Re: Low pass filter, cut off 500 Hz, short response time...
bill.sloman_at_ieee.org
Date: 02/11/05
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Date: 11 Feb 2005 06:35:38 -0800
Viram wrote:
> I'm trying to design a LP-filter with a cut off at around 500 Hz. My
> problem is getting a short response time and low/no resonance peak.
What
> order should the filter be? I'm currently trying with a simple
> LC-filter...
You need to get hold of a decent text on filter design - I use the
"Electronic Filter Design Handbook" by Arthur B. Williams and Fred J.
Taylor - ISBN 0-07-070434-1. Since then there has been a third edition
(ISBN 0-07-070430-9) which is supposed to be even better, but it seems
to be out of print - www.amazon.com offers three second hand copies for
about $130. You should be able to find it - or something very like it -
in a university library.
The usual choice for what you seem to want to be doing is a
"linear-phase" or "Bessel filter", though a "synchronously tuned"
filter (consisting of identical multiple poles) actually gives fastest
settling for a given number of poles, with the step getting to better
than 99% after five time constants. Adding more poles makes the step
steeper at the 50% point, and pushes the 50% point back from around one
time constant at one pole to closer to three at ten poles.
A simple LCR filter is two poles ...
---------
Bill Sloman, Nijmegen
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