Tom: What I meant was how clean (free of harmonic content) does each of a pair of audio tones (sinusoids) need to be if that pair is modulating an SSB transmitter in lieu of a speech signal? If I have a pair of audio sinusoids at, say, 2000 Hz and 2500 Hz, can each of them have 1% total harmonic distortion and still be useful in IMD testing? - JimT