[Elecraft] [K3] AGC White Paper

[Don Wilhelm]

Wes is correct.  Let me say it in a slightly different way.
The AGC simply reduces the gain of the receiver - and it will respond 
mainly to the strongest signal in the passband.  Actually it responds to 
the integral of all the signals in the passband, but if there is a 
single stronger one it will predominate.

When the receiver gain is reduced, ALL the signals will be 
proportionally reduced in amplitude - including the weaker ones.  This 
is often referred to as "AGC Pumping".

73,
Don W3FPR



On 3/6/2017 1:19 PM, Wes Stewart wrote:
> 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.
>