Direct Measurement and Representation of Common-mode Sources in Cable Harnesses

Predicting common-mode currents in cable harnesses is essential for predicting radiated emissions early in the design process. Using component-level tests to predict system-level emissions is difficult, however, as the common-mode current seen in the component-level test may differ dramatically from that seen in the system. A component-level measurement-based approach for characterizing common-mode sources is proposed here which may be used to predict common-mode currents for a variety of harness configurations. Common-mode source measurements were made by grouping sources together by the size of the loads they drive and measuring the effective common-mode source voltage and impedance for the group through a characterization board. Common-mode currents were predicted using these sources and transmission line models of the harness. The method was validated by characterizing sources in an engine controller from 20 MHz to 200 MHz and then predicting common-mode currents on harnesses of a variety of lengths, and thus for different common-mode impedances looking into the harness. The worst error between the predicted and measured common-mode current was less than 7 dB in the 20 MHz to 200 MHz frequency range.