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
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