Below the same with 100ps and 5000ps long waveform.
16.7mil points FFT, B-H window.
Again, the spectra are the "same", where the bottom one (5000ps long) has got much lower min frequency in the spectra, AND, it is much more detailed.
Fundamental is 500GHz, square-wave where the rise is 0.1ps and fall is 0.3ps.