Two audio signals of the same frequency shifted 90 degrees
Two non-harmonically related audio signals✓Correct answer
Two swept frequency tones
Two audio frequency range square wave signals of equal amplitude
Explanation
A two-tone test drives the transmitter with two audio tones of equal amplitude that are not harmonically related, typically something like 700 Hz and 1900 Hz. Choosing non-harmonic tones ensures that distortion products land on frequencies distinguishable from harmonics of the tones themselves.