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AGC to which stages? - Comments welcome - RF Cafe Forums
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Charl
Post subject: AGC to which stages? Posted: Mon Aug 21, 2006
7:58 am
Colonel
Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello all,
Looking at
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
(DC) amplfied?
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,
Charl
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languer
Post subject: Posted: Mon Aug 21, 2006
3:39 pm
Captain
Joined: Sun May 14, 2006 8:53 pm
Posts: 17
Location: Earth
Do you have a particular application
in mind?
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.
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fred47
Post subject: AGCPosted: Mon Aug 28, 2006 11:11 pm
General
Joined: Wed Feb 22, 2006 3:51 pm
Posts:
104
Hi!
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
very visible.
Good Luck!
Fred
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Charl
Post subject: Posted: Tue Aug 29, 2006 12:01 pm
Colonel
Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Thanks fred, that's very helpful. Confirms my suspicions! :)
Posted 11/12/2012
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