Influence of Variations in Imbalanced LISN Termination Impedances on Radiated Emissions

Radiated emissions measurements are often performed using a line impedance stabilization network (LISN) with a balanced termination, but in the real-world the common-mode characteristic of the termination may be highly variable. An asymmetric LISN allows testing under more realistic asymmetric termination conditions. The common-mode termination impedance of these LISNs must fall within set tolerances, but the impact of these tolerances is unknown. An analysis of the radiated emissions from a device under test (DUT) for a wide variety of power cable termination impedances is performed in the following paper to estimate the impact of allowed variations in imbalanced LISN termination impedances on the uncertainty or radiated emissions measurement. Methods are developed to rapidly predict emissions by first determining the S-parameters between ports located at the DUT, at the power cable termination, and at a "port"measuring radiated emissions. The S-parameters are then used in analytic equations to predict the radiated emissions for a variety of termination impedances, and the variation in radiated emissions caused by the allowed variation in asymmetric LISN termination impedances. Results demonstrate that previous recommendations allowing a ± 30° change in the phase and ± 10% change in the magnitude of the asymmetric LISN termination creates about 5 dB maximum variation in radiated emission from 30 MHz to 300 MHz among the tested configurations. According to the model used in this paper, the expanded standard uncertainty in the emissions measurements resulting from these variations is about 9.8 dB, which is well below the 15.5 dB limit suggested by CISPR 16-4-1.