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AGC to which stages? - Comments welcome - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: AGC to which stages? Posted: Mon Aug 21, 2006
Joined: Fri May 19, 2006 5:01 am
old schematics of tube radios, the AGC is usually wired to all stages:
RF preamp, RF mixer, and IF stages (sometimes just the IF stages). Would
I be able to get away with hooking it to just the RF preamp - possibly
I presume they used to connect the AGC to all
stages to get more negative feedback out of a low voltage swing: -1V
on one grid only brings gain down by so many dB, but -1V on 4 grids
brings it down by much more.
Thanks in advance,
Post subject: Posted: Mon Aug 21, 2006
Joined: Sun May 14, 2006 8:53 pm
Do you have a particular application
AGC is mainly there to keep everything linear, so if
the first thing to non-linear is the RF-amp that's where you need it
the most. Usually though the first thing is the IF. And because sometimes
you have strong unwanted signals within your passband, you may want
to start introducing loss (i.e. AGCing) in the IF so as to not affect
your NF (and thus not heavily impacting a low-power desired signal).
As I said before, it depends in your application.
Post subject: AGCPosted: Mon Aug 28, 2006 11:11 pm
Joined: Wed Feb 22, 2006 3:51 pm
Just a quick note - it's actually very hard to design
an amplifier with a gain that's variable over a wide range. That's why
many circuits apply AGC to several stages - to get a wider control range.
However, there are other considerations: intermodulation and noise
figure, for example. That leads designers to use "delayed AGC", where
some stages receive the AGC at levels that other stages don't receive
it at. There are different nonlinearities used to get this, some not
Post subject: Posted: Tue Aug 29, 2006 12:01 pm
Joined: Fri May 19, 2006 5:01 am
Thanks fred, that's very helpful. Confirms my suspicions! :)