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
