Long-cable effects on conducted emissions levels

A novel frequency-domain distributed-parameter circuit model of the setup for the measurement of conducted emissions (CE) in three-conductor systems is developed in order to predict the effects that an electrically-long power cord connecting the equipment under test (EUT) to the line-impedance stabilization network (LISN) plays on CE levels. To this end, a three-conductor lossless and uniform transmission line model is adopted for the description of the power cord, and modal decomposition of the line voltages and currents to common- and differential-mode voltages and currents is used to derive new closed-form expressions of the CE at the LISN ports. Suitable EUT-to-LISN matrix transfer functions are introduced and it is shown that CE measured at the entrance of the power outlet (i.e., at the LISN ports) may differ significantly from those flowing out of the EUT mains ports, due to propagation and coupling along the power cord. The role played by the mains cable cross section on CE propagation and conversion is investigated, and upper/lower bounds for the power cord effects on CE are derived analytically by applying an asymptote plot analysis to the magnitude of the EUT-to-LISN transfer functions. Direct numerical solution of the transmission line model is used to validate the obtained literal expressions of CE and related bounds, as well as to give explicit indications about the impact of long-cable effects on CE.