Radiated emissions, most often referred to as electromagnetic interference (EMI), are carefully regulated to ensure reliable operation and safety for users of electrical and electronic equipment. Regulations limit the allowable radiated emissions, and to keep their products within these limits, designers invest significant time and effort. One reason why EMI poses challenges is that emissions are usually a systems design issue.
EMI (also referred to as radio frequency interference, or RFI) is more concerned with a product interfering with existing radio, television, or other communications systems, such as mobile telephone. Outside the U.S. it also includes immunity to external energy sources, such as electrostatic discharge and power line transients. This usually applies to commercial, consumer, industrial, medical, and scientific products. Radiated emissions is usually measured at a 3 m or 10 m test distance.
Causes of EMI
There are typically three major factors that cause EMI, which are an energy source that creates harmonic signals, antenna-like structures such as an IO or power cable which radiates the harmonics, and some coupling path that connects the two. While it sounds simple enough to just take away the energy source, coupling path or antenna and the EMI is resolved, it’s not so simple. That said, modern oscilloscopes can play a vital role in troubleshooting EMI issues (Fig. 1).
Figure 1. Modern oscilloscopes can be used with near field probes to track down sources of EMI.
While most designers think of spectrum analyzers as the best tool for debugging EMI, today's fast oscilloscopes with advanced triggering and frequency domain analysis may be best suited for combining both time and frequency domain EMI analysis. One important clue when characterizing the EMI of a circuit is whether the harmonic content is broadband or narrow band. Broadband harmonics are largely from digital bus noise or DC to DC converters and appear as broad peaks in the frequency spectrum. Narrow band EMI is generated by processor USB or ethernet clocks, and generally appears as a narrow harmonically related series of spikes.
3 Steps to Identifying the Sources of EMI
Many product designers may be familiar with how near field probes may be used to identify EMI “hot spots” on PC boards and cables but may not know what to do next with this information. Using Tektronix Spectrum View found on 4, 5, and 6 Series B Mixed Signal Oscilloscopes as an example, here’s a three-step process to identify emission failures:
Step One – Use near-field probes – either H- or E- field – to identify energy sources and characteristic emission profiles on the PC board and internal cables. Energy sources generally include clock oscillators, processors, RAM, D/A or A/D converters, DC-DC converters, and other sources, which produce high frequency, fast-edged digital signals. If the product includes a shielded enclosure, probe for leaky seams or other apertures. Record the emission profile of each energy source.
Step Two – Use a current probe to measure high frequency cable currents. Remember, cables are the most likely structure to radiate RF energy. Move the probe back and forth along the cable to maximize the highest harmonic currents. Record the emission profile of each cable.
Step Three – Use a nearby antenna (typically, a 1 m test distance) to determine which of the harmonic signals actually radiate (Fig. 2). To do this, can use an uncalibrated antenna connected to a Tektronix 4/5/6 Series MSO spaced at least 1m away from the product or system under test to measure the actual emissions.
Figure 2. Some potential antennas and accessories.
Once the emission sources are identified from one or more of these three steps, you can use your knowledge of filtering, grounding, and shielding to mitigate the problem emissions. Try to determine the coupling path from inside the product to any outside cables. In some cases, the circuit board may need to be redesigned by optimizing the layer stack-up or by eliminating high-speed traces crossing gaps in return planes, etc. By observing the results in real time with an antenna spaced some distance away, the troubleshooting phase should go quickly when using the multi-domain analysis tools in the Tektronix 4/5/6 Series MSO Oscilloscopes.
For more detail on EMI troubleshooting, download the application note – Step by Step EMI Troubleshooting with 4, 5, and 6 Series MSO Oscilloscopes.