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Re: HUM_FORUM: HUM_FORUM: Re: HUM_FORUM: FWIW...

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  • Tom Becker
    ... for FM... Slope detection can happen, yes, and unintentional rectification (like through a corroded connection) can demodulate sufficiently-large signals
    Message 1 of 26 , May 10, 2010
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      > ... explains slope detection using only a diode and a tuned circuit
      for FM...

      Slope detection can happen, yes, and unintentional rectification (like
      through a corroded connection) can demodulate sufficiently-large signals
      (and, BTW, mix (intermodulate), or remodulate, and reradiate).

      Broadcast FM signals are not always just FM'd, though; they often carry
      a significant AM component, too. AM can be generated anywhere in the
      broadcast RF chain, from exciter to antenna. A mistuned broadcast
      transmit antenna, for example, will generate AM on an FM signal radiated
      from it by the transmit-equivalent of slope detection; the antenna
      behaves differently for different frequencies so, as the exciter shifts
      the carrier frequency as it should, the antenna works irregularly well,
      making AM. A similar effect can cause a broadcast directional radiation
      pattern to shift with modulation, also generating AM in receivers on the
      slopes of the moving pattern. And, a mistuned transmitter generates AM.

      So, the AM you hear on an FM signal might be transmitted that way - and
      today, many broadcasters (perhaps most) no longer have an engineer on
      staff, so the transmitter might not have been tuned for some time.

      Tom
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