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Paul Keinanen · June 26th 08, 09:32 AM posted to rec.radio.amateur.homebrew

On Wed, 25 Jun 2008 11:14:03 -0700 (PDT), wrote:

On Jun 25, 9:45*am, Tim Shoppa wrote:
I have been playing with homebrew crystal filters (following W7ZOI and
Bill Carver/K6OLG) for CW, as well as audio filters, and can tell you
that on CW the difference between a super-sharp-in-frequency
Chesbyshev filter (typical in ham equipment for a long time now) and a
more constant-delay (e.g. Gaussian to 6dB or 12dB, or equiripple
linear phase) filter is like night and day.

You wouldn't happen to know the group delay variance
of the filters you mentioned? Rough values are okay.

I notice you ask about a lot of digital modes but not CW. My ears have
been listening to CW for 30-some years now and I can do a lot of
processing in my brain. But what my brain cannot remove is horrible
filter ringing. I don't know how those other digital modes stack up...
maybe computers are better at removing horrible ringing than my brain.

I believe that in theory if the exact group delay profile is known,
then
a digital receiver can perform a certain amount of equalization. What
I'm curious about is how much varience can be introduced by the
filters in a receiver for various transmission types without needing
to equalize.

Excuse my ignorance, but why on earth do you do some crude analog
filtering and then continue with digital filtering, in which you have
much more alternatives ?

The only reason that I can think about using sharp IF crystal filters
is that the dynamic range of the following stages (product detector
and ADC) is not sufficient.

In a typical general coverage up converting receiver, the roofing
filter will define the bandwidth the ADC must handle. Also some gain
control (not necessary automatic) is needed to set the band noise well
below one LSB (LF/MF vs VHF/UHF and antenna efficiency on LF).

Even when designing an add-on unit for audio processing, why would
anyone use the receiver CW filters apart from dynamic range issues ?

Paul OH3LWR

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