FFT on a stored waveform

Hi everybody,

I have a time domain semi sinusoidal waveform stored in excell file in numerical format. Coul you please tell me how can I do FFT on that?

Regards,

Answer 1

[Image Analyst]

The xlsread() and fft() functions immediately spring to mind. Did you try them?

[Alzapoa]

Thands a lot. they work, but the fft() just do a numerical calculation could you tell me how to plot the elements. I tried the plot() command but it's just look like a line not a spectrum.

regards,

[Wayne King]

If xdft is the output of fft, then plot(abs(xdft)). The reason that plot(xdft) doesn't look right is because the output of fft() is in general complex-valued (as it should be).

[Image Analyst]

If the plot of the spectrum doesn't look like a line plot, what's it supposed to look like? You can use bar() if you want - would that be better? Of course you have to take either the real, imaginary, or magnitude like Wayne said. You might also want to plot the log of the amplitude if your signal looks just like one giant sharp spike, like it should for a mostly sinusoidal signal.