How can I get the frequencies of a signal converted from time domain to frequency domain via FFT command

Hi, I have a time domain signal which is saved in the attached picture. based on this time domain signal I need to convert it to frequency domain in the form of magnitude, frequency amd phase. I have used fft function in matlab and obtained the magnitudes and the phases , but I still need to find what are the frequencies related to the obtained magnitudes and the phases. Once I establish the data base of the magnitudes, phases and frequency for the time domain signal, I need to carry out further bode diagram analysis. Any suggestion how to obtain these frequencies in matalab?

Answer 1

[David Goodmanson]

If your frequency array runs from 0 to F, then F = 1/dt, where dt is the spacing between points in the time array. However, if you are taking the fft just of the time waveform in your plot, the sharp cutoff of the waveform at 600 sec is going to have a bad effect. To get the correct frequency components you need a waveform that decays all the way down to zero, if that is what it is eventually doing.

[Basim Touqan]

Thank you for your answer.

[Basim Touqan]

Mr. Goodmanson, If the signal in the time domain will not at all decay to zero and assume that it reaches the staedy state value of 0.035 (as per the attached picture) at time 600 sec. Is there still any possibility to have the frequencies of the frequecy domain of the signal? At the end I need the frequencies, amblitudes, and phases data base to plot bode diagram and have further analysis.

[David Goodmanson]

It appears that you are looking at the response of a low-pass filter to a step function input. If that's the case, if you Fourier transform the time derivative of the pressure waveform you should obtain the frequency response for a bode plot. There will also probably need to be a multiplicative factor as well, due to the step function height.