Wayne, For those of us ordering the cap through Mouser or DigiKey, can you reply with the SMD/SMT cap dimension code (e.g., 0603, 1608, etc.)
Tnx! Paul, W9AC ----- Original Message ----- From: "Wayne Burdick" <n...@elecraft.com> To: "Elecraft Reflector" <elecraft@mailman.qth.net> Sent: Sunday, December 04, 2011 8:50 PM Subject: [Elecraft] K3 hardware AGC experiment shows promise > 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 ______________________________________________________________ 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
