200Hz vs 250 Hz filter
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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: [hidden email] 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 |
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BTW, all this discussion about what each filter's bandwidth actually is vs.
what it's name is, raises a related point. With the K3, the DSP bandwidth at which a given crystal filter kicks in can be set to the nearest 50Hz. For example, if your "500 Hz" filter actually measures 565Hz at the -6dB point, then you can tell the K3 it is 550Hz wide. Same for the center frequency. If yours is off by a few Hz, that can be cranked into the Fc parameter for the filter. 73, Ed - W0YK _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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This is another reason I prefer the 500 Hz. When you use DUAL PB at the minimum width setting, the K3 sets DSP WIDTH = 150 and selects whichever roofing filter is active at 600 Hz. Since the 500 filter is actually 565 Hz, I set my CONFIG Menu to 600 (the point at which the 500 engages). So my 500 Hz is used instead of the next higher roofing filter when DUAL PB is set to minimum. Of course you could also set a 400 Hz to be selected at 600, but then you would have a deadband for normal operation between its 435 Hz bandwidth and the next higher filter engagement at 650. 73, Bill |
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In reply to this post by Bill W4ZV
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: [hidden email] > 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: [hidden email] 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 |
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