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Douglas C. Smith

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Technical Tidbit - March 2006
Predicting Cable Emissions from Common Mode Current
(with contributions from Mat Aschenberg an Charles Grasso)

common mode current measurement example

Figure 1.
Measuring Common Mode Cable Currents Generated by a Test Board

Abstract: Common mode current measurements are widely used to predict emissions from equipment cables.  Mat Aschenberg and Charles Grasso have written an analysis relating common mode current measurements to radiation comparing three different methods. Their treatment is unique in that it addresses the case where the cable is longer than one half wavelength. The results are presented as a pdf file.

Discussion:  Radiated emissions from a cable are directly related to common mode current flowing on the cable. Figure 1 shows the use of current probes to measure common mode current on cables. About 15 microamps of current flowing at the center of a one half wavelength dipole will result in radiated emissions that come close to international emission limits for class A equipment (industrial as opposed to consumer, or class B). By extension, one can say that a cable that is a significant fraction of a wavelength long may have the same emissions problem if the same current is flowing on the cable.  As a general rule of thumb, if a cable is carrying more than about 15 microamps at a frequency, one can expect that class A (industrial as opposed to consumer) radiated emissions requirements may be exceeded. This is somewhat of an oversimplification, but will serve as an example for this article.

Electromagnetic compatibility engineers often work in dB, so 15 microamps would be about 24 dB above a microamp or 24 dBuA. The current probes used to measure common mode currents are rated in transfer impedance, the ratio of the output voltage to the current through the probe. A popular probe used below 300 MHz is the Fischer F-33-1 probe. It has a transfer impedance of about 5 Ohms or about 14 dB above one Ohm or about 14 dBOhms. To relate the voltage out of the current probe for a 15 microamp current, one can multiply the current by the probe transfer impedance or add dBuA of the current to dBOhms of the transfer impedance to get the final level.

So for the F-33-1 we have:

Probe voltage output (dBuV) = 24 dBuA + 14 dBOhms = 38 dBuV

When adjusting a spectrum analyzer, I just put a display line at about 40 dBuV as the level I worry about.

Mat Aschenberg and Charles Grasso, both of Echostar, have performed an accurate analysis with supporting data comparing three different methods of relating common mode current to radiated emissions from a cable. They analyze the case where the cable is longer than a half wavelength, which is often true for many system cables. They have written a short paper with the results of their analysis in pdf format titled "Radiation from Common Mode Currents - Beyond 1 GHz." Click here to view or download the paper. In the paper, Mat and Charles address the relative accuracy of the three methods.

Summary: Common mode current measurements can be used to predict radiated emissions from cables. Such a measurement can avoid unnecessary failures during EMC testing. The linked paper by Mat Aschenberg and Charles Grasso compares three methods for relating common mode current to emissions and presents data to suggest a reasonable method to use.

Other articles on this website related to this topic are:
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Available now for private on-site delivery and as a public seminar: my latest single day seminar titled: Failure Analysis and Prevention in Electronic Circuits (Design Troubleshooting for the Lab and Field).

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