A scalar metric that quantifies the aggregate power contained in harmonic frequency components (integer multiples of a fundamental frequency) generated by a device or system relative to the power in the fundamental component; commonly computed as the ratio (or root‑sum‑square ratio) of harmonic amplitudes/powers to the fundamental amplitude/power.

Principle

Nonlinearities in a system driven by a single‑tone periodic input produce spectral energy at integer multiples of the input frequency; THD summarizes this energy relative to the fundamental and therefore measures deviation from linear, purely sinusoidal amplification or transmission for that input condition.

Demonstration

Illustrative calculation — Situation: single‑tone input at f0 into an amplifier that produces measurable 2nd and 3rd harmonics. Recognition: measure amplitudes at f0, 2f0, 3f0. Action: compute THD = sqrt(P2+P3+...)/P1 (or express as percentage or dB). Consequence: higher THD indicates stronger harmonic distortion under the tested tone and predicts audible or spectral quality degradation for similar periodic inputs.

Misapplication

Using THD measured with a single tone to characterize distortion for complex multi‑tone signals or digitally modulated waveforms without acknowledging intermodulation products and noise; THD only accounts for harmonics at integer multiples of the fundamental for that test signal.

Consequence

High THD implies energy diverted into harmonics, causing waveform distortion, spectral regrowth that can interfere with adjacent channels, reduced fidelity in audio systems, and potential failure to meet emission or fidelity specifications; mitigation typically requires more linear circuitry or feedback/compensation techniques.

Reversal

For multi‑tone or broadband signals, harmonic measures alone are insufficient—intermodulation distortion (IMD), THD+N (THD plus noise), and two‑tone IMD metrics are required. Also, with highly non‑sinusoidal intended signals (digital modulation), THD loses interpretive usefulness compared with EVM or error‑rate metrics.

Boundary

Clearly within: single‑tone, periodic excitation where harmonics are integer multiples of the fundamental and harmonic power is measurable. Boundary case: low‑level harmonics near the noise floor where measurement uncertainty affects THD accuracy. Clearly outside: nonperiodic, transient, or multi‑tone signals where distortion manifests as intermodulation or broadband products not captured by single‑tone THD.

Semantic Tension

Linearity versus efficiency: designing amplifiers for lower THD typically requires operating points or circuit topologies that reduce efficiency or increase cost; designers trade acceptable distortion versus power/area/cost constraints.

Synthesis

THD is a compact, input‑conditioned measure of harmonic distortion useful for single‑tone fidelity and spectral‑regrowth assessment, but it must be interpreted alongside other metrics (IMD, THD+N, EVM) when signals are multi‑tone or digitally modulated.