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AW: Bessel-filter problems

From: Bernd Heinze
Subject: AW: Bessel-filter problems
Date: Tue, 26 Apr 2011 21:10:36 +0100 (BST)

Hello Christian,

thanks for your reply. Unfortunately I'm not an filter expert, I played around 
with Diadem and decided that a Bessel filter with said parameters gives me the 
best results. I assumed if it works in Diadem it will also work in Octave.

> But are you sure that you really want to construct such a filter? Your ratio
> between corner frequencies and the Nyquist frequency (2.5 GHz ???) is quite
> high. Throw in a filter order of 10 and you will get a filter with
> coefficients that are numerically ill-behaved.

The data is taken from an oscilloscope which has a sampling rate of 5 GHz. As 
my signal contains many high frequencies this sampling rate is necessary. 
(Although it might be possible to reduce it by a factor of 2 or 4)

> Constructing a butterworth filter from your data shows what I mean: The
> resulting coefficients range from 1 to 1e5, i.e. they differ in scale by
> about 17 bits.

I assume this means that by using the filter I might get larger "errors" or 
imprecissions in my signal?


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