What signals are used to conduct a two-tone test?

A two-tone test is used to evaluate the intermodulation distortion (IMD) of a single sideband (SSB) transmitter. To perform this test, you inject two different audio frequencies into the microphone input simultaneously. It is crucial that these two audio signals are non-harmonically related. If they were harmonics of each other, it would be impossible to tell if the output distortion was caused by natural harmonic generation or by intermodulation between the two mixing tones. Using square waves or swept frequencies would introduce too many complex harmonics, muddying the test results. A proper two-tone test relies on clean, independent sine waves to clearly reveal any unwanted mixing products created by the transmitter's amplifier.