MATLAB: Fault in matlab help

errorfilter

Hello,

I am working on my thesis and I have to use a matlab filter for this. When I using the example on this page butter I tried the exact same filter. But mine was all but stable. How is this possible? Can somebody try this aswell? Thank you!

[z,p,k] = butter(9,300/500,'high');
sos = zp2sos(z,p,k);
fvtool(sos,'Analysis','freq')

Best Answer

Here's the result I get. Definitely does not match the documented example.

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MATLAB: Blackman FIR bandpass filter design

Hi olga,

There is a subtle difference between the specification of an IIR filter and an FIR filter. Because you are designing an FIR filter, you need to use 'N,Fc1,Fc2' instead of 'N,F3dB1,F3dB2'. Here is an example

h = fdesign.bandpass
h.Specification = 'N,Fc1,Fc2'
Hd = window(h,'window',@blackman)
fvtool(Hd)

HTH,

Honglei

MATLAB: Band-pass Butterworth filter

There is an error in your code that prevents you from calculating the passbands and stopbands correctly.

This calculation:

Wp = [1 49]/Fsp/2;

gives you these passbands:

Wp =
       0.0005       0.0245

However the passbands you want are:

Wp = [1 49]/(Fsp/2);
Wp =
        0.002        0.098

Those will give you the correct result. The parentheses around ‘(Fsp/2)’ make all the difference.

If you want to design a filter with rolloffs as steep as you want, a Butterworth filter is not the best option. I would use a Chebyshev Type II design.

The Code —

Fsp = 1000;                                     % Create Data
Fn = Fsp/2;
Wp = [1.0 49]/Fn;
Ws = [0.5 50]/Fn;
Rp=10;
Rs=30; 
[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs); 
[z,p,k] = cheby2(n,Rs,Ws);
[sos,g] = zp2sos(z,p,k);
figure(1)
freqz(sos, 2^16, Fsp)
set(subplot(2,1,1), 'XLim',[0 100])             % ‘Zoom’ X-Axis

set(subplot(2,1,2), 'XLim',[0 100])             % ‘Zoom’ X-Axis

The passband ripple in a Chebyshev Type II filter are irrelevant, since the filter has a flat passband. Allowing them to be 10 dB allows you to design a stable filter.

Best Answer

Related Solutions

MATLAB: Blackman FIR bandpass filter design

MATLAB: Band-pass Butterworth filter

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