MATLAB: What should be the value of ‘L’ while computing the FFT in MAT LAB

fft

What is L in a vibration signal acquired from picoscope.How its value can be used while computing FFT in MAT LAB..my signal file is attached here.Please clarify my doubt.

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I’m mind-reading here because you do not provide any context.

My guess is that your Question refers to scaling the fft (link). If my guess is correct, The ‘L’ you refer to is the length of the original signal, not the length of the padded fft (link) if ‘NFFT’ is greater than ‘L’.

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MATLAB: FFT of signal from text file

I don’t see the error in your code, but the code here I wrote (plotting half of the fft) works:

fidi = fopen('John Smith emg.txt', 'rt');
T_EMG = textscan(fidi, '%f%f', 'Delimiter',' ');
t = T_EMG{1};
EMG = T_EMG{2};
L = length(t);
Ts = mean(diff(t));                                 % Sampling Time
Fs = 1/Ts;                                          % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency
FEMG = fft(EMG)*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(FEMG(Iv)))
grid

MATLAB: Does length of signal matter in computing amplitude spectrum using fft

A bit more information would be nice, so guessing here.

The effect you are seeing is most likely the effect of doing a discrete Fourier transform with floating-point arithemtic. There is probably frequency ‘leakage’ in your fft results when the length is not a multiple of 100. You can most easily see this if you look closely at the area near zero amplitude for each peak. The peaks will look much more dispersed with some values of ‘L’ as compared to others. All the energy is still in the fft results. It is simply not as ‘concentrated’ at the peak frequencies for all values of ‘L’.

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MATLAB: FFT of signal from text file

MATLAB: Does length of signal matter in computing amplitude spectrum using fft

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