MATLAB: Hello all, I have a signal defined by the below. I want to take the Matlab FFT of this signal but I got a plot that does is not the Fourier Transform. Can someone direct me on how to be able to compute the FFT of the signal? Thanks Mates!

fft signal analysis

Signal defined by:
%%Time specifications:
   Fs = 8000;                   % samples per second
   dt = 1/Fs;                   % seconds per sample
   StopTime = 0.25;             % seconds

   t = (0:dt:StopTime-dt)';     % seconds
   %%Sine wave:
   Fc = 60;                     % hertz
   x = 3*sin(x) + sin(.5*x+40) + 2*sin(3*x-60);
   % Plot the signal versus time:
   figure;
   plot(t,x);
   xlabel('time (in seconds)');
   title('Signal versus Time');
   zoom xon;
The signal image is attached.
I did this for the FFT of the signal:
 Y = fft(x);% Plot single-sided amplitude spectrum.
plot(Y) 
title('Fast Fourier Transform of X graph)')
xlabel('Frequency (Hz)')
ylabel('Time (seconds)')
I figured since it was a simple function, I did not need elaborate coding but then I got the "FFT" attached.
Can someone tell me how to get the correct FFT please! Thanks.

Best Answer

Your stop time is so short that your signal is essentially a constant over that period. So the FFT is a just big DC spike as expected, which corresponds to a big "rect" function rather than the sum of 3 sine waves of different periods.
Set a stop time around 12*pi so you can get more of a varying waveform in there, especially more complete cycles instead of just a microscopic sample out of the waveform. That small sample is essentially a constant.
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