When testing the amplitude linearity of a single-sideband transmitter what audio tones are fed into the microphone input and on what kind of kind of instrument is the output observed?

When conducting a two-tone test on an SSB transmitter, it is crucial that the two audio tones are non-harmonically related. For example, using 700 Hz and 1900 Hz is effective, whereas using 1000 Hz and 2000 Hz would be problematic. If harmonically related tones are used, the natural harmonic distortion products generated by the amplifier can fall on the exact same frequencies as the intermodulation distortion (IMD) products you are trying to measure. This blending makes it incredibly difficult to accurately separate and identify the amplifier's intermodulation linearity. The output of this test is visually observed on an oscilloscope rather than a distortion analyzer. An oscilloscope provides a real-time visual representation of the RF envelope, allowing the operator to directly see amplitude distortions like flat-topping at peak power levels.