Wrongly demodulating AM...

Discussion in 'Wireless & RF Design' started by Externet, Apr 4, 2019.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Wrongly and poorly demodulating analog AM with a FM demodulator circuit; how is that called ? Slope detection or something else ?
    Read once tricks that removing the 10.7MHz crystal filter could somewhat yield an almost good AM audio; or was it shorting, or bypassing ? How is it done ?
    Trying to hear voice AM on an experiment at ~100 MHz frequency modifying/tweaking a plain 'FM' band receiver.
     
  2. bertus

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  3. AlbertHall

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    Jun 4, 2014
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    Yeah. The FM receiver has done its best to remove any AM before the signal gets to the demodulator.
     
  4. Externet

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    Thanks, gentlemen.
    Does the other way around has a name that I can search and learn about ?
     
  5. nsaspook

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    Aug 27, 2009
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    You mean an FM demodulator used as a envelope detector? Most are at least somewhat sensitive to amplitude fluctuations, that's why we use a limiter.

    The old Foster Seeley discriminator and most ratio detectors could be converted to bad envelope detectors by tuning off center carrier.
    [​IMG]
     
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  6. DickCappels

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    A slope detector is the other way around. With it an AM detector is tuned to one side of an FM signal and the output of the detector varies by the frequency change. Detecting Fm with an AM detector is slope modulation.

    As AlbertHall points, FM receivers have limiting (or clipping) stages that remove most of the amplitude modulation so there would be very little if any AM for an FM detector to detect, regardless of the tuning tricks used. The point of the limiting stages is to make the signal independent of amplitude variation, and in particular to reduce the susceptibility to electrical noise which is AM by nature.
     
  7. Externet

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    Thank you.
    The FM only receiver has no AM components populated shown here; nothing on pins 4, 7, 11, 12, 16
    Where is the limiting stage likely be... at pin 8 "FM IF" ?
    If the signal trace entering now to pin 8 from the "FM MIX" trough 10.7MHz filter was instead, connected to pin 7 (currently vacant/unused) would the AM detector work at the 10.7MHz IF instead, or it is not meant to handle other than 455KHz ?
    (IC is TA2003)
    upload_2019-4-4_16-42-26.png
     
  8. dendad

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    Feb 20, 2016
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    The easiest think to do is just get an AM receiver. An FM receiver is inherently the worst way to try to get AM as has been pointed out. It is designed to remove the AM!
    What frequencies are you trying to receive the AM on?
     
  9. Externet

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    100MHz and above.
     
  10. MisterBill2

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    Jan 23, 2018
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    If you already have a functioning FM receiver and you want also to detect an AM signal in the same frequency range, you can add a diode detector prior to the limiter stages. That is not very complicated, although you will need circuit information about the FM receiver circuit. The AM detector in an AM/FM radio will not help, usually, because it is for the 455 KHz AM section IF part of the radio.
    Note that the two circuits submitted so far will not be of any use.
     
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