How Regulatory Test Activity is Supporting the Rollout of mmWave Products

Author : By Ben Mercer, UL Solutions

15 June 2023

Figure 1: Example of a far-field anechoic chamber
Figure 1: Example of a far-field anechoic chamber

Regulatory radio frequency (RF) compliance testing is an established rite of passage for electronic products featuring wireless functionality - such those with Bluetooth or Wi-Fi technology incorporated. Increasingly, regulatory RF testing is also covering the millimetre wave (mmWave) spectrum. As this emerging wireless technology starts to mature, more product designers are likely to consider adopting it. However, they need to be aware that mmWave RF testing has particular challenges associated

As you would expect, mmWave technology uses wavelengths of less than a millimetre in length - operating in the frequencies that lie between 30GHz to 300GHz. This frequency band is uncongested compared to lower ones like 2.4GHz (into which many popular RF protocols are crammed). There is another advantage to mention though, as mmWave signals reduce in power more rapidly with distance. Now that might appear to be a disadvantage, but it does allow for reuse of frequencies by other devices within very short distances - thereby permitting higher concentrations of mmWave transmitting devices within a specific area than would be possible using lower frequency bands. 

Prospects for mmWave 
The potential of mmWave technology is already beginning to be exploited by a range of product types. One area of particular focus is radar. Here it can be used for presence and motion detection within industrial environments, etc. One example is radar modules that utilise mmWave frequencies to detect the rising levels of liquid in tanks. System designers are turning to mmWave technology because lower frequency bands are being shut down for radar transmissions in certain key markets (like the USA). Another growing mmWave application is for point-to-point wideband line-of-sight connections in areas like wireless backhaul. 

Regulatory compliance
Ensuring mmWave-enabled equipment comes to market smoothly is paramount, and regulatory compliance is the final hurdle in this process. Typically, these items will have undergone functional and field testing, but regulatory RF testing needs to follow different methodologies in controlled laboratory conditions. Checking compliance with mmWave regulations and getting certified by the relevant body (such as ETSI here in Europe) can be achieved, but the testing laboratory set-up and procedures must be appropriately customised for mmWave testing scenarios. 

Like any other RF testing and certification, mmWave device testing is done within a far-field anechoic chamber (such as the one shown in Figure 1). Housed within a large Faraday cage and lined with RF absorbent materials, this chamber blocks out all external radio signals and allows any wireless transmissions generated by a tested device to be seen in isolation, and thus properly examined. The RF absorbers reduce the reflections from the chamber walls, that would otherwise result in inaccurate measurements.  To receive and transmit mmWaves, moveable antennas are positioned around a device. The device being tested sits inside a rig on a turntable. This allows the test engineer to move the device around to get a 360-degree analysis of how it performs under regulatory test conditions.

Figure 2: An engineer using mmWave test equipment
Figure 2: An engineer using mmWave test equipment

The main challenge with mmWave regulatory testing in these chambers is how narrow the transmission beam is to detect and analyse. In the case of mmwave transmission, the analogy is the difference between being able to scope out the wide beam produced by a torch compared to one produced by a laser. 

Another challenge concerns the measurement instrumentation available to detect mmWave frequencies. To be able to see, and therefore determine, whether the device fits within the limits set by the various regulations, there needs to be a downconverter positioned between the receiving antenna and the signal analyser. This downconverter converts the mmWave frequency down to a lower frequency, which can then be measured by the test engineer via use of a spectrum analyser.

The obvious question is - why not build analysers that are applicable at high frequencies? Well, there are modern spectrum analysers that can support higher frequencies (up to 110GHz). However, these are of limited use in a highly controlled regulatory testing environment for several reasons. The most prominent one is that the analyser’s coaxial connection affects the testing process and means much of the signal is lost. This could be alleviated if the instrument was mounted behind the antenna, but putting such a heavy item up a metre and half high is impractical. By comparison, a downconverter is much smaller and lighter - allowing the test engineer to adjust the antenna in accordance with the regulatory test methodology with greater ease. 

Use of a downconverter for mmWave regulatory testing does come with a drawback, as the equipment generates false emissions. This means that when the engineer measures the signal, there will be more than one emission reading to examine. The solution is to employ extra testing technology to suppress those false signals and identify the genuine ones that must be measured against the regulatory criteria.

Logistical aspects 
Overall, mmWave regulatory testing is considerably time-consuming. To measure the much narrower beam widths, the turntable on which the device stands must be rotated more slowly to catch the emission. Similarly, testing for spurious emissions or harmonics takes a larger period of time than testing other wireless technologies. For a Bluetooth device, testing for spurious emissions may go up to 25GHz. In comparison, for a mmWave device, testing will need to be done across a wider range - from 30MHz to 200GHz or possibly beyond. This cannot be done in one go, but must be undertaken band by band, which requires a new test set-up each time bands are changed. As a result, the testing resource allocated is increased substantially. Potentially, automation and more versatile testing rigs may reduce the time involved in the future, but to achieve regulatory compliance and certification for the incoming wave of mmWave devices, this is the reality today. 

The good news is that, so far, the products that are coming in for regulatory testing do not tend to be seriously out of step with the standards set by the various regulatory authorities. However, this level of regulatory knowledge must be maintained, because engineers need to be designing a product that will not be seriously delayed at the regulatory testing phase. With mmWave technology still in its infancy, it can be easy to be inadvertently non-compliant. 

Figure 3: Setting an antenna in an anechoic chamber   (this image can be cut if space proves limited)
Figure 3: Setting an antenna in an anechoic chamber (this image can be cut if space proves limited)

Resolving issues before a new mmWave product goes into market is where the interaction between product design teams and test engineers becomes particularly valuable. If a regulatory test identifies non-compliance, there needs to be some trade-off in the design. Product designers will always want the highest output power they can achieve, because that gives them the best performance and functionality. But ramping the power up tends to incur more regulatory test problems. Pushing power amplifiers into saturation can lead to non-compliance in bandwidth or mask tests, and exceed other limits on spurious emissions. So, the interaction between a testing organisation and its customers should be to support them in how they work with the settings for their device - to produce as high an output power as can be achieved, while still keeping the product compliant. With mmWave technology, this means adjusting the firmware rather than making changes to the electronic componentry. 

Looking ahead
As mmWave technology continues to develop and demand grows, it will become even more important to design products with evolving regulations in mind. The current regulations are well settled, but there will be changes and refinements in the pipeline. For example, the revision of the European generic mmWave standard (EN 305 550) to cover the requirements of the Radio Equipment Directive never got formally published and was scrapped. This is now out for consultation, promising some imminent adjustments to those regulations. 

Regulatory testing for mmWave technology is tricky but possible. The test engineering skills and specialist laboratory facilities to help design engineers get their product to comply are very much available. Working with a testing laboratory aligned with changing regulations and new testing methods will be key to how new mmWave products come to market successfully in the coming years. 

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