A General Calibration Procedure for Measuring RF Voltages and Currents Applied to the EMC Analysis of Automotive High-Voltage Power Networks

This paper presents a general calibration procedure for probing RF voltages and currents in the time domain with conventional oscilloscopes using a directional coupler. It eliminates the need for a separate power calibration and takes advantage of the large dynamic range of a vector network analyzer during the calibration step. As a first application of this procedure, the electromagnetic disturbances on high-voltage (HV) lines in vehicles with hybrid or electric drive are precisely determined by applying an HV automotive-grade directional line coupler. In conjunction with the calibration algorithm, this coupler permits measuring the EMI relevant noise voltages and currents during real operation conditions of the vehicle without the need to modify the HV network by inserting LISNs or other artificial networks. The presented HV coupler offers a superior RF performance between its HV ports yielding an insertion loss of max. 0.8 dB and a return loss of min. 15 dB with respect to a typical low characteristic impedance of HV lines of 11 Ω over a frequency range from 150 kHz to 1 GHz. Thus, the coupler itself causes minimal influence on the original waveforms within the HV network and facilitates measurements very close to the noise sources, e.g., the power inverter, at a distinct calibration plane. Furthermore, it is shown how the accurate knowledge of these voltages and currents can be utilized to predict radiated emissions or to derive load impedances. Herein, the measurements are performed in a CISPR 25 related setup over a frequency range from 150 kHz to 500 MHz, achieving a good agreement to conventional measurement methods.