# Phase delay VS frecuency for IIR Butterworth

Started by November 17, 2008
```Hi Everyone!
I was hoping that someone could explain to me how I can calculate the
phase delay of an IIR filter.
I haven't found a way for doing it from H(z)...
Or if it is not possible, which would be in your oppinion the smartest way
to get it from H(s)? I have no code written for calculating poles and
zeroes in my program and would like to avoid doing it if possible...
Well, thank you very much for your attention and cheers!!

```
```On Nov 17, 11:36&#2013266080;am, "Calculadora" <pablo.martinez.lina...@gmail.com>
wrote:
> Hi Everyone!
> I was hoping that someone could explain to me how I can calculate the
> phase delay of an IIR filter.
> I haven't found a way for doing it from H(z)...

1. evaluate H(z) at z = e^(j*w).

2. compute the angle or arg{ H(e^(J*w)) }

3. add or subtract 2*pi at places where this angle has jump
discontinuities of greater than pi in magnitude.  this is called
"unwrapping the phase".

4.  multiply by -1.

5.  divide that function by w.  (5a.  for group delay differentiate it
w.r.t. w instead of dividing by w.)

now you're done.

r b-j

```