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What kind of interference or other receiver issues can cause a constant buzzing noise (picket fence lines in the audio spectrum) in the demodulated audio of a received NFM VHF signal?

With AM, it could be additive RF noise; but shouldn't a fully captured FM demodulation ignore that?

This particular noise is not due to a transmitter issue or PL tone or weak FM capture, as the buzzing appears and disappears depending on the location of the receiving station, not the particular transmitting station or receiver equipment (2M HT or SDR) used.

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    $\begingroup$ can you show us the spectrum? (I'll place a bet: it's nonlinearities in your receiver!) $\endgroup$ – Marcus Müller Jun 7 at 10:47
  • $\begingroup$ Due to signal strength of the FM carrier? Or IM with an out-of-band source? $\endgroup$ – hotpaw2 Jun 7 at 11:01
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    $\begingroup$ Again, this is just a wild guess, but if it's strong enough you can hear and clearly see it: probably clipping in your receiver (I should specify my bet, shouldn't I? If I meet you, and it was not clipping, a owe you a cold drink). But, again, very hard to say without a look at the spectrum and if possible at the original waveform. $\endgroup$ – Marcus Müller Jun 7 at 11:06
  • $\begingroup$ Is the station close when the buzzing happens? $\endgroup$ – Ben Madison Jun 8 at 1:20

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