1 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation Chelmsford Amateur Radio Society Advanced Course Receivers Part-4 – Receiver Demodulation

2 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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

3 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation AM Demodulator Diode D1 detects the peaks of the IF signal C1/R1 are a low-pass filter - filtering out the IF, leaving audio Sometimes called an Envelope Detector D1 R1C1 RF Input Demodulated AF Output Voltage Time Output Input Close-in Modulated RF & Audio Output Envelope Wider span

4 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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 Sideband Filter IF Amp Product Detector CIO ~ CIO = Carrier Insertion Oscillator IF Audio

5 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation USB Demodulation SSB demodulation is essentially mixing to baseband Amplitude Frequency MHz Lower Sideband Upper Sideband Amplitude Frequency MHz Upper Sideband Amplitude Frequency Upper Sideband -USB: Signals in Upper Sideband -Unwanted Lower sideband may contain noise and other signals -Mixing with the carrier frequency is product detection IF Filter Mixer

6 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation LSB Demodulation SSB demodulation is essentially mixing to baseband Amplitude Frequency MHz Lower Sideband Upper Sideband Amplitude Frequency MHz Lower Sideband Amplitude Frequency Lower Sideband -LSB: Signals in Lower Sideband -Typically the IF filter is not moved; the local oscillators are offset -When mixed down, the LSB spectrum becomes inverted IF Filter Mixer

7 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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 WaveBFO + Envelope Detector Beat Note - thus the BFO term

8 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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 Narrow CW Filter IF Amp Product Detector BFO ~ IF Audio

9 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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 C1 IF Input Demodulated Audio Output + C3 C2 D2 Ct L1 L2 L3

10 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation FM Demodulator Response Discriminator curve –Output voltage linear with frequency over limited range –Needs correct adjustment otherwise distortion results Frequency Output voltage Linear range IF Centre Frequency

11 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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

12 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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 Limiter Amp Product Detector Audio Tuned Circuit “Quad Coil” IF

13 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation FM Demodulator Output FM demodulators are based on the phase shift of a tuned circuit near resonance This gives ±45 degree phase shift over 3dB bandwidth This is known as the “S” curve

14 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 12: v1.4, 2-Dec-2012 (4) Receiver Demodulation 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 Audio IF Amp Analogue to Digital Fast Processor IF Digital to Analogue Spectrum Displays etc DSP demodulators