Skip navigation
All Places > Keysight Blogs > Better Measurements: The RF Test Blog > Blog > 2018 > April

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.


An 850-MHz signal with obvious harmonics on both sides.

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:

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.


The built-in harmonics measurement calculates the THD and results for up to 10 individual harmonics.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.

This week’s post is guest authored by Charlie Slater, Business Development and Operations Manager for Keysight Services.


These days, most organizations operate within one of two scenarios: cutting costs while delivering the same topline, or holding costs steady while increasing revenues. The third, less-common scenario is investing more to create a giant leap in output. If you’re in this fortunate group, confidence in future growth usually opens the door to major investments in plant, property and equipment (PPE)—and the “E” in PPE includes test equipment. Optimizing the management of test assets can help you create some semblance of order within the chaos.


Uncovering some unexpected side effects of rapid growth

Surprising problems can arise when your organization is moving at high speed. Several months ago I met with a manager in a high-growth company. Our purpose was to plan for onsite delivery of calibration services. When creating such a plan, key baseline information includes the location and condition of all in-hand test assets.


As we talked, it became clear that he had incomplete data about his company’s installed base of test equipment. Further discussion revealed the unexpected cause. The company’s engineers had extremely high purchasing authority and pallets of new network analyzers and spectrum analyzers were coming in every day. The manager had virtually no idea what was arriving and limited visibility into what his engineers were actively using or even if the equipment was in working order.


Gaining control of test assets and getting more from each one

During chaotic growth, sticking to the basics can help contain spending and restore order to an organization. For the company described above, accurate tracking of all new and existing RF equipment helped get its inventory under control. Today, better monitoring enables compliance with internal and external quality standards, and this includes staying up to date with test-asset calibration.


The underlying solution is real-time tools that provide centralized visibility. This enhances productivity by letting managers and engineers find and reassign unused instruments rather than waiting for delivery of new ones.


For any organization, real-time monitoring can pinpoint instruments that are underused or idle. In many cases, the most cost-effective way to refresh a languishing-but-viable test asset is an update or upgrade—and new functionality may be just a download away. For hardware upgrades that require installation, the turnaround time is usually shorter than the lead-time for a new instrument.


Exploring all three scenarios

To learn more, check out our latest resources, including a white paper about how to best enable 4G to 5G migration and a case study about how one company improved the health of their test assets.


Please chime in with any and all comments. How have you tried to optimize your situation? What worked best and why?

In a rock band, the drummer keeps the beat steady and the other musicians follow the rhythm. The drummer keeps the entire band in synch. The same concept is true when you integrate multiple instruments into a test system. The individual instruments need to be synchronized, especially when you are making multi-channel RF measurements. Like a drummer, a trigger and a reference clock communicate the “beat” to synchronize the instruments so they can make precise, time-aligned measurements. Let’s take a closer look at multi-channel measurements and how to achieve an accurate multi-channel test setup.


Multi-antenna RF techniques

Most modern wireless systems, whether in commercial applications or aerospace and defense, have adopted some kind of multi-antenna technique, such as MIMO (multiple input, multiple output), beamforming or phased-array radar. These techniques improve:

  • Spectral efficiency (bit/sec/Hz)
  • Signal quality
  • Signal coverage


For example, MIMO increases data rates by using two or more streams of data transmitted with multiple antennas. The antennas transmit the data on the same frequency and at the same time without interfering with one another, as shown in Figure 1. Spectral efficiency is improved using the same bandwidth.


Simplified 2x2 MIMO system

Figure 1. A simplified 2x2 MIMO system with two transmitters and two receivers.


Keys to synchronize multiple instruments

While MIMO and other technologies deliver increased data rates, they also increase the number of antennas in a device. And, as the number of antennas increases, test complexity increases significantly. For example, the latest IEEE WLAN technology, 802.11ax, use up to 8x8 MIMO. That means your test setup must have eight transmit channels and eight receive channels! And, it’s crucial that they are synchronized. 


To synchronize your test system, there are three key elements: the trigger, the sampling clock, and the event-signal effects.


An easy method to synchronize multiple instruments is to use a trigger. A trigger is a coordination signal that is sent to each instrument in a test setup. When the trigger signal is detected, each instrument performs its task. Using a trigger signal ensures all your instruments are in synch. However, there are two sources of error that must be addressed:

  1. Sampling clock: Even when all the instruments being triggered are identical, for example your signal generators, the initial phase of each instrument’s sampling clock is random. To align the sampling clock of each instrument, use the same reference frequency for all the instruments.
  2. Event-signal effects: Cabling and external devices can affect how long it takes your trigger signal to reach each instrument. This is called trigger delay. These event-signal effects need to be accounted for so that your instruments still transmit or receive at the same time. Using a channel skew control on your master instrument allows for precise time synchronization between all channels.


Figure 2 illustrates two arbitrary waveform generators (AWGs) that are in time alignment. Here’s a quick review of the setup:

  • First, use a common frequency reference to synchronize the timing clocks for all instruments.
  • Second, connect the master's "trigger out" to the slave's "trigger in" connector. The AWG will start generating the signal after a trigger event is detected.
  • Finally, remove the effects of master-to-slave trigger delay to align the two waveforms. The trigger delay can be measured with an oscilloscope or a digitizer. Then, add the delay time to the master AWG.


This process also applies to analyzers. You can use one splitter to distribute signals to a multi-channel analyzer and measure the time differences among the channels. The relative delays of each channel can be compensated by application software. Having the timing synchronized between the instruments allows you to build a multi-channel RF test system.


Two AWGs configured to generate time-aligned signals.


Figure 2. These two AWGs (master and slave) are configured to generate time-aligned signals. To remove the effects of master-to-slave delay, it is necessary to delay the signal generated by the master.


Modular instruments can make implementation easier

While the number of synchronized channels increases, the cabling between the instruments becomes much more complicated and achieving proper time-synchronization can take a significant amount of time. Modular instruments are based on standard instrumentation buses such as PXI, AXIe, and VXI. These instruments can share clocks and trigger signals through a backplane bus. This makes it easier to implement synchronization and makes the trigger events more repeatable because the test environment is controlled with minimal cabling.


For example, a PXI trigger bus consists of eight trigger lines spanning the backplane connectors. The trigger lines are divided into three trigger bus segments, slot numbers 1-6, 7-12 and 13-18. Figure 3 shows an easy PXI trigger setup with Keysight IO Library software.


PXI triegger setup using Keysight IO Library software


Figure 3. PXI trigger setup using Keysight IO Library software. In this example there are eight trigger lines (0-7) and three bus segments. The trigger routing direction between the segment of each trigger line can also be configured.


Figure 4 below shows two PXI chassis being used as a WLAN 802.11ax test solution that fully supports 8x8 MIMO. The PXI backplane bus routes trigger signals to target modules for eight-channel signal generation and acquisition. This system takes advantage of the PXI standards that minimize a chassis’ slot-to-slot trigger time and clock skew to hundreds of pico seconds.  This results in very accurate timing synchronization so you don't need to make any adjustment for MIMO transmitter and receiver testing.


WLAN 802.11ax test solution for 8x8 MIMIO.

Figure 4. WLAN 802.11ax test solution that fully supports 8x8 MIMO configuration in two PXI chassis.


Trigger and Time Synchronization Lead to Better Testing

To effectively test today’s multi-channel devices, you must perform tightly synchronized, multi-channel signal generation and analysis. With accurate triggering among the instruments, you ensure that all measurements start at precisely the right time. (If you require carrier phase coherency, you will also need to use a common local oscillator (LO) reference.) To simplify your test synchronization, consider a modular test system that allows easier integration of multiple instruments into a multi-channel test system.


If you’d like to know more about instrument interactions, refer to the following application note: Solutions for Design and Test of LTE/LTE-A Higher Order MIMO and Beamforming.


If you like this post, give it a like and feel free to share. Thanks for reading.