Re: freqz gives weird response

[Jurgen Defurne]

On Thu, 29 Sep 2011 09:34:07 +0200
Jurgen Defurne <address@hidden> wrote:

> 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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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