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From: Rune Allnor <allnor@tele.ntnu.no>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Obtaining the frequency response from a digital filter
Date: Sun, 8 Nov 2009 03:07:12 -0800 (PST)
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On 8 Nov, 11:18, "Ana " <filter_wo...@yahoo.com> wrote:
> Hi there,
>
> I am converting from analog to digital filter, a butterworth using the bilinear transformation. I am obtaining the num and the den of the digital function, but when am plotting using freqz the response is not as desired. don t know if i am doing something wrong prior to freqz or that freqz is not the right command to plot for digital filters.
...

Designing discrete-time filters from analog prototypes
via the BLT is a somewhat involved algorithm, that requires
some 8-10 individual steps. Get any one of those steps wrong,
and you will be lost. Your code above only hints at a couple
of those steps, so there is no wonder why you don't get the
required result.

The algorithm for designing filters is beyond the scope of
USENET, so pick up a book on digital signal processin (or take
a class) to learn.

> (ps. can i use this method to transform ANY other filter, like the  chebyshev, elliptic, thompson etc.... THANKS ALOT!!!)

The BLT is the core of a design method that can use several
analog IIRs as prototype. I know it can be used for the
Butterworth, Chebychev types I and II, elliptic filters;
maybe it works with other types of analog filters as well.
I don't know.

But again: You need to know what you are doing - and get
every single detail right! - to get the method to work.

Rune