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freqz gives weird response


From: Jurgen Defurne
Subject: freqz gives weird response
Date: Thu, 29 Sep 2011 09:43:02 +0200

Dear all,

I am busy learning poles and zeros, and I have the following function:

1((1 - lz^-1)(1 - l*z^-1))

where l and l* are a conjugate pole pair.

If I create two column vectors B and A:
B       A
0       0.5+0.5i
        0.5-0.5i

and use zplane(B,A) I get what I expect.

However, when I use freqz, instead of a low-pass filter
response I get a resonator response with one sharp peak.

I do not (really) think that freqz is incorrect, but more
that it is not really clear how to go about using zfreq.

I added some small sample code. 

B = [ 0 ];
A = [ 0.5 - 0.5i;0.5 + 0.5i];

figure;
zplane(B,A);

figure;
freqz(B,A);

Recreating this pole/zero plot with a small program call IIR
Explorer, I get a low-pass function.

Regards,

Jurgen


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