W4TV:
>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.
I agree.
>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.
Even any IMD benefit becomes very marginal. When
dealing with interfering signals at each edge of the
filter, the practical difference is 1/2 the -30 dB
bandwidth...i.e. the 500 Hz will adequately reject a
signal at 1092/2 = 546 Hz versus the 400 at 873/2 = 436 Hz
or a difference of only ~100 Hz from the center
frequency...which could easily be taken care of in
extreme cases by using SHIFT.
73, Bill W4ZV
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