Help With Starting Point for a Low Pass Filter

Prashant Kumar answered . 2025-07-12 18:00:41

[D,S] = xlsread('ExData.xlsx');
t = D(:,1);
s = D(:,2);

figure
plot(t,s)                                               % Plot Original Signal
grid

Ts = mean(diff(t));                                     % Sampling Interval
L = numel(t);
Fs = 1/Ts;                                              % Sampling Frequency
Fn = Fs/2;                                              % Nyquist Frequency
smc = s-mean(s);                                        % Subtract Mean (0 Hz)
FTs = fft(smc)/L;                                       % Fourier Transform
Fv = linspace(0,1,fix(L/2)+1)*Fn;                       % Frequency Vector
Iv = 1:numel(Fv);                                       % Index Vector

figure
plot(Fv, abs(FTs(Iv))*2)                                % Plot Fourier Transform
grid
xlim([0 1])

Ws = 0.09/Fn;                                           % Lowpass Stopband Frequency
Wp = 0.08/Fn;                                           % Lowpass Passband Frequency
Rp =  1;                                                % Passband Ripple
Rs = 50;                                                % Stopband Ripple & Attenuation
[n,Wp] = ellipord(Wp,Ws,Rp,Rs);                         % Elliptical Filter Parameters
[z,p,k] = ellip(n,Rp,Rs,Wp,'low');                      % Elliptical Filter Design (Specify Lowpass)
[sos,g] = zp2sos(z,p,k);                                % Second-Order-Section For Stability

figure
freqz(sos, 2^14, Fs)                                    % Plot Filter Characteristics
set(subplot(2,1,1),'XLim',[0 2.5])
set(subplot(2,1,2),'XLim',[0 2.5])

s_filt = filtfilt(sos,g,s);                             % Filter Signal

figure
plot(t, s_filt)                                         % Plot Filtered Signal
grid

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