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.
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