On May 13, 2007, at 11:18 PM, John, KI6WX wrote:
Don;
Don't forget Bessel filters. These are maximally flat for group
delay, but have a much slower rolloff.
A 5-pole 500-Hz 0.5 dB Chebyshev will be down 80 dB at 1 kHz and
will have a group delay of 16 ms at band center and 44 ms at -3
dB. A 5-pole 500-Hz Bessel filter will be down 40 dB at 1 kHz and
have a constant group delay across the passband of 10 ms.
You can have a flat group delay or a fast rolloff, but you can't
have both.
Right. TANSTAAFL (there ain't no such thing as a free lunch). Given
that we can get very steep skirts in DSP at the expense of
introducing more overall latency (delay) in the filter (usually not a
problem for what we are doing), then we can probably accept analog
roofing filters that don't provide the same ultimate attenuation in
the skirts.
Given the choice between the two filters I would probably opt for the
Bessel filter myself.
So, Elecraft, will you be offering a group of roofing filters
optimized for digital communications? How about your "brick wall"
filters in DSP?
73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
