# FFT vs Time Plot

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Sravantej il 6 Feb 2012
Modificato: Image Analyst il 2 Ott 2013
hi, i have a non stationary signal whose frequency is varying with time and i want to plot its FFT verses time of the signal. Can anyone explain me how to plot FFT vs Time
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Anton Semechko il 6 Feb 2012
What you want is called short time Fourier transform. Here is as example: http://www.mathworks.com/matlabcentral/fileexchange/1553-spectrogram-short-time-ft-log-magnitude
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Sravantej il 7 Feb 2012
hi anton, the link you provided was useful, but i have one doubt in it. When i use a code like
[tf1,f1,t1]=specgram(r1,2^nextpow2(50),120e6,50,20);
the output matrices i'm getting are of sizes t1=23*1, f1=33*1 and tf1=33*23. My question is how t1,f1 matrices sizes are determined i.e, what is the formula used to get the sizes of matrices based on window length and overlap. Here my signal size is 720*1, window length is 50, overlap is 20 samples. Could you please help me.
Anton Semechko il 7 Feb 2012
Look at the code to see how the size of the output arguments is determined with respect to the input ...

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