Current probes for differential mode conducted emission measurement

The current probe described in CISPR 16-1-2 is a well-established tool for common-mode measurements on cables, but limited attention has been paid to the practicalities of making differential mode measurements as outlined in figure B.4b of the standard. This paper presents an analysis of the additional errors involved in the differential mode configuration, and shows that for good high frequency performance the inductance of the necessary loops in the primary wires is important. We show that accuracy can be much improved by better management of such loops, but for use in the VHF range it is necessary to use a current probe of the smallest possible overall physical dimensions. We explain how to minimize errors in the application of existing current probes to DM measurements, and provide a guide to the design of improved probes and EUT wiring interfaces useful well into the VHF range.