MATLAB: FFT of a simple harmonic data

fft

Hi,

I have the two data vectors, Time and Displacement. When I plot them as plot(Time,displ), I can see the harmonic behavior but when I do the FFT using following code, I get strange plot and do not get frequency peak. I shall be grateful for the help. Here Fs=180 and L=180, the two being the sampling frequency and length of signal respectively. Both have dimensions 180×1. I just used the following code available from Matlab help for simple sine function.

Thanks…

Fs=180; 
L=180;
Y=fft(displ);
f = Fs*(0:(L/2))/L;
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
plot(f,P1)
title('Single-Sided Amplitude Spectrum of S(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')

Best Answer

It is difficult for me to follow your code. I always use (and recommend) the code from the R2015a documentation for fft (link). Especially note the code between the first (top) two plot figures.

Related Solutions

MATLAB: Obtaining result in Frequency Domain using FFT in Matlab

Here you go:

T = load('Rehan Rehan Time.mat');
D = load('Rehan Rehan displ.mat');
t = T.Time;
d = D.displ;
L = length(t);                                              % Signal Length (samples)
Ts = mean(diff(t));                                         % Sampling Interval (seconde)
Fs = 1/Ts;                                                  % Sampling Frequency (Hz)
Fn = Fs/2;                                                  % Nyquist Frequency (Hz)
dm = d-mean(d);
ft_d = fft(dm)/L;                                           % Normalised Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                         % Frequency Vector
Iv = 1:length(Fv);                                          % Index Vector
[pks,idx] = findpeaks(abs(ft_d(Iv))*2);
lbl = sprintf('Frequency = %.3f\nAmplitude = %.3f', Fv(idx), pks);
figure(1)
plot(Fv, abs(ft_d(Iv))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude (m)')
text(Fv(idx)+0.1, pks, lbl, 'HorizontalAlignment','left', 'VerticalAlignment','top')

MATLAB: Ask help to do the fft on the ‘untreatable’ data

Try this:

D = load('data.txt');
t = D(:,1);
s = D(:,2);
L = numel(t);
Ts = mean(diff(t));
Fs = 1/Ts;
Fn = Fs/2;
FTs = fft(s-mean(s))/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
[pks,locs] = findpeaks(abs(FTs(Iv))*2, 'MinPeakProminence',0.001);
figure
plot(t, s)
grid
xlabel('Time (s)')
ylabel('Amplitude')
title('Time Domain')
figure
plot(Fv, abs(FTs(Iv))*2)
hold on
plot(Fv(locs), abs(FTs(locs))*2, '^r', 'MarkerFaceColor','r')
hold off
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Frequency Domain')
xlim([0 1500])

Make necessary changes to get the result you want.

Best Answer

Related Solutions

MATLAB: Obtaining result in Frequency Domain using FFT in Matlab

MATLAB: Ask help to do the fft on the ‘untreatable’ data

Related Question