MEASURING THE INTERMODULATION DISTORTION OF LINEAR AMPLIFIERS

The measured distortion of a linear amplifier, normally called Intermodulation Distortion (IMD), is expressed as the power in decibels below the amplifier’s peak power or below that of one of the tones employed to produce the complex test signal.
A signal of three or more tones is used in certain video IMD tests, but two tones are common for HF SSB. The two-tone test signal provides a standard, controlled test method, whereas the human voice contains an unknown number of frequencies of various amplitudes and couldn’t be used for accurate power and linearity measurements. Separation of the two tones, for voice operation equipment, may be from 300 Hz to 3 kHz, 1 kHz being a standard adopted by the industry.