What does a two-tone test illustrate on an oscilloscope?

• A. Linearity of a SSB transmitter.
• B. Frequency of the carrier phase shift.
• C. Percentage of frequency modulation.
• D. Sideband suppression.

Answer: (A)

Two-tone testing probes how linearly the transmitter’s modulation path behaves. When two audio frequencies are applied at the input, a perfectly linear system would reproduce them at the output without creating new frequencies. If the path is nonlinear, those tones interact and produce intermodulation distortion—additional frequencies that are sums and differences of the original tones. Observing the output on an oscilloscope (or spectrum) shows whether extra frequencies appear. This directly reveals the transmitter’s linearity, which is crucial for SSB because nonlinearities distort the sidebands and can cause adjacent-channel interference. So the two-tone test illustrates the linearity of a SSB transmitter.