I agree that a narrower 8-pole filter would be very useful.
Meanwhile, those who are interested in a very narrow CW/DATA crystal
filter might want to consider our 5-pole model. This is one of the
filters Sherwood (an independent reviewer) used during his K3 receiver
testing. See the second to last column in his performance comparison
table:
http://www.sherweng.com/table.html
While it's true that the 5-pole 200-Hz filter is a bit narrower than
optimum for 170-Hz shift RTTY, many operators (including me) use it
for this all the time. The response shape is gaussian at the top, and
in practice the slight additional loss at the edges has little impact.
73,
Wayne
N6KR
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