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RE: Converter gain and noise..



Doug Braun wrote, about IF filter bandwidth vs. RF signal level:

But wouldn't the receiver's AGC take care of this?  If the signal
is S9+20, isn't the AGC is reducing its amplitude a lot, and the IF
is actually seeing roughly the same signal level it would on a S1 signal?

Wayne replies

Good question.  I'm not sure where AGC occurs in a typical receiver.  My experience with an Icom 751 IF notch filter suggests that most AGC in that radio occurred after the 150 kHz IF notch filter - possibly in the audio stage.

Another possible clue: AGC occurs AFTER the S-meter signal is derived.  Is the S-meter signal derived from the 455 kHz IF in most receivers?

Whatever RF stages AGC occurs in, I suspect that the AGC would be AFTER the input filtering for that stage so that the gain is controlled mostly by the desired signal and not too much by stuff that is filtered out.

I doubt that IF filters see roughly the same RF level for an S1 signal and an S9+20 signal, but I only know enough about receiver design to get myself into trouble.  Are there any receiver experts who can answer the "where is AGC" question?

Wayne Estes W9AE
Mundelein, IL, USA
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