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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
