Characterization of common-mode excitation at the driving point of a circuit

Common-mode currents are a leading contributor to the radiated emissions from electronic equipment. Such currents are particularly difficult to identify when designing circuits because conventional circuit analysis programs do not predict them. The currents flowing in the physical structure of a circuit can be found by means of full-wave analysis, but the results must be decomposed into their differential- and common-mode parts to identify the mechanism that excites the radiated emissions. The work described in this paper formulates a method for computing a common-mode impedance at the driving point of the circuit. This single figure of merit gives instructive insight into the proclivity of a particular circuit layout to produce common-mode emissions. Computing common-mode impedance values across a frequency range can expose resonances in the common-mode excitation to predict where peaks in the radiated emissions are likely.