Posted by
gm3sek on
Dec 05, 2011; 9:12am
URL: http://elecraft.85.s1.nabble.com/Fw-CQ-WW-DX-Contest-CW-2011-Unofficial-claimed-scores-tp7060148p7062232.html
Wayne Burdick wrote:
>Joe Subich, W4TV wrote:
>
>> Remember, IMD is driven by total *peak* signal level. The peak signal
>> level is not simply the scalar sum of the levels ... it is a vector
>> sum as the individual signals add in phase.
>
>And those peaks could be slightly modulating the AGC loop.
>
Maybe that should say "modulating the AGC loops" (plural)? The HAGC loop
and the DSP AGC loop interact strongly - especially around the HAGC
threshold - and this creates the possibility for each loop to modulate
the other.
When the HAGC begins to act, we know that the firmware automatically
adjusts the DSP AGC to give consistent audio levels and correct S-meter
readings. The firmware monitors for HAGC action by digitizing the
voltage on the HAGC1 line, but there has to be some delay before it can
become "aware" of any change. The dynamics of this process create the
possibility for the HAGC loop and the DSP AGC loop to modulate each
other.
As far as I'm aware, this feature is unique to the K3, which may help to
explain the "rig-specific" nature of the problem. If this is a valid
model, the effects would predictably be worst around the HAGC threshold
where the response is inherently nonlinear. The dynamic interaction
between the two loops would be quite sensitive to the rate(s) of
amplitude variation in the composite incoming signal, as individual
components are keyed on and off. It would also be affected by added
noise, due to its dependency on the instantaneous vector sum of *all*
incoming signal, noise and interference components hitting the HAGC
detector (as identified by W4TV).
The mathematics of two cascaded feedback loops are way beyond me, but
the benefit of increasing the HAGC time constant by a very large factor
(10-20) sounds at lot like avoiding competition between the dominant
poles in the two loops. But the same model also explains why this
wouldn't be a complete cure.
This is all speculation, because much depends on hidden detail in the
DSP AGC firmware... but might it make some sense?
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek______________________________________________________________
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