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Matlab code and filterDesigner give different plots for the same setting of Chebyshev II bandpass filter?

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Rui Zhang
Rui Zhang on 19 Oct 2020
Answered: Star Strider on 19 Oct 2020
Hello,
The image below shows the settings in the filterDesigner by using Chebyshev type II bandpass filter and the plots.
The image below shows my hand code in MATLAB using the same settings for Chebyshev II bandpass filter.
The plots are below and look different from the one in filterDesigner window:
Are there something wrong in my hand code of the MATLAB?

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Accepted Answer

Star Strider
Star Strider on 19 Oct 2020
I suspect that filterDesigner uses the second-order-section implementation of the filter.
You should, as well.
Change:
[b,a] = cheby2( ... );
to
[z,p,k] = cheby2( ... );
[sos,g] = zp2sos(z,p,k);
figure
freqz(sos, 2^16, Fs)
(where ‘Fs’ is the sampling frequency) and your filter Bode plot using freqz will likely approximate the filterDesigner implementation.

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