> Some ops have mentioned problems with signals much lower than this,
> which has always baffled me.
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. W8FN explained some of the
constraints in terms of peak to average power of multiple CDMA transmit
signals and IMD ... the same applies to receive signals and the peak
voltage handling capability in the IF amplifier/ADC. Six to 10 S4-S5
signals can create a greater peak voltage (if just for the shortest
time) than a single S9+40 signal.
It's been 25 years since I was exposed to the math behind this (as part
of work in dealing with multiplexed power amplifiers for analog TV) but
it would be instructive to convert the average power levels of a bunch
of S4-S5 signals to P-P voltage, sum those P-P values and convert the
peak voltage back to power levels to see the true impact. Solid state
IF amplifiers and particularly DACs don't have the soft compression
characteristics we were used to with remote cut-off pentodes or even
dual gate MOSFETs in the analog receivers of previous generations.
73,
... Joe, W4TV
On 12/4/2011 8:50 PM, Wayne Burdick wrote:
> 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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