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Technical Tidbit - January 2012
Measuring Common Mode Currents to Troubleshoot EMC Problems

Common mode current plot

Figure 1.
Typical Common Mode Current Plot
(Top of screen = 80 dBuV, Vertical 10 dB/div, Horizontal = 10 MHz/div,  Frequency Span from 0 Hz to 100 MHz)

Abstract: EMC testing usually comes at the end of project development. And when the equipment fails one of the tests, everything usually grinds to a halt. Often this results in engineers trying to diagnose the problem at the EMC test lab. Unless one is lucky, this generally does not produce the desired results. Use of common mode current measurements on cables is described and shown to be a powerful method to debug EMC problems as part of an EMC troubleshooting toolkit.

Discussion: The theory and method of common mode current measurements were discussed in my March 2006 Technical Tidbit, Predicting Cable Emissions from Common Mode Current. In this Technical Tidbit, I will discuss why common mode current measurements are so useful as a troubleshooting technique compared to troubleshooting at the EMC lab.

There are several reasons that common mode current measurements are useful:
  1. One can separate the cables (if possible, to minimize coupling between them) and measure the common mode currents on each separate cable. By knowing the common mode current on each cable, one can work on a single cable at a time to find the mechanism generating common mode current on the cable and fix the problem for that cable. At the EMC lab, the antenna measures the contribution from all cables and the equipment, so fixing just one cable may not produce measurable results when if fact you are making progress. Figure 1 shows common mode currents on several cables of a system. The 30 dBuV line represents the amount of common mode current that would just become a problem for Class B compliance (see the March 2006 Technical Tidbit for a discussion on this). Clearly the yellow trace is the problem at 30 MHz (third vertical line from the left) where the trace reaches 40 dBuV at that frequency.
  2. Common mode current measurements can quickly show the effect of equipment changes on the radiation profile of a cable. In fact, the plot in Figure 1 could just as easily be three different conditions for a single cable with the yellow trace being the original common mode current. The green trace looks like what a common mode choke or ferrite core on the cable might achieve, and the purple trace looks a lot like what a grounding change between the circuit board and the chassis might do.
  3. Common mode current measurements are so easy you can use them on early prototype equipment that you would not bother to book time in an EMC lab for. Using common mode current measurements on early prototypes can find EMC problems when you have the greatest number of low or no cost options for fixing the problem.
  4. Common mode current measurements can allow you to do EMC troubleshooting in your own lab where you have support equipment and people available and without EMC lab fees.
A couple of notes on making common mode current measurements:
  1. Set the spectrum analyzer on "Max Hold" and slide the current probe slowly down the first meter of cable to catch the current peak.
  2. Be sure to measure the cable common mode current before you turn on the equipment to make sure that a nearby radio transmitter or just lab noise may put too much noise in your cables to measure the contribution of your equipment. Depending on the size of the equipment being tested, a basement room or even an empty cube may be quieter than a development lab. Be suspicious of peaks around 900 or 1800 MHz as these can easily be a cellphone in the room, although most (but no all) common mode current related problems are at lower frequencies where the cables are the only system part large enough to radiate.

Summary: Common mode current measurements are an important troubleshooting tool in EMC work. Such measurements are one of several methods that can be applied in the development lab to quickly and inexpensively find and fix EMC problems.

Additional articles on this website related to this topic are:
  1. March 2006, Predicting Cable Emissions from Common Mode Current
  2. August 2011, Power Over Ethernet Injectors Can Affect Ethernet Radiated Emissions
    (High Frequency Measurement of 1000Base-T Ethernet Common Mode Currents)

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    Copyright 2012 Douglas C. Smith