Chelmsford Amateur Radio Society
Advanced Course
Receivers
Part-4 – Receiver Demodulation
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
1
Receiver Demodulators
Key names when referring to demodulators in receivers:• AM – the envelope detector
• SSB – the product detector
• CW – the BFO
• FM – the ratio detector
• Multimode transceivers have a demodulator for each mode and
the controls select which one to use
• This adds complexity - thus the more modern trend to DSP/SDR
• Terminology – may be referred to as detection or demodulation
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
2
AM Demodulator
D1
• Diode D1 detects the peaks
of the IF signal
Demodulated
AF Output
RF Input
• C1/R1 are a low-pass filter
- filtering out the IF, leaving audio
C1
R1
• Sometimes called an
Envelope Detector
Modulated RF & Audio Output Envelope
Voltage
Output
Time
Input
Close-in
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Wider span
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
3
SSB Demodulation
•
•
•
•
•
SSB = Single Sideband – without carrier and other sideband
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
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Product
Detector
Audio
~
CIO
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
4
USB Demodulation
• SSB demodulation is essentially mixing to baseband
Amplitude
- USB: Signals in Upper Sideband
Lower
Sideband
Upper
Sideband
Frequency MHz
- Unwanted Lower sideband may
contain noise and other signals
- Mixing with the carrier frequency is
product detection
Mixer
Upper
Sideband
Frequency MHz
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Amplitude
Amplitude
IF Filter
Upper
Sideband
Frequency
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
5
LSB Demodulation
• SSB demodulation is essentially mixing to baseband
Amplitude
- LSB: Signals in Lower Sideband
Lower
Sideband
Upper
Sideband
Frequency MHz
- Typically the 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
Advanced Licence Course
Anthony Martin M1FDE
Amplitude
Amplitude
IF Filter
Lower
Sideband
Frequency
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
6
CW Demodulation
• BFO = Beat Frequency Oscillator
• In older times a BFO was an oscillator that “beat” with the CW
carrier to make a signal that could be detected as AM…
+
Carrier Wave
Envelope
Detector
BFO
Beat Note - thus the BFO term
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
7
CW Demodulation
… nowadays we use the same product detectors as used for SSB
• BFO (Beat Frequency Oscillator) setting is an offset of the CIO
• This causes the CW carrier to be mixed down to almost zero
frequency so that it sounds like a tone
IF
Narrow
CW Filter
IF
Amp
Product
Detector
Audio
~
BFO
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
8
FM Demodulator
• The Ratio Detector
– True frequency detector
– Relatively unaffected by changes in signal level
• Common in valve and transistor receivers, but rarely used today
– The transformer is very specialised, nowadays it’s unobtainable
D1
L2
IF Input
C1
Ct
C3
L3
L1
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Demodulated
Audio Output
+
D2
C2
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
9
FM Demodulator Response
• Discriminator curve
– Output voltage linear with frequency over limited range
– Needs correct adjustment otherwise distortion results
Output voltage
Linear range
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Frequency
IF Centre
Frequency
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
10
FM IC Demodulator
• Typical FM 2nd IF
integrated circuit used
today includes:
– Local oscillator
– Mixer
– IF limiter amp
– Quadrature
detector
– Squelch filter
– Squelch trigger
– Audio Muting
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
11
FM Demodulator
• In the IC Quadrature detector circuit, a Product Detector acts as a
phase comparator
• Its output voltage depends on difference in phase between the inputs
IF
IF Limiter
Amp
Product
Detector
Audio
Tuned
Circuit
“Quad Coil”
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
12
FM Demodulator Output
• This is known as the
“S” curve
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Amplitude (mV)
• This gives ±45 degree phase
shift over 3dB bandwidth
800
600
400
200
300
Phase (degrees)
• FM demodulators are based
on the phase shift of a tuned
circuit near resonance
1000
80
60
40
20
0
-20
-40
-60
-80
300
Anthony Martin M1FDE
400
500
600
Frequency (kHz)
700
400
500
600
Frequency (kHz)
700
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
13
DSP demodulators
• DSPs (digital signal processors) are fast microprocessors
optimised for repetitive maths operations
• Signal processing is performed in the digital domain on numeric
representations of the signal (samples)
• Mathematical algorithms do functions like filtering, mixing,
oscillators and amplitude detection
• Can also do modulation, voice enhancement, noise reduction
IF
IF Amp
Analogue
to Digital
Fast
Processor
Digital to
Analogue
Audio
Spectrum
Displays etc
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 12: v1.4, 2-Dec-2012
(4) Receiver Demodulation
14