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## Re: freqz gives weird response

 From: Jurgen Defurne Subject: Re: freqz gives weird response Date: Thu, 29 Sep 2011 21:06:14 +0200

```On Thu, 29 Sep 2011 09:34:07 +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
> _______________________________________________
> Help-octave mailing list
> https://mailman.cae.wisc.edu/listinfo/help-octave

Mm, upon re-reading my post I see that there was an error in the
function, it actually is:

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

otherwise I don't have two poles!

Regards,

Jurgen

```