When you want to get the spectrum, the frequency domain result must be accurate to resolve 1MHz FWHM. This is because, The delta_f is inversely proportional to the signal length in time.
I actually prepared a long reply at the beginning. Unfortunately the forum does not work well as expected.
Later on, please be more specific like in your 2nd post so we can better answer you.
You are navigating away from the AIS Discovery experience