VNA Measurement of Common-Mode Impedance of Coupling/Decoupling Networks in Conductive Immunity Testing

Different methods to determine the common-mode impedance of coupling/decoupling networks (CDNs) are investigated. They are based on vector network analyzer (VNA) measurements by using a test adapter between the coaxial 50 $\Omega$ VNA test port and the equipment under test port of the CDN. It is shown that the influence of the test adapter should be taken into account above 50 MHz, which is far below the upper test frequency bound as defined in the IEC 61000-4-6 standard. Measurement results for the presented approaches are given for a CDN-M3 and a CDN-M5 type up to 500 MHz along with associated measurement uncertainties.