Re: Confused about how AGC works?

Jerry Gaffke

A poorly designed AGC circuit could oscillate exactly as you describe.
Whether or not it will oscillate depends on the delays through each element
and the gain of each element.  Both can vary depending on the frequency
at which they are calculated.

Jerry, KE7ER

On Thu, Apr 18, 2019 at 06:00 PM, digger AB3XU wrote:
To begin with I can follow along with the theory and it does makes sense. Very simply, the signal from Vol-Hi via Q1 is used to control the conduction of Q2 and how much Q3 shunts the IF signal. Ideally RF levels below S9 are left relatively untouched, however if greater than that, the action of Q2 conducting less and Q3 acting as a shunt  results in reducing clamping (reducing) the IF level.

So here's where I get lost.  As the IF level gets attenuated by the action of the AGC doesn't that in turn:
  1. attenuate the level you will see at Vol-HI
  2. which will then decreases the effect of the AGC
  3. leading to an increased IF signal level
  4. and increasing Vol-HI
  5. which then increases the AGC's effect
  6. reducing the IF signal level
  7. resulting in a feedback loop of the IF level going up and down...
Am I describing my confusion/question in a way that makes sense?

73, digger AB3XU

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