Publishable Summary for 15SIB04 QuADC Waveform metrology based on spectrally pure Josephson voltages

Overview

This project will develop measurement systems needed for fast metrology-grade waveform analysis. The new systems are centred on true alternating current (AC)-voltage quantum devices which will both operate at the highest level of accuracy and be simple enough for exploitation outside of national metrology institutes (NMIs). The term "true AC-voltage quantum devices" refers to the recently achieved breakthrough which has provided spectrally pure quantised Josephson AC-voltages exceeding, for the first time, the usability threshold of 1 V root mean square (RMS).

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Need

The need for the development of measurement systems based on true AC-voltage quantum devices (or Josephson devices) is driven by their fields of application, as sensing and measurement are increasingly dependent on fast analogue to digital conversion (ADC) (objectives 1 and 3). Recent Research & Development (R&D) in precision integrated circuits and measurement equipment has also brought about a step change increase in the sampling rates and accuracies available (objective 4). Although the direct traceability of direct current (DC) electrical metrology using quantum standards is well established, emerging measurement applications using high end equipment are placing new demands on the traceability of dynamic AC-voltage quantities, and this cannot currently be satisfied by existing electrical metrology approaches. For example, the company Fluke (a manufacturer of industrial electronic test equipment) has written "One of the barriers to reducing the uncertainty of these multifunction calibrators for AC voltage is the magnitude of uncertainty inherited in the traceability chain" (objectives 2 and 5).

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Objectives

The overall objective of this project is to provide for end-users direct, efficient, and highly accurate traceability of AC-voltages. The traceability will be to the SI (the volt) for dynamic measurements in the most relevant range of DC 1 MHz to 1 kV. The specific objectives of the project are:

 

  1. To develop a quantum-based real-time measurement system utilising the Josephson effect representation of the SI volt. Novel methods for biasing Josephson junctions, such as the use of optoelectronic devices, will be exploited to achieve larger voltage levels, as well as approaches for direct ADC in terms of the Josephson constant KJ = 2e/h. Specialised electronic circuits will be developed over the range of voltages and frequencies relevant in precision waveform metrology. They will be used for interfacing the sensitive and accurate low temperature Josephson devices to room temperature industrial precision waveform instruments.

  2. To develop a robust and end-user friendly quantum system as a practical realisation for providing direct traceability of the redefined base unit "volt" to end users, i.e. either national measurement laboratories or the next tier of end-users in the calibration and test sectors. This will also include automation techniques, Helium (He)-free cryogenic systems (that cool samples to 4 Kelvin (K)) and cost-effective components.

  3. To evaluate digital signal processing techniques with respect to their contribution to measurement uncertainty and to validate measurement methods for AC voltage calibration based on spectrally pure Josephson-AC-voltage references. The target uncertainty is 10 nV/V-level for frequencies up to 1 kHz and better than 10 µV/V up to 1 MHz. This will be validated via calibration of commercial instruments against a quantum standard and will be performed in collaboration with manufactures of precision instrumentation.

  4. To scale quantum waveforms up to 1 kV using voltage dividers or amplifiers. By measuring the divider output directly with a Josephson based digitising system the higher voltage waveform will be linked to the Josephson volt. The ultimate aim is to reach uncertainties ranging from 5 µV/V at 1 kV / 50 Hz to 25 µV/V at 120 V / 100 kHz.

  5. To engage companies in the project and to facilitate the take up of the technology and measurement infrastructure developed by the project, as well as supporting the development of new, innovative products, thereby enhancing the competitiveness of EU industry.

 

 

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Progress beyond the state of the art

This project will develop a new true AC-voltage quantum device measurement system that will be able to synthesise and measure AC waveforms at the 1 V level for an extended frequency range up to 1 MHz. In comparison with conventional thermal transfer standards, measurement times will be reduced from 1 hour to 1 minute, and the new system will achieve 100-fold better uncertainties. The 20 times faster real-time feedback loop will also allow the traceability of arbitrary waveforms. In addition, the project will produce newly developed voltage dividers with the lowest possible level of uncertainty for the wide frequency range DC to 100 kHz and will also enable the direct scaling of voltages up to 1 kV against the new quantum standard.

  • Based on the work in the preceding EMRP project SIB59 Q-WAVE, this project will go beyond the state of the art by providing direct traceability over the full range of parameters used to specify converters. The optically-driven Josephson Arbitrary Waveform Synthesizer (JAWS) will be used and it is anticipated that with no crosstalk 10 nV/V uncertainties will be achieved for the full frequency range DC to 1 kHz and a few µV/V up to 1 MHz within a minute. VTT has built an inexpensive pulsed laser which already has been successfully tested with Josephson arrays.

  • Based on the work in SIB59 Q-WAVE, this project will also go beyond the state of the art by providing for the first time a real-time measurement system with a 100 MHz update rate. The top-level design is complete and building further stages is underway.
     
  • The project will also enable a wider distribution of quantum standards in general, or for the quantum voltage digitiser, and it will be crucial for the standards to be made as turnkey systems. A turnkey system implies a user-friendly interface, with robustness against the environment and cryogen free operation. Several parts of it, such as device under test control, calibration data import, measured data export and connection to calculation toolbox have been completed at CMI.

  • The project will use two different approaches. One approach will use techniques that are more familiar in the radio frequency (RF) community, aiming for uncertainties of µV/V up to 100 kHz [for details see the first publications]. The other approach is to compensate the effect of the cables without any need for external calibration such as AC-DC measurements. METAS has finalized this load compensation bridge and its evaluation is planned.

  • Finally, new broadband resistive voltage dividers with buffer amplifiers will be developed by the project, to scale voltages down from 1 kV to 1 V, in a frequency range from DC to 1 MHz, with uncertainties of 5 µV/V for 1 kV / 50 Hz and 25 µV/V for 120 V / 100 kHz. The novel buffer amplifier already developed and built by CMI already has achieved perfect specifications. Voltage scaling is important for making AC quantum standards appropriate as replacements for current RMS AC methods. A new verification method based on pulse-driven Josephson arrays has been established at 10 nV/V level.

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Results

1. To develop a quantum-based real-time measurement system  the Josephson effect representation of the SI volt. Novel methods for biasing Josephson junctions, such as the use of optoelectronic devices, will be exploited to achieve larger voltage levels, as well as approaches for direct ADC in terms of the Josephson constant KJ = 2e/h. Specialised electronic circuits will be developed over the range of voltages and frequencies relevant in precision waveform metrology. They will be used for interfacing the sensitive and accurate low temperature Josephson devices to room temperature industrial precision waveform instruments.

The top-level system design is complete. Following the specification of a quantum-based real-time measurement system, a first prototype of the analogue delta sigma stages has been designed and modelled and testing is underway. The project finally will produce a quantum-based real-time measurement system for the DC to 1 MHz range, using the Josephson effect representation of the SI the volt. This will be a robust and user-friendly quantum system and will provide direct traceability of the redefined base unit "volt" to end users, such as national measurement laboratories or the next tier of end-users in the calibration and test sectors.  PTB has continued to design and fabricate suitable arrays as well as new carriers. JV together with HSN have further improved the photodiode mounting procedure such that it is now on an absolute reliable level. This technology has made it on the cover of IEEE Trans. on Components, Packaging and Manufacturing Technology.

 

2. To develop a robust and end-user friendly quantum system as a practical realisation for providing direct traceability of the redefined base unit "volt" to end users, i.e. either national measurement laboratories or the next tier of end-users in the calibration and test sectors. This will also include automation techniques, Helium (He)-free cryogenic systems (that cool samples to 4 Kelvin (K)) and cost-effective components.

The overall structure, properties and requirements of the user-friendly software is designed and described in a document. Several parts of it, such as device under test control, calibration data import, measured data export and connection to calculation toolbox have been completed at CMI. The user-friendly interface will be particularly important as quantum systems in industrial laboratories are operated by trained technicians rather than specialised experts. In addition, the new interface will evaluate digital signal processing techniques with respect to their contribution to the measurement uncertainty and validate measurement methods for AC voltage calibration based on spectrally pure Josephson-AC-voltage references.
 
A cryocooler feasibility study is underway at INRIM. This will be an important step for increasing the output voltage. So far, the output amplitudes of JAWS in a cryocooler is limited to 100 mV, however the project intends to develop 1 V JAWS in a cryocooler, which would allow a wider distribution of such quantum standards.

The costs of a setting up a complete Josephson synthesiser/digitiser system should also be markedly reduced by the development of the new pulse pattern generator (PPG) based on a pulsed laser. This should allow the project’s new quantum systems to be used by end-users outside of NMIs, instead of the currently available very expensive commercial PPGs. VTT has built an inexpensive pulsed laser which has been demonstrated to be able produce quantized DC voltages with room-temperature photodiodes driving a Josephson array.

 

3. To evaluate digital signal processing techniques with respect to their contribution to measurement uncertainty and to validate measurement methods for AC voltage calibration based on spectrally pure Josephson-AC-voltage references. The target uncertainty is 10 nV/V-level for frequencies up to 1 kHz and better than 10 µV/V up to 1 MHz. This will be validated via calibration of commercial instruments against a quantum standard and will be performed in collaboration with manufactures of precision instrumentation.

The consortium has started to develop a real-time system with 100 MHz update rate that can provide access to the SI for non-stationary voltages. In particular, traceability will be provided for arbitrary waveforms with known spectral composition. Fast ADC- and Field Programmable Gate Array (FPGA)-boards are selected by the consortium.  METAS has finalized on a load compensation bridge that will take into account the impedance and the leakage admittance of the leads and the device under test (DUT). A great advantage of this method is that it can compensate the effect of the cables without any need for external calibration. Investigations were carried out to find the best triaxial cables and connectors that should be mounted in the custom designed cryoprobe.

 

4. To scale quantum waveforms up to 1 kV using voltage dividers or amplifiers. By measuring the divider output directly with a Josephson based digitising system the higher voltage waveform will be linked to the Josephson volt. The ultimate aim is to reach uncertainties ranging from 5 µV/V at 1 kV / 50 Hz to 25 µV/V at 120 V / 100 kHz.

A new prototype divider using the split guard technique has been constructed and is being tested at RISE. Two new prototype dividers will be built using hermetically sealed resistors for improved stability. VSL performed initial tests on the prototype divider without buffer amplifier. Another new type of divider is constructed. The stability of integrating ADC is investigated at low frequencies using a programmable Josephson voltage standard at low frequencies.
A final version of the buffer amplifier has been developed at CMI. One piece of the buffer has been constructed and partly characterised. Measurements showed flatness below 0.1 µV/V up to 10 kHz, 2 µV/V at 100 kHz, and below 100 µV/V at 1 MHz. Input capacitance is below 1 pF, output impedance below 150 mΩ at 1 MHz.The work on the alternative buffer amplifier design at RISE has been stopped, as the CMI amplifier is perfect for the project.

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Impact

Impact on industrial and other user communities

This project will enable a step change in the delivery of traceability for time-varying quantities realised through electrical sensors. In particular, traceability for sampled electrical measurements, the basis of all modern instrumentation, will be provided to industrial end-users. As a first step, the consortium has started working on a quantum voltage digitiser as a new primary standard, opening more measurement capabilities of direct relevance to industrial communities. European instrumentation manufacturers have been invited to the first project meeting for a discussion on calibrating test devices against the quantum voltage digitiser. In this way, they will be directly involved in developing the measurement methodology for calibrating devices with this system and therefore be able to influence the future European quantum AC voltage calibration capabilities.
 
To facilitate further uptake of the project’s outputs there will be considerable engagement throughout the project with industrial stakeholders including manufacturers of AC voltage measuring devices as well as other end-users and calibration laboratories. To ensure that the project is aligned with industrial needs a number of industrial partners are participating in the project and, furthermore, a Stakeholder Committee has been established and two QuADC Newsletters have been distributed to inform interested stakeholders and parties about ongoing progress.
 
Organizing an international workshop as well as seminars at the national level has been discussed at project meetings. An ADC workshop will be held at NPL in early 2019. This workshop and seminars will be held to share project outputs and engage with the target end-user communities. Uptake of the new measurement capabilities developed by the project is expected as it will enable end-users to confidently demonstrate the performance of their products. In particular, uptake is expected amongst accredited laboratories and the manufacturers of ADC and spectrum analysers etc., and manufacturers of instrumentation relying on AC voltage measurements such as electrical power and power quality, and audio instrumentation.
 
Impact on the metrology and scientific communities

Conference presentations of project results are planned for the CPEM 2018, 8-13 July, Paris, France. Furthermore, collaboration agreements have been signed by the consortium with one NMI and one company. Such agreements enhance the working range of the project and, especially will draw more worldwide attention from the quantum standards community causing that the results of the project will directly impact the electrical quantum standards community, which is mainly formed by NMIs and high-level calibration laboratories. This community will be able to develop new measurement capacities based on the project’s quantum standards. AC quantum voltage standards affect around 70 % of NMIs’ calibration activity, so the outcomes of this project such as 10 nV/V uncertainties for AC voltages will contribute greatly to the future improvement of the European CMCs. The project will also have an impact in the electrical low-frequency community e.g. end-users involved in electrical sampling measurements and in dynamic quantities, and the testing of commercial devices and prototypes is planned within this project. A specification sheet for the novel quantum voltage digitiser is in preparation and will be distributed soon.

Impact on relevant standards


Finally, once the project has achieved first results it will support the metrological activities of key international and European committees such as the Consultative Committee for Electricity and Magnetism (CCEM) and EURAMET Technical Committee for Electricity and Magnetism (TC-EM) as well as standards organisations such as International Electrotechnical Commission (IEC) TC100 Audio and video multimedia systems and equipment and TC and IEC TC85 Measuring equipment for electrical and electromagnetic quantities and the Institute of Electrical and Electronics Engineers (IEEE) TC10 Waveform generation, measurement and analysis committee. This participation builds on activities already established by members of the consortium, who are highly influential in national and international metrology and standards committees, and will be used to facilitate greater awareness of the project results.

Longer-term economic, social and environmental impacts


The project will enhance the metrology for electrical voltage and other time-varying quantities by means of new techniques for the application of precision measurements. Calibration laboratories, other stakeholders, and industry will then profit by improved measurement capabilities in the next step. The world's new method can provide low logistical effort and downtime thanks to direct traceability to fundamental constants. In the longterm competitiveness of European calibration laboratories will be sustainably increased as a need for recalibration can be limited to a minimum or even eliminated.
 
Direct scaling of the Josephson defined waveforms to higher voltages will enable improved traceability of power quality measurements which will lead to an efficiency improvement in European power grids. Lower losses will generate e.g. less CO2 emission.
 
With the electrical instrument suppliers being the backbone of major advances in electronics and sensing equipment, the outcome of this project could have a vast impact on our society, economy, environment and even health. Advances in sensing technology by increasing performance, functionality and energy-efficiency of electronic devices could enable e.g. car companies to enhance their capability of building autonomous driving cars.


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