finding the dominant frequency via fourier transform

11 views (last 30 days)
Hi All,
I have been struggling to find the dominant frequency for a series of data that I have recorded in my experiments. Would be wonderful if you help me how to resolve my issue.
data series:
t=[40:0.1:300.2];
x=[6.54E-06
6.53E-06
6.52E-06
6.54E-06
6.58E-06
6.62E-06
6.65E-06
6.67E-06
6.67E-06
6.66E-06
6.65E-06
6.66E-06
6.69E-06
6.73E-06
6.75E-06
6.73E-06
6.69E-06
6.66E-06
6.65E-06
6.66E-06
6.66E-06
6.64E-06
6.61E-06
6.56E-06
6.51E-06
6.48E-06
6.46E-06
6.47E-06
6.47E-06
6.50E-06
6.53E-06
6.56E-06
6.55E-06
6.52E-06
6.52E-06
6.56E-06
6.62E-06
6.67E-06
6.69E-06
6.70E-06
6.71E-06
6.72E-06
6.74E-06
6.77E-06
6.80E-06
6.82E-06
6.83E-06
6.83E-06
6.82E-06
6.80E-06
6.77E-06
6.75E-06
6.76E-06
6.78E-06
6.77E-06
6.75E-06
6.73E-06
6.73E-06
6.76E-06
6.80E-06
6.83E-06
6.86E-06
6.87E-06
6.87E-06
6.86E-06
6.83E-06
6.83E-06
6.83E-06
6.83E-06
6.83E-06
6.83E-06
6.81E-06
6.79E-06
6.76E-06
6.74E-06
6.73E-06
6.72E-06
6.69E-06
6.65E-06
6.61E-06
6.57E-06
6.53E-06
6.49E-06
6.46E-06
6.45E-06
6.45E-06
6.44E-06
6.44E-06
6.44E-06
6.43E-06
6.43E-06
6.43E-06
6.42E-06
6.43E-06
6.43E-06
6.41E-06
6.41E-06
6.42E-06
6.41E-06
6.38E-06
6.33E-06
6.32E-06
6.33E-06
6.35E-06
6.36E-06
6.36E-06
6.33E-06
6.28E-06
6.23E-06
6.21E-06
6.20E-06
6.21E-06
6.24E-06
6.27E-06
6.30E-06
6.32E-06
6.32E-06
6.29E-06
6.26E-06
6.26E-06
6.28E-06
6.31E-06
6.32E-06
6.32E-06
6.33E-06
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6.32E-06
6.34E-06
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6.37E-06
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6.20E-06
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6.20E-06
6.22E-06
6.24E-06
6.24E-06
6.23E-06
6.22E-06
6.20E-06
6.18E-06
6.17E-06
6.19E-06
6.23E-06
6.27E-06
6.30E-06
6.31E-06
6.30E-06
6.30E-06
6.32E-06
6.35E-06
6.36E-06
6.37E-06
6.37E-06
6.38E-06
6.37E-06
6.37E-06
6.36E-06
6.37E-06
6.39E-06
6.40E-06
6.40E-06
6.40E-06
6.40E-06
6.37E-06
6.34E-06
6.29E-06
6.26E-06
6.24E-06
6.24E-06
6.23E-06
6.20E-06
6.17E-06
6.14E-06
6.13E-06
6.15E-06
6.19E-06
6.23E-06
6.27E-06
6.30E-06
6.32E-06
6.34E-06
6.33E-06
6.30E-06
6.27E-06
6.27E-06
6.28E-06
6.29E-06
6.31E-06
6.33E-06
6.35E-06
6.37E-06
6.41E-06
6.43E-06
6.44E-06
6.43E-06
6.39E-06
6.33E-06
6.27E-06
6.22E-06
6.19E-06
6.17E-06
6.18E-06
6.20E-06
6.23E-06
6.25E-06
6.27E-06
6.28E-06
6.29E-06
6.27E-06
6.26E-06
6.25E-06
6.25E-06
6.25E-06
6.27E-06
6.27E-06
6.26E-06
6.25E-06
6.22E-06
6.19E-06
6.17E-06
6.15E-06
6.15E-06
6.16E-06
6.18E-06
6.20E-06
6.20E-06
6.20E-06
6.19E-06
6.16E-06
6.13E-06
6.12E-06
6.12E-06
6.10E-06
6.08E-06
6.06E-06
6.06E-06
6.08E-06
6.12E-06
6.19E-06
6.24E-06
6.26E-06
6.26E-06
6.25E-06
6.22E-06
6.22E-06
6.22E-06
6.23E-06
6.25E-06
6.26E-06
6.27E-06
6.27E-06
6.27E-06
6.30E-06
6.34E-06
6.37E-06
6.39E-06
6.41E-06
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6.37E-06
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6.38E-06
6.35E-06
6.33E-06
6.33E-06
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6.34E-06
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6.22E-06
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6.24E-06
6.21E-06
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6.20E-06
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6.28E-06
6.32E-06
6.34E-06
6.34E-06
6.35E-06
6.36E-06
6.41E-06
6.46E-06
6.51E-06
6.53E-06
6.52E-06
6.49E-06
6.47E-06
6.48E-06
6.51E-06
6.56E-06
6.60E-06
6.60E-06
6.58E-06
6.53E-06
6.47E-06
6.42E-06
6.41E-06
6.42E-06
6.42E-06
6.39E-06
6.35E-06
6.29E-06
6.22E-06
6.16E-06
6.12E-06
6.11E-06
6.12E-06
6.13E-06
6.13E-06
6.12E-06
6.11E-06
6.11E-06
6.12E-06
6.15E-06
6.22E-06
6.30E-06
6.35E-06
6.36E-06
6.34E-06
6.33E-06
6.35E-06
6.38E-06
6.44E-06
6.49E-06
6.53E-06
6.54E-06
6.54E-06
6.52E-06
6.49E-06
6.47E-06
6.46E-06
6.45E-06
6.43E-06
6.41E-06
6.38E-06
6.35E-06
6.34E-06
6.36E-06
6.40E-06
6.44E-06
6.46E-06
6.46E-06
6.47E-06
6.48E-06
6.49E-06
6.50E-06
6.51E-06
6.53E-06
6.54E-06
6.53E-06
6.52E-06
6.52E-06
6.55E-06
6.60E-06
6.63E-06
6.66E-06
6.68E-06
6.70E-06
6.71E-06
6.71E-06
6.70E-06
6.69E-06
6.67E-06
6.64E-06
6.62E-06
6.62E-06
6.63E-06
6.65E-06
6.69E-06
6.73E-06
6.77E-06
6.80E-06
6.82E-06
6.82E-06
6.81E-06
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6.76E-06
6.76E-06
6.75E-06
6.74E-06
6.73E-06
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6.68E-06
6.65E-06
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6.42E-06
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6.46E-06
6.50E-06
6.53E-06
6.55E-06
6.54E-06
6.53E-06
6.53E-06
6.52E-06
6.51E-06
6.50E-06
6.48E-06
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6.48E-06
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6.48E-06
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6.35E-06
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6.31E-06
6.29E-06
6.25E-06
6.22E-06
6.22E-06
6.23E-06
6.25E-06
6.29E-06
6.32E-06
6.33E-06
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6.31E-06
6.30E-06
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6.40E-06
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6.45E-06
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6.32E-06
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6.31E-06
6.33E-06
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6.27E-06
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6.71E-06
6.72E-06
6.74E-06
6.74E-06
6.73E-06
6.71E-06
6.69E-06
6.65E-06
6.61E-06
6.59E-06
6.59E-06
6.60E-06
6.60E-06
6.59E-06
6.57E-06
6.54E-06
6.52E-06
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6.55E-06
6.60E-06
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6.00E-06
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6.10E-06
6.17E-06
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6.27E-06
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];
Fs = 10; % Sampling frequency
T = 0.1; % Sample time
L = length(t); % Length of signal
subplot(2,1,1);
plot(x);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(x,NFFT)/L;
f = Fs*linspace(0,1,NFFT);
subplot(2,1,2);
plot(f,Y)
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')
This result doesn't seem to be right. Because this dominant frequency is physically meaningless! Can anyone help me to figure this issue? Many thanks in advance. Hadi
  8 Comments
Hadi Mohammadi
Hadi Mohammadi on 7 Sep 2013
Thanks for your through comment. I don't have much background on this field. But, have been reading and watching several things today and am much more familiar to the topic since yesterday. Just one last question; by first harmonic you mean the first peak after Hz = 0 or sth else? thanks again
Image Analyst
Image Analyst on 7 Sep 2013
Harmonic means the first element. Second harmonic is the second element which is at twice the frequency of the first harmonic. The third harmonic is in the third element and has 3 times the frequency. These may or may not be a peak depending on what the original signal was.

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Accepted Answer

Youssef  Khmou
Youssef Khmou on 7 Sep 2013
Your data is sampled with Fs=10 Hz, Your Frequency spectrum is :
Fs=10;
N=length(x);
fx=fft(x)/N;
f=(0:N-1)*Fs/N;
figure, plot(f(1:end/2),10*log10(abs(fx(1:end/2))))
  3 Comments
Youssef  Khmou
Youssef Khmou on 7 Sep 2013
Mohammadi, It is clear that the dominant frequency is 0 Hz, repeat the experiment, and analyse the data for second time for verification, try to verify if your data needs some pre-processing like filtering low, high, or bandpass, or verify the devices for some adjustment .

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