Chelmsford Amateur Radio Society Advanced Course Receivers Part4

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Chelmsford Amateur Radio Society Advanced Course Receivers Part-4 – Receiver Demodulation Chelmsford Amateur Radio

Chelmsford Amateur Radio Society Advanced Course Receivers Part-4 – Receiver Demodulation Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 1

Receiver Demodulators Key names when referring to demodulators in receivers: • AM – the

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 M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 2

AM Demodulator D 1 • Diode D 1 detects the peaks of the IF

AM Demodulator D 1 • Diode D 1 detects the peaks of the IF signal Demodulated AF Output RF Input • C 1/R 1 are a low-pass filter - filtering out the IF, leaving audio C 1 R 1 • Sometimes called an Envelope Detector Modulated RF & Audio Output Envelope Voltage Output Time Input Close-in Chelmsford Amateur Radio Society Advanced Licence Course Wider span Anthony Martin M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 3

SSB Demodulation • • • SSB = Single Sideband – without carrier and other

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 Advanced Licence Course Product Detector Audio ~ CIO Anthony Martin M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 4

USB Demodulation • SSB demodulation is essentially mixing to baseband Amplitude - USB: Signals

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 M 1 FDE Amplitude IF Filter Upper Sideband Frequency Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 5

LSB Demodulation • SSB demodulation is essentially mixing to baseband Amplitude - LSB: Signals

LSB Demodulation • SSB demodulation is essentially mixing to baseband Amplitude - LSB: Signals in 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 M 1 FDE Amplitude IF Filter Lower Sideband Frequency Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 6

CW Demodulation • BFO = Beat Frequency Oscillator • In older times a BFO

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 M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 7

CW Demodulation … nowadays we use the same product detectors as used for SSB

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 M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 8

FM Demodulator • The Ratio Detector – True frequency detector – Relatively unaffected by

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 D 1 L 2 IF Input C 1 Ct C 3 L 1 Chelmsford Amateur Radio Society Advanced Licence Course + D 2 Demodulated Audio Output C 2 Anthony Martin M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 9

FM Demodulator Response • Discriminator curve – Output voltage linear with frequency over limited

FM Demodulator Response • Discriminator curve – Output voltage linear with frequency over limited range – Needs correct adjustment otherwise distortion results Output voltage Linear range Chelmsford Amateur Radio Society Advanced Licence Course Frequency IF Centre Frequency Anthony Martin M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 10

FM IC Demodulator • Typical FM 2 nd IF integrated circuit used today includes:

FM IC Demodulator • Typical FM 2 nd 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 M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 11

FM Demodulator • In the IC Quadrature detector circuit, a Product Detector acts as

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 M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 12

FM Demodulator Output • This is known as the “S” curve Chelmsford Amateur Radio

FM Demodulator Output • This is known as the “S” curve Chelmsford Amateur Radio Society Advanced Licence Course Amplitude (m. V) • This gives ± 45 degree phase shift over 3 d. B 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 M 1 FDE 400 500 600 Frequency (k. Hz) 700 Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 13

DSP demodulators • DSPs (digital signal processors) are fast microprocessors optimised for repetitive maths

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 M 1 FDE Slide Set 12: v 1. 4, 2 -Dec-2012 (4) Receiver Demodulation 14