MATLAB: MATLAB FFT (y-axis values)

fftMATLAB

Hi all,
I am analysing the noise of a signal using FFT. I am reading the data (time and voltage/amplitude) from a text file. I used the code below to plot the FFT. How can I know that the plot is correct? What is the label for the y-axis? I did search for FFT plots on google and most of them have the y-axis in the thousand range. Why are my y-axis values so low?
Any help would be highly appreciated. Thanks.
fidi = fopen('data.txt', 'rt');
sig = textscan(fidi, '%f%f', 'Delimiter',' ');
t = sig{1};
amplitude = sig{2};
L = length(t);
Ts = mean(diff(t));                                 % Sampling Time
Fs = 1/Ts;                                          % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency
F_sig = fft(amplitude)*2/L;                         % Fourier Transform (Normalised)
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                 % Frequency Vector
Iv = 1:length(Fv);                                  % Index Vector
figure(1)
plot(Fv, abs(F_sig(Iv)))
grid
zoom on

Best Answer

You don’t say what your signal is, but perhaps the values are low because the amplitude of your signal is low. If you take the standard deviation of your signal, it should have a similar magnitude:
stdev_amplitude = std(amplitude)
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