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Old 30th Oct 2017, 12:58 pm   #21
G8HQP Dave
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Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Interesting Armstrong AM detector circuit.

Yes, it is strange that the envelope detector is so poorly handled by most textbooks. I have lots of radio books, and so far I have only found one which gives the formula for the ideal voltage-fed envelope detector. Both Terman and Langford-Smith (the two 'bibles' of radio design) move too quickly to non-ideal detectors without properly dealing with the ideal case. One of my hobby-horses is that you cannot properly understand the real thing until you have first understood the ideal thing.

The one exception I have found so far is 'Radio Communication' by D C Green. He derives a formula giving the upper limit for the time constant:
CR <= sqrt(1-m^2)/(m 2 pi fm)
where C and R are the envelope detector components, m is the AM modulation index, fm is the modulation frequency. This shows that distortionless demodulation is not possible for 100% (m=1) AM, because the CR upper limit for this would be zero but there is also a lower limit imposed by the carrier frequency. Any external loading on the output makes things worse.

For 50% modulation the ideal envelope detector starts to distort for modulation frequency above 3/(4 pi RC) = 0.24/RC. 90% would be 0.033/RC - so for normal AM modulation and IF frequencies the window between the upper and lower limits for CR is quite narrow. For good IF filtering we need fc >> 1/(2 pi RC) - say, fc > 1/(RC). 470kHz IF then requires RC > 2.1us. 5kHz modulation at 90% requires RC < 6.7us. In practice it seems that many sets use a high RC value and accept some treble distortion - the frequency chacteristics of music and speech come to our aid so full modulation is unlikely above a few kHz. Modern broadcast audio compression may not help!
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