On Tue, 2007-06-19 at 14:51 -0400, Bill Cotter wrote:
> Jerry,
>
> Taking your point regarding filter response shape (ideally, rounded
> edges, steep slopes, and flat passband), and looking at the
> response curves for the Inrad CW filters, they look pretty incredible.
>
> http://www.qth.com/inrad/graphs/759.gif
>
> http://www.qth.com/inrad/graphs/750.gif
>
> http://www.qth.com/inrad/graphs/763.gif
>
> I don't know if these are actual plots of sampled 400Hz filters, or
> 'marketing' plots. But, using these filters, I can say that they
> are very, very good for weak signal listening in noisy conditions.
> They don't have the incessant roaring (noise power) in the passband
> like the TT 500Hz filter.
>
> 73 bill n4lg
>
Yes, they do look nicely rounded, no sharp corners, no vertical slopes,
no close in notches, all of which cuts way back on ringing. And it only
took 30 years of fussing about ringing filters to make them come about.
That's a failing of the Collins S-line with any summertime static about,
it folds up because of filter ringing while other receivers without the
sharp corners continue to copy. The difference between a Tchebychev and
Gaussian amplitude response can be very dramatic when hit by short
spikes, even with the same set of crystals.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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