Re: Confused about how AGC works?

Jerry Gaffke

Do a google search for "AGC  AVC"
Let me know if you find any hits that draw a distinction.
I only see entries that state they are synonyms.

My 2015 ARRL Handbook has no index entry for AVC,
Perhaps yours is an older edition?

AGC systems, oscillators, and PID algorithms are all closely related as feedback systems.
Learning about one can help understand the others.

Jerry, KE7ER

On Fri, Apr 19, 2019 at 08:16 PM, MadRadioModder wrote:

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”…



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