frequency domain to time domain
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nur syafiqah
on 17 Dec 2021
Commented: Walter Roberson
on 20 Dec 2021
i have frequency domain data in excel in term of frequency and amplitude. I need to convert frequency domain to time domain in MATLAB.
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Accepted Answer
Walter Roberson
on 17 Dec 2021
If you know the sampling frequency and the duration, then you can discretize each frequency to get a bin number for fft purposes.
Sampling frequency times duration gives you the number of original samples.
Now create an spectra that is the same length as the number of original samples.
Take the bin number information relevant to each frequency, and at that offset, store the entire magnitude.
Now,
spectra(end:-1:floor(end/2)+1) = spectra(2:floor(end/2));
which extends the frequency list symmetrically into the negative frequencies.
Now you can ifft(spectra)
The result will be a composition of pure sine (or cosine) waves as if the phase were 0 for all frequencies.
If you want a more accurate approximation, then when you discretize the frequencies to get bin numbers, allocate the magnitude proportionately between floor(bin_number) and ceil(bin_number)
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Walter Roberson
on 20 Dec 2021
Let L (signal length) = Fs * duration . Then the one-side spectrum has frequency bins that are at frequencies Fs * (0:L/2)/L so the difference between them is Fs/L which is Fs/(Fs*duration) = 1/duration Hz
So take floor(frequency/duration)+1 to get the bin number to write the absolute magnitude in to.
[You will stop have to copy the flipped one-side spectra to become the two-sided spectra before doing the ifft)
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