Extending voltage range to 10 V rms in AC–DC difference measurements with AC programmable Josephson voltage standard

Abstract

The larger-scale programmable Josephson voltage standard (PJVS) chip composed of 524 288 niobium nitride-based overdamped Josephson junctions enables us to generate stepwise-approximated AC waveforms with a root-mean-square (rms) amplitude of up to 10 V. In this study, we have extended the voltage range of the differential sampling system based on the AC-PJVS to an rms value of 10 V with the larger-scale PJVS chip. The sampling digital voltmeter (DVM) that is employed to measure differential voltages is modified to use a 10 MHz reference clock instead of an internal clock. This modification substantially improves the uncertainty of AC-DC difference measurements at low frequencies, which is caused by the internal clock error of the sampling DVM, by preventing the incoherent sampling of the differential voltages. The expanded uncertainties (k = 2) of the AC-DC difference measurements are reduced from 7.0 µV/V to 1.8 µV/V for a frequency of 62.5 Hz and from 8.0 µV/V to 4.4 µV/V for a frequency of 1 Hz. The improvements in the uncertainty are more significant compared with a conventional method at low frequencies, where the accuracy of a thermal voltage converter is reduced.