Re: Confused about how AGC works?

Jerry Gaffke
 

All the AGC circuits proposed in this forum (and there have been many) have a fast attack and then a relatively slow decay.
That way when the QRO guy across town keys up, it saves our ears as much pain as possible, and keeps the gain throttled
back in case he decides to utter yet another syllable.  The fast attack and slow decay thing is non-linear, and thus harder
to model with equations.  

The PID algorithm is worth knowing about, an ideal approach to many feedback loops for us punters:
    https://en.wikipedia.org/wiki/PID_controller
No nasty math, easy to understand. 
You get three knobs, and you twiddle them till the system starts to behave itself.
A good example of where you might want to use PID would be on a heater for an oven-controlled-crystal-oscillator, or OCXO.
A processor such as the Nano would be entirely adequate for implementing the algorithm.
The goal would be to add enough heat when the oven is cold to quickly bring it up to temperature,
but not leave the heater on so long that the oven temperature overshoots and gets too hot.
Note that there will be a time lag between when the heater shuts off and when the
sensor near the quartz crystal sees the temperature stop rising.
This system could easily oscillate if the knobs are set wrong,
with the temperature alternating between too high and too low. 


> The PID loop would likely not be as beneficial in an AGC circuit when your neighbor down the street
> keys up his kilowatt with his beam pointed your way, at there is nothing gradual to anticipate.


In that case, you want the "D" knob turned way up, for a very fast AGC attack.
However, since the PID loop is a linear system, the AGC decay will be equally fast, and that is not what you want.

Jerry, KE7ER


On Fri, Apr 19, 2019 at 07:30 PM, Tom, wb6b wrote:
Many AGC circuit go even further by causing the AGC to cut in fast but restore the gain at a slower rate. 

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