Frequency spectrum at specific frequencies from extracted data using matlab

[peponas]

Hello,

I've made a code extracting the values of electric field versus time (measured in ns). I have to find the frequency spectrums for two different time windows from the same timeseries and compare them.
I would like to find the frequency spectrum for both the time sets, but calculated at the same frequencies. So far, I can find the frequency spectrum but for arbitary frequency (not GHz that corresponds to ns) and not for the exact same frequencies.


Any ideas?

 

[FvM]

If both windows have same lenght (number of samples), they'll map to the same frequency points.

 

[peponas]

If both windows have same lenght (number of samples), they'll map to the same frequency points.

Ok, I undestand. But how frequency points (which are discrete points) can be matched to the real frequency?

I have as an example OriginLab Pro 's FFT, which is taking into account not only the field points( Y axis) but the time points as well (X axis) to extract the frequency spectrum. I suppose I can use the time points to find the frequency spectrum to the right scale, because if I find only the frequency points and not their units, it's not so right (I suppose)