Chelmsford Amateur Radio Society
Intermediate Course
(4) Receivers
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
1
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
2
Crystal Receiver
• 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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
3
Crystal Receiver Circuit
• 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
X1
C3
100pF
R1
100k
Crystal
Earphone
RF Earth
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
4
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
RF
amplifier
Demodulator
AF
amplifier
BFO
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
5
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
Mixer
Local
oscillator
Chelmsford Amateur Radio Society
Intermediate Licence Course
IF
amplifier
Demodulator
AF
amplifier
BFO
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
6
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
RF 145MHz
~
LO 123.6MHz
Chelmsford Amateur Radio Society
Intermediate Licence Course
145MHz–123.6MHz=21.4MHz
IF frequency
Image frequency is
123.6MHz-21.4MHz=102.2MHz
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
7
AM Envelope Detection
• Demodulation of a modulated audio signal
• Detector output follows envelope of RF
– Otherwise known as an “envelope detector”
Envelope
Modulated RF
Demodulated Audio
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
8
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
Voltage
Output
D1
RF Input
C3
100pF
Time
Demodulated
R1
100k AF Output
Input
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
9
Diode Detector Operation
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
Cathode
Diode conducts on peaks only
400
D1 current
200
0
0
0.5
Chelmsford Amateur Radio Society
Intermediate Licence Course
1
1.5
2
2.5
Anthony Martin M1FDE
3
3.5
4 Time/µSecs
Slide Set 7: v1.3 28-May-2009
(4) Receivers
10
CW Demodulation - BFO
• BFO = Beat Frequency Oscillator
– Slightly offset BFO added to carrier to generate the “beat note”
+
Carrier Wave
Envelope
Detector
BFO
Beat Note
…the origins of the “BFO” !
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
11
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
IF
Sideband
Filter
IF
Amp
CIO = Carrier Insertion Oscillator
Chelmsford Amateur Radio Society
Intermediate Licence Course
Product
Detector
AF
Amp
~
CIO
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
12
USB Demodulation
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Amplitude
Amplitude
IF Filter
Upper
Sideband
Frequency
Slide Set 7: v1.3 28-May-2009
(4) Receivers
13
LSB Demodulation
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Amplitude
Amplitude
IF Filter
Lower
Sideband
Frequency
Slide Set 7: v1.3 28-May-2009
(4) Receivers
14
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
Chelmsford Amateur Radio Society
Intermediate Licence Course
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
15
AGC
• AGC = Automatic Gain Control
• AGC samples the signal strength at the detector and then feeds
back a control signal to adjust the gain of the earlier stages
• AGC keeps audio volume fairly constant if signals level vary
RF
Amp
Mixer
LO
Chelmsford Amateur Radio Society
Intermediate Licence Course
IF
Amp
Detector
AF
Amp
AGC
Circuit
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
16
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
Mixer
IF
amplifier
Discriminator
Local
Oscillator
Chelmsford Amateur Radio Society
Intermediate Licence Course
AF
amplifier
Squelch
circuit
Anthony Martin M1FDE
Slide Set 7: v1.3 28-May-2009
(4) Receivers
17