Differential sampling measurement of a 7 V rms sine wave and a programmable Josephson voltage standard

A 10 V programmable Josephson voltage standard has enabled sine waves with rms voltages up to 7 V to be accurately measured with differential sampling methods. This paper reviews the challenges and limitations of differential sampling that arise when rms voltages greater than a few volts are measured. Preliminary measurements confirmed the capability of the NIST 10 V Josephson array to perform this task and emphasize the need for highly stable and low-phase-noise ac sine wave voltage sources in order to further reduce the measurement uncertainty¹.

I. Introduction

NIST recently completed development of a 10 V programmable Josephson voltage standard (PJVS) [1] that allows rms voltage measurements to be extended up to 7 V. The differential sampling method was previously implemented for measuring 1.2 V rms amplitudes for a new electrical power standard [2] [3]. This straightforward sampling method can characterize any sine wave source signal by use of a stepwise-approximated sine wave that is synthesized with the PJVS. Each step provides a quantum-accurate voltage reference for the different sample periods. The residual differential voltage is measured with a sampling voltmeter in a null-detector configuration. Particularly suited for waveform frequencies below 1 kHz, the differential sampling method avoids the limitations induced by the PJVS transients that occur when it switches between quantized levels [4] [5]. Increasing the sine wave source's rms voltage up to 7 V opens new options for the direct calibration of instruments such as ac calibrators and ac digital voltmeters, without the use of a thermal transfer standard [2], [6].