Direct-reading absolute calibration of AC voltage ratio standards

A new system for AC voltage ratio absolute calibration has been developed at IEN. The system is based on the simplest possible application of the bootstrap method: during the step-up, the voltage differences are read directly with a vector voltmeter, purposely built to have an extremely high common-mode rejection ratio, exceeding 180 dB at 1 kHz. The system is built around a new two-stage divider, having k:10, k:11 and 1:100 ratios, a twostage bootstrap transformer (1:10 and 1:11 ratios) with a guarded one-turn secondary stage having adjustable cross-capacitances and a two-secondaries (k:10 and k:11) transformer for easy guarding. The method is insensitive to several possible systematic errors of the voltmeter or coming from the bootstrap operation. The system has been employed during the participation of IEN in the CCEM-K7 key comparison.