The myth: Conducted EMI emission profiles are always attributable to conducted currents propagating from the product power connections to the Line Impedance Stabilization (artificial mains) Networks (LISNs)
The reality: Effects from other systems can result in common-mode potentials that when measured at the LISNs appear to be propagated from the product power terminals.
Conducted EMI emission processes in power lines typically involve two characteristics of EMI: differential-mode and common-mode. Differential-mode currents emanating from power supplies in systems-products set up a line-to-line circulation of current. This current spectra then interacts with the series impedances of the LISNs, resulting in a differential-mode voltage spectra between the lines. Since the measurement of the conducted voltages (in dBuV) is typically single-ended to Earth reference, the differential-mode potential in the single-ended test is seen as one-half the value of the amplitude that is actually between the typical power pair.
Common-mode potentials are also measured in the single-ended process. These are superimposed upon any differential-mode spectra. Common-mode potentials typically circulate from both lines of the power pair at approximately equal levels in common-phase, as referenced to Earth. At frequencies below approximately 5 MHz, common-mode potentials may circulate from the power pair back to the earth safety wire. In this case, the current will be out of phase with the safety wire compared to the power pair. At frequencies above approximately 5 MHz, however, any interface connections to the product may become involved in common-mode circulations to the power pair. In effect, common-mode EMI emission from the interface cable(s) can circulate through either direct-conducted or distributed impedance paths of Earth reference back to the power pair. Inversely, common-mode EMI from the power pair can circulate through to the interface. When common-mode potentials sourced from the interface circulate to the power pair, the measurement effect may be seen as higher frequency narrowband voltages at the LISNs in power at “logic frequencies”.
To evaluate the propagation modes and separate the effects, use of current probes is effective. Differential-mode effects can be investigated by measuring the currents separately on each line of the power pair, then measuring the two together in the probe aperture. Differential levels will show some degree of cancellation for the combined pair, compared to the amplitudes on either wire. Common-mode circulations between the pair that circulates to the Earth safety wire will show approximately equal currents from the pair (combined in the probe) to the safety wire separately, and will suggest a phase cancellation when the safety wire is combined within the probe with the pair. Conducted circulations that are sourced from the interface can be evaluated by placing the probe around the interface cable(s) at the frequency (or spectra) of interest, then comparing this profile to that displayed on the whole of the power cable. When the interface source effect occurs, the solution to narrowband emissions on the power cable may be found in shielding or otherwise suppressing the common-mode currents in the interface rather than attempting to suppress the power entry (which may be futile, since that was not the source). 
ICM wishes to acknowledge the first appearance of W. Michael King’s “Myth vs. Reality” series with NTS – Silicon Valley.
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W. Michael King is a systems design advisor who has been active in the development of over 1,000 system-product designs in a 50 year career. He serves an international client base as an independent design advisor. Many terms used for PC Board Layout, such as the “3-W Rule”, the “V-plane Undercut Rule”, and “ground stitching nulls”, were all originated by himself. His full biography may be seen through his web site: www.SystemsEMC.com.Significantly, he is the author of EMCT: High Speed Design Tutorial (ISBN 0-7381-3340-X) which is the source of some of the graphics used in this presentation. EMCT is available through Elliott Laboratories/NTS, co-branded with the IEEE Standards Information Network. |
