The cutoff frequency of the lowpass filter is too high.
Try these —
LD = load('T35.mat');
T35 = LD.T35;
sampling_interval = 10;
Fs = 1/sampling_interval;
L = numel(T35);
t = linspace(0, L-1, L)/Fs;
[FTT35,Fv] = FFT1(T35, t);
figure
semilogx(Fv, abs(FTT35)*2)
grid
xlabel('Frequency')
ylabel('Magnitude')
title('T35: Fourier Transform')
xlim([0 Fs/2])
Fco = 1E-5;
T35_filt1 = lowpass(T35, Fco, Fs, 'ImpulseResponse','iir');
Fco = 8E-7;
T35_filt2 = lowpass(T35, Fco, Fs, 'ImpulseResponse','iir');
figure
plot(t, T35, 'DisplayName','Original (Unfiltered)')
hold on
plot(t, T35_filt1, 'DisplayName','Filtered (F_{co} = 1\times 10^{-5})', 'LineWidth',2)
plot(t, T35_filt2, 'DisplayName','Filtered (F_{co} = 8\times 10^{-7})', 'LineWidth',2)
hold off
grid
legend('Location','best')
function [FTs1,Fv] = FFT1(s,t)
s = s(:);
t = t(:);
L = numel(t);
Fs = 1/mean(diff(t));
Fn = Fs/2;
NFFT = 2^nextpow2(L);
FTs = fft((s - mean(s)).*hann(L), NFFT)/sum(hann(L));
Fv = linspace(0, 1, NFFT/2+1)*Fn;
Iv = 1:numel(Fv);
FTs1 = FTs(Iv);
end
.
