Guarded vector voltmeter for AC ratio standard calibration

A new setup for the calibration of the ratio of inductive voltage dividers against a ratio standard is under development at IEN. The setup is based on the direct measurement of voltage vector differences between the device under test and the standard, with a purposely built vector voltmeter described here. The uniqueness of the voltmeter is the extremely high common-mode-rejection-ratio, greater than 180 dB at 1 kHz, obtained with a bootstrapping-guarding technique. The voltmeter is battery-operated, has variable gain and a dual-channel synchronous demodulating stage with high harmonic rejection, and is operated with a personal computer through an optical-fiber interface.