[R-390] Spectrum Analyzer

[Randy and Sherry Guttery]

On 3/15/2011 9:10 AM, Barry wrote:
>   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.
Yes -
>   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.
Yes - so - as indicated above - as the sweep signal 
approaches the passband - at some point the IF's bandwidth 
will allow some signal to pass. (In real life) from that 
point - and "up to" -100db - a spectrum analyzer will show 
the signal being passed.
>   When the signal is inside the bandpass, then the AGC voltage increases and thus decreases the gain.
What complicated matters in the example given - the passband 
is arbitrarily given as -100db -- so that as soon as the 
sweep signal "crosses" into the passband - the AGC limits 
the gain so that the passed signal never exceeds -100db...  
resulting in a "square" pass band (flat topped)...

>  If that's correct, then why would the IF output pop to full output?

It won't  - it is flat BUT *at  -100db* since that's the 
maximum signal allowed by the agc (again - in this 
theoretical example - real life ain't that perfect).

best regards...

-- 
randy guttery

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