[Elecraft] AGC

[Ron D'Eau Claire]

> In looking over the schematic of the K2 I noticed that the AGC is
> derived by converting to a lower IF and then producing the necessary
> AGC voltages.  Interesting design.  Any immediate advantages to doing
> it in this fashion?  No big deal, just curious.
> --
> Helmut Usbeck, WB2ADT

Yes, there is a significant advantage.  Using a signal at an i-f is common
of most "high performance" AGC systems.

One of the major things it does is to provide a very fast "attack" or
response to turn down the gain when a very strong signal arrives .The AGC
can do this in a few cycles of signal, which when using an i-f signal as
several Megacycles is very fast indeed.

The common alternative for a less expensive system is what is used in the
K1 - audio-derived AGC. That also responds in a few cycles, but now it is
looking at audio frequencies instead of the much higher i-f, so the speed
with which it can turn down the gain is much, much slower. So slow, in fact,
that it produces the characteristic "pop" ops have report that the K1 makes
on strong signals. That is common with virtually all audio-derived AGC
systems.

Of course, the AGC system could use the same i-f as the main signal path,
but that means that great care must be taken to keep the BFO out of the agc
circuit! Otherwise, the AGC will respond to it instead of the on-air signals
coming through. That's why AGC's were not commonly used on older receivers
and if you do turn it on with the BFO on, the receiver normally desenses.
The AGC thinks the BFO is a very strong signal and reacts accordingly.

The easiest way to keep the BFO out of the AGC is to put the AGC detector on
a whole different frequency from the BFO, as they did in the K2.

Ron AC7AC
K2 # 1289