Yup, I knew that would cause confusion. That's NOT what I meant. I meant that
if you listen to ONE signal at a time, their apparent amplitudes do not reflect
reality, because of the AGC. If BOTH signals are present in the passband at the
same time, then my comments DO NOT apply. I was ONLY talking about hearing ONE
signal AT A TIME.
NONE of my discussion EVER presupposed more than ONE signal being received at a
time.
THANK YOU. :^)
I am beginning to believe that it's impossible to shed any more light on this
topic. It continues to be one of the most misunderstood areas of all, and I'm
not sure if any effort can clear up the confusion without adding more confusion
of its own. At least, I know I've failed! The next time someone asks a question
about AGC, I'm going to chomp down on a towel, duct tape my arms to the chair,
and start my breathing exercises: deep breath.... hold..... exhale.....
again.....
Al W6LX
________________________________
From: Wes Stewart <wes_n...@triconet.org>
To: elecraft@mailman.qth.net
Sent: Monday, March 6, 2017 10:19 AM
Subject: Re: [Elecraft] [K3] AGC White Paper
In Al's very nicely done paper he has this paragraph:
"The purpose of Automatic Gain Control (AGC) is to reduce the range of the
signals seen by the sensitive stages in the receiver. The AGC stage is
designed to vary its gain depending on the input signal; stronger signals
get less gain, and this has the effect of compressing the amplitude range.
This is the desired response."
I believe this paragraph and the accompanying graphic can be misleading to the
unwary. AGC does not compress the range of signals, it simply lowers the gain
through the receiver. The range (difference between) signals might well be 130
dB at the input but it better be 130 dB everywhere else in the receiver too. If
lower level signals are driven into the internal noise level because of gain
reduction, so be it; that should be the only reduction in range.
Al continues:
"But signals above this threshold will be acted on by the AGC. Even though
in real life an S9 signal is 5 S-units stronger than an S4 signal, because
of the AGC it will sound only 11.1 dB louder – less than 2 S-units louder.
This is because, reading from the above graph, an S4 (-103 dBm) signal
produces -15.3 dBV of audio output and an S9 (-73 dBm) signal produces -4.2
dBV, a difference of 11.1 dB. A five S-unit difference has been reduced to a
less-than-two S-unit difference."
Some are going to read this and mistakenly believe that while receiving both an
S9 signal and an S4 signal, AGC is going to reduce the ratio between them from
5
S-units to two S-units.
Hence, I suspect that "mush" the proponents believe that AGC somehow magically
reduces the amplitude of the stronger signal that activates the AGC but allows
the weaker ones to have full gain, so that they "catch up" and become
indistinguishable from the stronger one. That's a limiting receiver, nice for
FM, not so nice for CW/SSB.
All IMHO, of course,
Wes N7WS
On 3/5/2017 10:50 PM, Al Lorona wrote:
> I spent a little bit of time this weekend and put together yet another K3 AGC
> (YAKA) "white paper" to put some measurements and discussion of the results
> down in the hope that others may benefit. Maybe it'll help folks understand
> AGC better by demystifying some of the K3 idiosyncrasies. I hope it makes you
> think about a few things in a way that maybe never occurred to you. I put the
> document in my Dropbox at the following link:
>
>
> https://www.dropbox.com/s/drfujqupr4pcet3/Getting%20the%20Most%20Out%20of%20K3%20AGC%20System.pdf?dl=0
>
>
> I'll leave the file there for several days in case any of y'all are
> interested.
>
> 73,
>
> Al W6LX
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