Column: EMC
Testing radiated emissions By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK O
f all electromagnetic compatibility tests, the radiated emission test is one of the most important, since it aims to prove
a unit does not electromagnetically interfere with nearby equipment. Assessing the radiated emissions of
a product is challenging, firstly due to ambient noise coming from nearby radio and TV broadcast transmitters, handheld devices like two-way radios, the equipment and machinery used for the assessment, and ESD events, among others. Secondly, reflections caused by metal structures, including racks, cabinets, junction boxes, conduits and pipes, pose additional problems. If testing is not set up and performed
correctly, there can be a significant difference in results between chamber and benchtop testing – as much as 20dB. Therefore, it is essential to carefully consider and address these challenges during benchtop testing, to ensure the accurate assessment of a unit’s radiated emissions. Figure 1 shows some of the equipment that is often involved in performing both near- and far-field measurements.
Step 1: Near-field assessment A near-field measurement of the subsystems is essential to ensure that all clock frequencies and their harmonics are recorded, since they may also appear in the far-field measurement. In cases where the product will fit in a transverse
electromagnetic (TEM) cell, engineers prefer that, since it is a quicker method. However, a product doesn’t always fit in a TEM cell, which calls for near-field probes – both magnetic and electric field loops. This method “sniffs” the subsystem and records spurious levels that could potentially radiate in the far field. It’s worth noting that the purpose of
these measurements is not to correlate the results in the far field, but to determine the frequencies of critical spurious emissions in the far-field results. Similarly, near-field measurement results should not be used to directly predict far- field emissions. This is because near-field readings are highly dependent on the geometry of the source and its properties, making it difficult to provide correlations
Figure 1: Near- and far-fi eld measurement tools
10 September 2023
www.electronicsworld.com
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