Non-Intrusive Impedance Characterization of Two-Port Electrical Devices by Inductive Probes

Many industry applications related to power electronics, including electromagnetic interference (EMI) filtering, control system stability analysis, and electrical equipment monitoring, require broadband impedance measurements. To this aim, inductive probes have been proposed as non-intrusive signal couplers, clamped on wiring harness to carry out in-circuit impedance measurements while the system under test is normally operating. Previous contributions on this subject include either one-port methods, unsuitable for multiport systems, or multiport methods developed for admittance-matrix measurement. Besides, those methods were assessed up to 30 MHz, while the current CISPR-25 automotive standard extends conducted-EMI verifications up to 110 MHz. In this paper, an inductively-coupled measurement procedure is formulated for two-port impedance-matrix characterization. After discussing probe selection criteria, the proposed test setup is validated by ad-hoc passive networks, whose impedance matrix can be independently and accurately evaluated. Comparison with the obtained measurement data proves good accuracy from 150 kHz to 110 MHz. Results obtained by using different pairs of probes and the role played by the number of cable turns are also discussed. As a real automotive application example, a lead-acid battery is characterized in terms of physical and modal impedance parameters.