How to Make Total Harmonic Distortion (THD) Measurements

Learn how to measure your system's total harmonic distortion.
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Transcript:
From simple continuous waves to complex digitally modulated signals, every real signal has some amount of distortion. It is important to characterize this in your systems because distortion results in energy at unintended frequencies, which could cause interference. Today, we’ll discuss how you can determine the ‘Total Harmonic Distortion’ of your system using a signal analyzer.

Hi, I’m Ally, and welcome to Keysight’s Rapid Measurement Series.
Total harmonic distortion is the measurement of the harmonic distortion present in a signal and is the ratio of the sum of the powers of all surrounding harmonic components to the power of the fundamental signal – your device’s signal.

A good example of distortion is speakers. A high-quality speaker will have minimal distortion and sound better. A low-quality speaker will have higher distortion. And the difference between the two is the quality of the amplifiers and filters in the speakers. To quantify this, we need to calculate the ‘Total Harmonic Distortion’ of our system, in dBc. When designing a system, you want your total harmonic distortion to be as low as possible.

Measuring harmonic distortion can be quite time consuming if you have to do it manually –you have to identify all the harmonic power levels and add them together, and then find the ratio to the power of the fundamental signal.

It’s much easier with this signal analyzer, though.

By using the ‘Harmonics’ measurement, the total harmonic distortion and up to ten harmonics are calculated for us, shown in dBc.

The harmonic distortion measurement supports signals from simple CW all the way up to complex multi-carrier communication signals.

By knowing the total harmonic distortion of my signal, I can now evaluate whether or not my signal will cause any interference with systems operating in other channels.