On Jun 26, 3:32*am, Paul Keinanen wrote:
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 ?

I kind-of have the same questions too. For a homebrew project, having
all the software-designed-radio complexity in addition to the tight-
analog-filtering-from-DC-to-daylight complexity seems to just... make
everything too complicated and not fun anymore.

But the best ham receivers couple impressive front ends with
effectively tight roofing filters with SDR aspects effectively (and
quite usably) and I can see why someone would want to try their hands
at their own competitive design. But, wow, it's a lot of effort. And a
lot of ham receivers - especially the first and second generation
designs - combined all these technologies into radios that are
actually painfully complicated to use. (When the QST review starts
contrasting menu option 73 submode 4 with menu option 105 submode 13,
that's a real turn-off to me. At the same time, other younger
operators just love that sort of complexity!)

On the other hand, a truly simple analog front end (e.g. Softrock)
combined with a computer is a hell of a lot of fun. You spend a lot
more time looking at a computer screen and less listening but that's
what some like.

Tim N3QE.