Measurement Method for Mixed-Mode S-Parameters of Termination Structure in Differential Communication Line

Bulk current injection (BCI) tests are considered an important method of evaluating the immunity of a differential communication system. Malfunctions during BCI tests are partly caused by common-mode reflection and mode conversion of electronic control units (ECUs). Their characteristics are determined by termination structures (such as connector, enclosure, and ground wiring) and components on a printed circuit board (PCB) housed in ECUs. Conventional measurement methods using current probes cannot separate the characteristics of the termination structures for ECUs on one side because the measurement results include the effects of cable length resonances and reflections at the ECUs. Furthermore, the methods cannot measure mode conversions between the differential and common modes because currently, the current probe measures only the common mode. Thus, we cannot obtain an accurate model of the ECU on one side in the differential communication system. This study proposes a method to measure mixed-mode S-parameters of the termination structures for the ECU on one side using a current probe. We designed the load simulator on the other side to suppress the reflection and mode conversion using matched termination and balanced circuit. Consequently, we measured only the equipment under test (EUT) characteristics without cable length resonances. In addition, we placed two ports on the PCB instead of an IC to measure transmission through the EUT. Subsequently, we measured the characteristics including mode conversions. Furthermore, the effectiveness of this method was verified by SPICE simulation and experiment. In the experiment, experimental and theoretical results were compared considering measurement errors.