T HD
Understanding total harmonic distortion measurements THD is the ratio of the sum of the powers of all harmonic
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.
components to the power of the fundamental frequency. Properly
DAVID HERRES | CONTRIBUTING EDITOR
not by the random frequency, broad-spectrum distortion that is
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 known as noise. THD + noise is a separate though important metric. The familiar sine wave is comprised of a single frequency, while nonsinusoidal 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.
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