[R-390] Spectrum Analyzer

[Barry]

Hmmm, I think I need to chew on that one a while.  I'm not sure all of that makes sense to me just yet.

I do appreciate the explaination, Don.

Thanks,
Barry - N4BUQ


 On Mon 14/03/11  3:56 PM , 2002tii bmw2002tii at nerdshack.com sent:
> Barry wrote:
> 
> >how does the AGC affect the bandpass, particularly
> its shape.  I >thought all the AGC did is change the gain at the
> various stages so >the shape would be the same just translated "up" or
> "down" depending >on the signal.
> 
> 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).
> 
> A real-life AGC won't distort your results to quite that degree, but 
> receiver AGCs do have lots of gain and very flat slopes, so they 
> distort the apparent passband shape significantly.  Also, their 
> response is dynamic (and generally asymmetrical, attack to release) 
> so the envelope distortion will depend on how fast you sweep in 
> relation to the AGC time constants.
> 
> It's just like trying to measure the frequency response of an audio 
> amplifier with a speech compressor in the measurement chain.  Bad idea.
> 
> If you are puzzled why the AGC does that when you sweep, but not when 
> you listen:  Actually, it does do that when you listen, with some 
> particular signals.  With phone (or other wide-ish signals), or in a 
> noisy band, the AGC responds to whatever is loudest and everything 
> else is related to that according to its relative received signal 
> strength and the IF passband shape.  But when there is a quiet band 
> with just one CW signal (like your sweep tone, which is at just one 
> frequency at any particular time), that signal DOES pop up to pretty 
> much full output when it is still well down the IF response skirt, 
> doesn't change much as you tune through it, and stays at pretty much 
> full output until it is well down the IF response skirt on the other
> side.
> Best regards,
> 
> Don
> 
> 
> Copyright (c) 2011.  Not for redistribution
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390Help: http://mailman.qth.net/mmfaq.htmPost: R-390 at m
> ailman.qth.net
> This list hosted by: http://www.qsl.netPlease help support this email list: http://www.qsl.net/donate.html
> 
>