IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO. 4, APRIL2009 791 Josephson-Voltage-Standard-Locked Sine Wave Synthesizer: Margin Evaluation and Stability Blaise Jeanneret, Frédéric Overney, Luca Callegaro, Alessandro Mortara, and Alain Rüfenacht Abstract—This paper describes a Josephson-voltage-standard- locked synthesizer where a commercial digital-to-analog converter is used as a sine wave generator. The output amplitude of the gen- erator is controlled by the calculable fundamental of the stepwise waveform generated by a SINIS Josephson junction array. Such a system combines the versatility of a commercial source with the stability and accuracy of the Josephson standard. By discarding the measurements performed during the transients, broad voltage steps of 1.7 mA were obtained up to frequencies of 500 Hz. Index Terms—AC measurements, Josephson devices, Josephson voltage standard, waveform synthesizer. I. I NTRODUCTION S INCE THE first development of series arrays of over- damped Josephson junctions in the mid-1990s [1], a large variety of applications have been found for the programmable Josephson voltage standard (PJVS) (see [2] and [3] for recent reviews). These applications span an important range of the high-precision electrical calibration area: digital voltmeter (DVM) linearity measurements [4], Zener standard calibra- tions, fast reversed dc measurements of thermal converters [5], [6], potentiometric systems, quantum voltmeters, and watt bal- ance experiments [7]. In addition to all these applications, the original motivation was to develop a waveform synthesizer with a calculable RMS value by rapidly switching between the different voltage steps that are available at the array’s output. It was already clear from the beginning that the transients occurring during these step transitions would limit the accuracy of the PJVS. However, it was recently shown that a 1-V stepwise-approximated sine wave can be generated with an un- certainty on the order of one part in 10 7 at frequencies of below 200 Hz [8]. Quantum waveform synthesizers suffer from a number of different limitations. 1) The total RMS value of the synthesized waveform de- pends on the shape and the transition time between Manuscript received June 4, 2008; revised August 25, 2008. First published November 17, 2008; current version published March 10, 2009. The work within this EURAMET joint research project was supported by the European Community’s Seventh Framework Programme, ERA-NET Plus, under Grant Agreement 217257. The Associate Editor coordinating the review process for this paper was Dr. Yi-hua Tang. B. Jeanneret, F. Overney, and A. Mortara are with the Swiss Federal Office of Metrology (METAS), 3003 Bern-Wabern, Switzerland (e-mail: blaise. jeanneret@metas.ch). L. Callegaro is with the Istituto Nazionale di Ricerca Metrologica (INRIM), 10135 Torino, Italy. A. Rüfenacht is with the National Institute of Standards and Technology, Boulder, CO 80305 USA. Digital Object Identifier 10.1109/TIM.2008.2006963 Fig. 1. Schematic of the synthesizer. Using a sampling system, the amplitude of the commercial DAC is locked to the amplitude of the fundamental of the Josephson voltage through a feedback loop. The main components of the system are synchronized to an external 10-MHz source. A 1 and A 2 are buffer amplifiers. the different voltage steps [9], [10]. This dependence increases with frequency. 2) Due to the stepwise nature of the generated signal, many harmonics are present in the spectrum, and the apparent RMS value will depend on the bandwidth of the signal path up to and including the load [11]. 3) Finally, any current drawn from the Josephson array by the load will affect the quantization of the voltage steps. Therefore, external current sources are needed to drive low-impedance loads [8]. A synthesizer that avoids these limitations while still main- taining a quantum accuracy is described in this paper. The main idea (see Fig. 1) is to use the Josephson array to stabilize the output voltage of a high-spectral-purity digital-to-analog converter (DAC). There are three major requirements for such a synthesizer to work. 1) The ac Josephson voltage measurements have to be inde- pendent of the various bias parameters (array bias current, microwave power, and chip temperature) over a certain range. The determination of the operating margins is called a flat-spot measurement. 2) The short-term stability of the DAC source has to be high enough to ensure that the feedback can guarantee the required accuracy. 3) The synthesizer accuracy must be validated by perform- ing a highly accurate comparison with a state-of-the art thermal converter. In the succeeding paragraphs, a careful study of the first two points previously mentioned will be presented. 0018-9456/$25.00 © 2008 IEEE Authorized licensed use limited to: Frederic Overney. Downloaded on March 27, 2009 at 09:01 from IEEE Xplore. Restrictions apply.