Amplifier - harmonic simulation

I'm an RF beginner and I would like to ask you about amplifier simulation:

- if we take as a source signal a harmonic signal like sinus waveform the result output signal won't have anymore a sinus form. In freq domain we'll get a spectrum of n harmonics. I guess that products like n*freq_source are not the ones we want (they could affect in a negative way other devices etc), so should we just filter them or maybe analyze amp in a different way (including something else then harmonics)?

- how can we get rid of numerical noise (if it's it) by using the function "fs" in ADS transient symulation. Why does the spectrum is so rich, while it should have just the fundamental tone and harmonics (n*f_fun_tone)?

Thanks!
ikkam