Mapping the effect of a circuit impedance on the common-mode excitation of electromagnetic interference

The control of electromagnetic radiation from electronic products in order to comply with the various governmental standards regulating electromagnetic interference around the world is a challenging part of the product design process. A principal reason is the relative difficulty of accurately modeling the way the common-mode portion of the radiation is produced. In Jerse (1999) a common-mode impedance measured at the driving point of a circuit was derived along with a numerical method to compute it. This impedance can be used as a single figure-of-merit to assess the proclivity of a configuration to generate common-mode emissions. As a result, design tradeoffs can be made without performing extensive full-wave analyses and decomposing the derived currents into their modal components. The work described in this paper extends the technique to permit the inclusion of a variable external circuit element and takes advantage of this single figure-of-merit to gauge the effect of the impedance of a transmission line load on the common-mode excitation of an example circuit.