Feature: Test and measurement


Figure 1: Coexistence scenario with typical radio communications services. Radio communications service pairs in the same frequency band or adjacent bands (red, orange) are critical. This is where coexistence testing becomes important


product liability risks or simply to obtain market access, especially when products are used in the healthcare sector.


The problem with the blocking tests Blocking tests were identified as suitable for coexistence- capability testing, and have been included in European standards associated with the RED. Performing the test involves establishing a connection between the DUT (device under test) and a radio communications tester with specified parameters such as frequency and signal level. A defined interference signal generated by an external signal generator is superimposed on the signal. A robust receiver can still operate as intended even in the presence of strong interference signals. Otherwise, follow-up work will be needed. However, tests that simply comply with standards cannot


ensure that a device will be as immune to interference in regular use as suggested by the test results. Te reason is that standards can’t keep up with the market as it evolves, and as rapid advances are made in radio technology, tests only cover the bare minimum. A completed and properly documented test may create legal certainty for major product faults, but the market will remain unimpressed, and users will have false confidence in a faulty product. Actual product performance in real-world EM environments is what ultimately matters. ETSI EN 300328 V2.2.2 for devices operating in the 2.4GHz


band is one example of a European standard with room for improvement. Coexistence is especially critical in this densely- occupied band in which WLAN, Bluetooth, household microwaves and other devices jostle for space. Te standard blocking test is fairly easy to pass, but hardly depicts the situation in the real world. Tis idealised interference scenario uses a CW interference signal with a constant signal level, and a noise floor limited to twice the signal bandwidth. However, these CW interference signals are not realistic and actual broadband modulated signals can degrade reception quality even at lower levels. Complete blocking of communications by interferers is unlikely.


Instead, data throughput drops because individual data packets get lost and must be requested again. Tis means a device can have insufficient interference immunity despite passing a coexistence test; see Figure 2.


Figure 3: Radio communication is penetrating nearly all aspects of our lives, making coexistence increasingly important


Figure 2: The greater the frequency offset of the interference signal from the wanted signal, the larger the permissible interference level, as illustrated here with a noisy interference signal (AWGN)


Realistic approaches Not all standards lag behind current requirements. One example is EN 303340 version 1.1.2. for DVB-T and DVB-T2 broadcast receivers, which demands the use of various types of interference signals, including simulation of a fully-occupied LTE base station signal. Te standard uses state-of-the-art interference signals but has other weaknesses. It is virtually impossible to judge how well a receiver works in


practice simply based on whether it passes a test in line with this standard, since the primary value for measuring degradation is the frequency with which picture failure points occur. If these occur no more than every 15 seconds when interference signals are applied, the device is standard-compliant and can be sold. Whether the customer is happy with this level of quality is another question.


Ensuring coexistence in the IoT era Te growing omnipresence of wireless products means coexistence will be a key issue for the industry; see Figure 3. However, all devices cannot be lumped together, and distinctions must be made. A wireless pacemaker or an automotive emergency call system need much more accurate testing and greater quality certainty than a WLAN-controlled toy, for example.


www.electronicsworld.co.uk May 2021 29


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