Velocity spectra for Time and Velocity data with FFT

Hello I have an evenly spaced time series data as attached. I want to find velocity spectra (as the below figure) from it.

How to start?

[Fei Deng]

Hi Sayeed, you can get velocity spectrum via fft analyses by doing following:

Fs = 1000;            % Sampling frequency
T = 1/Fs;             % Sampling period
L = 1000;             % Length of signal
t = (0:L-1)*T;        % Time vector
S = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t); % Signal
Y = fft(S);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
plot(f,P1)

P1 is the spectrum that you need.

Refer the link for more details: http://www.mathworks.com/help/matlab/ref/fft.html?searchHighlight=fft