What kind of input signal is used to test the amplitude linearity of a single-sideband phone transmitter while viewing the output on an oscilloscope?

To properly test the amplitude linearity of a Single Sideband (SSB) transmitter, you must use a two-tone test, which requires feeding two audio-frequency sine waves into the microphone input. This simulates the complex, varying amplitude of a human voice while providing a stable, repeating pattern that can be analyzed on an oscilloscope. A single sine wave is insufficient because, in an SSB transmitter, a single audio tone produces a single, constant-amplitude radio frequency (RF) signal. This steady carrier would not exercise the amplifier's ability to handle changing power levels. Meanwhile, normal speech changes too rapidly to produce a stable waveform for visual inspection, and a square wave contains multiple harmonics that would clutter the output. When two distinct sine waves are applied, they interact to create a recognizable "bow-tie" or intersecting envelope pattern on the scope display. By observing the peaks and valleys of this pattern, you can easily spot "flat-topping" or crossover distortion, which are key indicators of a non-linear amplifier.