[Elecraft] [K3] AGC White Paper

[Wes Stewart]

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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