7. Wave Analyzers and Distortion Analyzers

7.1 Wave analyzer

A wave analyzer is essentially a tunable narrowband voltmeter. It mixes the input down to an intermediate frequency, passes it through a narrow filter, and displays the magnitude. By tuning the local oscillator, you measure the amplitude at one frequency at a time. Used historically for harmonic analysis (set the analyzer to $f_0, 2f_0, 3f_0,\ldots$ and read each harmonic's level).

Largely supplanted by spectrum analyzers and FFT software, but the architecture (mixer + filter) is identical to a swept-tuned spectrum analyzer.

7.2 Harmonic distortion analyzer (THD)

Total Harmonic Distortion is defined as: $\text{THD} = \frac{\sqrt{V_2^2 + V_3^2 + V_4^2 + \ldots}}{V_1}$ where $V_1$ is the fundamental and $V_n$ are the harmonics. A harmonic distortion analyzer has a notch filter tuned to the fundamental. The total RMS of the input is measured first; then the fundamental is notched out and the residual RMS is measured. The ratio gives THD+N (THD plus noise), which is what most published audio specs report.

For audio amps, a THD of 0.01% is excellent; for switching power supplies, line current THD might be 30%. The number tells you a lot about how clean a circuit is and how much it deviates from a pure sinusoidal response.