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.