FM radio cancellation

Discussion in 'Wireless & RF Design' started by swawrzyniak, Nov 19, 2003.

  1. swawrzyniak

    Thread Starter New Member

    Nov 19, 2003
    1
    0
    I tune 1 FM radio to a station fairly high on the FM band - about 107MHz. I tune another FM radio with no volume to a lower point on the FM band - about 97MHz. At the right point, the volume on the first radio goes off. I have even used this to cancel static. I teach Digital Electronics but know nothing about radio.
     
  2. Battousai

    Senior Member

    Nov 14, 2003
    141
    44
    This sounds like a shielding/harmonic distortion problem.

    If you have an a very nonlinear amplifier it will put out frequencies other than those it was designed for.

    For example if you have a nonlinear amplifier running at 2mhz, it also puts out frequencies at 4, 6, 8, ... mhz.

    There are more complex forms of distortion, like intermodulation distortion where if you drive a nonlinear amplifier with two frequencies it will put out frequencies that are linear combinations of the two input frequencies.

    For example if you have a nonlinear amplifier and you input two signals at 2mhz and 3mhz, the amplifier will put out frequencies at 2, 3, 3-2=1, 3+2=5, 2(3)+2=8, etc...

    So it may be the case that one 97mhz signal is interfering destructively with the other 97mhz signal.

    I wonder if one antenna reacts to the other antenna? If one antenna picks up a signal at 97mhz currents, voltages, and b-fields appear on that antenna. I wonder if some of this signal is then transmitted back out and picked up by the second antenna? I'm really not sure about this theory... :unsure:
     
  3. PRR

    New Member

    Dec 16, 2003
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    0
    > I tune 1 FM radio to a station fairly high on the FM band - about 107MHz. I tune another FM radio with no volume to a lower point on the FM band - about 97MHz. At the right point, the volume on the first radio goes off. I have even used this to cancel static.

    A superheterodyne radio has a Local Oscillator. The difference between the LO and the signal becomes the Intermediate Frequency. For FM radios the IF is usually 10.7MHz. So if your second radio is tuned to 107.0-10.7= 96.3MHz, its LO may be at 107.MHz, on top of where your first radio is tuned.

    The LO is not supposed to leak outside the case, or not much, but sometimes it does.

    An FM radio getting two signals will "lock" onto the stronger signal. The weaker signal is almost completely suppressed.

    So you have your jazz on 107.0, and switch-on a second (leaky) radio at 96.3MHz, the leaky radio's LO at 107.0 "locks" the first radio. Now you are listening to the audio on the second radio's LO, which is nothing (maybe a little power-buzz). The first radio plays the near-silence instead of the jazz. This works just as well when the "station" is atmospheric static: any stronger signal will "lock" the FM detector and the weak signal will go away.

    :unsure: I have some trouble with my explanation. The numbers don't line up; they may if you check them carefully. And on FM radios the LO is "usually" -lower- than the signal, not higher (though it can work either way).
     
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