[TenTec] AGC in Omni C discovery

[Steve Ellington N4LQ]

For you AGC buffs, I've been impressed with how the Omni C seems to be imune
to QRN and have found what may be a contributing factor.

The AGC in the Omni C (and maybe older Omni's) is like nothing I've seen
before. I would appreciate it if someone would check their Corsair schematic
and see if it matches what I'm about to describe.

The gain of the 2 stage IF amplifiers is FIXED. Yes you read it right!
FIXED. Only the RF gain knob can vary the IF amp gain. The agc does not
change the bias on the IF amp's transistors like most receivers do. Instead,
the agc voltage developed by a pair of diodes is applied to a pair of
darlington transistors and fed to 3 diodes which act as shunts to RF. These
diodes are on the inputs an output of each amplifier. When agc voltage is
developed, the diodes are forward biased and simply shunt a portion of the
RF to ground. The gain of the IF amplifiers remains constant and according
to the manual "fixed biased for maximum dynamic range". BTW: these are PIN
diodes.

I checked my Argosy and Paragon II. They use the conventional system.

Now here's the questions:

1. TenTec implies that there is a certain bias that provides maximum dynamic
range. Is this true.

2. What effect does madly varying gain have on an amplifier as a result of
applying agc bias? Wouldn't the frequency of the variations mix and produce
some extra products?

Any comments welcome!


Steve N4LQ


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