WP1: Josephson Impedance Bridges


This aim of this work package is to develop Josephson impedance bridges in both fundamental ways and also to reduce their complexity and cost. Josephson bridges represent an innovation in metrology: an “intrinsically referenced measuring method”. As such, they do not require calibrating at each frequency of operation unlike today’s bridges, but still offer uncertainties that equal today’s most accurate coaxial bridges.

Within this work package, these quantum based bridges will be developed further, optimised for precision calibrations and automated. Uncertainties for different waveforms will be investigated in Task 1.1 to understand limitations caused by transients between quantised voltage levels. Based on these findings the aim in Task 1.2 is to build Josephson bridges in a four-terminal-pair configuration using two completely separated quantum systems. The defining conditions for four-terminal-pair impedances can then be fulfilled and the uncertainty is expected to be a few parts in 109 (k=1) at 1 kHz. The aim of Task 1.3 is to take a step towards the dissemination of the impedance scale using Josephson bridges. A Josephson ratio bridge will be assembled and optimised using less equipment than historically used. Typically, the revised bridge will only use instruments available in laboratories operating a single programmable Josephson system for DC calibrations. Furthermore, this Josephson bridge will use a cryocooler which will make the system mobile and independent from a liquid helium supply. At the end of the JRP four Josephson impedance bridges will be operating in Europe namely the two-Josephson-array systems for low and medium frequencies at SP and PTB, the cryocooler based system at MIKES and the one-Josephson-array system at CEM.



WP2: Digital bridges


Start month: June 2013, End month: January 2016

The aim of this workpackage is to develop and implement digital bridges suitable for primary impedance metrology. Digital bridges apply mixed-signal electronics, the basis of modern electronics test and measurement instrumentation.

The bridges will cover the audio frequency range between 20 Hz and 20 kHz, and be able to measure impedances which have non‑decadic values and arbitrary phase angles over the entire complex plane. As a result, the transfer of traceability to commercial impedance meters will be improved.

Several competing technologies and bridge topologies have the potential to reach the performance required by primary metrology, by creating impedance scales with a base uncertainty better than 0.5 ppm which is lower than achievable from top-class commercial instruments. Two examples of this base uncertainty are the measurement at 1592 Hz of capacitances between 10 pF and 1 nF, or of resistance standards between 10 kΩ and 100 kΩ at frequencies between 1 kHz and 10 kHz. The workpackage will therefore investigate the most promising of these technologies.

The digitally-assisted (DA) and fully-digital (FD) impedance bridges targeted in this WP are modular in nature. One important module is the signal generation, which will be based on polyphase digitally‑synthesised sources (DSS) which are addressed in Task 2.1. Task 2.2 and Task 2.3 concentrate on the remaining modules required to build DA and FD bridges, respectively. Due to this modularity of the bridges, integration between the activities of the JRP-Partners and REGs will be achieved by exchange of information, prototype components, algorithms and software, and testing and characterisation results. There is a very high probability that a “good” solution, achieved on one bridge mesh, can be quickly implemented in other bridges.

DA bridges base their accuracy on calibrated passive reference devices acting as ratio arms (ratio transformers). The source accuracy, although important, does not directly limit the bridge accuracy.

FD bridges do not employ passive ratio arms. The bridge flexibility is highly improved and the frequency range is not limited by the ratio transformer. The DSS source accuracy is a direct limitation on the overall bridge accuracy. The targeted bridge base accuracies of ≤ 1 ppm can only be achieved by employing high-performance DSS sources, or with sampling using high-performance ADCs.

This WP will develop its own DSS sources for both types of bridges. Initial versions of the less accurate sources for DA bridges will be available much more quickly than the more accurate electronics required by FD bridges. Other parts of the bridges, for example the output stage optimised for inductive loads and the balancing strategies, can be developed with DA bridges and then simply adopted by FD bridges.

The expected performance of DA and FD bridges (target base accuracy of 0.1 ppm) will go beyond the current state of the art in measurement capability (magnitude and frequency range, measurement of non‑decadic standards, measurement at arbitrary phase angles), measurement speed, ultimate accuracy, and the workload of the operator. The key aim of all the tasks in this WP is to develop instruments suitable for implementing new traceability chains within the NMIs, to provide upgraded calibration services to customers, and to extend measurement capabilities.



WP3: New impedance standards


Start month: June 2013, End month: December 2015

The aim of this work package is to develop a series of new impedance standards to improve and extend the dissemination of the impedance units over the whole complex plane, i.e. with arbitrary values of the amplitude and the phase angle. This is an important issue for industry where the impedances encountered are not necessarily close to the axes. In addition, advances in micro- and nano-electronics require measurements of extremely low capacitances. Therefore, capacitance standards with very small values, as small as 10 aF, will be developed together with a programmable capacitance standard ranging from 10 aF to 10 pF.

In this workpackage, the JRP-Partners will collaborate during the design phase of the different standards and the fabrication of the standards will be then distributed among the JRP-Partners. TUBITAK and MIKES will develop passive impedance standards with non decadic values and phase angles other than 0° and +/- 90°. METAS will develop an active impedance standard able to simulate impedances with arbitrary value and arbitrary phase angle. LNE will develop passive capacitance standards with very small values and will also develop a programmable capacitance standard in collaboration with NIST.

These new impedance standards will be used in WP4 to compare and validate the performance of the new measuring systems developed in WP1 and WP2.



WP4: Towards dissemination of expanded impedance capabilities


Start month: March 2014, End month: April 2016

The main aim of this workpackage is to validate that measuring systems developed in the other WPs (WP1: Josephson bridges, WP2: Digital bridges and WP3: New impedance standards) function with the targeted uncertainties.

This work is particularly important as it constitutes the preliminary condition for a dissemination of impedance measurements in the whole complex plane (i.e. arbitrary phase angles and ratios) and in the extended frequency range (10 Hz to 20 kHz) addressed in this JRP.

Comparisons to “traditional” manual state-of-the-art bridges for measurements of nearly pure impedances (capacitance and resistance) are an integral part of the development of the new standards and are thus incorporated in WP1 and WP2. Task 4.1 will compare Josephson bridges to digital bridges (DA bridges and FD bridges) and also FD bridges to DA bridges. These comparisons will be performed at intermediate or arbitrary phase angles and for non-decadic impedance ratios using the new impedance standards specifically developed for this task in WP3. All NMIs/DIs involved in the JRP will participate in these comparisons

Furthermore, the expected performance of the novel impedance bridges developed in WP1 and WP2 will enable the investigation of the problem of frequency dependence of capacitors in Task 4.2. Uncertainties below parts in 107 for capacitance measurements will become available below 500 Hz. As mentioned in B1.b, the differences in capacitances reported by CCEM could directly impact the recommendation for the “mise en pratique” of the farad and the ampere. Moreover, the frequency range between 50 Hz and 500 Hz is of great interest for industry. The new bridges will be also used to investigate the frequency dependence of high capacitance values in this range.

Task 4.3 directly addresses the lack of standards for providing traceability for capacitance values below 1 pF. The small and ultra-small capacitance standards developed in Task 3.2 will be characterised and calibrated. This task will involve comparison measurements with collaborators NIST and Andeen-Hagerling.

This work package will demonstrate the advances in impedance metrology brought about by the whole JRP. On the one hand it will show the great flexibility of the new bridges for measurements in the whole impedance complex plane with uncertainties comparable to the “classical” manually-operated bridges, and on the other hand will demonstrate the great leap forward achieved in the extension of the impedance scales and in the dissemination of the impedance units. An additional goal in this WP is to establish measurement procedures for impedance values where none currently exist, primarily due to the lack of standards with the required stability and the lack of standards for arbitrary ratios, intermediate phase angles, or for sub-pF capacitance.



WP5: Creating Impact


Start month: June 2013, End month: May 2016

The activities within this work package will ensure that the benefits of the JRP outcomes will be implemented. On the one hand, this JRP will impact the very top hierarchy of the metrology pyramid for the dissemination of the international system of units. On the other hand, the JRP will impact the wider scientific and metrological communities, as well as stakeholders from industry. Research groups from universities, instrument manufacturers, industrial and accredited calibration laboratories, sensor developers and other stakeholders are primarily interested in fast and reliable calibrations at a given uncertainty level, which is not necessarily the best that can be achieved.

This breadth of “end-user” communities will require a variety of methods to publicise the results of the JRP. As an example, the JRP-Partners not only plan to present the JRP achievements at scientific conferences such as CPEM or the north American NCSLi, but also at national meetings of the calibration laboratories such as the PTB Seminar for the German accreditation service (DAkkS), the Congreso Español del Metrología in Spain or the ltalian Congresso dell’Associazione Gruppo di Misure Elettriche ed Elettroniche.

The dissemination of the outcomes in the JRP will be carried out in the international metrology community, within and beyond Europe, in order to promote international harmonisation of the applied techniques and methods developed in this JRP. The dissemination and appropriate linkage to the “end-user” community, stakeholders and standards committees will be monitored at each project meeting.

Opportunities for knowledge transfer of the standards, the measurement methods and set-ups developed in the JRP will be explored in line with the results achieved. In particular the knowledge transfer and exploitation activities will be reviewed at the midterm where appropriate additional activities will be initiated.



WP6: JRP Management and Coordination


Start month: June 2013, End month: May 2016

The project will be managed within six work packages (WPs). Each work package will be managed by an experienced WP leader and the JRP-Coordinator, PTB, will also lead WP6. It is recognised that the management process has to be assisted and therefore sufficient labour resources have been allocated to this task for WP leaders.

The structure of the JRP and the associated management hierarchies are determined by the work packages and by their task sub-structure. The JRP-Coordinator will be supported by the project management board consisting of the leaders of each workpackage. The members of the project management board will guide the JRP, attend the project meetings (accompanied by additional specialists, if needed), organise the progress meetings at their local institutes, synchronise the project’s progress with the milestones specified in the Gantt chart, and call additional meetings if needed to ensure the overall project’s success. The project management board will report to the JRP-Coordinator. Regular contacts by phone and email (if suitable using video conference facilities) as well as progress meetings will aid in managing the project. Each meeting will be attended by at least one representative from each JRP‑Partner and the REGs.