Chelmsford Amateur Radio Society Intermediate Course (4) Receivers

1Chelmsford Amateur Radio SocietyIntermediate Licence Course

Anthony Martin M1FDE Slide Set 7(4) Receivers

Chelmsford Amateur Radio Chelmsford Amateur Radio Society Society

Intermediate CourseIntermediate Course (4) Receivers (4) Receivers



Receivers

Summary

– Block diagrams of receivers

– The Superhet receiver

– Detection of AM, FM, and SSB

– How a Diode AM Detector works

– Use of BFO and CIO

– AGC



Crystal Receiver

• Block diagram– Single tuned circuit; poor selectivity– No gain; poor sensitivity– Requires large aerial and earth; only receives strong signals

Tunedcircuit

Diodedetector

Earphone



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ANTENNA

D1AA119

R1100k

C2500pF

C3100pF

L1COIL

X1

EARPHONE

C1100pF

EARTH

CRYSTAL

Crystal Receiver Circuit

• L1, C1 - Tuned circuit – selects signal• D1 – Detector diode – demodulates• C3, R1 – Low-pass filter for audio



TRF Receiver

• Block diagram– RF amplifier gain increases sensitivity– One or more tuned circuits– All the gain is at one frequency – feedback is a problem– AF amplifier provides more power for loudspeakers

RFamplifier

DemodulatorAF

amplifier

BFO



Superhet Receiver

• Block diagram

– Mixer changes variable RF frequency to fixed IF frequency

– IF amplifier provides selectivity with several tuned circuits

– Fixed IF can use non-tunable crystal or ceramic filters

Localoscillator

MixerAF

amplifierIF

amplifierDemodulator

BFO



Mixer as a Converter

• Mixer may be used a frequency converter

• Changes the selected RF frequency to the IF frequency using a tunable LO signal.

• Mixers have spurious responses – image frequency, half the RF…

• LO can be above or below the RF

• IF can be above or below the RF

Mixer 145MHz–123.6MHz=21.4MHzIF frequency

RF 145MHz

~

LO 123.6MHz

~ Image frequency is123.6MHz-21.4MHz=102.2MHz



Diode Detector Circuit

• Diode D1 rectifies AC into DC

• C1/R1 is a lowpass filter - filters out the RF

• D1 conducts on positive half-cycle only

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D1

R1100k

C1100pF

RF Input DemodulatedAF Output

Output

Vol

tage

Time

Input



Diode Detector Operation

• Accurate SPICE simulation of diode detector

Dio

de V

olta

ge /

V

-2

-1

0

1

2

200

00 Time/µSecs0.5 1 1.5 2 2.5 3 3.5 4

Dio

de C

urr

ent

/ µ

A

400

600

800

D1 anode input

D1 cathode output

D1 current

Diode conducts on peaks only



Envelope Detection

• Demodulation of a modulated audio signal

• Detector output follows envelope of RF

– Otherwise known as an “envelope detector”

Demodulated Audio

Modulated RF

Envelope



CW Demodulation - BFO

• BFO = Beat Frequency Oscillator

– Slightly offset BFO added to carrier to generate the “beat note”

+Carrier Wave

BFO

EnvelopeDetector

Beat Note…the origins of the “BFO” !



SSB Demodulation

• SSB filter selects only the wanted sideband

• Product detector mixes to baseband

• CIO is at the frequency where carrier would have been

• Product detector is a balanced mixer

AFAmp

SidebandFilter

IFAmp

ProductDetector

IF

CIO = Carrier Insertion OscillatorCIO

~



USB Demodulation

• SSB demodulation is essentially mixing to baseband

IF Filter

Mixer

Am

plitu

de

Frequency MHz

LowerSideband

UpperSideband

Am

plitu

de

Frequency

UpperSidebandA

mpl

itude

Frequency MHz

UpperSideband

• Unwanted sideband may contain noise and other signals

• Mixing with the carrier frequency is product detection



LSB Demodulation

• SSB demodulation is essentially mixing to baseband

• Typically, IF filter is not moved; the local oscillators are offset.

• When mixed down, the LSB spectrum becomes inverted.

IF Filter

Mixer

Am

plitu

de

Frequency MHz

LowerSideband

UpperSideband

Am

plitu

de

Frequency

LowerSidebandA

mpl

itude

Frequency MHz

LowerSideband



Demodulation Summary• CIO = Carrier Insertion Oscillator

– A fixed local oscillator used to demodulate SSB– It reinserts the carrier that was removed in the transmitter– Wanted sideband is directly translated directly to Audio

• BFO = Beat Frequency Oscillator– A slightly variable local oscillator used to demodulate CW– CW demodulated by envelope detector

• BFO/Detectors can resolve SSB. A product detector can resolve CW

• Two Issues– Input Signal Levels can vary greatly– Detection methods described so far are for Amplitude based modulations, not FM



AGC

• AGC = Automatic Gain Control• AGC adjusts the gain to keep carrier level constant• Signal level varies widely, but audio volume stays constant

MixerIF

AmpDemod

RFAmp

AFAmp

LOAGC

Circuit



FM Receiver

• Uses FM Discriminator to detect small frequency deviations• FM receivers use IF limiter amplifiers; no AGC needed• Squelch detects high-frequency noise and gates audio

LocalOscillator

MixerAF

amplifierIF

amplifierDiscriminator

Squelchcircuit

Chelmsford Amateur Radio Society Intermediate Course (4) Receivers

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