Quantum voltage standards based on the Josephson effect currently ensure the traceability to DC and low-frequency AC voltages. The aim of the JRP Q-WAVE is to provide direct and efficient traceability for precision devices generating or measuring arbitrary waves at frequencies up to 10 MHz. The demand for these significant improvements of high-precision voltage measurements is caused among other things by the rapid progress of semiconductor industry offering analogue-to-digital converters (ADC) and digital-to-analogue converters (DAC) with higher and higher sampling rates and accuracy.
Digital metrology is now the method of choice in the instrumentation sector with sensing and measurement becoming increasingly dependent on analogue-to-digital conversion of sampled measurements. One example is dynamic measurement where sensor-electrical signals are digitised and then processed using digital algorithms to recover the measured parameters. Another is electrical power measurement in the presence of harmonics where signal-processing techniques are used to extract the measured power. The scale of digital metrology is already large and is expected to continue to increase. A key benefit is that once the electrical signal has been digitised, quantities such as the basic root mean square (RMS) value, peak value, crest factor and harmonic content can all be calculated from one data set whereas previously, each quantity required a special instrument feature or range and each of these ranges had to be separately calibrated.
Recent industrial R&D in precision integrated circuits and measurement equipment has brought about a step change in the sampling rates and potential accuracy available. However, the present methods for disseminating the SI volt for non-stationary or alternating waveforms (AC) are not keeping pace with requirements. They rely on the equivalence of direct and alternating current (DC and AC) via a thermal transfer device with the consequence that DC and AC are seen as separate quantities. Whilst DC measurements have enjoyed a quantum basis for many years, AC measurements have not. Furthermore, AC measurements are based on single frequency waveforms and the thermal transfer methods give no information on harmonic content or phase.
The challenging aim of this JRP is to provide direct and efficient traceability to the SI volt for ADC and DAC operating in the frequency range from DC to 10 MHz. These procedures comprise the investigation and measurement of arbitrary waveform signals. Therefore, the JRP addresses the following scientific and technical objectives:
The project will result in a new capability for the provision of traceability for sampled electrical measurements directly to the SI using the voltage references based on the Josephson effect. The dissemination methods developed in the project will have an impact on the specification, calibration and testing of the latest generation of analogue-to-digital and digital-to-analogue converters according to the published IEC and IEEE standards. The modeling of sampled electrical measurements and estimation of measurement uncertainty will enable proper traceability for converters used in research, test and calibration and the instrumentation sector. Converters recently released to the market have a performance at the limit of what can be measured with currently available test instrumentation. This JRP, through investment in the development of the SI for sampled electrical measurements, will unlock the door to direct traceability for this fast moving sector.
The JRP will ensure that the work carried out within it is of relevance to the widest possible range of stakeholders and potential end-users. The JRP results will be shared through a combination of knowledge transfer, training and dissemination activities. Knowledge transfer includes activities like the set-up of a Stakeholder Group, publications of scientific papers, conference presentations, and a JRP website. In addition, practical trainings and demonstrations will be performed. Furthermore, new procedures will furthermore be disseminated via good practice guides.
Key highlights
The project has made a good scientific progress in the first 24 months:
Measurement system based on the Josephson effect for dynamic calibration
Suitable test arrays for operation by photodiodes have been developed and fabricated at PTB. Commercially available InGaAs photodiodes with a 20 GHz bandwidth have originally been selected. Because of problems with operating these photodiodes, lensed photodiodes have been identified as an alternative solution; in addition, they allow much easier illumination conditions. NPL, JV, PTB, and REG(HBV) have jointly developed and fabricated special high-frequency carriers for the photodiodes and mounted some photodiodes on the carriers for further investigations. INRIM and PTB have demonstrated preliminary measurements of cryocooler operation of Josephson arrays; first results were published and presented at the international conference EUCAS 2013.
Josephson arrays of the first generation (single arrays of double-stacked Josephson junctions) were also designed and fabricated at PTB, and some arrays were selected after characterisation for further investigations. An RMS output voltage of about V = 178 mV (peak voltage V = 502 mV) have been reached with an array containing 9,000 triple-stacked Josephson junctions. In addition, first sine waves with RMS output voltages of 1 V were demonstrated at PTB by series operation of four chips containing 63,000 Josephson junctions altogether with a commercial 8-channel pulse pattern generator. NPL has configured an opto-electronic system to deliver a GHz rate optical pulse stream imaged to a spot size approaching 10 µm in diameter.
A first order delta sigma feedback loop has been designed and built at NPL as a main component of the Josephson Analogue-to-Digital Converter. This includes the analogue stages (difference amplifier, integrator and comparator) and the first part of the digital data processing stage. Successful operation of the delta sigma loop has been demonstrated at NPL using direct feedback from the comparator (i.e. without using a Josephson junction array). The next stage will be to integrate the Josephson junction array and opto-electronics.
Dissemination methods based on state of the art instrumentation and converters
At INRIM, a synchronous detection approach has been realized and tested to directly transfer high purity quantum waveforms to a high speed semiconductor DAC based synthesizer beyond the acoustic band. INRIM supported by TUBITAK also developed a prototype of a synthesiser based on high-update rate DACs for static and dynamic characterisation of wideband ADCs, employing a phase interpolator to improve the spectral purity of wideband DACs. Using an equivalent time sampling approach, an oversampling method was implemented at METAS to perform a waveform reconstruction that will allow an extension of the measurement bandwidth. In addition, the Programmable Josephson Voltage Standard test bench has been completed at METAS, which allows the systematic evaluation of the integral non-linearity, of the Allen variance and of the amplitude of the harmonics of the Fourier spectrum.
A transconductance amplifier from NMIA has been used at PTB to measure the combined system of binary-divided and pulse-driven arrays with a thermal converter at the 1 V level within the frequency range from about 150 Hz to 1 kHz. The same signals (time domain and frequency spectrum up to 500 kHz) have been measured at PTB with a commercial fast ADC which was directly calibrated using the Programmable Josephson Voltage Standard and different waveforms. A first version of the filter for the Delta-Sigma Programmable Josephson Voltage Standard has been built and evaluated at SP. Several sampling strategies to enable estimation of the frequency response of ADCs based on the step response were tested at VTT and an optimal solution was found. At VTT, a new semiconductor DAC based arbitrary waveform generator prototype has been designed and built, and it has shown excellent short-term stability below 1 kHz frequencies. This high stability allowed the optimisation of the usage of the AC Quantum Voltmeter at PTB. Three mercury-wetted step generators have been built and evaluated up to 50 kHz at VTT.
Digital signal processing techniques
A synchronous detection approach has been realized and tested to direct transfer high purity quantum waveforms to high speed semiconductor DAC based synthesizer beyond the acoustic band at INRIM. A first version of the filter for the Delta-Sigma Programmable Josephson Voltage Standard has been built and evaluated at SP. A list of synchronous and asynchronous sampling techniques was compiled by SIQ and VSL; it contains some yet unused but promising techniques amongst others. TUBITAK, CMI, SIQ and VSL have prepared a detailed list of identified error sources. They performed simulations and experiments to identify those error sources that result in most dominant effects in generating and sampling systems. A list of identified effects caused by identified errors and possible techniques for their mitigation has been then compiled by TUBITAK, CMI, SIQ and VSL. CEM and SIQ have identified parameters of high accuracy ADC that require improved characterisation and have written a report as a basis for further discussions. A software toolbox is being prepared by CMI supported by SIQ. VSL and SIQ have compiled a list containing identified cable resonance phenomena and possible solutions. Detailed calculations and preliminary tests at VSL have then shown that operation and measurements at frequencies above about 100 kHz are limited due to standing waves in the cables connecting the Josephson array at 4.2 K and the measurement electronics at room temperature.
Stage 3 REG (HBV) Hogskolen i Buskerud og Vestfold in Toensberg, Norway, started work on 1 November 2013. Their work is presently focused on the mounting of photodiodes and optical fibres in a robust way. This work will allow cooling the photodiodes to 4 K without destroying the electrical connections or misaligning the laser. Within the framework of an RMG, Tezgül Coşkun Öztürk (TUBITAK) worked for about 3 months at PTB. In joint cooperation a number of experiments have been performed to investigate uncertainties in the time domain and related effects in the frequency domain especially of the combined Josephson system consisting of programmable and pulse-driven voltage standards. The Instituto Nacional de Tecnologia Industrial (INTI), Argentina, the National Measurement Institute of Australia (NMIA), the D. I. Mendeleyev Institute for Metrology (VNIIM), Russia, and the Universidad de Málaga (ISIS - DTE - UMA), Spain joined the JRP as Collaborators.
The first newsletter was published in June 2014, the second newsletter in February 2015. Several papers describing first achievements of the JRP were submitted to the international Conference on Precision Electromagnetic Measurements (CPEM 2014) and have been presented at the conference in Rio de Janeiro, Brazil, 24 - 29 August 2014. Further papers have been presented at other national and international conferences and workshops (cf. 
Publications and Presentations). Several papers have additionally been submitted among other to the journal IEEE Transactions on Instrumentation and Measurement at CPEM 2014 and will be published in 2015.