Find the frequency for signals by using FFT

Hello every body,

Thank in advance. I have several signals. They have been created by using a collections of images for one week. Anyhow, the siganls are available in the attached excel sheet. The interval time between each reading is 1-minute. My question is how we could the FFT to calculate the domenant frequecny (or the frequency) for each signal.

Thank you again.

[dpb]

See the documentation for fft at

doc fft

It has example of the FFT-based PSD laid out.

[Mohanned Al Gharawi]

Thank you for your answer, but I got confused!!!

That's my code:

s=data;

t = 1:length(s);

Ts = 1/(1*60); % Sampling Frequency in Hz

Fs = 1/Ts ; % Sampling Frequency

Fn = Fs/2; % Nyquist Frequency

L = length(s);

fts = fft(s)/L; % Normalised Fourier Transform

Fv = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector

Iv = 1:length(Fv); % Index Vector

amp = abs(fts(Iv))*2; % Spectrum Amplitude

phs = unwrap(angle(fts(

[Mohanned Al Gharawi]

I got zero value for the frequency!!!

Answer 1

[Star Strider]

One approach (I recognise my code from some previous Answer, so I’ll supply the rest of it):

T1 = readtable('signals.xlsx', 'VariableNamingRule','preserve');

t = T1.('time (minutes)');
L = numel(t);
s = T1{:,2:9};
Ts = mean(diff(t))*60;                                      % Sampling Interval in Seconds
Fs = 1/Ts;                                                  % Sampling Frequency in Hz
Fn = Fs/2;                                                  % Nyquist Frequency
s_mean = mean(s);
fts = fft(s - s_mean)/L;                                    % Normalised Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                         % Frequency Vector
Iv = 1:length(Fv);                                          % Index Vector
amp = abs(fts(Iv))*2;                                       % Spectrum Amplitude
phs = unwrap(angle(fts));                                   % Spectrum Phase

figure
subplot(2,1,1)
plot(Fv, abs(fts(Iv,:))*2)
xlim([0 2]*1E-4)
grid
subplot(2,1,2)
plot(Fv, phs(Iv,:))
xlim([0 2]*1E-4)
grid

I subtracted the mean (d-c- offset) to make the other peaks more visible.

The frequencies all appear to be the same. I leave the analysis of the individual signals to you.

[Mohanned Al Gharawi]

It worked, I really appreciate your help. Thank you Star Strider!!

[Star Strider]

As always, my pleasure!