MATLAB Central Newsreader - Obtaining the frequency response from a digital filter

Obtaining the frequency response from a digital filter

Ana — Sun, 08 Nov 2009 10:18:01 -0500

Hi there,

I am converting from analog to digital filter, a butterworth using the bilinear transformation. I am obtaining the num and the den of the digital function, but when am plotting using freqz the response is not as desired. don t know if i am doing something wrong prior to freqz or that freqz is not the right command to plot for digital filters.

disp(' --------------------------------------------');
disp('| Design of a Digital 2nd order Butterworth Low-pass Filter |');
disp(' --------------------------------------------');
fs=input('Sampling frequency: ');% around 20e3;
s = tf('s'); % The analog form
num = 1;
den = [1 1.414 1 ];

%Converting to digital form
[numd,dend] = bilinear(num,den,fs)

zplane(dend,numd)
pause;
freqz(dend,numd,fs) % plotting the response.
pause;

Thankyou very much for any help . REally appreciate it.

Regards,
Ana

(ps. can i use this method to transform ANY other filter, like the chebyshev, elliptic, thompson etc.... THANKS ALOT!!!)

Re: Obtaining the frequency response from a digital filter

Rune Allnor — Sun, 08 Nov 2009 11:07:12 -0500

On 8 Nov, 11:18, "Ana " <filter_wo...@yahoo.com> wrote:
> Hi there,
>
> I am converting from analog to digital filter, a butterworth using the bilinear transformation. I am obtaining the num and the den of the digital function, but when am plotting using freqz the response is not as desired. don t know if i am doing something wrong prior to freqz or that freqz is not the right command to plot for digital filters.
...

Designing discrete-time filters from analog prototypes
via the BLT is a somewhat involved algorithm, that requires
some 8-10 individual steps. Get any one of those steps wrong,
and you will be lost. Your code above only hints at a couple
of those steps, so there is no wonder why you don't get the
required result.

The algorithm for designing filters is beyond the scope of
USENET, so pick up a book on digital signal processin (or take
a class) to learn.

> (ps. can i use this method to transform ANY other filter, like the chebyshev, elliptic, thompson etc.... THANKS ALOT!!!)

The BLT is the core of a design method that can use several
analog IIRs as prototype. I know it can be used for the
Butterworth, Chebychev types I and II, elliptic filters;
maybe it works with other types of analog filters as well.
I don't know.

But again: You need to know what you are doing - and get
every single detail right! - to get the method to work.

Rune

Re: Obtaining the frequency response from a digital filter

Ana — Sun, 08 Nov 2009 12:19:01 -0500

Would you suggest any particular book? Thanks alot.

Regards,
Ana

Re: Obtaining the frequency response from a digital filter

Rune Allnor — Sun, 08 Nov 2009 13:56:49 -0500

On 8 Nov, 13:19, "Ana " <filter_wo...@yahoo.com> wrote:
> Would you suggest any particular book? Thanks alot.

Filter design is fairly standard, but this one contains
a lot of details that are left out of most books:

http://www.amazon.com/Digital-Signal-Processing-Signals-Systems/dp/0071454241/ref=sr_1_1?ie=UTF8&s=books&qid=1257688535&sr=8-1

Rune