Posted by
Joe Subich, W4TV on
Dec 11, 2007; 4:03pm
URL: http://elecraft.85.s1.nabble.com/FILTER-SETTINGS-tp456756p456757.html
Bill,
> >By this simple calculation (or by plotting on graph paper) one
> can see that the 200 Hz filter should do a BETTER job as the
> most "narrow" filter. It remains "tighter" to about -35 dB
> and the five pole design should result in less pulse stretching
> (ringing in the presence of static) than the 250 (really 350 Hz)
> filter.
>
> Excellent post Joe (as usual). Another reason one
> might want the 200 versus the 250 is the relative difference
> between the 400 and 500 filters. From the published data:
>
> Filter BW(-6dB) Shape Factor
> 200 224 4.0
> 250 370 2.1
> 400 435 2.1
> 500 565 3.1
>
> Practically there is little BW difference in the 250 and
> 400 8-pole filters. If one wanted both a very narrow
> and moderately narrow filter, I would choose the 500
> and 200 5-pole filters. They have at least an octave
> difference in total bandwidth...plus the group delay
> issues you cited in your last paragraph above.
Expanding the 200/250 analysis to 200/250/400/500 as I did
privately for someone else:
200 250 400 500
-------------------------------------------------
- 6dB 224 370 435 565 Hz
-60dB 896 777 913 1751 Hz
slope 6.22 3.77 4.43 10.98 Hz/dB
-10dB 274 400 470 653 Hz
-20dB 398 475 559 873 Hz
-30dB 522 550 647 1092 Hz
-40dB 647 626 736 1312 Hz
-50dB 771 702 825 1531 Hz
I really think the 200/500 pair is the best value. The
only possible benefit of the 400 over the 500 would be
an improvement of 500 Hz spaced IMD. However, as you
have pointed out, with strong interfering signals at
500Hz and 1 KHz, the problem is far more likely to be
radiated phase noise and clicks from those signals than
receiver generated IMD.
I'm looking forward to information on the variable CW
filter, something like a 400 Hz to 1 KHz range would
be ideal in place of either the 400 or 500 Hz filter.
73,
... Joe, W4TV
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