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 0.5-V Josephson junction array devices for quantum voltage standards**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFF02000402).

The design, fabrication, and the characterization of a 0.5-V Josephson junction array device are presented for the quantum voltage standards in the National Institute of Metrology (NIM) of China. The device consists of four junction arrays, each of which has 1200 3-stacked Nb/NbxSi1−x/Nb junctions and an on-chip superconducting microwave circuit which is mainly a power divider enabling each Josephson array being loaded with an equal amount of microwave power. A direct current (dc) quantum voltage of about 0.5 V with a ∼1-mA current margin of the 1st quantum voltage step is obtained. To further prove the quality of NIM device, a comparison between the NIM device with the National Institute of Standards and Technology (NIST) programmable Josephson voltage standard (PJVS) system device is conducted. The difference of the reproduced 0.5-V quantum voltage between the two devices is about 0.55 nV, which indicates good agreement between the two devices. With the homemade device, we have realized a precise and applicable 0.5-V applicable-level quantum voltage.

Using a Programmable JVS for evaluation of Zener voltage standards stability and secondary uncertainty under controlled temperature variation

A study was carried out with two 732B model Zener-diode based electronic voltage standards, in order to analyze the influence of external temperature variation in their outputs. The temperature was controlled and the external variations were monitored. Inmetro Programmable Josephson Voltage Standard (PJVS) and Secondary systems were used for this purpose. Studies have shown that, despite the internal temperature control, there is a strong correlation between the external temperature and the internal thermistor value, for both Zeners. Inmetro Zener calibration uncertainty budget was revisited and the achieved uncertainties are ±0.09 μV (at 1.018 V) and ±0.4 μV (at 10 V), k=2.

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.

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.

Heat transfer analysis of a programmable Josephson voltage standard chip operated with a mechanical cooler

We are developing a 10V programmable Josephson voltage standard (PJVS) system using a 10K mechanical cooler that enables liquid-helium-free operation. In our previous experiments, we identified a problem that the optimum bias operating points may be affected by the heating effect of the chip depending on the output voltage. In this study, we have observed cross-sectional and in-plane structures of the PJVS chip module in order to determine the cause of the heating effect. We perform a heat transfer analysis using the finite element method with several models based on the observation results. We confirm a temperature increase of approximately 100mK at the chip surface, due mainly to unintended voids in the InSn solder.

Traceability of DC high voltage measurements using the Josephson voltage standard

This paper introduces a new methodology for obtaining high voltage DC measurements traceability to the International System of Units (SI) at the Egyptian National Institute for Standards (NIS). The traceability has been achieved via the NIS automated 10 V DC Josephson Voltage Standard (JVS). A 100kV DC voltage divider with a nominal voltage ratio of 10,000:1 is being used with its display in parallel with a high sensitive digital voltmeter. The traceability has been realized by calibrating this digital voltmeter via the JVS system and then it has been used to calibrate the divider display readings. Moreover, the divider ratio has been accurately calibrated using a traceable calibrator source on its high voltage side and the calibrated digital voltmeter on its low voltage side. Accurate and traceable high voltage values have been obtained associated with their expanded uncertainties. Highly improved uncertainties have been achieved using this new calibration technique.

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.

Low-Frequency Characterization in Thermal Converters Using AC-Programmable Josephson Voltage Standard System

We have measured the low-frequency characteristics of thermal voltage converters down to 1 Hz using a differential sampling measurement system based on ac-programmable Josephson voltage standard (AC-PJVS) system. The measured ac-dc transfer difference of a planar multijunction thermal converter using our system is evaluated to be <; 1 μV/V above 10 Hz and <; 35 μV/V at 1 Hz with lower uncertainties compared to the conventional method. The estimated overall uncertainty measured by using our system is 3.1 μV/V ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> = 1) at the frequency of 10 Hz and root-mean-square voltage of 3 V. Our measurement results above 10 Hz are in good agreement with the results obtained by the conventional method within a standard deviation of the mean. Our differential sampling measurement system using AC-PJVS is a useful tool for low-frequency ac voltage metrology.

Cryogen-Free Operation of 10 V Programmable Josephson Voltage Standards

Given the recent shortages of liquid helium, cryogen-free operation of superconducting devices, such as programmable Josephson voltage standard (PJVS) systems, has become preferable worldwide, and a necessity in some locations. However, reliable operation on a cryocooler is heavily dependent on the ability to create a constant temperature that is low enough to allow the PJVS junctions to operate uniformly. In this work, we systematically investigated as a function of temperature the performance of NIST 10 V PJVS chips employing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Nb/Nb}_{x}\hbox{Si}_{1-x}/\hbox{Nb}$</tex></formula> superconducting junctions. Additionally, we addressed the major factors limiting the performance of a cryocooled PJVS: adequate attenuation of the coldhead temperature oscillations and the minimization thermal gradients between the chip and the cryocooler. Through the development of a robust and reproducible method for soldering chips to a Cu carrier (package), we increased the thermal conductances within the packaging to their practical maximum values. This, in addition to the incorporation of a passive two-stage thermal filter, allows us to confidently predict that the required cooling power for the successful cryogen-free operation of the NIST 10 V PJVS is <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 0.5 W at 4 K.

Direct comparison between the NIST 10 V Conventional Josephson Voltage Standard and 2.5 V Programmable Josephson Voltage Standard

In the present paper, the intercomparison results of the NIST 10 V Conventional Josephson Voltage Standard (NIST10) and 2.5 V Programmable Josephson Voltage Standard (PJVS) Systems have been discussed. The two systems were directly intercompared at 1.018 V and 2.511 V from September 2006 to February 2007. The differences between the two systems (i.e. NIST10 — PJVS) at 1.018 V and 2.511 V were 0.21 nV and −0.95 nV respectively. The intercomparison results reveal that the noise of digital voltmeter (DVM) affects the measurement results significantly. Even with DVM of the same model, their noise rejection capability may be different when accuracy of a few nanovolt (nV) is required, although for Zener reference standard measurement, it is sufficient because the measurement uncertainty is dominated by the noise and non-linear drift of Zener reference standard.

Automation of Direct Josephson Voltage Standard Comparison and an Application

A direct comparison between two Josephson voltage standard (JVS) systems has been used in the Bureau International des Poids et Mesures key comparisons or regional supplementary comparisons to ensure the uniformity of the representation of the volt based on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J - 90</sub> with a relative uncertainty of a few parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> at a nominal voltage of 10 V. Most direct JVS comparisons are manually operated. Hardware and protocol for an automatic direct JVS comparison have been developed to verify the performance of a JVS while retaining the same level of uncertainty attained by a manual comparison. The automatic JVS direct comparison significantly improves the efficiency of the data acquisition. A case study examining the offset related to the digital voltmeter polarity in an automatic direct JVS comparison is presented.

Experimental Determination of the Voltage Lead Error in an AC Josephson Voltage Standard

The National Research Council (NRC) of Canada has recently established an alternating-current Josephson voltage standard (ACJVS) system based on the National Institute of Standards and Technology pulse-driven Josephson junction arrays. This paper describes the efforts undertaken at the NRC and the experience that was gained. An experimental method of measuring corrections for the voltage probe lead errors is described, and first results are reported. By introducing the ACJVS, the NRC will be able to reduce the uncertainties of the thermal transfer standard calibration by threefold on the 200-mV range and five- to tenfold on the 20-mV range, in comparison with thermal-converter- and micropotentiometer-based calibrations.

Practical Operation of Cryogen-Free Programmable Josephson Voltage Standards

Cryogen-free operation is rapidly becoming the preferred implementation of most superconducting electronics systems including programmable Josephson voltage standard (PJVS) systems. There are strong operational incentives for using the smallest possible cryocooler in order to minimize acoustic noise, system footprint, and power consumption. In addition, Nb/NbxSi1-x/Nb junction technology, which operates near 4 K, offers better yield than NbN/TiNx/NbN technology, which can operate at 8.5 K, thus making lower temperature operation near 4 K desirable. As junction density increases, however, self-heating of the junctions can create significant thermal gradients between the arrays and coldhead. Thus careful design of the overall system is required to maintain acceptable operating margins. We have developed a calorimetric measurement technique to characterize the system variables and used it to evaluate several different PJVS configurations. This technique uses the PJVS subarrays as both heat sources and temperature sensors, in conjunction with a time gated measurement technique, to characterize the thermal response of the system. A passive thermal filter incorporating a Pb thermal mass is used to reduce the temperature oscillations of the cryocooler. Our results suggest that, with appropriate system design, operation of a practical 10 V PJVS on a small (nominally 100 mW capacity at 4.2 K) cryocooler may be possible.

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.

Single-Chip 10-V Programmable Josephson Voltage Standard System Based on a Refrigerator and Its Precision Evaluation

We precisely evaluated a single-chip 10-V programmable Josephson voltage standard (PJVS) system based on a closed-cycle refrigerator by directly comparing it with a conventional Josephson voltage standard system. The PJVS chip fabricated using NbN/TiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /NbN junction technology was used. The result of the measurement showed a good agreement with a combined standard uncertainty of 3.1 parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> at 10 V.

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.

Microwave Packaging for Voltage Standard Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Improved packages for Josephson Voltage Standard (JVS) circuits have increased operating margins, reliability, and longevity of JVS systems. By using the “flip-chip-on-flex” technique, reliable chip and cryoprobe mounting have been demonstrated. The microwave structures on these packages have been improved such that more power can be delivered to the JVS chip over a wider frequency range: DC to over 30 GHz. Detailed finite-element simulations were performed to optimize the chip-to-flex launches as well as the on-flex transmission lines. It was found that coplanar waveguide transmission line designs had improved insertion and return losses compared to those of the microstrip transmission line designs, in large part due to the large discontinuities associated with through-substrate vias for microstrip ground connections. The improved coplanar-waveguide package/probe yielded insertion loss dominated by the <formula formulatype="inline"><tex Notation="TeX">$\sim$</tex></formula>0.25 dB/GHz cable loss and VSWR better than 2:1 for the entire 0–30 GHz band. Substantially larger JVS system operating margins were measured using the coplanar-waveguide package; for a 5120 junction array a quantized voltage step greater than 1 mA bias current range is shown for a 10–30 GHz band. </para>