Re: Confused about how AGC works?

Jack, W8TEE

Yep...seems to be a lot of confusion on this point...

Jack, W8TEE

On Friday, April 19, 2019, 2:21:43 PM EDT, MadRadioModder <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.





From: [] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Friday, April 19, 2019 11:14 AM
Subject: Re: [BITX20] Confused about how AGC works?


Here's my shot at a quick summary regarding AGC circuits for the Bitx40 and uBitx
Opinions may vary.

When tuning across the band with a stock radio, you might run across a weak station
and turn the volume up to hear it.  Then a local QRO station responds and
you have to rip the headphones off to protect your ears, or be very quick in reaching
for that volume control.  An AGC circuit (automatic gain control)  turns the volume down
for you when a strong station shows up like that.

This is an early version of Don's (ND6T) AGC circuit:
Still viable, works well enough, simple and easy to build from scratch.
Really strong local signals can still be annoyingly loud, though much less so.

Don refined that FET shunt attenuator design by adding a FET series attenuator
to get additional dynamic range, keeps even the strongest stations down in volume.
It is a bit more complicated, but performs better.
Was kitted up by Kees (no longer available),
and is now available from

Either of the above AGC circuits will work on the Bitx40 or the uBitx.

Don's design attenuates the RF signal as it comes in from the antenna,
using a control signal derived from the audio. 

Most of the other AGC designs presented in this forum (there have been a
half dozen or so) attenuate the audio after the audio pre-amp, simple and
works well enough to protect the ears, and you avoid the tricky business of 
attenuating the RF path.  Here's a very early example of this approach:

Attenuating the RF path is preferred, since the stock receiver dynamic range
is limited primarily by that audio pre-amp, and also to some extent by the IF amps
and mixers.  Don's design reduces the signal right where it comes in from the
antenna jack, so nothing in the radio will get overloaded and cause distortion.  

So an AGC design with attenuation in the audio stage works to protect the ears,
but strong signals may be distorted enough to be unintelligible.  With RF 
attenuation, strong signals remain undistorted.

All successful AGC circuit designs presented in the forum thus far detect the
incoming signal level just before the volume control.  Signal detection must occur
after the 12mhz crystal filter, otherwise we would also detect strong nearby stations
that the crystal filter rejects.  Detection of audio is not quite ideal since it takes time for
the signal to arrive there, so there will be a brief pop when a strong signal shows up
before it is properly attenuated.  But detection of 12mhz RF between the crystal filter and the
demodulator is difficult, because the unshielded 12mhz BFO on CLK0 will sneak into
our AGC signal detector and look like a very strong signal.

Here's an example of a well regarded AGC scheme for a high performance receiver:
It's about as complicated as an entire uBitx receiver.

Jerry, KE7ER

On Thu, Apr 18, 2019 at 06:43 PM, Curt wrote:

check this out - this is a similar version of the same circuit

the main idea is sampling the audio, feeding the transistor to boost this signal.  the 2 diodes develop a rectified signal based upon the audio, and this is used to bias a MOSFET used as a variable resistor. 



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