I agree with this 100%. I suspect that the only reason Elecraft is offering the 8-pole filters is that it is easier just to let people buy them than to try to explain why they don't need them. :=)
Al N1AL On Tue, 2008-02-05 at 03:50, Bill Tippett wrote: > PA3CEV: > > >On 40 the band is very busy over here in Europe. Since my CW skills > are not the best, a good filter makes it easier for me to "decode" > Morse in a crowded band. Now I have a 400 8 pole and I think a 200 Hz > 5 pole or 250 Hz 8 pole will improve the rx and possibly take some > workload of the DSP? > > There is very little bandwidth difference between the > 400 and 250 8-pole filters: > > Filter BW(-6dB) Shape Factor > 200 224 4.0 (BTW my 200 is about 210 Hz) > 250 370 2.1 > 400 435 2.1 > 500 565 3.1 > 1000 1063 1.6 > > I chose my filters so there was an octave difference > in bandwidth between them (e.g. 500 and 200 in your case). > > There is also virtually no difference in 2 kHz IMD performance: > > Filter 20kHz 10kHz 5kHz 2kHz > 200 Hz, 5 pole 100+ 100+ 100+ 95 > 250 Hz, 8 pole 100+ 100+ 100+ 95 > 400 Hz, 8 pole 100+ 100+ 100+ 95 > 500 Hz, 5 pole 100+ 100+ 100+ 94 > 1 kHz, 8 pole 100+ 100+ 100 94 > > W2VJN of Inrad says the area of interest for roofing > filters is -30 dB on the skirts and above. Shape factor below that > point is not critical. From measurements by OH9NB on the 5-pole > 200 and by Elecraft on the 8-pole 250, it appears both have about > the same -30 dB bandwidth (about 500 Hz). So I would choose the > 200 over the 250 because: > > 1. It has a narrower -6 dB bandwidth (224 vs 370 above). > 2. It has similar -30 dB bandwidth (both about 500 Hz). > 3. It is less expensive. > > >But which one to choose, how does one weigh the importance of steeper > skirts against a filter with less bandwidth? > > Answered above...shape factor is not critical below -30 dB. > > >Why doesn't a 200 Hz 8 pole exist, is there a problem in group > delay time, or ringing? > > Probably all of the above! Remember that Orion shares a > very similar front-end architecture (down conversion, 9 MHz 1st IF > and narrow roofing filters). Ten-Tec filters are all 4-pole! There > is NO good reason to use 8-pole filters in rigs like Orion or the > K3 if there is a 4/5-pole available for your desired bandwidth. I > have all 5-poles in mine except for the 1.0k which had no 5-pole > alternative. > > There is also no good reason to use gold-plated speaker > wire but if it makes you feel good and you want to spend extra money, > please do so...Elecraft and the filter manufacturers will always be > happy to take your money. ;-) > > 73, Bill W4ZV > > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
