EMC TESTING
Designing for EMI testing D
esigning a product is already a daunting task without increasing the time-to- market pressure. Testing EMI can and should be done as early as possible in the product development cycle to avoid large
costs and lost market share due to delayed product launches. Nearly 50 per cent of products fail EMC compliance the first time. Every day spent on debugging, isolating and correcting the EMI problem increases the time to market. The time lost could have been used to work on another project or on improving the design instead. Testing EMI requires a different approach than
the usual time domain-related measurements and other types of general RF tests. Due to the unpredictable interactions of interference sources that may come from one or multiple components on the device-under test (DUT), it is imperative for R&D engineers to choose the best tools for locating, capturing and analysing those signals. Most R&D engineers use oscilloscopes throughout
their product design cycle to capture waveforms, verify serial buses and debug unwanted signals. In the past, oscilloscopes were not commonly associated with EMI testing due to FFT limitations. However, that has changed with the R&S RTO/RTE digital oscilloscopes. Adding the capability of using the oscilloscope to perform EMI testing would eliminate the need to purchase other equipment. Oscilloscopes, spectrum analysers and test
receivers tackle EMI problems from different perspectives and angles. Each offers a different approach and diagnostic techniques that can certainly complement each other at different stages of the product development cycle. There are two ways to use an oscilloscope: set the
frequency span that you are interested in or start by setting a wide-span scan. Setting the right frequency band helps observe the spectrum of interest. The objective is to obtain an overview of the spectrum and distinguish between narrowband and broadband signals. The user can then narrow down to the frequency span of interest for further analysis.
42 /// Environmental Engineering /// June 2018
❱❱ The new R&S RTC1000 oscilloscope series features instrument integration in a compact form factor
AVOID OVERLOADING To obtain correct results with the spectral analysis function, it is important to make sure the oscilloscope is not overloaded – when the measured signal can no longer be fully displayed on the screen. The time domain signal should always be monitored on the screen in addition to the spectral signal. In case of overloading, the oscilloscope’s vertical sensitivity should be reduced. Intensity grading reveals signal details such as the
frequency of the occurrence signal. It can easily detect and distinguish continuous wave signals, random emissions, bursts and pulses. Commonly occurring frequency components are
displayed in a different colour to distinguish them from rarer frequency components. This allows the user to tell at a glance whether a given emission originates in a clock line with constant frequency or if it is associated with sporadic disturbances. Generally, EMI/EMC emissions can be
To complement the launch of its new R&S RTC1000 oscilloscope series, Rohde & Schwarz has published a step-by-step guide to EMI testing. Andy Pye highlights key messages
categorised as either current-dominant or voltage- dominant. Current-dominant emissions are usually associated with differential mode conditions and radiate primarily in the magnetic field (H-field). Voltage-dominant emissions are normally associated with common-mode conditions and are detected most strongly in the electro field (E-field). A dominant H-field emission indicates a lower
source impedance, whereas predominantly E-field radiation shows the impedance is high. Since the H-field dominant emission fades more
quickly with distance from the source and would be much more difficult to detect in far-field tests, it is important at this stage to locate these unwanted emissions in the near-field using H-field probes. The H-field dissipates more quickly with
distance, but this does not eliminate its significant contribution to far-field performance when combined with other emission sources. With the oscilloscope’s advantage of the time and
frequency real-time single view, the near-field probe should also be used in combination with voltage/current/digital probes on other channels to provide further clarification. EE
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