EMRP-SIB53: Automated Impedance Metrology extending the Quantum Toolbox for Electricity

• Forschungsstelle: METAS
• Projektnummer: F-5212.30024
• Projekttitel Englisch: EMRP-SIB53: Automated Impedance Metrology extending the Quantum Toolbox for Electricity

Schlüsselwörter (Englisch)

Impedance metrology, sampling technique, AC coaxial bridge

Kurzbeschreibung (Englisch)

This project aims to improve impedance metrology at the lowest uncertainties by making the transition from measurements performed at predefined values and relative phase angles for selected frequencies to any ratio over the complete audio frequency range (20 Hz to 20 kHz). This will be achieved with two different types of impedance bridges: Josephson bridges and digital bridges.
Impedance is central in electrical metrology and also in many other disciplines that use impedance changes in sensors for their measurements. The present dissemination is based on manually operated coaxial bridges that can only be used over a limited range of frequencies. This project will develop a recent advance in electrical metrology, an intrinsically referenced measurement for electrical impedances that does not require calibration. This intrinsically referenced method employs quantum standards, which are already well established in voltage and dc-resistance metrology. Quantum standards are independent of time, place and environmental conditions. They allow extremely low measurement uncertainties and highly robust operation while only requiring comparatively simple tests to ensure their quantized behaviour. Similarly, this innovative intrinsically referenced measuring method only requires a simple test to check the correct operation of the Josephson arrays independently of the frequency of the measurements. Josephson bridges offer an unprecedented combination of low measurement uncertainty, extended frequency range and speed of operation for this performance.
When applied in this way, the Josephson arrays “simply” deliver two waveforms of precisely known amplitudes and defined relative phase. The ratio transformer(s) which has been used to date for this purpose is no longer required. As a result, the bridge no longer needs to be recalibrated for each signal frequency. The additional limitation for the frequency range of the bridge is also eliminated. Conventional digital signal synthesizers could also fulfil this role, but despite the continuous improvement of semiconductor electronics, they still lead to deterioration in the accuracy of an impedance ratio bridge by one or more orders of magnitude. However, uncertainties in the region of parts in 10⁷ seem possible with the latest technology and new ideas for modifying the amplitude of the fundamental frequency component generated in a targeted manner. Novel electronic signal generators to fulfil the requirements of this level of uncertainty in fully digital bridges will be developed.
These two independent measurement methods for determining impedances will significantly extend the coverage of the impedance complex plane, whilst also providing a measurement infrastructure which due to its automated operation will also allow the use of such setups in metrology laboratories that do not have at their disposal extremely highly-skilled metrology personnel. As with all new developments, these novel bridges will be validated against tradi-tional measurements during their development in the project.

This project is part of the European Metrology Research Programme (EMRP, http://www.euramet.org/index.php?id=emrp); it is partly funded by the European Union on the basis of Decision No 912/2009/EC.

Projektziele (Englisch)

The direct impact of the project will be mainly regarding traceability and NMI capabilities. New routes will be established where there are none at the moment (for impedance values commonplace in nanotechnology). The extension of NMI calibration capabilities to intermediate phase angles will also allow confirmation of industrial calibration of standards. Automated impedance bridges for unprecedented accuracies will also reduce the costs of calibrations. At least nine NMIs will operate automated bridges by the end of the project; five of them based on Josephson arrays. Four partners will develop and test fully digital bridges that also operate over a comparable frequency range and without the need for a highly skilled operator. Additional potential for reductions in the complexity and costs associated with today’s maintenance of the impedance scales will be realized through the programmable impedance simulator developed in this project.
Specific Objectives for METAS:- 

• To develop a four terminal-pair bridge, based on sampling technique, to compare im-pedances ranging from 10 ohm to 1 Mohm at frequencies from 20 Hz to 20 kHz.
• To  develop a four terminal-pair  impedance simulator for |Z| between 10 ohm and 1 Mohm with a phase angle between -90 deg and 90 deg in the frequency range from 50 Hz to 20 kHz.

Abstract (Englisch)

The purpose of the project was to improve electrical impedance metrology at the lowest uncertainties by making the transition from measurements performed at predefined values and relative phase angles for selected frequencies to any ratio over the complete audio frequency range (20 Hz to 20 kHz). The work of METAS within the project was twofold: First to develop further an existing sampling-based bridge to decrease the uncertainties by a factor 10 for the comparison of any kind of impedance standards. The second was the development of an impedance simulator (iSimulator) allowing the calibration of measuring instruments (LCR-meters) over the entire complex plan.
At the end of the project, the performances of the new sampling-based bridge have been improved by a factor 10 for the comparisons of impedances into a 1 to 1 ratio and by a factor 2 to 5 for other impedance ratios.

Umsetzung und Anwendungen (Englisch)

Further developments are now undertaken to use this new bridge for calibration services.
The development of the iSimulator is terminated and its validation has been performed either at METAS or during the on-site demonstration at esz AG in Eichenau, Germany, in February 2016.
New Calibration and Measurement Capabilities (CMCs) for the calibration of LCR-meters at any impedance between 1 Ω and 1 MΩ with any phase angle and at any frequency between 50 Hz and 20 kHz have been submitted. In 2017, METAS is the first laboratory to offer such calibration services.

Publikationen / Ergebnisse (Englisch)

1. F. Overney and B. Jeanneret, “"RLC" Bridge Based on an Automated Synchronous Sampling System,” IEEE Trans. Instrum. Meas., vol. 60, no. 7, pp. 2393–2398, Jul. 2011.
2. F. Overney, “Sampling Based RLC Bridge: Realization of the inductance scale,” in VDI-Berichte Nr. 2216, VDI Wissen., VDI Verlag GmbH, Düsseldorf, 2013, pp. 195–206.
3. F. Overney and B. Jeanneret, “Impedance Simulator for Automatic Calibration of LCR Meters: Proof-of-Principle Experiment,” imeko.org, pp. 156–160, 2013.
4. F. Overney and B. Jeanneret, “A state-of-the-art impedance simulator,” METinfo, vol. 2016, no. 2, pp. 26–31, 2016.
5. F. Overney and B. Jeanneret, “Calibration of an LCR-Meter at Arbitrary Phase Angles Using a Fully Automated Impedance Simulator,” IEEE Trans. Instrum. Meas., pp. 1–8, 2017.