Method for Ensuring Accurate AC Waveforms With Programmable Josephson Voltage Standards

The amplitudes of stepwise-approximated sine waves generated by programmable Josephson voltage standards (PJVS) are not intrinsically accurate because the transitions between the quantized voltages depend on numerous conditions. We have developed a method that ensures that the total rms output voltages of arbitrary ac waveforms synthesized by the PJVS are accurately referenced to the quantized Josephson voltages. This is accomplished by digitizing the output waveform, utilizing the quantized voltages to correct digitizer gain, noise, and nonlinearity, and then utilizing measurements of the bandwidth, rise time, and harmonic content to precisely tune the PJVS bias parameters. Our goal is to develop an AC standard source that can directly synthesize voltages with the accuracy expected of a quantum-based standard without the use of a thermal voltage converter.

I. Introduction

Since the introduction of series arrays of intrinsically shunted Josephson junctions in the mid-1990s [1]–[3], PJVS systems have utilized selectable sets of junction arrays to generate stepwise-approximated waveforms with the intent of bringing quantum-based accuracy to the field of ac voltage metrology. Significant progress has been made for a number of ac metrology applications, most usefully with sampled comparisons [4]–[10] and for (50–60 Hz) power metrology [11], [12]. Unfortunately, the inherent error in the output voltage caused by the transitions between the quantized voltages [13], [14] has remained a fundamental and unsolved weakness. Consequently, thermal voltage converters (TVCs) remain the primary standards for ac voltage worldwide. Various measurement schemes have been investigated that utilize combinations of TVCs and stepwise PJVS synthesized waveforms working together [15], [16]. However, the previous techniques that have exploited PJVS waveforms have not provided the required accuracy at frequencies above 100 Hz without utilizing a TVC.