how to filter a signal

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rohith bharadwaj 2017-9-9

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https://ww2.mathworks.cn/matlabcentral/answers/355847-how-to-filter-a-signal

评论： Star Strider 2021-2-3

I have a signal and I filtered the signal using a cheby1 filter. the frequencies of the signal range from 0.058 to 349Hz. I had to remove frequencies above 0.7Hz. after filtering the signal again when I find the frequencies I'm getting frequencies above 0.7Hz.

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Answer 1

Star Strider 2017-9-9

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https://ww2.mathworks.cn/matlabcentral/answers/355847-how-to-filter-a-signal#answer_280910

I showed you how to correctly design a filter here. If you want to design a filter to remove all frequencies above 0.7 Hz, design a lowpass filter, specify the passband frequency as 0.7/Fn and the stopband at 0.72/Fn. Use a Chebyshev Type II filter for this, instead of a Type I, since you now want a relatively flat passband.

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Wietse van Geel 2021-2-3

编辑：Wietse van Geel 2021-2-3

EDIT: found your previous post in the link! I'll try that first

Thank you for this helpful post. I'm designing a filter for the first time, and find it quite challenging.

Could you clarify the above answer by explaining how the last part, ending with:

[soslp,glp] = zp2sos(z,p,k);

can be implemented as a filter?

This part works fine for me:

[b,a] = cheby2(1,0.5,0.7/325,'low');

fil=filter(b,a,y);

And playing around with the numbers and plotting the results give some idea of what happens.

Best,

Wietse

Star Strider 2021-2-3

在 MATLAB Online 中打开

Wietse van Geel — My pleasure!

I have switched over to elliptic filters, since they are much more computationally efficient and generally perform much better. The code would now be:

Fs = 650
Fn = Fs/2;                                              % Nyquist Frequency (Hz)
Wp = 0.7/Fn;                                            % Passband Frequency (Normalised)
Ws = 0.72/Fn;                                           % Stopband Frequency (Normalised)
Rp =   1;                                               % Passband Ripple (dB)
Rs =  50;                                               % Stopband Ripple (dB)
[n,Wp] = ellipord(Wp,Ws,Rp,Rs);                         % Filter Order
[z,p,k] = ellip(n,Rp,Rs,Wp,'low');                      % Filter Design
[soslp,glp] = zp2sos(z,p,k);                            % Convert To Second-Order-Section For Stability
figure(5)
freqz(soslp, 2^16, Fs)                                  % Filter Bode Plot