Frequency spectrum at specific frequencies from extracted data using matlab

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?

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)