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The Envelope detector is over 100 years old .In its classic form it is a simple half wave diode that peak detects the carrier with the load resister and load capacitor time constant chosen to allow the output to follow the envelope of the carrier .The Envelope detector is well documented elsewhere.Would an averaging type of detector be better in terms of noise performance ? Was the peak detector chosen for simplicity or maximum output voltage or was it the first thing that worked?

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  • \$\begingroup\$ And how would you make the average detector? The peak detector is really simple, as you have said. As technology has improved, AM has almost disappeared, so I don't know if anybody would care to improve AM detection. Amplitude modulation is used in QAM, but QAM demodulation has nothing to do with ye olde diode detector. \$\endgroup\$ Commented Aug 19, 2017 at 6:54
  • \$\begingroup\$ @ Claudio Avi Chami .I had prototyped for fun an active AM detector that is full wave and develops the rectified 455KHz volts across a 1K2 test resistor .This was for me to check waveform accuracy on the scope .If I were to use this circuit in say a car radio would there be any benefit of peak Vs average .The active detector is functionaly equiv to the diode envelope det but has less distortion. \$\endgroup\$
    – Autistic
    Commented Aug 19, 2017 at 8:46
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    \$\begingroup\$ I think synchronous detection of AM improves SNR by 3dB, though improving AM is rather like adding lipstick to a pig. \$\endgroup\$
    – Neil_UK
    Commented Aug 19, 2017 at 9:20
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    \$\begingroup\$ Are you talking about peak detection verses low pass filtering the full wave rectified version of the modulated carrier? \$\endgroup\$
    – Andy aka
    Commented Aug 19, 2017 at 9:22
  • \$\begingroup\$ @ Andy aka .I am asking about LPF of fullwave rectified waveform Vs the standard peak detector .I do not think that there will be a big difference because the 455KHz incoming Modulated IF is band limited anyway .Maybe there is a small weak signal SNR benefit because of the averaging effect of impulse noise .I am hoping this question sparks debate .Asking questions is a risky activity for me . \$\endgroup\$
    – Autistic
    Commented Aug 19, 2017 at 9:30

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You cannot demodulate an AM signal by averaging it - Assuming that's what you're asking.

If you filter an AM signal with a LPF the amplitude of the carrier becomes suppressed, thereby also suppressing the envelope of the carrier, hence loosing your modulation signal. This should make sense when looking at the transient depiction of an AM signal. Amplitude modulation
The only way of demodulating AM is by envelope extraction, and the simplest electronic circuit for that would be a rectifier. There are better ways of envelope extraction that give better SNR but at the cost of more circuit complexity.

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    \$\begingroup\$ Averaging an AC signal, produces the DC component, which is zero. \$\endgroup\$
    – Whit3rd
    Commented Aug 20, 2017 at 5:35
  • \$\begingroup\$ @Whit3rd Yes, though for applied electronics averaging would be a LPF, that suppress the AC amplitude. As the amplitude of the above signal goes to zero so does its envelope, hence you loose both the carrier and modulation. Same is true for FM, but not true for PWM/PDM. In order to demodulate AM signals you must first take the envelope (rectifying the carrier, or do an actual envelope extraction with signal processing), and then low pass filter it to remove the carrier. \$\endgroup\$
    – Martin
    Commented Aug 20, 2017 at 15:17

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