Digital Josephson Impedance Bridge

• Forschungsstelle: METAS
• Projektnummer: F-5212.30079

Schlüsselwörter (Englisch)

Josephson Voltage Standard, Impedance Bridges, Pulse Driven Arrays, Digital Bridge

Kurzbeschreibung (Englisch)

Besides the traditional SIS Josephson junction arrays which provide the reference for DC voltages, the quantum AC voltage metrology is based on two different systems: the programmable standard (PJVS) and the pulse driven standard (ACJVS). The PJVS generates an AC voltage by timely switching the number of active Josephson junctions. The major drawback of this approach is that the voltage is not precisely defined during the switching process. These transients limit the bandwidth of the system to 1 kHz. The ACJVS is based on a more complicated approach which is relying on a modulation of the microwave frequency driving the Josephson junctions. This system is not plagued by transient voltages and its bandwidth can extend at least to a MHz. However, raising its output voltage up to a volt has been a real challenge which necessitated almost 20 years of development.
Recently the idea of using the PJVS to perform impedance measurements has emerged and PTB is working on such a Josephson bridge using two PJVS. This approach is working well to compare impedances of the same kind. However, the transients strongly complicate the comparison of impedances of different kind. Therefore the natural idea is to use a transient free system, namely the ACJVS. Only NIST has this ability and only METAS has the appro-priate impedance bridge to perform this task. During the project JB-DAB, both capabilities were associated for the first time. This project was extremely successful and its results have been summarized in a final report and a publication. However, this development has not reached maturity yet, and the proposal is to pursue the effort in the framework of a new pro-ject consisting of optimizing the concept now named “Digital Josephson Impedance Bridge (DJIB)”.
The aim of this project is to improve and extend the services in the area of impedance and ac voltages calibration at METAS. The first step in the project is to further develop and test the Digital Josephson Impedance Bridge (DJIB) with a dual AC Josephson Voltage Standard (ACJVS) source from NIST, which was first constructed during the project JB-DAB. The results of this evaluation will form the basis of the decision to purchase the ACJVS system from NIST.

Projektziele (Englisch)

The project objectives are:
- Systematic test of the DJIB bridge at METAS using the NIST dual ACJVS source.
- Purchase of a dual ACJVS source from NIST in order to develop and offer new and improved impedance measurement services at METAS.

Abstract (Englisch)

The project performed a full characterization of a Dual Josephson Impedance Bridge (DJIB) at frequencies up to 80 kHz by using the DJIB to compare the best available impedance standards that are (a) directly traceable to the quantum Hall effect, (b) used as part of international impedance comparisons, or (c) believed to have calculable frequency dependence.
The heart of the system is a dual Josephson Arbitrary Waveform Synthesizer (JAWS) source that offers unprecedented flexibility in high-precision impedance measurements. The JAWS sources allow a single bridge to compare impedances with arbitrary ratios and phase angles in the complex plane. The uncertainty budget shows that both the traditional METAS bridges and the DJIB have comparable uncertainties in the kilohertz range. This shows that the advantages of the DJIB, including the flexibility which allows the comparison of arbitrary impedances, the wide frequency range and the automated balancing procedure, are obtained without compromising the measurement uncertainties.
These results demonstrate that this type of instrument can considerably simplify the realization and maintenance of the various impedance scales. In addition, the DJIB is a very sensitive tool for investigating the frequency-dependent systematic-errors that can occur in impedance construction and in the voltage provided by the JAWS source at frequencies greater than 10 kHz.

Umsetzung und Anwendungen (Englisch)

The first and most important application of the DJIB is the direct realization of the farad from the ohm using a graphene quantized Hall resistance. This topic is the goal of the GIQS project, which will end May 31, 2022. By the end of GIQS, many spectacular improvements in the realization of the impedance and the customers services will have been achieved:

• Replacement of the quadrature bridge streamlining the establishment of traceability in the impedance area and extending its frequency range.
• Improvement in the calibration of key components (10:1 ratio transformer, main injection transformer) of the bridge currently used for customer calibrations (Autobridge), and the calibration of 12.9:10 ratio transformers.
• Comparison of any two impedance standards at the highest level of accuracy up to 80 kHz, with at least a factor 10 improvement over the present measurements uncertainty.
• Direct realisation of non-decadic ratios of interest to customers (such as 10 to 12.906).
• Productivity gain in capacitance calibration through direct R-C comparison over a wide frequency range and determination of the frequency dependence of capacitance standards

In a later development stage, starting in 2023 once the JAWS system is completely under control at METAS, the JAWS will have a strong impact in AC voltage too:  it will make AC voltage measurements directly traceable to the primary quantum standard of AC voltage leading to new and/or improved services like:

• Direct calibration of AC-voltmeters over a broad frequency range up to 100 kHz (using the results of the EMPIR project QuADC, where METAS has investigated and mitigated the effects of cable frequency response on AC-Josephson-based voltage measurements).
• Calibration of the integral nonlinearity of 24-bit ADCs up to 100 kHz.
• Direct calibration of inductive voltage dividers.
• Direct calibration of lock-in amplifiers down to the mV range.