0
$\begingroup$

I am trying to design a digital ButterWorth filter for the given specifications.

rp=3;
rs=15;
FS=1;
wp=0.5*pi;
ws=0.75*pi;
pwp=2*FS*tan(wp/2);
pws=2*FS*tan(ws/2);
[n,wn]=buttord(pwp,pws,rp,rs,'s')
[b,a]=butter(n,wn,'s');
[bn,an]=bilinear(b,a,FS);%error
freqz(bn,an,512,FS);

If I design a filter manually i am getting different values for bn and an but Matlab gives different answer. Can anybody please help me?

Improve this question
$\endgroup$
1
  • $\begingroup$ Do you get the same filter order? Are both filters meeting the specification? $\endgroup$
    – Oscar
    Jan 22, 2015 at 14:51

1 Answer 1

Reset to default
1
$\begingroup$

I cannot run your code because I have not matlab on this pc, but I can try to give you some advice.

The first thing I will check is the way you defined the cut-off frequencies. The Matlab function uses normalized frequencies (look at the examples here http://it.mathworks.com/help/signal/ref/buttord.html), which means you have to define it in this way: lets assume you want to cut your signal at 5 Hz and your sampling frequency is $Fs$

cut_freq = 5;
Wn = cut_freq/(Fs/2);

Of course if you have a band-pass filter you have to define 2 cut-off frequencies and therefore $Wn$ will be a vector of 2 elements.

Hope this helps. As soon as I get my laptop I will try to run your code and see what happens.

Improve this answer
$\endgroup$
4
  • $\begingroup$ thanks for your advice.I checked the examples.. But there ,they are directly designing digital filter...But what i am trying to do here is given the analog specifications of a butterworth filter, designing digital butterworth filter using Binilear Tranformation teqniq..For more clarity i will post original Q. below $\endgroup$
    – spectre
    Nov 22, 2014 at 15:29
  • $\begingroup$ Design a digital filter,given the equavalent analog filter with following specifications Passband ripple=3.01 db Passband edge=500 hz Stopband attenuation =15 db Stopband edge=750 hz Sample rate=2000 hz, and design using Bilinear Tranformation. $\endgroup$
    – spectre
    Nov 22, 2014 at 15:34
  • $\begingroup$ Sorry I did not specify, my answer was related to the first part of your code where you used the 'buttord()' function. There I do not understand the way you are defining the frequencies 'pwp=2*FStan(wp/2); pws=2*FStan(ws/2); [n,wn]=buttord(pwp,pws,rp,rs,'s')' $\endgroup$
    – Rhei
    Nov 22, 2014 at 15:35
  • 1
    $\begingroup$ To be more clear, $pwp$ and $pws$ are not normalized frequencies, but buttord() asks for normalized values of frequencies. So I think the problem could be in the way you are using the buttord() function $\endgroup$
    – Rhei
    Nov 22, 2014 at 15:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.