# Spectrogram of known frequency signal

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GCats on 24 May 2021
Answered: Walter Roberson on 27 May 2021
Hi all,
I need to generate a spectrogram for a frequency signal which I constructed by selecting only certain parts of diffrerent sine waves at different frequencies. In other words, I have a signal in the frequency domain but I do not have the time domain corrisponsent to input in
spectrogram(x)
. Can I generate a spectrogram from an already known frequency domain signal?
Thanks!
Nagasai Bharat on 27 May 2021
Hi ,
Can you let me know the problem you are facing in code for me to reproduce the issue?

Walter Roberson on 27 May 2021
If you have a list of frequencies and corresponding amplitudes, then:
If practical, find the greatest common divisor (GCD) between the frequencies. If you can do that, then each frequency will be an integer multiple of the GCD. Divide each frequency by the GCD to get its multiple; call that list of multiples M and the corresponding amplitudes A .
Now create a vector,
temp(M) = A;
So for example if the 7th frequency was 4 times the greatest common divisor, then temp(4) would be assigned the amplitude associated with the 7th frequency.
Now construct
ix = [0, temp, fliplr(conj(temp))];
time_signal = ifft(ix);
And now you should be able to
spectrogram(time_signal, [], [], [], GCD)