[R-390] Spectrum Analyzer

[Barry]

 On Mon 14/03/11  3:56 PM , 2002tii bmw2002tii at nerdshack.com sent:
> AGC doesn't affect the passband shape, but it affects your VIEW of 
> the passband as you sweep through it.  Imagine a perfect AGC with 
> infinite slope and a threshold 100 dB below peak output -- as soon as 
> your sweep signal entered the edge of the bandpass (-100 dB), the IF 
> output would pop all the way to full output and remain there until 
> the sweep left the -100 dB bandpass at the other edge.  It would 
> appear to you that your IF had a perfectly square shape (flat top 
> with an infinite cutoff rate) the width of the IF's -100 dB 
> bandwidth.  So, instead of the actual funky-looking, slope-shouldered 
> IF response of, say, 4 kHz at the -6 dB points, you would see an 
> amazing, perfectly rectangular response say 15 kHz wide (or wherever 
> the -100 dB points of your IF filter is).

The above is the part I don't quite understand.  I thought that the AGC would attempt to keep the IF output as constant as possible.  When the sweep signal is outside of the bandpass, then the reduced AGC voltage would result in an increase in the gain in the IF stages.  When the signal is inside the bandpass, then the AGC voltage increases and thus decreases the gain.

If that's correct, then why would the IF output pop to full output?  It would seem the opposite would be true.  I'm not saying it doesn't, but I don't quite understand why it would act that way.

BTW, when you say "perfect AGC with infinite slope", do you mean an AGC with an instantaneous response time?

(from a later message)
> 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.

Yes, that part makes sense to me.  I just can't seem to make this agree with the explaination above.

Thanks again,
Barry - N4BUQ