Which of the following best defines total harmonic distortion THD and how it is measured?

• A. THD is the ratio of the fundamental power to the sum of all harmonic powers; expressed as a percentage.
• B. THD is the ratio of the noise floor to the signal and is always in dB.
• C. THD is the ratio of the sum of all harmonic powers to the fundamental; expressed as a percentage or in dB.
• D. THD is the ratio of DC offset to the AC amplitude.

Answer: (C)

THD measures how much of the signal’s energy lives in harmonics rather than the pure fundamental tone. To quantify it, you analyze the output waveform with Fourier analysis to separate the fundamental component from its harmonic components. Then you compare the energy (or power) in all the harmonics to the energy in the fundamental. The ratio of the sum of all harmonic powers to the fundamental power, expressed as a percentage or in decibels, is THD. This emphasis on harmonic energy relative to the main signal is what distortion is about.

The other ideas aren’t correct because THD isn’t about the noise floor (that’s more like signal-to-noise ratio) and it isn’t determined by DC offset, which is not a harmonic of the fundamental. Also, the ratio isn’t the fundamental relative to harmonics; it’s harmonics relative to the fundamental.