Programmable Josephson Voltage Standard System Research Articles

Simulation Analysis of Phase Jitter in Differential Sampling of AC Waveforms Based on the Programmable Josephson Voltage Standard

The effect of phase jitter on differential sampling using the programmable Josephson voltage standard (PJVS) system is studied in this paper. A phase jitter model is established for the measured signal, and compensation coefficients for phase jitter removal are derived for three different post-processing methods based on the discrete Fourier transform algorithm (DFT). Based on our analysis, the phase jitter compensation coefficients are determined by the phase jitter angle distribution and harmonic order. Furthermore, after analyzing and simulating various common distributions, the phase jitter compensation coefficients have been verified. The simulation shows that when the standard deviation of the phase jitter angle is 20 ns, and the frequency of the measuring waveform is 3.46 kHz, the influence of the phase jitter is 1 × 10−7. The results of the simulation indicate that, in the differential sampling of AC waveforms using a PJVS system, phase jitter is one of the error terms for an uncertainty budget that cannot be neglected, particularly as the frequency of the measured waveforms increases.

Development of a PJVS system for quantum-based sampled power measurements

The paper deals with recent progresses at INRiM towards the development and characterization of a programmable Josephson voltage standard (PJVS) operating in a small liquid helium dewar as well as with its integration for the realization of a practical quantum sampling electrical power standard. The PJVS is based on a 1V superconductor-normal metal-superconductor (SNS) binary-divided array of 8192 Josephson junctions. To ensure proper operating conditions of the PJVS chip, a custom short cryoprobe was designed, built and successfully tested. The overall system is being developed in the framework of EMPIR project 19RPT01-QuantumPower. The goal is to establish a new quantum power standard (QPS) based on a single Josephson voltage standard for sampled power measurements and to gain confidence in running PJVS for precise calibration of digital sampling multimeters and arbitrary waveform digitizers used in the ac-voltage and power metrology community.

Stepwise compensation waveform generation by using Josephson voltage standards for joule balance

Flux linkage difference in the new joule balance can be obtained by the time integration of the induced voltage u(t) from the suspended coil relative to a moving magnet driven by a dc motor translation platform. Due to the finite acceleration of the dc motor, the transition and waveform of u(t) are finite and not regular. To accurately measure the time integration of u(t), a compensation waveform with precision time integration should be synthesized. In this paper, a stepwise compensation waveform is synthesized by a programmable Josephson voltage standard (PJVS) according to u(t). The accuracy of measuring the stepwise waveform with multiple transitions can be improved by reducing the ratio of the time integrated value of the total transitions to the total waveform less than one part in 102 in the joule balance. The time integration of the rise/fall transition is measured by a synchronized reference square wave generated by another PJVS system. With the total time integration more than 20 Vs, the uncertainty of the generated stepwise waveform is within 5.2 × 10−8 VsV−1s−1. The result confirms that the PJVS has the capability to generate a stepwise compensation voltage for flux linkage difference measurement.

Uncertainty Evaluation of a 10 V RMS Sampling Measurement System Using the AC Programmable Josephson Voltage Standard

We have evaluated the uncertainty in the differential sampling measurement of the rms amplitude of 10 V waveform synthesis based on an ac programmable Josephson voltage standard system with a compact cooler and a modified digital voltmeter (DVM). The error due to a sampling DVM timer is much reduced with an external 20-MHz frequency reference for the sampling DVM. The thermal electromotive force voltage of our reverse-polarity relay switch is sufficiently small and stable for the application of a differential sampling system. We have also performed a variety of measurements to evaluate this differential sampling technique. The evaluation results show that the overall expanded uncertainty is 1.3 parts in $10^{6}$ ( $k = 2$ ) for the frequency of 62.5 Hz at an rms amplitude of 10 V.

Precision square waves synthesized by programmable Josephson voltage standards for induced voltage compensation in a Joule balance

A programmable Josephson voltage standard (PJVS) can be used to generate a precision square wave for induced voltage compensation to measure the mutual inductance between the coils in a joule balance. In this paper, the influence of the transitions between quantized voltages in the synthesized square waves is analyzed in detail. The ratio of the time-integrated value of the transitions to the total waveform is reduced to several parts in 104 to improve the measurement accuracy. The influence of different configurations of the integrating digitizer is discussed. The result shows that when the voltages are in a quantum state, the time-integrated agreement between the measured and theoretical differences for two PJVS systems is within 4 × 10−9 V s V−1 s−1. For the total time integration of a voltage waveform larger than 2 V s, the combined relative uncertainty is less than 5.9 × 10−8 V s V−1 s−1. The result confirms the capability of the PJVS to generate a precision square wave for the joule balance.

Evaluation of Linearity Characteristics in Digital Voltmeters Using a PJVS System With a 10-K Cooler

We have evaluated the linearity of commercial digital voltmeters (DVMs) using a programmable Josephson voltage standard (PJVS) system. The PJVS system is composed of a 524 288-niobium nitride-based Josephson junction array that has the ability to generate arbitrary output voltages up to 17 V with a resolution of 12 bits. In our system, the junction array is operated in a 10-K compact cooler without a liquid-helium coolant. Up to now, we have succeeded in generating accurate dc voltage levels of up to approximately 13 V using this system. In this paper, we demonstrate our PJVS system as a tool for investigating the linearity characteristics of commercial DVMs, with an accuracy level of 0.1 $\mu \text{V}$ /V or better, and discuss the uncertainty sources in these measurements.

NIST 10 V Programmable Josephson Voltage Standard System Using a Low-Capacity Cryocooler

The recent shortage and increasing cost of liquid helium provides motivation for cryogen-free operation of superconducting devices such as NIST programmable Josephson voltage standard (PJVS) systems. However, operation on closed-cycle cryocoolers must not compromise the performance of the PJVS system. New cryogenic packaging and cryostat integration are presented that have been optimized for the NIST 10 V PJVS, demonstrating improved attenuation of coldhead temperature oscillations, and improvement of thermal conductances in the junction-to-coldhead path. When combined with an improved design of 10 V PJVS devices employing Nb/Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> /Nb junctions with increased operating margins, we have operated a NIST PJVS at 10 V with over 1.32 mA current margins on a nominal 200 mW cryocooler using a 3.0 kW water-cooled compressor. The new cryo-package, in conjunction with the improved generation of chips, eliminates the need for liquid cryogens in applications using NIST 10 V PJVS systems.

Calibration System for Zener Voltage Standards Using a 10 V Programmable Josephson Voltage Standard at NMIJ

We have developed a calibration system for the Zener voltage standards using a programmable Josephson voltage standard (PJVS) for liquid-helium consumption reduction and improvement in the calibration performance. The PJVS array is composed of 524 288 niobium nitride (NbN)-based overdamped Josephson junctions and is cooled by a 10-K compact cooler. To verify the validity of the system, indirect and direct comparisons of the PJVS system and our conventional Josephson voltage standard system have been carried out. As a result, an agreement within a relative expanded uncertainty of $8.4\times 10^{-10}$ was obtained for a nominal voltage of 10 V. We also discuss the uncertainty sources in the calibration of the Zener voltage standards using our PJVS system.

Direct comparison of two NIST PJVS systems at 10 V

Two NIST programmable Josephson voltage standard (PJVS) systems are directly compared at 10 V using different nanovoltmeters. These PJVS systems use arrays of triple-stacked superconducting niobium Josephson junctions with barriers made of niobium silicide. Compared with the voltages produced by Josephson voltage standards based on hysteretic junctions, PJVS systems using damped junctions produce predictable voltage levels. However, in order to guarantee the quantization of the voltages and to minimize the errors at the room-temperature voltage output, additional precautions are required. We report several experimental results of voltage measurements that contain significant systematic errors. The generated voltages appear reproducible but they are, in fact, inaccurate. When proper measurement procedures are followed, the results of a direct comparison using an analogue detector show that the two independent PJVS systems agree within 2.6 parts in 1011 at 10 V with a relative total combined uncertainty of 3.4 parts in 1011 (k = 1). Investigations show that the largest systematic error and most significant contribution to the uncertainty budget is caused by the leakage resistance of each PJVS to ground. This paper describes a measurement procedure to characterize this leakage resistance and one approach to including the resulting voltage error in the uncertainty budget.

A 10 V programmable Josephson voltage standard and its applications for voltage metrology

The concept of a programmable Josephson voltage standard (PJVS) was first proposed in 1997. Since then a significant amount of research and development work has been devoted to the fabrication of the programmable Josephson junction array and its deployment in a voltage standard system. This paper reports the recent development of a 10 V PJVS system at the National Institute of Standards and Technology (NIST) and its voltage metrology applications. The superior stability of the voltage step of the new 10 V PJVS enables it to perform the same tasks as the conventional Josephson voltage standard (JVS) that uses hysteretic voltage steps and to improve the efficiency and effectiveness of a JVS direct comparison. For the first time, a comparison between a conventional JVS and the NIST 10 V PJVS was performed in order to verify the performance of the NIST 10 V PJVS. The mean difference between the two systems at 10 V was found to be −0.49 nV with a combined standard uncertainty of 1.32 nV (k = 1) or a relative combined standard uncertainty of 1.32 parts in 1010. Automatic comparisons between the 10 V PJVS and a 2.5 V PJVS at 1.018 V were performed to monitor the long term accuracy and stability of the 2.5 V PJVS and to support the NIST electronic kilogram experiment. By matching the voltages of the two PJVS systems during a comparison, the type B uncertainty can be minimized to a negligible level. The difference between the two PJVS at 1.018 V was found to be −0.38 nV with a combined standard uncertainty of 0.68 nV (k = 1) or a relative combined standard uncertainty of 6.7 parts in 1010. Issues encountered during the PJVS comparison and potential challenges for 10 V applications are also discussed.

Evaluation of Low-Frequency Characteristic of a Thermal Converter Using Programmable Josephson Voltage Standard

We are developing a programmable Josephson voltage standard (PJVS) system based on a compact cryocooler. The system is capable of generating two arbitrary waveforms by use of a dual-output PJVS (DO-PJVS) device. Owing to this dual-output feature of the PJVS system, the low-frequency characteristic in the ac-dc transfer difference of a thermal converter has been investigated using the 90° addition method. The evaluation of the low-frequency characteristic of a thermal converter is of great importance for connecting conventional ac-dc transfer standards and new Josephson-based ac voltage standards. To fulfill this purpose, a dual-heater thermal voltage converter (DH-TVC) with improved low-frequency performance has been developed. The low-frequency ac-dc transfer difference of the DH-TVC was evaluated to be smaller than 1 μV/V at frequencies of as low as 1 Hz.

Precise Measurement of a 20-V Programmable Josephson Voltage Standard System

We demonstrated the precise measurement of a 20-V programmable Josephson voltage standard (PJVS) system with an accuracy of 10-9. A 20-V PJVS chip including 524288 NbN/TiNx/NbN Josephson junctions (JJs) was used and operated in a cryocooler. By optimizing a microwave frequency, the widest current margin of a voltage step greater than 1 mA was obtained at 15.85 GHz. The maximum voltage of 17 V was also obtained at 18.00 GHz although we could not obtain voltages exceeding 20 V due to defective JJs. We also successfully compared the PJVS system with a 10-V Zener diode with an accuracy of 10-8.

Error and Transient Analysis of Stepwise-Approximated Sine Waves Generated by Programmable Josephson Voltage Standards

We are developing a quantum-based 60 Hz power standard that exploits the precision sinusoidal reference voltages synthesized by a programmable Josephson voltage standard (PJVS). PJVS systems use series arrays of Josephson junctions as a multibit digital-to-analog converter to produce accurate quantum-based dc voltages. Using stepwise-approximation synthesis, the system can also generate arbitrary ac waveforms [i.e., an ac programmable Josephson voltage standard (ACPJVS)] and, in this application, produces sine waves with calculable root mean square (rms) voltage and spectral content. The primary drawback to this ACPJVS synthesis technique is the uncertainty that results from switching between the discrete voltages due to finite rise times and transient signals. In this paper, we present measurements and simulations that elucidate some of the error sources that are intrinsic to the ACPJVS when used for rms measurements. In particular, we consider sine waves synthesized at frequencies up to the audio range, where the effect of these errors is more easily measured because the fixed transition time becomes a greater fraction of the time in each quantized voltage state. Our goal for the power standard is to reduce all error sources and uncertainty contributions from the PJVS-synthesized waveforms at 60 Hz to a few parts in 107 so that the overall uncertainty in an ac power standard will be a few parts in 106.

Demonstration of a 10 V programmable Josephson voltage standard system based on amulti-chip technique

We have demonstrated a programmable Josephson voltage standard (PJVS) operation upto 10.84 V using a multi-chip technique. We combined two PJVS chips fabricated usingNbN/(TiNx/NbN)2 junction technology. Each PJVS chip was mounted on a single chip carrier using bondingwire, and the two chip carriers were connected by a simple Cu lead wire, and mounted on acryocooler. High-precision measurements confirmed flat voltage steps for all 22 cells, with apeak-to-peak variation of 100 nV and wide margins of at least 0.35 mA. We also confirmedthe stability of the voltage steps in spite of a temperature and RF frequency variation of ± 0.1 Kand ± 0.1 GHz, respectively.

4 K cryocooler implementation of a DC programmable voltage standard

NIST and HYPRES, Inc. have been collaborating to develop a 1 Volt DC programmable Josephson voltage standard (PJVS) that operates on a closed-cycle refrigerator. The goal of this work is to construct a platform that will allow the chip to work at 4 K without liquid helium, thereby making the system more convenient and eliminating the need for users to handle liquid cryogens. In our existing PJVS systems, the Josephson chip temperature is the only parameter that is not computer controlled. The addition of a cryocooler will allow automated warming and cooling of the Josephson device and enable an intrinsic voltage standard system in which every control function is automated, and the only required user input is the desired output voltage. The cryocooler package is designed to allow PJVS chips to be easily interchanged between the cryocooler and liquid helium cryoprobes.