Characterization of current transformers for impedance measurements in automotive immunity test setups

The most commonly used automotive immunity test methods are described in the ISO 11452 series. The bulk current injection (BCI) and absorber line shielded enclosure (ALSE) apply different coupling mechanisms to assess the immunity of a device under test (DUT). The correlation between these methods is often poor, in spite of using the same cable harness and loads. It can be improved by enforcing the equivalence between the currents injected into the terminal units attached to both harness ends. Such procedure requires knowledge about the impedance at each harness end, which is directly associated with the impedance at any position along a test harness (loop impedance). Estimating the loop impedance using a RF current transformer (CT) proves to be a practical approach, due to the simple application without the need for major modifications of the test setup. In this paper, first in accordance to the lumped element model, the transformation phenomena including the input impedance, the transfer impedance, and the insertion impedance are derived. Furthermore, a wide band modelling procedure for CTs based on de-embedding a calibration fixture is developed, which provides an individual dataset suited for extracting the transformation properties and allows the expansion of the application for the impedance measurement. By focusing on the sensitivity of the input impedance as a primary quantity for measuring the loop impedance, pros and cons of using typical EMC current probes for the impedance measurement are assessed experimentally.