The AGC is the "magic" of Corsair II. Having experimented in the circuit I can
say the parts selection seems to represent a precarious balance. There must
have been some trial and error in the development. It really makes me wonder
who figured it out and how much time they had to spend on it. It is weird but
in a genuis kind of a way, I think. The previous audio AGC Ten Tec transceivers
like OMNI C suffered from some loud gunshot upon occasion, would put me under
the desk when it went off. Corsair II does not suffer this. My ideal AGC is
*none* all the way up to S9 then just enough attenuation after that to keep out
of overload. Corsair II fits the bill. ;-)
Re: [TenTec] Corsair II AGC circuit
from [Steve Hunt] [Permanent Link][Original]
To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] Corsair II AGC circuit
From: Steve Hunt <steve@karinya.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sat, 23 Apr 2011 14:30:55 +0100
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Barry,
That sounds right.
U2A is included as an audio phase inverter - that allows *full-wave*
rectification of the audio using the D6/D7 + D8/D9 diode pairs.
Full-wave rectification provides a faster attack time - the Achilles'
Heel of most audio-derived AGC systems.
The inclusion of R46 across C34 provides a leakage path which allows the
AGC to follow a very slowly-varying signal; if you don't have it, the
AGC voltage pretty much "freezes" at its highest value until the hang
circuitry discharges it.
The inclusion of R54 establishes a sensible discharge time for C34 once
the hang circuitry activates - it sounds very unpleasant if it's not
included.
U2B provides a very high input impedance so as not to shunt the main
capacitor C34; Q6 then buffers its output to provide low impedance drive
to the IF stages and the S-meter.
In the SLOW position C33 is effectively in parallel with C32; in the
FAST position C32 alone is in operation and establishes the hang period
in combination with R49 which is the charge path. Positive-going audio
half-cyles which exceed the Vbe threshold of Q8 cause it to conduct and
discharge C32/C33; absence of audio allows C32/C33 to charge via R49.
Once the voltage on C32/C33 has risen high enough, Q9 is cut off and C34
discharges via R54 and D10.
But you'd probably already worked all that out :)
73,
Steve G3TXQ
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|