Tips for troubleshooting EMI issues

Throughout the world, almost all governments attempt to control the harmful electromagnetic interference (EMI) that emanates from electronic goods produced in their countries (figure 1). Very specific rules and regulations cover the design of electronics in order to provide a level of protection and safety for users.

This is a good thing, of course. But it does mean that companies must expend a great deal of time and effort on product design and testing in order to minimise their EMI signatures and pass official EMI certification tests. The bad news is that even after employing good design principles, selecting high quality components, and carefully characterizing the product, when it comes time for compliance testing, an EMI failure can still throw a major monkey wrench into the launch schedule if the testing does not go smoothly in all phases.

Often companies try to protect themselves from this scenario by performing "pre-compliance" measurements during the design and prototyping stages. It's much better to identify and remediate potential EMI issues before the product is ever sent out for compliance testing.

Of course, most companies' labs do not contain the test house conditions needed for making absolute EMI measurements. The good news is that it is entirely feasible to identify and resolve EMI issues without duplicating test house conditions. This article discusses some of the techniques you can use to reduce the risk that a product will fail the final full EMC compliance evaluation at the test house. It also includes an example on determining signal characteristics and coincidence in order to zero in on a source of EMI emission.

Understanding the EMI report
Before diving in to the troubleshooting techniques, a word about EMI test reports would be useful. At first glance, the EMI reports appear to provide straightforward information about a failure at a specific frequency. It might look like a simple matter to use the report data to identify which component of a design contains the offending source frequency and apply some attenuation in order to pass the test on the next pass. However, while many of the test conditions are explicit in the report, some important things to think about may not be so apparent. Before sifting through the design to try to determine the source of the problem, it can help to understand how a test house produces the report.

Consider the EMI test report in figure 2, which shows a failure at around 90MHz.

The corresponding tabular data report, shown in figure 3, details the values for test frequency, measured amplitude, calibrated correction factors, and adjusted field strength. The adjusted field strength is compared in the next column to the specification to determine the margin, or excess, shown in the far right column.

In the margin column shown, you can see that there is a single peak that is above the limit for this specific standard, at 88.7291MHz, with a -2.3 margin difference from the spec.

You're finished, right? Not so fast.
You're finished, right? Not so fast. Don't let all those digits lead you to believe that this is the precise frequency of the problem EMI source. In fact, it is very unlikely that the frequency given in the test report is exactly the frequency of the source. According to the Special International Committee on Radio Interference (CISPR), when performing radiated emissions testing, different test methods must be used depending upon the frequency range. Each range requires a specific resolution bandwidth filter and detector type, as shown in Table 1. The filter bandwidth determines the ability to resolve the exact frequency of interest; this means that the frequency ranges vary in how well they can hone in on the offending source.