Design World – Power Transmission Reference Guide June 2019

eeworldonline.com | designworldonline.com 6 • 2019 DESIGN WORLD — EE NETWORK 33

THD

Understanding total

harmonic distortion measurements

Most electronics engineers and technicians have a

good understanding of total harmonic distortion

(THD). But there are a few elusive details that

come into play during THD measurements.

DAVID HERRES | CONTRIBUTING EDITOR

THD is the ratio of the sum of the powers of all harmonic

components to the power of the fundamental frequency. Properly

speaking, the fundamental frequency is the first harmonic, but THD

discussions frequently don’t acknowledge this fact. THD considers

distortion contributed by second-order and higher harmonics but

not by the random frequency, broad-spectrum distortion that is

known as noise. THD + noise is a separate though important metric.

The familiar sine wave is comprised of a single frequency, while non-

sinusoidal waveforms are made up of two or more sine waves that

can be added together on a point-by-point basis

moving along the time-domain X-axis. Breaking

down a complex non-sinusoidal waveform’s sine

wave components is a mathematically difficult

process but became practical with the advent of

the Fast Fourier Transform in the 1960s. Today,

one simply imports the nonsinusoidal signal into

a spectrum analyzer or, using Math Mode in an

oscilloscope, presses FFT. Then, displayed on the

screen in real time, is the signal at the channel

input in the frequency domain.

Scopes and THD meters aren’t the only

instruments capable of gauging harmonic

content. Power analyzers, such as the

PA3000 from Tektronix, are optimized for

characterizing power sources, including

their harmonic content.

Herres — Test and Measurement HB 06-19.indd 33 6/7/19 1:47 PM