Evaluation of Influence to Radiated Emission Measurement in Consideration of Connection Direction of Unbalanced Power Line Termination Device to Outlet in Test Sites

The impedance characteristics of the outlet that supplies power to the EUT at EMC test sites influence emissions from the power cable of the EUT, resulting in poor correlation of measurement results between test sites. To address this problem, a VHF-LISN (Very High Frequency Line Impedance Stabilization Network) is considered as a power line termination device for determining outlet impedance. Currently, two types of VHF-LISN are proposed: balanced termination and unbalanced termination. Round-robin tests (conducted to verify the respective effects of the proposed VHF-LISN) showed that emissions from the power cable varied depending on the connection direction of the power plug of a differential mode (DM) signal source to the unbalanced VHF-LISN. We created a simpler power line termination device and performed evaluation using a highly reproducible method. The evaluation proved that radiated emissions of the balanced power line termination device remained unchanged irrespective of the power plug connection direction in the case of either a common mode (CM) or differential mode (DM) signal source. However, radiated emissions of the unbalanced power line termination device changed with differential mode (DM) signal sources.