Evaluation of the Common Mode and the Differential Mode Components From Conducted Emission Measurements

The design of the power supply electromagnetic interference filter needed to mitigate the conducted emission of electronic modules can be performed best if the magnitude of the common mode (CM) and that of the differential mode (DM) interference are known. In common test setups, the two terms can be obtained from the signals measured at the line impedance stabilization networks (LISNs) output ports using DM and CM rejection networks or through the postprocessing of the output signals in the time domain. Both these approaches rely on the perfect matching of the LISNs internal filters, which is not realistic. Indeed, the LISNs mismatch allow the DM to be measured as CM and vice versa. In this work, the influence of the LISNs mismatch on the separation of CM and DM is investigated and a fast and accurate method to do that is proposed.