Change:
[b,a] = butter(n,Wn);
to:
[z,p,k] = butter(n,Wn);
and eliminate the tf2zp call.
Then the filtfilt call should be:
filtered_signal = filtfilt(sos,g, noisy);
See the documentation on the zplane function for details on how to use it. Use the ‘sos’ output as the argument for zplane and impz.
I have no idea what ‘I need to get magnitude start from 40db’ means. Note that ‘Rp’ (passband ripple or magnitude) and ‘Rs’ (stopband ripple or magnitude) are defined in terms of dB, so change them to do what you want.
Testing the signal —
LD = load(websave('bai2','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1234397/bai2.mat'))
noisy = LD.noisy;
Fs = LD.fs;
L = numel(noisy);
t = linspace(0, L-1, L)/Fs;
filtered_signal = lowpass(noisy, 2500, Fs, 'ImpulseResponse','iir');
figure
subplot(2,1,1)
plot(t, noisy)
grid
subplot(2,1,2)
plot(t, filtered_signal)
grid
ylim([-1 1]*0.015)
The filtered signal (here the output of an elliptic lowpass filter simply to be certain that there was something left after filtering it) has a very small amplitude, so plot it separately as I did here in order to see it. Your result should look something like this result.
EDIT — (17 Dec 2022 at 15:24)
Minor corrections.
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