Re: Confused about how AGC works?


I tend to use the definition of AGC from the Radio Amateurs Handbook and other IEEE sources that pertain to the Radio-Electronics industry specifically rather than Wiki… which is typically diminished science for the masses from random writers who may or may not actually understand the topic.  The Wiki article you refer to discusses AGC applied to Audio systems, Power systems (generators), Process Control systems (like PID controllers), and even biological systems. While a tiny bit of this definition is applicable to radio systems (feedback with variable gain in some form), it’s by no means specific or useful.  Historically (that being from the late 1930’s and early 40’s when the first patents for AGC were granted), AGC was defined as a closed-loop system that provides attenuation to one or more RF-IF gain stages of a receiver.  The goal was to keep the output level of the receiver constant and prevent gain stages from saturating at high RF front-end input while increasing gain for weak (low voltage) signals yet irrespective if the input.  The underlying algorithms to do this generally consist of signal measurements in the IR-RF chain and post detector, filter and integrator for response shape, to be fed back to a variable attenuator of some type.  Our ears are logarithmic in response (a second outcome of Fletcher-Munson) and since we can (and do often) define signal strength as logarithmic voltage, the feedback algorithm can be linear or semi-linear (logarithmic).  This is called Input-VGA or IVGA (attenuator-amplifier) as opposed to Output-VGA (which is fixed gain amplifier followed by a passive attenuator).   I believe Zenith held one of the first patents on AGC, but it was contested early and proliferated by others while in litigation… and it was evident on a myriad of AM receivers starting in the 50’s and forward.  This is what I know with 45+ years of designing AGC circuits for commercial and military receivers.


I don’t know much about AVC other than it was used in the audio industry and proliferated there for recording and delivery (dBX had a compression form of it).


I have no idea what “Transmit AVC is”…



From: [] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Friday, April 19, 2019 1:58 PM
Subject: Re: [BITX20] Confused about how AGC works?


"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. 




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