Nice Greeting,
I am trying to write a MATLAB code for cpsd ,but I could not figure out what does nfft mean....could you please assist me to understand how could get nfft for the data signal?
thanks in advance
May be this link will be useful
http://stackoverflow.com/questions/29439888/what-is-nfft-used-in-fft-function-in-matlab
Hi, the previous answer is fairly comprehensive, I would only add that you should choose NFFT depending on your problem. Since the FFT frequency resolution Δf = Fs/NFFT, you should choose a NFFT value that is a power of a prime number in order to speed up your operation for big arrays, or you can choose a value that simply best suits your analysis needs.
Suppose you are monitoring the ac line voltage and you want to measure its amplitude, you will expect a single pulse at the frequency of 50 Hz with a height equal to the sine amplitude. Unfortunately you will get different results depending on the value of NFFT. If you use nextpow2(L), you will experience a power leakage between contiguous FFT bins (e.g. 48.83 Hz, 49.8 Hz, 50.78 Hz) that results in an amplitude error of almost 6% if you measure the maximum. Conversely if you use NFFT = L, you will have a frequency bin in 50 Hz that will contain all the power. Run the following code to evaluate yourself
Fs = 1000; % Sampling frequency
T = 1/Fs; % Sample time
L = 1000; % Length of signal
t = (0:L-1)*T; % Time vector
y = sin(2*pi*50*t); % Normalized 50 Hz Line voltage
NFFT2 = 2^nextpow2(L); % Next power of 2 from length of y
Y2 = fft(y,NFFT2)/L;
f2 = Fs/2*linspace(0,1,NFFT2/2+1);
NFFT1 = L; % Same length of y
Y1 = fft(y,NFFT1)/L;
f1 = Fs/2*linspace(0,1,NFFT1/2+1);
% Plot single-sided amplitude spectrum.
H = figure('Name','Single-Sided Amplitude Spectrum of y(t)')
subplot(1,2,1),plot(f2,2*abs(Y2(1:NFFT2/2+1)),'r')
title('NFFT=nextpow2(L)'),xlim([0 100]),xlabel('Frequency (Hz)'),ylabel('|Y(f)|')
subplot(1,2,2),plot(f1,2*abs(Y1(1:NFFT1/2+1)),'b')
title('NFFT=L'),xlim([0 100]),xlabel('Frequency (Hz)')
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