Back to EMC testing basics

Full EMC conformance certification requires a number of different tests
Jonathan Newell spoke to TÜV SÜD Product Service about getting back to basics on the complex subject of Electromagnetic Compatibility. With more products able to transmit and receive signals, the advent of an exponential rise in connected devices as part of the Inteet of Things and the proliferation of highly miniaturised, densely packed circuitry in our everyday lives, the subject of radio frequency interference is certainly not going to get any easier. The electronics industry is bristling with challenges as profusely as the antennas that give its products the means of achieving those all-important wireless communications. Wireless is no longer a convenience but a necessity that enables technologies that promise to make profound changes to the way we live and work, such as driverless cars and Industry 4.0. For such technologies to work, there has to be no interference, no spurious radio waves crossing from one antenna to another or finding their way into pseudo-antennas such as the circuitry on a PCB. In this context, the standards covering emissions and interference are regularly reviewed and subject to changes, such as the Radio Equipment Directive (RED), which came into force earlier this year. To lea more about the phenomenon of EMC and the tasks faced by the test house, I spoke to Peter Dorey, Principal Consultant EMC at TÜV SÜD Product Service. Product Proliferation I asked Dorey what EMC testing means to manufacturers and why there is such focus on the phenomenon at the moment. Electromagnetic Compatibility deals with the issues of Radio Frequency (RF) disturbances or emissions, sometimes referred to as Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI), “Generally, there is a proliferation of radio or wireless products, which are operating in environments with sensitive equipment and this equipment has to be protected,” Dorey says. All of these radio or wireless products must now comply with the Radio Equipment Directive (RED), which applies to all equipment capable of transmitting or receiving signals. The purpose is to protect devices from emissions in the range of 150kHz to 6GHz. This is driven by a list of standards, including RED but bespoke tests can also be done for product performance verification or to meet specific standards associated with particular types of equipment. Assailants and Victims There are two ways of looking at any product from an EMC perspective and that’s as either an assailant emitting RF signals that might interfere with other products or as a victim receiving signals that interfere with its function. Assessing potential victims is immunity testing for commercial products. The defence sector chooses to use the word susceptibility rather than immunity, reflecting a more pessimistic or pragmatic attitude to EMC. “This glass half full – glass half empty difference reflects the defence industry’s pragmatic view that EMC will affect their products and they want to know to what extent. Everything is susceptible and they want to quantify this,” Dorey told me. The testing performed is designed to check intended product functions in an environment where there is a known transmitter. EMC Testing Testing is performed at all levels of the final product bill of materials, including components sub-assembly and system levels. RED affects everything that has the ability of transmitting or receiving radio signals but there are also a lot of standards that relate to specific industries, such as automotive manufacturing and the highly regulated defence and healthcare industries. On top of that, there is a whole different set of standards and regulations for the aerospace industry because of safety. “These industries have their own sets of standards in addition to those which are generally applicable. The EMC directives don’t relate to safety and are more aligned towards stopping equipment from polluting the radio spectrum,” explained Dorey. Mains connected equipment also needs to be checked for harmonics of up to 2kHz travelling back into the mains plus voltage loads and flicker, so a comprehensive test plan is needed for all new products to make sure all these test requirements are catered for without making repeated trips to the test laboratory. Emission testing To measure emissions, the product under test is placed into an anechoic chamber to block out any environmental factors that could influence the test results. According to Dorey, the controlled environment provided by the anechoic chamber consists of a faraday cage with blocks out exteal factors and absorbent interior walls to stop radio reflections from the equipment itself. The wedge shaped absorbers on the walls give the facility the appearance of an acoustic chamber. “EMC chambers do absorb sound reflections so they seem to be acoustic in nature but this is really just a side effect of the design of the RAM (Radio Absorbent Material) wedges which are tuned for radio absorbance, not sound,” Dorey explained. EMC receiving antennas are arranged around the product to measure the radiated interference. These specialist antennas are designed to have a very wide frequency response unlike a tuned TV or mobile phone antenna. Tests are performed in frequency ranges up to 6GHz as well as sometimes in the 6-18GHz or 18-40GHz range, which require different antennas. For standard testing, radiated frequencies are measured between 30MHz and 6GHz. The equipment is also wired into the network to measure conducted emissions. “Radiated frequencies lower than the 30MHz that the antennas can pick up are tested using conducted levels of interference since one implies the other,” explained Dorey. The signals received by the antennas are fed into test receivers and spectrum analysers to build an EMC emission profile of the product under test. Immunity testing Immunity testing requires a reverse set up to that of emission testing using a signal generator, power amplifier and antennas used to transmitting rather than receiving a signal. These antennas need to produce signals across a wide frequency range at the appropriate power level, which is much higher than the power levels in emissions tests. Emission testing is about spectrum protection and the levels are measured in microVolts, Immunity testing requires much higher levels, measured in Volts. Product performance needs to be monitored against criteria supplied by the manufacturer. Although it’s feasible to use the same antennas for both types of tests, there are sufficient differences to necessitate using different antennas for immunity testing. Similarly, the emissions testing chamber is normally different to that used for immunity testing as they need different anechoic materials. Other equipment needed includes a tutable and mast, which are demanded by the standards, as well as equipment for other tests, such as transient interference, such as lighting and Electro Static Discharge (ESD). Sometimes tests can be performed on site in the open air if the product is too big to be placed in a chamber. Emissions testing can be performed like this but background noise needs to be accounted for. This can be done using subtractive analysis by measuring the background interference and making the same measurements with the product but since emission thresholds are often very low level, they can be drowned out by the background interference, rendering the test ineffective. It isn’t possible to do immunity testing across the full spectrum in this way because it isn’t legal to transmit signals in this way outside of a chamber environment. It’s only possible to do this using specific frequency bands using legal equipment like mobile phones but the scope is very limited.

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