April 20, 2018

The Four Ws of Harmonic Distortion

Posted by nickben Apr 20, 2018

In the previous edition of The Four Ws, I reviewed the fundamentals of adjacent channel power (ACP). This time I’m discussing the WHAT, WHY, WHEN and WHERE of harmonic distortion measurements. Measuring harmonic distortion will help you validate the proper functioning of your device’s components and, in turn, avoid interference with systems operating in other channels.

What is harmonic distortion?

From simple continuous waves (CW) to complex digitally-modulated signals, every real signal has some amount of distortion. One type of distortion to consider is the total harmonic distortion (THD). The THD value indicates how much of your device’s signal distortion is due to harmonics. These harmonics are energies created at various multiples of the frequency of your signal where none previously existed or should exist. This extra energy is frequently caused by nonlinearities in the transfer function of a circuit, component or system. In practical systems, nonlinearities are due to gain compression, transistor switching or source-load impedance mismatches.

Figure 1: A basic swept measurement made with an X-Series signal analyzer shows an 850-MHz signal with obvious harmonics on both sides.

To calculate THD you need to determine the ratio of the sum of the power of all surrounding harmonic components to the power of your device’s fundamental signal:

The resulting THD is stated in dBc.

Why and When to measure THD

THD is typically characterized during design validation and troubleshooting when you are confirming that your signal is behaving as expected. Your THD will indicate if your device’s surrounding harmonics will affect your signal quality or interfere with another device.

You want the THD to be as low as possible. This implies that your device has a nearly pure signal making it unlikely that it’s harmonics will cause interference. On the other hand, a high THD means that you may need to rework your design because the distortion could negatively affect your signal quality or create interference in other channels.

Measuring THD can also be an effective indicator of overall signal performance. In an amplifier, for example, excessive THD indicates issues like clipping, gain compression, switching distortion, or improper transistor biasing or matching.

An example of Where distortion shows up and how you measure it

A simple, real-world example of harmonic distortion is found in audio speakers. Let’s say you’re playing a song from your phone and you hook it up to a speaker. If the speaker’s internal components – amplifiers and filters – give us an accurate reproduction of the song, then the speaker has a low amount of distortion. On the other hand, if the speaker’s internal components give you a misrepresentation of the song then it has a high amount of distortion. Therefore, you want your device’s THD value to be as low as possible to maintain good signal quality.

Another issue harmonic distortion can cause is interference with other signals. Since harmonic distortion is unwanted energy at the harmonics (integral multiples) of the fundamental frequency, the distortion can interfere with another device that is operating in the same band as the harmonic. Therefore, a low THD value is also a good indicator that interference is less likely to occur.

Seeing your signal’s harmonics can be difficult to observe and measuring them can be quite time consuming if done manually. You’d have to identify all the harmonic power levels, sum them, and then find the ratio to the power of your device’s signal. That is a hassle.

However, some signal analyzers provide a built-in measurement that will automatically calculate THD for you. This can shorten your measurement time and ensure an accurate calculation.

Figure 2. The built-in harmonics measurement on an X-Series signal analyzer quickly calculates the THD for the same 850-MHz signal seen in Figure 1. In addition to THD, the measurement shows results for up to 10 individual harmonics.

Using the harmonics measurement shown in Figure 3, you can calculate the total harmonic distortion and the results for up to ten harmonics, automatically.  All you have to do is set the fundamental frequency and the measurement takes care of the rest.

At each cycle, the analyzer performs an accurate zero-span measurement of the device’s signal and each of its harmonics. It calculates the level of each harmonic, as well as the total harmonic distortion of the signal, both of which are shown in dBc. The harmonic distortion measurement used in our example supports signals from simple CW to complex multi-carrier communication signals.

Wrapping up

Knowing the total harmonic distortion of your signal can help you evaluate if your device will cause any interference with its own signal or with systems operating in other channels. If you identify troublesome harmonics, you’ll have to rework your design and use something like a filter to tune them out.

THD is just one of nine RF power measurements made easy with PowerSuite, a standard feature on the X-Series signal analyzers. If you’d like to learn more about power measurements, check out the PowerSuite page and the Making Fast and Accurate Power Measurements application note.

I hope my fourth installment of The Four Ws provided you with some worthwhile information. Please post any comments – positive, constructive, or otherwise – and let me know what you think. If this post was useful give it a like and, of course, feel free to share.

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