East Kent Radio Society EKRS
1
Intermediate Course
(4) Receivers
Karl Davies
Receivers
2
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
3
 Block diagram



Single tuned circuit; poor selectivity
No gain; poor sensitivity
Requires large aerial and earth; only receives strong signals
Tuned
circuit
Diode
detector
Earphone
Crystal Receiver Circuit
4
 L1, C1 - Tuned circuit – selects signal
 D1 – Detector diode – demodulates
 C3, R1 – Low-pass filter for audio
Antenna
D1
AA119
C1
100pF
L1
Coil
C2
500pF
RF Earth
X1
C3
100pF
R1
100k
Crystal
Earphone
TRF Receiver (Tuned Radio Frequency)
5
 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
RF
amplifier
BFO
Demodulator
AF
amplifier
Superhet Receiver
6
 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
Mixer
Local
oscillator
IF
amplifier
BFO
Demodulator
AF
amplifier
Mixer as a Converter
7
 Mixer may be used as 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
RF 145MHz
~
LO 123.6MHz
145MHz–123.6MHz=21.4MHz
IF frequency
Image frequency is
123.6MHz-21.4MHz=102.2MHz
AM Envelope Detection
8
 Demodulation of a modulated audio signal
 Detector output follows envelope of RF

Otherwise known as an “envelope detector”
Envelope
Modulated RF
Demodulated Audio
Diode Detector Circuit
9
 Diode D1 rectifies AC into DC
 C1/R1 is a lowpass filter - filters out the RF
 D1 conducts on positive half-cycle only
Voltage
Output
D1
RF Input
C3
100pF
Time
Demodulated
R1
100k AF Output
Input
Diode Detector Operation
10
Diode Voltage / V
 Volts and Current in the diode detector (SPICE Simulation)
2
D1 cathode output
1
0
D1 anode input
-1
Diode Current / µA
-2
800
Anode
600
Diode conducts on peaks only
400
D1 current
200
0
0
0.5
1
1.5
2
2.5
3
3.5
4 Time/µSecs
Cathode
CW Demodulation - BFO
11
 BFO = Beat Frequency Oscillator

Slightly offset BFO added to carrier to generate the “beat note”
+
Carrier Wave
Envelope
Detector
BFO
…the origins of the “BFO” !
Beat Note
SSB Demodulation
12




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
IF
Sideband
Filter
IF
Amp
CIO = Carrier Insertion Oscillator
Product
Detector
~
CIO
AF
Amp
USB Demodulation
13
Amplitude
 SSB demodulation is essentially mixing to baseband
Lower
Sideband
Upper
Sideband
Frequency MHz
• Unwanted sideband may contain
noise and other signals
• Mixing with the carrier frequency
is product detection
Mixer
Upper
Sideband
Frequency MHz
Amplitude
Amplitude
IF Filter
Upper
Sideband
Frequency
LSB Demodulation
14
Amplitude
 SSB demodulation is essentially mixing to baseband
Lower
Sideband
Upper
Sideband
Frequency MHz
• Typically, IF filter is not moved;
the local oscillators are offset.
• When mixed down, the LSB
spectrum becomes inverted.
Mixer
Lower
Sideband
Frequency MHz
Amplitude
Amplitude
IF Filter
Lower
Sideband
Frequency
Demodulation Summary
15
 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
16
 AGC = Automatic Gain Control
 AGC adjusts the gain to keep carrier level constant
 Signal level varies widely, but audio volume stays constant
RF
Amp
Mixer
LO
IF
Amp
Demod
AGC
Circuit
AF
Amp
FM Receiver
17
 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
Mixer
Local
Oscillator
IF
amplifier
Discriminator
AF
amplifier
Squelch
circuit