Wayne Burdick wrote: >It *is* difficult to build very narrow 8-pole filters with low loss. >That's one reason Elecraft offers a 5-pole, 200-Hz unit. > >5 poles is entirely adequate at these narrow widths. Our 2-kHz IMD >dynamic range using the 200-Hz filter is outstanding. > Of course it is; but this discussion is about QRM at much closer spacings, sometimes occurring within the passband of a 350-400Hz filter.
It is also about obtaining the maximum possible performance from the K3 under conditions of extreme QRM, which is sometimes strong enough to make the hardware AGC flicker on and off... at which point, the K3 fares less well. Under these extreme conditions, the DSP needs every possible protection from a tight roofing filter. I do have to keep emphasizing that word "extreme". Many people will never experience such levels of QRM because they have the good sense to run away... but contesters need to stay and tough it out. >We've had some requests to offer a 270 or 300-Hz 5-pole filter, which >would be better optimized for narrow-shift RTTY than the 200-Hz filter. It was I who initiated that request, for precisely the reasons outlined above (and further back in the archives [1]). Wayne kindly provided suggestions for a modification to increase the bandwidth of the 200Hz filter to about 270Hz, in order to accept more of the FSK sidebands and allow a few Hz more tolerance for calling stations. That filter has proved to be a true winner [2]. However, contesters are always looking for an extra edge, and a true 250-270Hz 8-pole filter with a steeper transition from the passband into the stopband would be the next step to try. As Joe, W4TV so rightly says, it would fill a genuine unmet need. 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek [1] http://lists.contesting.com/pipermail/rtty/2009-August/029397.html [2] The modification is to change both C3 and C5 in the KFL3-200A to 830pF (680 + 150pf SMD, in parallel). The filter schematic is in Elecraft's K3 schematics file. Details of the wider passband are at: <http://www.ifwtech.co.uk/g3sek/misc/kfl3-200_mod260.gif> Although Wayne provided helpful advice, please note that this modification is not officially supported or recommended by Elecraft. -- ______________________________________________________________ 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
