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I am generating a signal as a function of time. This is an irregular signal generated by using the superposition principle and random phase. Thus I actually have already all the frequency components of the signal. And from the frequency components I know that there is also a frequency which has the highest amplitude, implying that this is the peak frequency. For the sake of argument lets say the peak frequency is 0.2 Hz.

With these frequency components I generate the signal with random phases, which is 300s long. If I now transform the signal from time domain to frequency domain by applying the Fourier transformation I get all the frequency components again. From the results I can see that the peak frequency is 0.2 Hz - the same peak frequency as the signal was generated with.

But now when I only take the first 30 seconds of the signal, should the peak frequency still be 0.2 Hz? Or is 0.2 Hz the peak frequency for the whole signal (300s)?

I hope you understand my question :)

Thanks!