Investigation on effectiveness of very high frequency line impedance stabilization network (VHF-LISN) for measurement reproducibility

At present, radiated emission measurement standards do not specify the common mode impedance for the outlet terminal of mains network to the equipment under test (EUT) in the test site, i.e., do not define the common mode impedance as the terminating condition on the far end of the EUT power cable. Because of the difference in the terminating condition of the EUT power cable which dominantly causes radiated emission in the frequency range between 30 MHz and 300 MHz, radiated emission measurements of a tabletop EUT are poor in reproducibility. In an effort to resolve this issue, we investigated the effectiveness of some devices through inter-laboratory comparison measurements in which a very high frequency line impedance stabilization network (VHF-LISN) and a common mode absorbing device (CMAD), one at a time, were inserted between the mains network and the EUT power cable for the purpose of defining the terminating condition. As a result, we found that both VHF-LISN and CMAD are effective to improve the reproducibility of radiated emission measurement in comparison with the case of no such device, but we also found that their effectiveness differs with the type of EUT.