[Elecraft] 200Hz vs 250 Hz filter

[Alan Bloom]

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 
> 
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