[R-390] Spectrum Analyzer

[Ben Loper]

I picked it up and it's very clean and no errors on start up, so I'll see
how it goes

On Mon, Mar 14, 2011 at 9:05 PM, Barry <n4buq at knology.net> wrote:

> 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
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
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