A portable VNA-based system for grid impedance measurements

Conducted emissions are strongly dependent on the grid and EUT impedance, to measure in live conditions. A portable system based on a low-cost VNA and two internal or external current probes is presented. It was verified in the lab against various calibration standards (50 Ω, mismatched termination, 150 Ω coupling-decoupling network and 100 pF capacitor) using a Agilent E5061B as reference. Results of calibration and the effects of probes non-linearity are provided, having considered both saturation (with an external bias circuit) and signal-to-noise ratio (by injecting external noise). The maximum observed spread of impedance magnitude is 1% for matched condition (50 Ω) and 10% in mismatched conditions. Performance can always be improved by using a more expensive and bulkier VNA.