Re: Confused about how AGC works?

Jerry Gaffke

"Automatic gain control (AGC), also called automatic volume control (AVC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, ..."
A google search for "AGC vs AVC" shows they are considered synonomous in radio design (but not for power grids), and that AGC is the more modern term.

In this forum, all such circuits for receivers have been getting referred to as AGC.
For some reason, the term AVC has only been used in this forum for automatic level control while transmitting
(Except for these posts from MRM.)
Works well for me, as few of us care that much about AVC while transmitting.
I suggest we keep it that way. 

Anyways, regarding the various AGC schemes:

There is this AGC kit from Sunil, it detects audio, and uses that to somehow attenuate the signal in one of the IF amps.
Unfortunately, nobody seems to know how to hook it up:
I'm sure it's easy enough to resolve with a quick email to Sunil (he monitors this forum).
And is definitely a contender, especially if you are already ordering one of the excellent Bitx40/uBitx enclosures from Sunil.

There have been several other AGC kits made available.
Let us know if any are still active, other than Sunil's and the one from

An AGC scheme that attenuates audio is a perfectly fine solution.
As stated in my previous post, one that attenuates RF (or IF energy) instead of audio
can give better dynamic range, but is in some ways more complicated and error prone.
An audio only AGC scheme could be mounted on the back of the volume pot, with the only
additional wire required being one to supply power.  No worries about coax and such. 
If you never hear stations so strong that they are still distorted after you turn down the volume knob
on your stock uBitx, you don't need the extra dynamic range offered by an RF (or IF) attenuated AGC.

And if you occasionally do, that could be taken care of with a manual RF gain control,
which could be just a 1k pot in the received RF from the antenna.

Jerry, KE7ER

On Fri, Apr 19, 2019 at 11:21 AM, MadRadioModder wrote:

As discussed before…


“An AGC circuit (automatic gain control)  turns the volume down for you when a strong station shows up like that.”


That’s known as AVC or “Automatic Volume Control”… not necessarily AGC.



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