# How to get a segment of the magtitude response of a filter?

Started by June 15, 2009
```Hi,
I am designing a decimation multirate filter with CIC and the
decimated rate is very large . It could be larger than 1,000,000. The
difficult I face is that when the decimated rate is too large , I
can't view the details of the magnitude response of the passpand. In
order to get the details of the magnitude response , the freqz
function in matlab need to carry out a FFT with millions of points .
It's beyond the capability of my PC. I am thinking whether there is
some methods , which can get needed segment of the magnitude response
Best Regards.
```
```On Sun, 14 Jun 2009 20:30:06 -0700, agump wrote:

> Hi,
>      I am designing a decimation multirate filter with CIC and the
> decimated rate is very large . It could be larger than 1,000,000. The
> difficult I face is that when the decimated rate is too large , I can't
> view the details of the magnitude response of the passpand. In order to
> get the details of the magnitude response , the freqz function in matlab
> need to carry out a FFT with millions of points . It's beyond the
> capability of my PC. I am thinking whether there is some methods , which
> can get needed segment of the magnitude response with relatively small

Try doing it symbolically, with a pencil and paper.  A CIC filter has to
be just about the easiest 'real' filter out there to do the math on.

--
www.wescottdesign.com
```
```On 6&#26376;15&#26085;, &#19979;&#21320;12&#26102;51&#20998;, Tim Wescott <t...@seemywebsite.com> wrote:
> On Sun, 14 Jun 2009 20:30:06 -0700, agump wrote:
> > Hi,
> > &nbsp; &nbsp; &nbsp;I am designing a decimation multirate filter with CIC and the
> > decimated rate is very large . It could be larger than 1,000,000. The
> > difficult I face is that when the decimated rate is too large , I can't
> > view the details of the magnitude response of the passpand. In order to
> > get the details of the magnitude response , the freqz function in matlab
> > need to carry out a FFT with millions of points . It's beyond the
> > capability of my PC. I am thinking whether there is some methods , which
> > can get needed segment of the magnitude response with relatively small
>
> Try doing it symbolically, with a pencil and paper. &nbsp;A CIC filter has to
> be just about the easiest 'real' filter out there to do the math on.
>
> --www.wescottdesign.com

Hi , Tim,
Maybe I forget to mention that I need to put a FIR behind the
CIC . The difficult focuses on analyzing the overall frequency
Best Regards.
```
```On Jun 15, 3:44&nbsp;am, agump <agump...@gmail.com> wrote:
> On 6&#26376;15&#26085;, &#19979;&#21320;12&#26102;51&#20998;, Tim Wescott <t...@seemywebsite.com> wrote:
>
>
>
>
>
> > On Sun, 14 Jun 2009 20:30:06 -0700, agump wrote:
> > > Hi,
> > > &nbsp; &nbsp; &nbsp;I am designing a decimation multirate filter with CIC and the
> > > decimated rate is very large . It could be larger than 1,000,000. The
> > > difficult I face is that when the decimated rate is too large , I can't
> > > view the details of the magnitude response of the passpand. In order to
> > > get the details of the magnitude response , the freqz function in matlab
> > > need to carry out a FFT with millions of points . It's beyond the
> > > capability of my PC. I am thinking whether there is some methods , which
> > > can get needed segment of the magnitude response with relatively small
> > > calculation. Please give some advise , thanks. Best Regards.
>
> > Try doing it symbolically, with a pencil and paper. &nbsp;A CIC filter has to
> > be just about the easiest 'real' filter out there to do the math on.
>
> > --www.wescottdesign.com
>
> Hi , Tim,
> &nbsp; &nbsp; &nbsp; &nbsp; Maybe I forget to mention that I need to put a FIR behind the
> CIC . The difficult focuses on analyzing the overall frequency
> Best Regards.- Hide quoted text -
>
> - Show quoted text -

Still as Tim stated, use the symbolic form of the CIC filter's
response. Then multiply it by the FIR's response.

Clay

```
```
agump wrote:

> Hi,
>      I am designing a decimation multirate filter with CIC and the
> decimated rate is very large . It could be larger than 1,000,000. The
> difficult I face is that when the decimated rate is too large , I
> can't view the details of the magnitude response of the passpand. In
> order to get the details of the magnitude response , the freqz
> function in matlab need to carry out a FFT with millions of points .
> It's beyond the capability of my PC. I am thinking whether there is
> some methods , which can get needed segment of the magnitude response
> Best Regards.
```
```Thanks you all . I'll try as you suggest.
Best Regards.
```