From: Jens Tingleff
Subject: Re: What filer has equal ripple phase response, and where can I find the pole positions for a 4th order filter?
Date: 20 Jan 2003 01:24:06 -0800
Organization: Newsguy News Service [http://newsguy.com]
X-Newsreader: Direct Read News 4.20
In article , "Peter says...
>> Bessel filters have maximally-flat phase response, but what filter is a
>> general case, with equi-ripple phase response.
>> I'm asking because I would need to use an all pass network based on 6th
>> Bessel poles (plus the 6 matching zeros of course) whereas I would hope I
>> get by with 4th order if I relax the spec to equi-ripple phase (say +/-3
>> Tony (remove "_" from email address to reply)
>Although such filters are indeed feasible [low pass with equiripple group
>are no "standard" approximations or common tabulations of such filters.
>Your only practical recourse is to select or design an appropriate low pass
>then to approximate and synthesize an all-pass delay equalizer to place in
>with the filter. The all-pass delay equalizer would be designed, say using
>exchange algorithm, to equalize the group delay of the filter-equalizer
>cascade to be
>a prescribed constant plus or minus the prescribed equiripple amount.
Two references I like to quote. Involves different circuit structures, though.
B J Bennet. A new filter synthesis technique the hourglass. IEEE Transactions
CAS, 35(12):1469 1477, December 1988.
D R Huard, J Andersen, and R G Hove. Linear phase analog filter de-sign with
arbitrary stopband zeros. In Proceedings of the International Symposium on
Circuits and Systems, pages 839 842, 1992.
[Doesn't someone want to know how to make one of those filters on an Integrated
Key ID 0x09723C12, email@example.comfirstname.lastname@example.org
Analogue filtering / HIPERLAN / Mdk Linux / odds and ends
http://www.imaginet.fr/~jensting/ +44 1223 211 585
"I don't think you *can* dig your way off a planet.." D Adams