Posted by
wayne burdick on
Dec 05, 2011; 1:50am
URL: http://elecraft.85.s1.nabble.com/Fw-CQ-WW-DX-Contest-CW-2011-Unofficial-claimed-scores-tp7060148p7061538.html
Hi all,
The K3's hardware AGC time constant is a compromise between recovery
time and IMD due to modulation of the loop. C238, on the bottom of the
RF board near the front, sets this time constant.
We did a little experiment today (thanks to Tree, N6TR) that suggests
increasing the size of C238 substantially might be a worthy change. In
the case of signals just large enough to tickle the hardware AGC, the
first IMD products were reduced by something like 18 dB. This will
also increase the recovery time for very strong signals, so the jury
is out on whether this is OK for the average user.
For the experimentally inclined: C238 is easy to get to; just remove
bottom cover A (the front half). C238 is a large-ish surface mount
capacitor nestled between two 20-pin connectors. The present value is
0.1 uF. Tree tacked a 1-uF cap on top of it. Then he tacked another
one on, which improved things by another few dB.
Some ops have mentioned problems with signals much lower than this,
which has always baffled me. But I got to thinking: Suppose you're
listening to a bunch of S4-S5 signals in your DSP passband. You could
have larger signals outside the DSP, but inside the crystal filter. Or
you could have clicks from strong signals that get inside the crystal
filter but you can't hear because you're using a narrow DSP filter. Or
you could have noise spikes. Any of these could ping the hardware AGC
just enough to cause IMD between all of the signals in the passband.
My point is that increasing the loop time constant could have a more
general benefit when a band is busy and/or noisy.
Let me know if you try this and whether the results are of interest.
(I live in an RF-free zone, it seems, so I can never recreate the
problem here. Frustrating!)
73,
Wayne
N6KR
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