the local oscillator is tuned by a variable capacitor

ganged to those of the tuned r.f . circuits.

It is tuned to oscillate at a frequency which differs from that to which the r.f.

circuits are tuned by a fixed amount for every position of the tuning dial.

The local oscillator output is mixed with the r.f. carrier. The fixed frequency

difference (the i.f.) is part of the output of the mixer.

AFAmplifier

LocalOscillator

The process of mixing or beating two frequencies together to get a different

frequency is called "heterodyning."

The result of this mixing is a frequency which is above the audio-range—in other

words, a supersonic frequency. That is why the receiver was originally known as

the "supersonic heterodyne."

The particular superhet you will study will have a tuned-grid type oscillator

operating at 465 kc/s above the r.f. frequency. The i.f. is therefore 465 kc/s. The

variable capacitor in the oscillator tuned-circuit is ganged with the tuning capacitor

in the aerial tuned-circuit, as shown in the illustration on page 5.61.

As the receiver is tuned to an incoming signal, the local oscillator is also varied

to keep it at a frequency of 465 kc/s higher than the signal to which the aerial

circuit is tuned. The table below gives examples of typical operating frequencies.

SOME TYPICAL OPERATING FREQUENCIES FOR THE SUPERHET

Frequency to Frequency I.F.

which R.F. Circuits of Difference

are tuned Local Oscillator Frequency

in kc/s in kc/s in kc/s

550 1015 465

710 1175 465

880 1345 465

1440 1905 465

5.50 [§ 6

The Local Oscillator (continued)

There are several types of oscillators which can be employed as local oscillators.

But the types most frequently used are modifications of the Armstrong and of the

Hartley oscillators.

An ideal local oscillator should possess the following characteristics:

1. The frequency of its output should be stable, and free from drift at all settings.

2. It should be capable of delivering sufficient voltage to the mixer.

3. The amplitude of the output should be constant over the entire frequency range.

4. The oscillator should have minimum inter-action with other tuned-circuits. (If

the oscillator inter-acts with other tuned-circuits, there will be a change in

oscillator frequency every time the other circuits are tuned.)

5. The oscillator should radiate a minimum of energy into space.

The oscillators found in re-

ceivers used for medium broad-

cast band reception are usually

designed to produce a signal

whose frequency is higher than

the frequency of the incoming

radio wave.

The tuning capacitor of the

oscillator is ganged with the

capacitor of the r.f. tuned-

circuit so as to maintain a con-

stant difference in frequency as

the receiver is tuned across the

band. This is known as "track-

ing." A condition of perfect

tracking occurs when the oscil-

lator tuned-circuit is resonant

exactly the right number of kc/s

(i.f.) higher than the r.f. tuned-

circuits, for all settings of the

tuning dial.

The process of adjusting the

tuned-circuits so as to maintain

this constant difference at both degrees rotation of ganged capacitor

the high and low ends of the tuning bands is known as "aligning."

The process of adjusting a receiver to obtain good tracking will be dealt with

more completely in the Section later on in this Part dealing with the alignment and

adjustment of superhet receivers.

§6] 5.51

The Local Oscillator (continued)

There are two ways of designing the oscillator tuned-circuit so that it will produce

a signal the right number of kc/s higher than that of the r.f. circuit.

One method employs a special kind of ganged capacitor. The plates of the

oscillator section of this capacitor are made smaller than the plates of the r.f.

section. Since the capacitance of the oscillator section is less than that of the

r.f. section, the oscillator section will resonate at a higher frequency.

In addition, the plates of the oscillator section are so shaped as to produce correct

tracking as the plates are meshed or unmeshed.

© Ganged capacitors with

sections of differing sizes (DGanged capacitors with

identical sections

When both sections of the capacitor are identical, the total capacitance of the

oscillator tuned-circuit is reduced by placing a capacitor, called a "padder" capaci-

tor, in series with the oscillator tuning capacitor. This padder is often an adjust-

able mica capacitor. As a result of this reduction in capacitance, the oscillator

circuit resonates at a higher frequency. The capacitance of the padder is usually

between 500 and 1000 pF. In the process of alignment, the padder capacitor is

adjusted for best tracking at the low-frequency end of the band.

In order to align the superhet receiver at the high-frequency end of the band,

trimmers are placed in parallel with each section of the tuning capacitor, just as

they are in TRF receivers. These trimmers will usually have a capacitance varying

between 2 and 20 pF.

The end vanes of each section of the ganged capacitor are usually split to provide

tracking adjustment at intermediate points.

TYPICAL RFCIRCUIT

TYPICAL OSCILLATORCIRCUIT

5.52 [§ 6

How the Mixer Stage Works

The mixer works on the following principle. If two different frequencies are

mixed or combined in a valve, the output will contain many different frequencies,

of which the four principal ones are:

1. The modulated r.f. signal from the r.f. amplifier or the aerial.

2. The unmodulated local oscillator r.f. output.

3. The sum of 1 and 2.

4. The difference between 1 and 2.

The difference frequency is the desired i.f. signal.

The signals resulting from the mixing of a modulated carrier with the unmodulatedoutput from the oscillator will have exactly the same modulation shape as the

original carrier wave. The tuned-circuits of the i.f. amplifier are used to select the

desired signal, and to discriminate against the others.

V.+® Modulated RF Signal Input

Local Oscillator Output

Mixed Signal

V.-

® Modulated IF Signal (Desired Output From Mixer)

§6] 5.53

How the Mixer Stage Works (continued)

Of the several frequencies present in the anode circuit of the mixer valve—the

original signal, the oscillator signal, a signal whose frequency is the sum of the

first two signals, and another signal whose frequency is equal to their difference

—

only the latter, or i.f., signal must be passed on to the next stage.

This is accomplished by using the primary of a tuned i.f. transformer as the

anode load. The primary and secondary coils are tuned to the intermediate

frequency which, in the receiver you will consider in this Part, is 465 kc/s. In

this manner, maximum response is obtained for the i.f. signal.

This i.f. signal is passed on to the following i.f. amplifier stage, while the other

signals are rejected by the selective action of the tuned i.f. transformer.

HOW THEMIXER STAGE

WORKS . . -

1000 Kc/s

ToDetector

Oscillator OSCILLATOR SIGNAL1465 Kc/s

5.54 [§ 6

Mixer Valves and Frequency Changers

The three types of mixer circuit you will meet most frequently are illustrated

below.

The first employs a pentode as the mixer valve, and in the circuit illustrated the

r.f. signal is injected at the control grid and the oscillator signal at the suppressor

grid. The oscillator signal could equally be coupled inductively or capacitively

to the cathode, to the control grid, or to the screen grid of the mixer.

HEPTODEMIXERIsUPPRESSOR

(be

- OSCILLATOR *The second type of mixer circuit employs a heptode—a valve

with seven electrodes. Heptodes are of two kinds, one of whichis designed simply as a mixer valve; the other type combines

the functions of mixer valve and oscillator valve. Valves of

this latter kind are known as "frequency-changers."

In the heptode mixer, the r.f. and oscillator signals are injected

at grids G-l and G-3. In the heptode frequency-changer, the

cathode, G-l and G-2 act as a triode in an oscillator circuit, and

the r.f. signal is fed to the "injector grid" G-3.

-HT+ ..rf

- -^GRID

\&iiE-3

m¥OSCILLATOR

The third and most common type is a triode-hexode fre-

quency-changer valve circuit. The triode-hexode consists of a

triode and a hexode (six-electrode valve) enclosed in one en-

velope and sharing a common cathode. The triode portion is

used as the valve in the oscillator circuit, and the r.f. signal is

applied to G-l.

-HT+

RFAMP

I

Trl-Al"!

IFAMPZ=!~

TRIODE HEXODEHEXOOE ANOOE SCSEEN

s+ s. GRIDS

2HPSIGNALGRID E:

zk

OSCILLATORSECTION

§ 6] 5.55

The I.F. Transformer

I.F. transformers may be tuned to the correct frequency by adjusting small mica

trimmer capacitors. This process of adjustment will be described later.

The coils and capacitors are mounted in small metal cans which act as screens.

Small holes in the tops of the cans make it possible to vary the value of the capaci-

tors by turning adjusting screws without removing the screening-can.

AdjustingScrews

TrimmerCapacitor

SecondaryCoil

Primary Coil

Primary Leads

Secondary Leads

Many i.f. transformers have powdered iron-dust cores and fixed mica capacitors.

Tuning is accomplished by turning a set screw which moves the dust core in or

out of the coil. This type of transformer is known as a "permeability-tuned"

transformer.

No matter what method is used to tune the transformer, you will find that nearly

all ii. transformers are double-tuned. This means that both primary and secondary

are tuned to the intermediate frequency. This produces a very high degree of

selectivity. -

5.56 [§6

The I.F. Amplifier

The intermediate-frequency amplifier is permanently tuned to the theoretically

constant difference in frequency between the incoming r.f. signal and the local

oscillator.

The tuning of an i.f. amplifier stage is accomplished by means of two tuned i.f.

transformers. The one associated with the grid circuit of the amplifier is called

the "input" i.f. transformer, while the one associated with the anode circuit is called

the "output" i.f. transformer.

The valves employed in i.f. amplifiers are normally variable-mu pentodes.

Since this amplifier is designed to operate at only one fixed frequency, the i.f.

circuits may be adjusted for high selectivity and maximum amplification. It is in

the i.f. stage that practically all the selectivity and voltage amplification of the

superhet is developed.

Simple superhet receivers may contain only one i.f. amplifier, while more com-plex receivers contain as many as three i.f. amplifier stages.

The choice of intermediate frequency is a compromise between the desire for

high selectivity and the need to reduce the possibility of second-channel inter-

ference. Use of a low intermediate frequency, such as 175 kc/s, results in high

selectivity, but increases the possibility of second-channel interference. A high

intermediate frequency reduces the possibility of second-channel interference, but

reduces the selectivity.

The choice of 465 kc/s as the intermediate frequency for the receiver described

in this book represents a compromise between these two undesirable extremes.

-HT+

IF Output

Detector

IF Input

§6] 5.57

The Detector and First Audio Stage

The conversion of the i.f. signal into an audio signal is accomplished by means

of a diode or crystal detector.

The detector circuit in the superhet receiver will sometimes be combined in one

valve with the first stage of audio amplification. The receiver's manual volume

control and automatic gain control are also often included in this part of the circuit.

The valve employed for this purpose may be a double-diode triode. The diode

section acts as the detector, and the triode section as the audio amplifier.

Since a detailed explanation of the operation of diode detectors has already been

given, the operation of the diode detector which is shown in the accompanying

circuit diagram will be only briefly described.

HT+

The diode acts as a rectifier, and conducts current during that half of the signal

cycle in which the anode is made positive with respect to the cathode. During

the other half-cycle, when the anode is negative, no current flows.

This produces a pulsating d.c. which contains two components, one of which is

audio frequency and the other intermediate frequency. The filter circuit, consist-

ing of the 47-K resistor and the two 200-pF capacitors, filters out the i.f. component.

The audio component of the pulsating d.c. produces an a.f. voltage across the 47-Kfixed resistor and the 500-K potentiometer.

The a.f. voltage is applied to the grid of the first audio amplifier, and amplified

at the anode as shown. RL and C2 are part of the Automatic Gain Control (AGC)circuit which you will study later.

5.58 [§6

The Detector and First Audio Stage (continued)

The audio signal developed across the 500-K potentiometer is taken off the sliding

arm, and applied to the grid of the first audio amplifier. The potentiometer is

connected as a voltage divider, and functions as a detector-output type of volume

control. The triode acts as an audio amplifier, which increases the voltage of the

a.f. signal and passes it on to the last stage, which is known as the "second audio,"

or "output" stage.

The purpose of this stage is to amplify the signal output of the first a.f. stage

until it is strong enough to operate a loudspeaker. Power output is the main con-

sideration in this stage.

How Automatic Gain Control Works

Atmospheric conditions may sometimes cause fading of signals coming from

certain stations. The resulting output of the receiver may at one moment be loud

enough to blast the listener from his seat, while at the next moment it may fade

to the point of inaudibility.

Moreover, as you tune from one station to another, the signal strength may vary

in the same way.

One method of preventing this is to have the operator continually adjust the

manual volume control in such a way as to keep the output constant despite varia-

tions in signal strength. A better way is by the addition of a circuit which will

accomplish this task automatically—an automatic gain control or AGC circuit.

The function of the AGC circuit is to vary the sensitivity or gain of the receiver

in accordance with the strength of the signal. It reduces the sensitivity when a

strong signal comes in, and increases the sensitivity when the signal becomes weaker.

The result is that the output of the receiver remains fairly constant despite variations

in signal strength.

Two signals

of unequali strength

RECEIVERWITHOUT

AGC

'#'l|||l'i||IM||M|||.i

/\/\/\Ay\

COMPENSATES FOR VARIATIONS IN SIGNAL STRENGTH

§6] 5.59

How Automatic Gain Control Works {continued)

The AGC circuit most frequently encountered is incorporated in the diodedetector stage. It requires that at least one, and preferably all, of the precedingi.f

. amplifier, mixer, or r.f. amplifier stages employ the variable-mu type of valve.

It also requires some means of transferring the negative voltage which is de-veloped by the AGC circuit to the control grid of these variable-mu valves.

Ao*PlifU

In the diagram above, the resistor

R-l is the diode load, the i.f. filter

being omitted for clarity. When the

anode is positive with respect to the

cathode, current flows through R-l

from (a) to (b). Thus (a) becomes

negative with respect to (b).

The waveform appearing at the

negative end of R-l (a) is actually an

audio wave with a negative d.c. com-

ponent. The negative d.c. component varies with the signal strength.

The AGC filter circuit, consisting of R-2, C-2, filters out the audio, and C-2

charges up to the negative d.c. component. It is this negative voltage that is applied

through the AGC line to the grids of the variable-mu valves in the preceding stages.

The amount of negative voltage developed at (a) will vary with two factors. One

is the relatively rapid variation in strength and amplitude produced by the audio

signal at the transmitter during the process of modulating the carrier wave. The

second is the slower variation in negative AGC voltage produced by variations in

signal strength due to atmospheric conditions.

If the rapid variations produced by the audio modulating signals were allowed

to travel down the AGC line to the preceding i.f. or r.f. stages, undesirable effects

would be produced. The AGC filter circuit, consisting of R-2 and C-2, is added

to remove these audio frequency variations of the negative AGC voltage.

The slower variations in signal strength which show up as a slowly-varying

negative d.c. voltage are not bypassed, but pass down the AGC line to the grids

of the preceding amplifier stages.

5.60 [§6

How Automatic Gain Control Works (continued)

Since these preceding i.f. and r.f. stages employ variable-mu valves, the amount

of gain produced in each stage is dependent on the amount of bias present on the

control grid.

When the signal increases in strength, a high negative AGC voltage is developed

between one end of R-l and earth. This negative voltage is applied through the

AGC filter circuit and the AGC line to the control grids of the preceding stages,

thus increasing the negative bias on these valves.

Because of this increased bias, there is a considerable decrease in the amount of

amplification or voltage gain. In other words, the sensitivity of the receiver has

been reduced.

On the other hand, when a weak signal enters the receiver, a much smaller

negative AGC voltage is developed. The bias on the amplifier valves is reduced,

resulting in considerably greater receiver sensitivity and voltage amplification for

the weak signal.

As far as the human ear is concerned, these variations in receiver sensitivity as

the signal strength varies occur almost instantaneously, thus producing an output

whose volume is reasonably constant.

IF AMPLIFIER, DETECTOR AND FIRST AUDIO AMPLIFIER

§6] 5.61

Complete Circuit Diagram of a Superheterodyne Receiver

* The stages shown below include the following: a mixer, a local oscillator, one

i.f. amplifier, a diode detector, an audio voltage amplifier, an audio power ampli-

fier, and a rectifier.

»

5.62 [§6

Receiving C.W. Signals

You may recall from your study of transmitters that there are several methods

of impressing intelligence upon a carrier wave. One of these methods is known as

"amplitude modulation." The superhet receiver we have considered up to this

point is designed for use with amplitude-modulated (AM) signals.

Another method of conveying intelligence involves the interruption of a carrier

wave in accordance with a code such as the Morse Code. These signals are called

"interrupted continuous wave" or "CW signals."

Since there is no modulation in this type of signal, it cannot be detected by crystal

or diode detector circuits. In order to hear the signal, it is necessary to use a

detector which employs the heterodyne principle.

The heterodyne principle involves mixing the CW signal with a signal obtained

from an oscillator. The result of this mixing is an AM signal which is interrupted

in the same manner as was the original CW signal.

This AM signal can then be detected, and the familiar "dit-dah" sound of code

will be heard in the earphones or loudspeaker.

NO SOUND DAH-DIT-DAH

§ 6] 5.63

The Heterodyne Principle

You may have observed that when two adjacent piano keys are struck at the

same time, a distinct throbbing sound can be heard. This throbbing sound, known

as a beat, has a frequency equal to the difference between the frequencies of the

two notes struck.

If the two notes struck have frequencies of 264 and 297 cycles respectively, the

beat frequency will be equal to the difference between them, or 33 cycles.

Similarly, when two alternating voltages of slightly different frequencies are com-

bined in a detector, the resultant voltage produced in the output will include a

frequency which is equal to the difference between the frequencies of the two original

voltages. This is the basis of the heterodyne principle.

For example, if two inaudible r.f. waves whose frequencies are 465 kc/s and

466 kc/s respectively are applied to a detector valve, the smaller wave (A) will

add and subtract from the larger wave (B) to make the amplitude of the larger

wave (B) vary in the manner shown below. The rate of variation of the amplitude

of wave B is the difference between the frequencies of the two waves—in this case

1 kc/s.

Observe that wave B, because of the introduction of wave A, has been trans-

formed into an amplitude-modulated wave. The audio modulation can be heard

by detection of this AM signal.

llllllliitiiiiiiiiiiiiiii______l____HKR!19K___wl

Wave A j\ A A A:\j\jv

+6 6KC/S

Wave Bf-f -f-4 -1— * t-

lKc s Difference Frequency

Wave B ' \/|Modulated i

J

/ ! 1 Volt

/ 1 Volt— -+- - *—

I A 1A ^ A iA A, r P\ * (\. )t\

\ U_U.j4.44-H -4-4-

!• i voit

s466 Kc/s

5.64 [§ 6

The Beat Frequency Oscillator

In superhet receivers the reception of CW signals is accomplished by means of a

separate oscillator called a "beat frequency oscillator" (BFO), capacitively coupled

to the diode detector or into the previous i.f. amplifier.

The BFO may be a Hartley oscillator tuned to a frequency 1 kc/s above or below

that of the intermediate frequency. Thus, if the i.f. is 465 kc/s, and the frequency

of the BFO is 466 kc/s, a 1-kc/s audio signal will be produced in the diode detector.

The frequency of the BFO is variable over a small range, making it possible to vary

the pitch of the resulting beat note until a satisfactory tone is produced.

Coupling between the BFO and the detector or i.f. amplifier is sometimes achieved

through the stray capacitance of the wiring, and the coupling capacitor is then

omitted.

HT+

Analysis of the Local Oscillator Stage

5.65

THE

To mixer grid

200p1I

HT+

005

500p

ll-wvwiok \—y

SOOp

LocalOscillator

V5 6C5GT/G

22K

620pPadder

Now that you have seen the complete circuit of a superhet, it will be worth

your while to spend a little time analysing the functions of the circuit components

used in the oscillator, the mixer, and the detector stages.

The local oscillator circuit is basically that of an Armstrong oscillator. Feedback

is accomplished inductively, using coil T-4. The variable tuning capacitor is

ganged to the variable tuning capacitor of the mixer stage. The 620-pF capacitor

is a padder capacitor, used to make adjustments in the process of aligning the

oscillator tuned-circuit. It also serves to reduce the total capacity of the oscillator

tuned-circuit so that the oscillator resonates at a frequency higher than that of the

incoming signal.

The 500-pF capacitor is a grid capacitor used to couple the tuned-circuit to the

grid, while the 22-K resistor is the grid-leak resistor. The r.f. choke is the anode load

;

it also prevents r.f. from going towards the power supply. The 005-fj.F capacitor

couples the r.f. output of the anode circuit back to the coupling coil, while effectively

blocking the flow of direct current.

Finally, the 200-pF capacitor is used to couple the output of the oscillator to the

suppressor grid of the mixer. The 10-K resistor connected to the valve grid helps to

keep the oscillator frequency stable.

5.66

Analysis of the Mixer and I.F. Stages

[§6

*7/ie Mvxjgn Staae

To AGC 39 tfrnpLfden Stage

T-l is the aerial coil used to couple the aerial to the control grid of the mixer.

The variable tuning capacitor is used to tune the receiver to the desired station. It

is ganged to the variable capacitor of the oscillator tuned-circuit.

The signal from the oscillator is impressed on the suppressor grid, and the 22-Kresistor is used to provide a path to earth for electrons which may collect on the

suppressor grid. The 680-ohm resistor is a cathode bias resistor, while the 0-l-[xF

capacitor in parallel with it is used to bypass the r.f. signal round the cathode bias

resistor.

The 100-K resistor and 01-(xF capacitor connected to the bottom portion of the

secondary winding of the aerial coil act as a decoupling network whose function is

to keep the r.f. signal out of the AGC line.

The 100-K resistor and 01 -[xF capacitor connected to the screen grid function as

the screen grid voltage dropping resistor and decoupling capacitor respectively.

T-2 is the input i.f. transformer which couples the 465-kc/s i.f. signal found in the

anode circuit of the mixer to the grid circuit of the following i.f. amplifier.

The 680-ohm resistor and 0-1 -[xF capacitor in the cathode circuit of the i.f. amplifier

serve as the cathode bias resistor and bypass capacitor respectively.

The 100-K resistor in the screen grid circuit is the screen grid voltage droppingresistor, while the 1-[xF capacitor in the screen circuit is the screen grid decoupling

capacitor.

T-3 is the output i.f. transformer used to couple the i.f. amplifier with the diode

detector.

Both i.f. transformers are permanently tuned to the intermediate frequency of

465 kc/s.

§6]

Analysis of the Diode Detector and First Audio Stages

5.67

The two 200-pF capacitors function as the detector filter capacitors. Their purpose

is to bypass the i.f. component of the signal to earth round the 47-K and 500-K diode

load resistors.

The 47-K resistor is part of the filter network, while the 500-K potentiometer also

acts as a bleeder resistor across the filter. It controls the amount of detector output

delivered through the 01-jxF coupling capacitor to the grid of the first audio

amplifier, and thus serves as a volume control.

The 1-meg. resistor and 1 -[xF capacitor in the AGC line filter out the relatively

rapid variations in AGC voltage produced by the audio component of the signal.

They allow the slower variations in AGC voltage produced by variations in signal

strength to pass unimpeded down the AGC line.

The 1-meg. resistor connected to the control grid serves as a path to earth for any

electrons that may accumulate on the grid. The 270-K resistor acts as the anode

load of the first audio stage, while the 001-(xF capacitor in the anode circuit couples

the ouipui of the first audio amplifier to the grid of the audio power amplifier.

5.68 [§6

What Alignment Is

For optimum performance, the superheterodyne receiver must be adjusted almost

as carefully as a jeweller adjusts a watch. The process, called "alignment," is the

same for all superheterodyne receivers.

The purpose of alignment is to get the maximum gain in the superhet receiver

for any setting of the main tuning dial. When the dial is set to receive a station

transmitting at 980 kc/s, you want the receiver to give the greatest gain at 980 kc/s.

The same must be true for every setting on the dial. The tuned-circuits—r.f., local

oscillator, and i.f.—must be adjusted so as always to give the maximum output.

How does the superhet circuit have to be tuned to give the greatest gain for each

dial setting? The following aims must all be achieved:

1. The i.f. transformers must be tuned to the fixed i.f. frequency.

2. The r.f. tuned-circuit must be tuned to the frequency on the dial.

3. The local oscillator must be tuned to give an output at each setting of the maindial which is above the dial setting, or r.f. frequency, by a difference exactly

equal to the i.f. frequency.

If you examine the superhet circuit diagram below, you will see which parts of

the circuit have to be adjusted in the alignment procedure.

§ 6] 5.69

Sensitivity Measurements

Sensitivity measurements are used to determine how sensitive a receiver is. Areceiver may be operating normally as far as your ear can detect; but if the overall

gain of the set is low, you may not be able to receive some weak signals.

This failure would only show up by measuring the overall gain of your receiver,

and comparing the results with the specification laid down by the manufacturer.

If a receiver was tested and found to have low sensitivity, the cause would be

determined by checking the gain of each amplifier stage and comparing the results

with the specification, thereby determining which stage has the low sensitivity.

Consider a typical medium broadcast-band receiver. Broadcast receivers are

not designed to be very sensitive, since very powerful stations are relatively close to

the receivers. In these receivers, a loss of sensitivity would mean that you would

turn up the volume control and nothing more. Therefore, sensitivity measurements

are not necessary.

Only when reception becomes so poor that it is uncomfortable or impossible to

hear a station would you attempt to repair the receiver.

5.70 [§6

Sensitivity Measurements (continued)

In some receivers, sensitivity measurements are very important. In a radar

receiver, lack of sensitivity would mean that distant targets which should be detected

would not be noticed at all. Decreased gain in a communication receiver wouldmean that weak signals could not be heard.

If any of these devices have low sensitivity, you could not (unless you are a very

experienced operator) discover the fact merely by operating them, since you usually

have no way of obtaining all the necessary data. You cannot tell that a distant

target is present unless you pick it up; you cannot tell that a weak, distant trans-

mitter is calling you unless you hear the message.

The best check on the performance of the receiver is through sensitivity checks.

Here is a typical way sensitivity measurements would be made with receiving

equipment.

An output meter is used to measure the output of the last stage of the receiver.

The instruction book for the piece of equipment will tell how many micro-volts are

required as the input to this receiver for a standard output as measured on the

output meter. Using a signal generator which has a calibrated output, you inject

a signal of the proper frequency into the receiver input. You adjust the signal

generator output until you read the standard amount of output on the output meter.

By comparing the input you needed with the instruction book's data, you can tell

if the receiver is up to specification.

If the input you used is larger than that stated in the instruction book, yourreceiver has too low a sensitivity. You would then take stage-by-stage sensitivity

measurements to determine the weak stage.

Starting with the last stage of the receiver, you inject a signal of correct frequency,

and adjust the audio signal generator output until the standard receiver output is

obtained. If the input you used compares well with the instruction book data,

the last stage of the receiver is working properly.

You repeat this procedure for each stage, working backwards from the last stage.

That stage which requires a larger input than that specified in the instruction bookis the stage which has too low a gain.

DIRECTION OFSIGNAL INJECTION

(I&fH *f ifadU

TABLEA-F INPUTS

INPUT TO VOLTS

OUTPUT2ndAF

^ 1st AF

1.5

0.8

0.15

* 1,000 CYCLES OF^RDDOEj

ttZT INSTRUCTION BOOK^- DATA

§6] 5.71

Aligning the I.F. Section

If the gain of the i.f. amplifier is low, realignment may be necessary. The

procedure for this is as follows.

First stop the oscillator from oscillating by removing the valve, or by shorting

its grid to earth. This prevents any signal other than that of the signal generator

from entering the i.f. amplifier. You must also short the AGC line to earth, since

the AGC circuit, if operative, would tend to broaden the receiver response and thus

make it more difficult to align the receiver sharply.

The output meter leads are then connected across the speaker, and the signal

generator test leads are applied to the various test points in the i.f. section.

With the manual volume control at maximum, a modulated 465-kc/s signal is

injected into point 1, the grid of the i.f. amplifier. Using a trimming tool, adjust

the trimmers on the i.f. output transformer for a maximum output.

Next, inject the i.f. signal into point 2, the grid of the mixer stage, and adjust

the trimmers of the i.f. input transformer for a maximum output.

The trimmers on both the i.f. transformers are then again retrimmed slightly to

obtain optimum alignment of the i.f. section.

A further test which would also be carried out at this stage is the measurement

of the i.f. amplifier bandwidth.

5.72[§ 6

Aligning the Mixer and Oscillator

With the i.f. amplifier aligned, the r.f. tuned-circuits in the grid of the mixer andthe local oscillator are the next to be aligned.

Replace the oscillator valve (or remove the short from grid to earth), but leave

the AGC circuit shorted to earth. Then apply signal generator output betweenthe aerial terminal and earth (chassis); and set the signal generator to give a modu-lated r.f. output of 1500 kc/s.

The receiver dial is then set to 1500 kc/s, and a signal is observed on the outputmeter. With the trimming tool adjust the oscillator and r.f. trimmers to give maxi-mum output.

Now the r.f. circuit is tuned to resonate at 1500 kc/s, and the oscillator is tuned to

oscillate at 1965 kc/s.

§ 6] 5.73

Aligning the Mixer and Oscillator {continued)

The mixer and oscillator tuned-circuits must now be aligned at the low end

of the band.

Set the signal generator to 600 kc/s and the receiver dial to 600 kc/s. Thenadjust the oscillator padder capacitor to give maximum output. Now adjust the

oscillator to oscillate 465 kc/s above the incoming signal of 600 kc/s.

Although the dial is set at 600 kc/s, there is no assurance that the r.f. tuned-

circuit is tuned to 600 kc/s. The ideal alignment for maximum output is to have

the r.f. tuned-circuit exactly resonant at 600 kc/s, with the oscillator tuned to 465

kc/s above 600 kc/s.

First note the reading of the output meter, and then tune the receiver in one

direction slightly away from a receiver dial reading of 600 kc/s. Now readjust

the padder for maximum output. If the output reading is greater than it wasbefore, you have changed the setting of the tuning dial in the right direction. If the

output reading is less, you must tune the receiver in the opposite direction from the

600-kc/s dial reading.

Having found the right direction, continue to vary the setting of the tuning dial

and to adjust the padder until a maximum output is obtained. At this point the

r.f. circuit is tuned exactly to 600 kc/s, with the local oscillator tuned to 1065 kc/s.

Even so, however, the pointer on the tuning dial may not yet be opposite the

600-kc/s mark on the dial. It is necessary, therefore, to move the pointer relative

to the dial without moving the tuning capacitor spindle. The pointer is loosened

(e.g., unscrewed) and moved until it is opposite the 600 kc/s mark on the dial. It

is then re-tightened.

The final step in the alignment procedure is to re-check the alignment at the highend of the frequency band, and to re-adjust where necessary.

5.74

REVIEW—Superheterodyne Receiver

Superheterodyne. A type of re-

ceiver in which the r.f. signal is con-

verted to a lower frequency r.f., and

then amplified before detection. It

has much higher sensitivity, selectivity

and stability than has the TRF.

[§6

Mixer. This is the circuit in a

superhet which takes the r.f. signal

and beats it against the signal gene-

rated by a local oscillator. The resul-

tant constant-frequency signal is lower

in frequency than the r.f., and is thus

easier to amplify.

Local Oscillator. This circuit is

tuned simultaneously with the r.f.

tuned-circuits in such a way that its

output frequency is always a fixed

amount greater or less than the fre-

quency of the signal being received.

Its output is combined with the r.f.

signal in the mixer.

I.F. Amplifier. This is the section

of the superhet which selects and

amplifies one of the signals coming

from the mixer. Its input and output

are usually coupled by transformers of

which the primary and secondary are

both tuned. This results in high

selectivity.

Detector and A.F. Amplifier. These

circuits perform the same functions as

in the TRF receivers. In the superhet,

the diode detector is often combined

with the first a.f. amplifier stage.

30p" "JSOOp ^22K

Local

Oscillator

620p

IFInput

kz:200 200

S MIXER GRIDS AGC LI*

§«1

REVIEW—Superheterodyne Receiver (continued)

Automatic Gain Control (AGC).

This circuit compensates for variations

in signal strength. A diode rectifies

the signal, and the negative d.c. com-

ponent is applied to the r.f. and i.f.

amplifier grids. When the signal in-

creases, the diode output increases

—

thus putting more negative bias on the

r.f. and i.f. amplifiers and lowering

their gain.

Tracking. When the difference

between the local oscillator frequency

and the r.f. signal frequency is con-

stant over the entire tuning range of

the superhet, it is said to have perfect

tracking. This, however, is never

achieved in practice.

Beat Frequency Oscillator (BFO).

This is an oscillator used when it is

desired to receive CW signals with the

superhet. Its output is tuned close to

the frequency of the i.f., and is fed into

the detector or i.f. amplifier. It beats

with the incoming signal, producing a

beat note in the audio range. With a

BFO, a CW signal is heard as a pure

tone. Without a BFO, CW signals

are heard as a soft hiss, or not at all.

Amplifier

5.75

Diode Detector

AGC /± R-2 /t '

jC-2+_/

To first

AF amplifier

RF / Oscillator

Padder

O Trimmer

V ^#TRACKING : 465 Kc/s difference

over entire tuning range

I DFO

Signal

Second Channel Interference. If

the i.f. is 465 kc/s, then two signals

(one 465 kc/s above, and the other

465 kc/s below, the oscillator fre-

quency) will both send a signal through

the i.f. amplifier and to the loud-

speaker. One of them is the desired

signal; the other is an image. The

purpose of a tuned aerial coil and

tuned r.f. amplifiers is to eliminate this

second channel interference.

550KC/8

5.76 §7: FAULT-FINDING

Introduction

It is now time for you to tackle the problems of finding the various types of fault

which are likely to occur in electronic equipment.

Before you begin to practise fault-finding of any kind on actual equipments, how-

ever, there are two matters of the utmost importance to which you must give your

careful attention.

(i) You must learn to develop a logical approach to the whole problem of fault-

finding in general,

(ii) You must learn how to select and how to use various types of test instrument.

THINK ABOUTFAULTFINDING. . . .

A LOGICALPROCEDURE

LEADS TO AQUICK SOLUTION

Though you will learn how to set about the logical finding of faults on an actual

piece of equipment—the superhet—which you have already studied, remember that

the general priciples of fault-finding which you will learn can also be applied to

electronic equipments of any other kind. Fault-finding is merely problem-solving.

Learning how to use test instruments properly, however, is essentially a matter

of practice, and cannot be satisfactorily learnt from a book. Full instructions on

how to use any particular test instrument, however, are given in the appropriate

manufacturer's handbook.

5.77§7]

Fault-finding Procedure

Step I—Collate the SymptomsThe first step in fault-finding is to ascertain the symptoms of the fault.

Before you can do this, you must obviously be in a position to recognize thenormal state of the equipment. In the case of the superhet on which you will doyour fault-finding practice, for example, the equipment is considered to be in thenormal state when signals can be received over the whole of the tuning range, andwhen the volume of the output can be varied without causing noticeable distortion.Under service conditions, you will either be told the symptoms of a fault by the

operator of the equipment, or you will have to find them for yourself. In the lattercase you find out how the equipment differs from its normal state by operating it,

and by using built-in metering facilities where they exist.

In more complicated and larger equipments, extensive metering and monitoringfacilities are normally provided—for example, a switched meter or meters may bebuilt into a large transmitter which can be used to measure various currents in thecircuit. On radar equipments it is often possible to use one of the cathode raytubes of the set to display waveforms at different points in the circuit.

I"1

!Ih.'llll.

Step 2—Decide which Stage is Faulty

It may be possible at this point, from, your knowledge of the symptoms, todecide in which stage or group of stages the fault is likely to be.

Always, however, think carefully about the symptoms before you actually doanything to remedy them.

5.78 B 7

Fault-finding Procedure (continued)

Step 3—Inspect the Equipment

Many defects can be found at once by using your senses of sight, hearing, touch,

and smell. Once you have heard a transformer sizzle and smelt the smoke, you

will be able to spot a burned-out power transformer without even turning the chassis

over

!

Loose ,.. _ _„ _.vajve JUtSsT/5^?^ Discoloured

resistor

SSSi«« Leaking transformerconnection

Visual inspection does not take long. In about two minutes you should be able

to see the trouble, if it is the kind that can be seen at all.

Start your inspection with the equipment switched off, and look for:

(i) Loose Valves—A valve which is not properly seated in its socket may not be

making proper contact with the rest of the circuit. Push all valves firmly

into place,

(ii) Shorts—Any terminal or connection which lies close to the chassis or to another

terminal should be examined for the possibility of a short. Look for and

remove any stray blobs of solder, bits of wire, nuts or screws.

It sometimes helps to give the chassis a not-too-vigorous shake (if that be

possible), listening for any tell-tale rattle.

Remember to correct any condition which may cause a short circuit. If it isn't

causing trouble now, it will begin to do so in the future.

(iii) Loose, broken or corroded connections—These could quite easily be the source

of the trouble.

(iv) Damaged Components—Look for discoloration, melting insulations, leaks from

oil-filled transformers. Remember, however, that these components may have

been damaged by the fault, and may not necessarily be the cause of it.

The component may be protected by a fuse. Check whether the fuse has

blown—and if it hasn't, find out why it hasn't. A fuse with too high a rating

may have been fitted, or there may be a short across the fuse holder.

After inspecting the switched-off equipment and remedying all obvious defects,

switch on and continue your inspection.

After switching on the equipment, look for:

(i) Overheating parts—If any part smokes, or if you hear anything which sounds

like boiling or spluttering, switch off immediately. There is a short circuit

somewhere which you have missed in your first inspection.

§7] 5.79

Fault-finding Procedure (continued): Step 3—Inspect the Equipment (continued).

Use your ohmmeter, if necessary, to locate it, beginning in the neighbour-hood of the smoking part,

(ii) Cold Valves—In some valves it is possible to see the glow of the heater fila-

ment; with others it is necessary to wait until the filament has had time to

warm up, and then to touch the valve to see if it is warm.If there is no heater glow and the valve is cold, either the valve is unservice-

able or there is a break in the heater connections.

Remove suspect valves, and test for continuity across the heater pins. If theohmmeter reading is very high, or infinity—the valve is unserviceable.

If the ohmmeter indicates continuity through the heater, the fault lies in theheater supply circuit. Check that the valve is making proper connection withits socket, and then use an a.c. voltmeter to find the break in the path fromthe heater voltage source to the valve base,

(iii) Sparking—Tap or shake the chassis. If you see or hear sparking you havelocated a loose connection or a short.

Smoking parts

Sparking

Cold valves

Remember that even though you do find and repair a defect, you must still proveto yourself that the equipment is operating properly and that there are no otherdefects. Usually, there will be only one fault in a piece of equipment, unless thefaulty component has become unserviceable because of some other fault.

When you find a fault by inspection, try to imagine another fault which couldhave caused the one you have located. If you merely replace the faulty componentand then turn the equipment on, the replacement part itself will very likely bedamaged. The most obvious example of this is a fuse which burns out, is re-placed, and then the replacement burns out.

You must locate the cause of the trouble before you replace faulty parts.

5.80 B 7

Fault-finding Procedure (continued)

Step 4—Signal Injection and Tracing

Devices such as radar and communications equipment are very complex. If,

therefore, you attempted to do fault-finding on a radar receiver, for instance, by

means of voltage and resistance checks alone, you would have a long and tiresome

task ahead of you. There would be literally hundreds of voltage, current and resis-

tance checks for you to perform—not to mention valves and tuned-circuits to be

tested.

And then there would always be a good chance that none of your checks would

show you what was wrong; for static testing will rarely show up such faults as

misaligned tuned-circuits, certain valve defects, or defective automatic control

circuits.

The procedures of signal injection and signal tracing, however, enable you to find

the fault quickly and easily by greatly reducing the number of points to be tested.

By these procedures, you can locate the stage which contains the fault; and

sometimes, depending on the nature of the fault, the faulty part itself. In this

way you can quickly narrow down the possible causes of trouble, with a minimum

number of checks of those stages which are functioning properly.

If in step 2, for instance, you decided which stage was likely to contain the fault,

it is only necessary to inject an appropriate signal into that stage, and check the

output, to confirm or disprove your deduction. If, however, you were unable to

deduce from the symptoms which stage was faulty, you must check the complete

equipment.

The general procedure is as follows:

1 Test each section of the equipment by putting in a signal, and by checking either

the signal at the equipment output or the signal at the section output.

2. Once you know the section which contains the fault, you can isolate the trouble

to a particular stage within the section by injecting signals of the proper frequency

and amplitude into the grids of the various valves, starting at the output and

working back towards the input. The stage at which the signal disappears, or

becomes distorted, is the place to look for trouble.

Step 5—Voltage and Resistance Tests

Once the defective stage has been found, the defective component can be isolated

by using voltage and resistance checks.

§7] 5.81

Fault-finding by Signal Tracing

Signal tracing and signal injection are basically the same thing. Each has someadvantages over the other for the testing of different types of circuit.

The basic purpose of both these methods is to locate the exact area of trouble.

Any break or short in the signal path can be located, because the signal will dis-

appear at that point. If the trouble is due to an incorrect voltage on a valve, or to

a faulty valve, the signal will not pass (or will be distorted) between the grid andanode circuit of the valve. If the trouble is of this nature, it can be localized

immediately to the specific valve; and then the exact fault can be located by voltage

and resistance checks, or by trying a valve known to be a good one.

In the procedure for signal tracing, the normal signal input for a piece of equip-

ment is connected to the input terminals. An oscilloscope or meter is then used to

trace the signal from the input towards the output. The point at which the signal

disappears or becomes distorted is the point to look for the fault.

Signal tracing can be used with practically every type of circuit that you will

come across; but in receivers, the tracing of signals is difficult because of the lowvoltage r.f. signals present in the early stages of the circuit.

Sequence U4ed Ut Su^ud *7*actHy

Fj

H Signal S3GeneratorH H

| Input |

O

/

±11

N| 1 Meter H3 B tn-

^H 1 Oscilloscope M

5.82 [§ 7

Fault-finding by Signal Injection

In the procedure for signal injection, an oscilloscope or output meter is perma-

nently connected to the output of a piece of equipment.

A signal generator is used to inject a signal of the proper amplitude and fre-

quency into the various test points, starting at the output and working towards

the input.

Signal injection is used mainly with receivers, and with other similar equipment

containing high frequency amplifiers whose output cannot easily be checked.

Signal injection solves this problem by using a signal generator to inject signals

into various parts of the equipment. The amplifiers in the equipment under test

will give a large enough gain so that the signal can be observed at the output.

The first stage to check is the last stage of the piece of equipment. If this last

stage is operating normally, the next-to-last stage is checked by feeding a signal

into that stage, and then by checking the output at the same point as before.

It is because you are always observing the output of the equipment as a whole in

signal injection that the last stage in the equipment is the first one to be checked.

Just as in signal tracing, the point where the signal becomes distorted or dis-

appears is the point to look for the fault. The last stage, for example, may be

checked and found correct. So the signal is put on the next-to-last stage; and if

the output of the equipment as a whole then ceases to be normal, you can be sure

that the trouble is in the next-to-last stage.

§ 7] 5.83

Testing Within Stages

Having localized the fault to a single stage, the last step in fault-finding pro-cedure is to make detailed checks within the stage to find the faulty component..The types of test made within stages are:

(i) Voltage and Current Measurements. Equipment handbooks and data sheets

will give correct operating voltages and currents for the stage,

(ii) Resistance and Continuity Tests. Resistances from various points in the stageto earth may be quoted in the equipment handbook. If such information is notgiven, it may be deduced from the circuit diagram.

Where a resistor lies in one of a number of parallel paths, it is better to

disconnect one end before measuring its resistance.

Where the wiring of the stage is suspect, continuity tests with an ohmmetercan be used to find the fault,

(iii) Substitution. It is sometimes more convenient to check whether a componentis fulfilling its function by substituting a known good component.The best example of this is the valve of a suspect stage. Where voltage or

current checks indicate the valve as a possible source of trouble, the substitu-tion of a known good valve will quickly confirm or refute your suspicion.

5.84 [§7

Fault-finding in the Superhet Receiver

Y

V

Detector AFAmplifier

LocalOscillator

a nHeater Voltage HT+Voltage

PowerSupply

1. The Power Supply

A power supply unit consists of at least three stages; mains transformer, rectifier,

and filter. Your procedure for fault-finding should therefore be either:

(i) Check the input and output of each stage, beginning with the a.c. mains input

to the primary; or preferably

(ii) Check from the d.c. output back towards the a.c. mains input.

2. The Audio Amplifier

When fault-finding in the audio amplifier stages of a receiver, it is better to use

the signal injection method, because you will have to use that method for the rest

of the receiver. The 'scope or output meter should be connected across the loud-

speaker at the output transformer secondary. An audio signal is injected into the

various test points from the loudspeaker towards the detector. The point at which

the signal disappears or becomes distorted is the place to look for the fault.

3. The Detector

The detector takes a modulated r.f. (or i.f.) signal and separates the audio from

the r.f. component. The high-frequency component is bypassed to earth and the

audio signal is connected to the audio amplifier.

§7] 5.85

Fault-finding in the Superhet Receiver (continued)

When checking a detector, therefore, the procedure is to inject a modulated r.f.

(or i.f.) signal into the detector input. If an audio signal corresponding to the

modulation does not appear at the output, there is a fault in the detector.

4. The I.F. Amplifier

The i.f. amplifier is an r.f. amplifier operating at a fixed frequency of 465 kc/s.

The operation of the i.f. amplifier is similar to that of the r.f. amplifier described

in the amplifier section—the only difference being that the i.f. amplifier operates at

a fixed frequency, and may for this reason be designed for a much higher gain.

By injecting a modulated 465-kc/s signal, you can first test the i.f. output trans-

former, then the valve, and finally the input transformer. In all cases an audio

signal should appear at the output.

5. The Mixer and the Oscillator

The mixer stage selects the desired modulated r.f. signal from the aerial, andmixes it with the unmodulated signal from the local oscillator. The local oscillator

and the mixer tuning circuit have mechanically-ganged tuning capacitors whichkeep the frequencies of the selected signal and of the oscillator 465 kc/s apart. Asa result of the mixer valve action, a modulated 465-kc/s signal is fed into the i.f.

amplifier no matter what the frequency of the selected r.f. signal.

The mixer is tested by first injecting a modulated 465-kc/s signal into the grid.

If this signal passes through the mixer and appears as an audio signal at the final

output, the mixer valve is operating correctly.

Then a modulated r.f. signal is injected at the same point, and the receiver is tuned

to this signal. An audio signal should appear at the output. If no signal appears,

there is a fault in the oscillator circuit.

The methods of testing an oscillator stage to verify that it is oscillating are

described on page 5.90.

6. The Aerial Input Circuit

If the mixer and oscillator stages are proved correct, the final step is to test the

aerial circuit by injecting a modulated r.f. signal at the aerial input terminal andtuning the receiver. If no audio output is obtained, the fault lies between the input

terminal and the grid of the mixer valve.

5.86 [§ 7

Test Instruments

In the fault-finding procedures described in the preceding pages, a number of

requirements for test instruments have been mentioned. You need not at this stage

know the circuits and principles of the test instruments you will use; but you should

be familiar with the facilities they offer.

Before you attempt to use test instruments in fault-finding, you must be familiar

with the operating instructions issued by the maker, and also know how to inter-

pret the results indicated by the instrument.

The facilities offered by the more common test instruments are summarized below.

R.F. Signal Sources. These are called Signal Generators, and give an r.f. output

whose frequency is variable over a wide range. The output is accurately calibrated

in terms of voltage, but the instrument should not be regarded as an accurate

frequency standard.

The instrument also includes facilities for modulating its own output—either by

amplitude modulation (AM) or by frequency modulation (FM), for which see Part 6.

Frequency Standards. The instrument used to check the frequency of a trans-

mitter or to inject a signal at a precise r.f. frequency into a receiver is called a

Wavemeter or a Frequency Meter. Such an instrument normally has a built-in

calibration checking system.

The wavemeter does not usually provide modulated r.f.

A.F. Sources. The instrument which provides an a.f. signal is called an Audio

Signal Generator. One source of a.f. commonly used is the Beat Frequency Oscil-

lator (BFO).

The output of such instruments can be varied in amplitude and frequency over

the whole of the a.f. range.

Instruments for Tracing Signals. The three instruments most commonly used for

tracing signals are the CRO, the Output Meter, and the Valve Voltmeter. Thefacilities offered by a CRO have already been described in Part 4 of Basic Electricity.

Output Meters. In an output meter, a.f. power is dissipated in a fixed impedance

within the instrument. The a.f. voltage across this fixed impedance is measured

by a meter across a rectifier bridge. The meter is calibrated to read power directly.

The fixed impedance is matched to the output impedance of the circuit in which

the power is being measured by a tapped transformer incorporated in the instrument.

The AVO Model 7, when switched to the power range, can be used as an output

meter of this type.

Valve Voltmeters. Voltmeters of the kind described in Part 1 of Basic Electricity

cannot be used to measure accurately the voltage across very high impedances or

resistances, because of the shunting effect of the meter. In such cases (for example,

when measuring the voltage on the grid of a valve) a valve voltmeter, which itself

has a very high input impedance, is used instead.

#o<> #oooooooo

§ 7] '5.87

Valve Testing

Although it is sometimes convenient to find out whether a valve is functioning

properly in a circuit by substituting for it a known good valve, there will be timeswhen it will be necessary to test valves. For this purpose you will use instrumentsknown as "valve testers."

Since burned-out filaments cause the majority of valve failures, it is usuallypossible to discover such defective valves by removing them from the equipmentand testing them with an ohmmeter.

In general, however, the most satisfactory method of determining whether someof the valve electrodes are shorted, and whether the emission or mutual conductanceare normal for its type, is to use a tester.

Note, however, that the valve tester cannot always be looked on as a final

authority for determining whether or not a particular valve will operate satis-

factorily in a given equipment. This is because the valve might be operating in

the equipment on a portion of its characteristic curve which is not covered in thetester; or it might be operating in the equipment with voltages much higher or muchlower than those used in the tester.

The check for filament continuity and for shorted valve electrodes is generallyperformed as the first part of the testing procedure. If the filament is found to beopen-circuited, it is useless to attempt further testing of that valve. If shortedelectrodes are discovered, it is not advisable to test further, as the shorted electrodesmay blow fuses or damage the tester.

Filament continuity and shorted electrodes are indicated by the lighting of asmall neon or pilot lamp on the instrument panel, or by an indication on a meter.The next step is to test the mutual conductance of the valve. When doing this,

the tester simulates the normal operation of the valve by applying a known signalto the grid, and measuring the strength of the amplified signal in the anode circuitby means of an output meter. Since this procedure is performed under conditionswhich resemble the actual operating of the valve in an equipment, the results ob-tained give a good indication of the valve's serviceability.

Outputmeter

Valve conductance indicated bystrength of amplified signal

Manufacturers supply, in addition to instructions on the use of a valve testertesting data for a very wide range of valves.

5.88 [§7

Some Examples of Fault-finding in the Superhet

Let us now go through the correct procedure to locate four typical faults in the

superhet receiver.

The first fault is an open-circuited coupling capacitor between the anode of the

first a.f. amplifier and the grid of the output valve.

When an audio signal is applied to the grid of V-4, an output will be observed

on an oscilloscope or output meter. When an audio signal is applied to the grid

of V-3, no output signal is observed.

Using a 001 -pF blocking capacitor in the lead from the audio signal generator

(in order to prevent H.T. being fed back and damaging the a.f. generator), apply the

signal to the anode of V-3. No signal is observed at the output. The fault area is

therefore between the anode of V-3 and the grid of V-4.

The coupling capacitor is immediately suspected, as it is the only a.c. connecting

link between anode and grid. Substitution of a known good capacitor will show

that the fault has been cleared.

AUDIO SIGNAL SCOPE PICTURE

POINT©POINT (2

C/RCUITJ CONCLUSION

The second fault is a short to earth (chassis) from the slider of the volume control.

Using signal injection, an output is observed when an audio signal is placed on

the grid of V-4 and the anode of V-3. The same signal applied to the grid of V-3

also produces an output.

SCOPE PICTURE

§7] 5.89

Some Examples of Fault-finding in the Superhet (continued)

It is as well at this point to check the action of the volume control. Injecting

the a.f. signal at the top-end of the volume control results in no output, and rotating

the control has no effect. With the receiver switched off, a resistance check from

the potentiometer slider to earth reads zero with the control in any position. This

proves that the slider is shorting to earth.

A visual inspection of the component may reveal the fault, or failing that a

replacement volume control will restore the receiver to a working condition.

The third fault is a short-circuited i.f. trimmer capacitor.

By injecting modulated signals, it is found that when a 465-kc/s signal is applied

to the detector anodes of V-3 an output is observed. Injecting a modulated signal

of 465 kc/s on to the grid of V-2, however, gives no output. Applying a signal to

the anode of V-2 (through a d.c. blocking capacitor) still gives no output. The

fault has then been isolated to the i.f. amplifier stage V-2.

The valve is replaced, but the fault still remains.

Next, d.c. voltage readings are taken, which appear quite normal. Suspicion is

now localized to the i.f. transformer.

The receiver is switched off, and a resistance check is made from anode to

H.T.(+). The result is a reading of zero, instead of the correct reading of 6 ohms.

This confirms that the i.f. transformer is faulty.

A replacement transformer, after being correctly aligned to 465 kc/s, would cure

the fault; but if a new ii. transformer is not available, it is necessary to remove the

screening can from the faulty one and (by disconnecting the trimming capacitor from

the coil) to check by resistance measurement whether the capacitor or the coil is

short circuited.

In this case it is a faulty capacitor—and replacement of this component is much

cheaper than is the cost of a complete i.f. transformer.

RB RF SIGNAL SCOPE PICTURE

0®Us!

IRfl 465k<ys

kSS powt (2)

B SI RESISTANCE READING

^^.^ ^PT@TO PT(3) On

m ^ coN(FUSION

5.90 [§7

Some Examples of Fault-finding in the Superhet {continued)

The fourth fault is an unserviceable local oscillator valve.

Signal injection shows that a modulated 465-kc/s signal applied to the control

grid of the mixer valve V-l gives an output. A modulated 1000-kc/s signal applied

to the same point, however, gives no output when the tuning capacitor is varied

over its range.

If the oscillator V-5 were working properly there would be a point, as its tuning

capacitor was varied over the tuning range, at which the difference between the

oscillator frequency and the applied modulated signal of 1000 kc/s would be465 kc/s; and this would pass through the superhet and be observed at the output.

Since no signal appears, however, the oscillator stage is suspect.

Three methods which can be employed to verify if an oscillator stage is

oscillating are:

1. Disconnect the earthy end of the grid resistor and check if there is any d.c.

grid current, using a suitable milli- or micro-ammeter.

2. Check if there is any d.c. voltage on the grid of the valve.

3. Check if there are a.c. oscillations present at the grid or anode of the valve.

Checks (2) and (3) above have to be carried out with a valve voltmeter. Theimpedances from grid to earth, and from anode to earth, in an oscillator circuit are

high. The input impedance of a meter such as the AVO 7 is comparatively low;

and if such a meter were connected to an oscillator circuit, it would shunt the anodeor grid circuit—thus seriously impairing the performance of the oscillator, or even

stopping it oscillating altogether.

The valve voltmeter, however, has a high input impedance; so it can be connected

to an oscillator circuit without these adverse effects.

Using any of the three methods given, the result in the case we are considering

would be:

The most likely

1. No grid current.

2. No d.c. voltage on the grid.

3. No oscillations at the anode; so the stage is not oscillating,

cause is a faulty valve.

A change of valve will therefore make the receiver operative in this particular case.

RF SIGNAL 1 SCOPE PICTURE

POINT(J)

465 kc/S

POINT (T)lOOOkc/s

DC VOLTAGECHECK

POINT (2

READNG

OV

CONCLUSION

§7]

REVIEW—Fault-finding

5.91

The correct sequence of steps in fault-finding is as follows:

(i) Collate the symptoms of the fault.

(ii) Consider the symptoms and deduce a possible cause,

(iii) Inspect the equipment for obvious defects.

(iv) Test the suspect stage (or every stage in the equipment if the symptoms

do not lead you to suspect one particular stage).

(v) Having located the faulty stage, make voltage, current and/or resistance

checks to detect the faulty component.

(vi) Replace the faulty component and check that the equipment is operating

normally.

Steps in Fault Finding

1. Isolate defective staqe by siqnal injection.2. Check d.c. voltaqes from valve pins to earth.3. Check resistance from valve pins to earth.

As you gain experience in fault-finding, you will be able to make more use of the

evidence and will have to test fewer stages before you find the faulty one.

Fault-finding is not a matter of gambling on a chance—it is a matter for logical thought.

5.92 §8: GENERAL REVIEW OF RECEIVERS

Aerial. The purpose of a receiving aerial

is to pick up electro-magnetic waves radiated

by transmitting aerials. These waves, in

cutting the aerial, induce voltages in it, causing

currents to flow. The currents flow into the

input of the receiver, where they generate

signals which are amplified by the receiver

circuits.

Electromagnetic Waves

lllllli»M|||ll

V)) jvy

TRANSMITTER

Directional Characteristics. The position

of a receiving aerial relative to the transmitting

aerial will determine the strength of signal

picked up. If the frame aerial of a receiver

is parallel to the frame aerial of a transmitter,

the signal picked up will be of maximumamplitude. If the receiving aerial is turned so

that its edge faces the broad side of the trans-

mitting aerial, a very weak signal will be picked

up. Therefore, the aerial is said to have

directional characteristics.

R.F. Amplifier Stage. An r.f. amplifier

stage in a receiver improves the sensitivity and

selectivity of the receiver. The added sensi-

tivity results from the amplification of the

desired signal, and the added selectivity

results from the use of tuned-circuits which

discriminate between the desired and undesired

signals.

Audio Amplifier Stage. An audio amplifier

stage in a receiver amplifies the detected audio

signal. Audio stages, which precede the last

stage, are voltage amplifiers whose sole

function is to increase the amplitude of the audio

to the level where it is large enough to drive the

last stage. The last stage, called the "power out-

put stage," supplies the large current variations

necessary to drive the loudspeaker.

§8]

GENERAL REVIEW-

5.93

-Receivers (continued)

Detectors. The function of a detector in a

receiver is to remove the audio component from

a modulated r.f. signal so that it can be ampli-

fied by a.f. stages. A simple detector consists

of a tuned-circuit, a rectifier, and a filter.

Grid-leak Detector. This type is basically

a diode detector with amplification added.

The grid and cathode form the diode detector,

with the grid acting as the anode. The rectified

signal developed across the grid-leak resistor is

amplified in the anode circuit. This detector

s more sensitive than the diode type.

Anode-bend Detector. This detector em-

ploys a triode or pentode, biased near cut-off.

Rectification takes place in the anode circuit,

since the negative half of the modulated r.f.

grid signal drives the valve into cut-off.

TRF Receiver. This receiver employs r.f.

amplifiers, a detector, and a.f. amplifiers. The

tuned-circuits are ganged-capacitor tuned. Ashortcoming of the TRF is that since the tuned-

circuits are not fixed-tuned, constant sensivity

and selectivity cannot be realized over a tunable

band.

' s y \ / \RF

stagesDetector AF

stages

rrRF RECEfVEFAll tuned circuits ganged

5.94

GENERAL REVIEW—Receivers (continued)

Superheterodyne Receiver. The disadvan-

tage of the TRF is overcome in the superhet

receiver, in which all desired r.f. signals are

converted to the same fixed lower frequency

signal (called the "intermediate frequency"))

where the signal is amplified by fixed tuned-cir-

cuits before it is detected. To accomplish this,

the superhet incorporates a mixer, a local

oscillator, and an i.f. amplifier in addition to

the usual TRF stages.

[§8

1*1....

\Wnr

AmptafUr imlH

Obtaining the I.F. Signal. The fixed i.f.

signal is obtained by beating the incoming

signal with the signal from a local oscillator

which is always a fixed amount away from the

incoming signal. This is accomplished by

ganging the capacitors of the oscillator and the

r.f. amplifier so that the difference between

the r.f. resonant frequency and the oscillator

resonant frequency is constant for all settings

of the tuning dial. The oscillator resonant fre-

quency is said to "track" the r.f. resonant

frequency.

To Mixer Grid

620p

Automatic Gain Control. The superhet

receiver incorporates an AGC circuit whose

function is to equalize the receiver output for

both strong and weak incoming signals. It does

this by using a filter circuit which charges up to

the d.c. level of the rectified r.f. wave. This

d.c. voltage (negative with respect to earth)

is then applied as bias to the grids of the i.f.

and r.f. stages, all of which may employ

variable-mu valves. In this way the bias volt-

age, and therefore the gain, of these stages is

directly related to the intensity of the received

signal.

IFAmplifier

iHT*

Diode Detector

;—Lj

c-'t fTo first

AF amplifier

Aligning. A superhet is said to be "aligned" when it is giving optimum performance

over its frequency range.

When aligning a superhet the i.f. stages are adjusted first. Then the trimmers of the

r.f. tuned-circuits and local oscillator are adjusted at the high end of the band. The

adjustment of the low frequency end of the band is made with a padder capacitor.

§9: MISCELLANEOUS ELECTRONIC CIRCUITS 5.95

Most of the very large number of electronic circuits which have been devised to

perform an almost bewildering variety of duties in modern industry and modern

defence equipment employ principles already familiar to you from your study of

the three-stage transmitter and the superhet receiver.

They can nearly all be understood if you remember the principles of the three

basic valve circuits—the rectifier, the amplifier, and the oscillator.

THESE ARE BASIC TO ALL ELECTRONIC EQUIPMENT

glliliS^*^

Here are three particular circuits you are likely to meet in working with electronic

equipment:

(i) The cathode follower,

(ii) A typical time-base generator,

(iii) A typical voltage stabilizer.

5.96

The Cathode Follower

The circuit of the cathode follower is illustrated below:

-HT+

B»

*-HT-

The input signal is applied between grid and earth, and the output taken fromacross the resistor in the cathode of the valve. The undecoupled cathode intro-duces negative feedback; so the output signal (which is the fluctuating componentof the cathode current) is in opposition to the incoming signal.

Fout is a faithful reproduction of KiN , but is of lower amplitude.In other words, the cathode follows the grid—hence the name of the circuit.

A cathode follower has the following properties:

1. The voltage gain is always less than one (normally of the order of 0-9). That is

to say, the cathode follower does not amplify the input signal.

2. The circuit has a low output resistance and a high input resistance.

You will find cathode followers in use where it is necessary to match a highimpedance into a low impedance without the need of amplification. Such a systemwould be required if the output of an amplifier of, say, 1-megohm output impedancewas required to feed into a cable of 300-ohm impedance. The .cathode followerwould be inserted between the amplifier and the cable.

You saw, when you were learning about video amplifiers, how important it wasto maintain a good square or rectangular waveform. One use of the cathodefollower is to maintain a waveform shape while at the same time matching twoimpedances.

§9]5.97

A Typical Time-base Generator

A time-base generator generates the voltage which is applied to the X plates of a

CRO, with the object of causing the spot to move from left to right across the tube,

and then to fly back again. The voltage waveform required for this purpose is

illustrated in the first diagram below.

Voltage applied j)k

to X plates

Time«« *^v^-| /^Fly back time

Time occupied by sweep \J_/

Now consider the simple circuit illustrated below:

-&**£ »-HT+

Voltage

applied to

X plates

*-HT-

The valve is a gas-filled triode, similar to the gas-filled diode described in Part 1

of Basic Electronics except that its striking voltage can be controlled by varying

the negative potential on its grid.

When H.T. is applied to the circuit, the capacitor C begins to charge through

the resistor R. It continues to charge until the voltage across C—and therefore

across the gas-filled triode—reaches the valve's striking voltage, which is in turn

controlled by the negative potential on the grid.

Once the valve has struck, its resistance becomes negligible; and C discharges

through resistor r and the valve.

This process is repeated, the gas-filled valve remaining cut-off until its striking

voltage is reached again.

The time-constant for charging the capacitor (RC) and the time-constant for dis-

charging the capacitor (rC) are very different; and the output waveform across C is

as illustrated below.

*-T

You can see that the waveform of this voltage is not identical with the ideal

time-base voltage waveform illustrated above. A circuit whose output approaches

more nearly to the ideal is described on the next page.

5.98

A Typical Time-base Generator {continued)

the cir<

[§9

The output voltage of the circuit illustrated below is the voltage across thecapacitor C.

-*~HT+

Synch

*~HT-

When H.T. is applied to the circuit, C begins to charge, its charging currentflowing through the pentode V-l.

You have already learnt that the current through a pentode is constant over awide range of anode voltages. The magnitude of the current through the pentodeis determined by the voltages applied to the control grid and screen of the valve.Therefore the pentode acts as a constant current device, and C charges at a linearrate.

When the voltage across C reaches the striking voltage of the gas-filled triode V-2,this valve strikes; and C discharges rapidly.

The process is repeated, producing the "saw-tooth" output illustrated below.

The control P varies the grid voltage of V-l, and therefore controls the currentthrough V-l and through capacitor C. It therefore controls the slope of the saw-tooth waveform.

The control Q varies the striking voltage V-2, and therefore the amplitude ofthe saw-tooth waveform.

§9] 5.99

A Typical Voltage Stabilizer Circuit

In Part 1 of Basic Electronics you learnt how gas-filled diodes could be used to

stabilize the output of a power-supply unit.

The output voltage of a power supply unit using gas-filled diodes as stabilizers

is, however, only stable within narrow limits—principally because of the current

limitations of gas-filled valves.

A stabilizer circuit which will operate between wider limits is illustrated below.

+ 6UnstabHiscd

6 +Stabilised

dc output

voltage

If the output voltage in the above circuit increases, the voltage at the grid of V-2

will increase also, since it is taken from a resistance-chain across the output. This

will cause an increase of current through V-2, and the voltage at its anode will

decrease. This anode is connected directly to the grid of V-l, which acts as a

variable resistance.

As the voltage on the grid of V-l is decreased—that is to say, made more negative

—the current through V-l is reduced, and the output voltage is restored to its

previous value.

Similarly, if the output voltage falls, the "resistance" of V-l is decreased, current

through the valve increases, and the output voltage attains its correct value.

The variable control VR-1 determines the voltage applied to the grid of V-2, and

hence determines the value of the stabilized d.c. output voltage.

The gas-filled diode V-3 keeps the cathode V-2 at a constant potential, so that

only changes at the grid of V-2 affect the current through V-2—and therefore the

"resistance" of V-l.

You will learn more about these and other special circuits when you get on to

your study of Basic Radar.

s.100 §10: FREQUENCY MODULATION:TRANSISTORS

You have now completed your study of the basic circuits used in electronics:

namely, rectification and amplification. You have also learnt how the valve ampli-

fier can be used as an oscillator, and you have seen how all these circuits can be

combined to form an AM wireless communications system.

Before you leave what may be called the "Basic Fundamentals Area" of this

fascinating subject, however, and pass on to the study of such practical applications

as Telecommunications Equipment, Radar, Echo-sounding, Fire Control Equip-

ments, Missile Guidance Systems, or Servo-mechanisms, you should first learn

something of two other subjects: Frequency Modulation and Transistors.

Frequency Modulation (or FM, as it is usually abbreviated) can be simply

described as another method of modulating wireless waves in order to transmit

intelligence. In this method of modulation, the frequency of the carrier wave is

varied at a rate depending on the frequency of the modulating wave, and to an

extent depending on the amplitude of the modulating wave. This method of modu-lation offers some advantages over AM, particularly in the matter of freedom fromstatic interference; and it is being increasingly used in commercial and military

communication networks.

Transistors were invented as recently as 1948—by three American scientists,

Shockley, Brattain and Bardeen. They are being used to replace valves in manyelectronic circuits; since they

offer economies in space,

weight and power output

which make them very suitable

for use in equipments where

these factors are of import-

ance.

It is these two new topics

—

Frequency Modulation and

Transistors—which form the

subject-matter of Part 6 of

Basic Electronics.

Part 6

FREQUENCY MODULATION

AND

TRANSISTORS

INDEX TO PART 5

(Note: A cumulative index covering all six Parts in this series will be found at the end of

Part 6)

Aerials, receiver, 5.13, 5.20

selecting and installing, 5.16

types of, 5.14

A.F. amplifier, 5.41

in the superhet receiver, 5.57, 5.67

in the TRF receiver, 5.38, 5.42

tone control, 5.39

volume control, 5.40

Alignment, 5.68, 5.71, 5.72

Anode-bend detector, 5.35

Automatic gain control (AGC), 5.58

Band switching in receivers, 5.24

Capacitors

gauged, 5.25

padder, 5.65, 5.73

trimmer, 5.26, 5.72

Cathode follower, 5.96

Crystal detector, 5.30

Crystal receiver, 5.10

Detector, 5.37

anode-bend, 5.35

crystal, 5.30

diode, 5.32

grid-leak, 5.33

in the superhet receiver, 5.57, 5.67

in the superhet receiver for CWworking, 5.64

in the TRF receiver, 5.29

Diode detector, 5.32

Fault finding, 5.76, 5.91

in the superhet receiver, 5.84

procedure, 5.77

signal injection, 5.82

signal tracing, 5.81

testing within stages, 5.83

Fidelity, in a receiver, 5.9

Frequency changing valves, 5.54

Grid leak detector, 5.33

Heterodyne principle, 5.63

].F. amplifier, 5.56, 5.66

I.F. transformer, 5.55

Local oscillator, 5.49, 5.65

Mixing, 5.52

mixer stage in the superhet receiver, 5.53,

5.66

mixer valves, 5.54

Oscillator, beat frequency, 5. 64

Oscillator stage in the superhet receiver,

5.65

Receiver, 5.92

crystal, 5.10

fidelity, 5.9

introduction to, 5.1

selectivity, 5.8, 5.46

sensitivity, 5.7

superhet, 5.12, 5.43, 5.61, 5.74

the jobs performed by, 5.5

TRF, 5.11, 5.21

R.F. amplifier, 5.41

in the superhet receiver, 5.47

in the TRF receiver, 5.22, 5.28

R.F. transformer, 5.23

Selectivity in a receiver, 5.8, 5.46

Sensitivity in a receiver, 5.7

measurements, 5.69

Superhet receiver, 5.12

complete circuit of, 5.61

fault-finding in, 5.84

Test instruments, 5.86

Time base generator, 5.97

Tone control, 5.39

TRF receiver, 5.11, 5.21

Valve testing, 5.87

Voltage stabilizer circuit, 5.99

Volume control

automatic (AGC), 5.58

manually operated, 5.27, 5.40

WIGANCENTRAL]LIBRARY.

621