[Elecraft] [K3] AGC White Paper

[Joe Subich, W4TV]

On 3/6/2017 2:29 PM, Don Wilhelm wrote:
> Actually it responds to the integral of all the signals in the
> passband, but if there is a single stronger one it will predominate.

In truth, depending on the AGC *attack time constant* the AGC system
responds to the *instantaneous peak voltage* of all signals in the
passband.  Even for two signals where one signal is *only* 10 dB
stronger than the other, the *instantaneous peak voltage* can be as
much as 20 dB greater than the stronger signal alone.

With multiple signals, e.g. a large pile-up, and [relatively] slow
attack times, the instantaneous peak voltage can drive the ADC into
clipping before the AGC can react while with very fast attack and
decay times AGC can "pump" the receiver gain at a syllabic rate and
further "muddy" the recovered audio.

Steady state (single signal, single tone) measurements can show the
general AGC behavior but it is still an "art" to find the best
combination of attack time, decay time, threshold, slope (gain
reduction) above the threshold, "hold" times and *overall gain*
to keep peak signal voltages below the ADC clipping point and
simultaneously keeping the AGC response from "pumping" receiver
gain in such a way as to generate IMD (mush).

The key here is the *DYNAMIC* response of the AGC system.

73,

    ... Joe, W4TV


On 3/6/2017 2:29 PM, Don Wilhelm wrote:
> 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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