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From: "Peter O. Brackett"
Subject: Re: Asymmetrical frequency response of lumped element bandpass filters
Date: Mon, 14 Oct 2002 20:47:51 -0400
Organization: MindSpring Enterprises
X-Server-Date: 15 Oct 2002 00:47:42 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2919.6600
Your observation is true of bandpass filters derived from low pass
prototypes through the *classic* low pass to band pass transformation and of
certain other kinds of band pass filters. But...
your observation about band pass filters is simply not true in general.
It is entirely possible to design lumped element bandpass filters of
arbitrary prescribed symmetry.
To do so one has to start at the "approximation" stage in the band pass
domain and do the complete filter design/syntheses directly in the band pass
domain without using any symmetry affecting transformations.
The whole technique is outlined fully in several classic textbooks on filter
design. See for instance:
A. S. Sedra and P. O. Brackett
"Filter Theory and Design: Active and Passive"
Matrix Publishers, Portland, OR 1978.
You should find this book shelved in your local technical library at:
LC Shelf Call: TK7872.F5S42 -or-
Most advanced computer programs for filter design can be used to design
bandpass filters in this way. The program Filtor developed by myself and
several others at the University of Toronto and illustrated by some designs
in the above book is one example of such a program.
Indialantic By-the-Sea, FL.
"The Technical Manager" wrote in message
> Why is it that lumped element LC bandpass filters have an asymmetrical
> frequency response where the slope between the passband and stopband on
> the high frequency side is less steep than the slope on the low
> frequency side ? Filters constructed from distributed elements normally
> have perfectly symmetrical frequency responses.
> Is it something to do with the admittance vs frequency response of
> distributed elements being periodic with frequency whereas the
> admittance vs frequency response of an LC resonator isn't periodic with
> frequency and exhibits an admittance of either zero or infinite ohms -
> depending on whether it is parallel or series LC circuit - only at
> DC and infinity Hz ?
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