What are harmonic distortion components in a nonlinear amplifier, and how are they measured?

• A. Harmonics are random noise; THD measures their level.
• B. Harmonics are fractional multiples of the fundamental; THD uses their phase relations.
• C. Harmonics are integer multiples of the fundamental; THD is the RMS of the harmonic content relative to the fundamental.
• D. Harmonics are the same as intermodulation products; THD measures total intermod distortion.

Answer: (C)

Nonlinear amplification turns a pure sine input into a waveform that contains extra frequency components. These components are harmonics—integer multiples of the fundamental frequency (2f, 3f, and so on)—because the distortion injects new Fourier terms into the signal. The way we quantify this distortion is Total Harmonic Distortion: we drive the amplifier with a clean sine, look at the output spectrum, and compare the RMS level of all the harmonic components to the RMS level of the fundamental. THD is computed as the square root of the sum of the squares of the harmonic amplitudes divided by the fundamental’s amplitude, typically expressed as a percentage. The phase of the harmonics isn’t part of THD, it relies on magnitudes. Intermodulation products, which arise when more than one frequency is present, are measured separately.