[R-390] Spectrum Analyzer

[2002tii]

Barry wrote:

>I'm not sure all of that makes sense to me just yet.

AGC tries to hold the IF output level constant, regardless of the 
signal input.  It does that by measuring the IF output after all filtering.

The important take-home point is that with the AGC on, as you track 
the signal source across the IF passband, the level you are measuring 
(coming out the IF output jack, or the diode detector) depends on TWO 
things -- (i) the shape of the IF passband, and (ii) the IF gain (and 
the RF gain, if you are injecting signal at the antenna).  AGC 
changes the IF gain as the signal sweeps through the passband, trying 
to hold the IF output constant even though the filter is attenuating 
it when it is not at the center of the passband.  Since you are 
trying to see the shape of the passband, you would like level you are 
measuring to depend on only ONE thing -- the shape of the passband.

Hope this is clearer.

Don


Copyright (c) 2011.  Not for redistribution