838 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO. 4, APRIL2009 Progress Toward a 1 V Pulse-Driven AC Josephson Voltage Standard Samuel P. Benz, Senior Member, IEEE, Paul D. Dresselhaus, Alain Rüfenacht, Norman F. Bergren, Joseph R. Kinard, Senior Member, IEEE, and Regis Pinheiro Landim, Member, IEEE Abstract—We present a new record root mean square (rms) out- put voltage of 275 mV, which is a 25% improvement over the maxi- mum that is achieved with previous ac Josephson voltage standard (ACJVS) circuits. We demonstrate the operating margins for these circuits and use them to measure the harmonic distortion of a commercial digitizer. Having exceeded the threshold of 125 mV rms for a single array of Josephson junctions, we propose and discuss the features of an eight-array circuit that is capable of achieving 1 V rms. We investigate the use of a resistive divider to extend the ACJVS voltage accuracy to higher voltages. By the use of a switched-input measurement technique, an integrating sampling digital voltmeter, a resistive voltage divider, and ACJVS synthesized sine waves as reference voltages, we characterize the stability of a commercial calibration source for a few voltages up to 2.7 V. Index Terms—Digital–analog conversion, Josephson arrays, quantization, signal synthesis, standards, superconductor– normal–superconductor devices, voltage measurement. I. I NTRODUCTION S INCE the invention of the pulse-driven Josephson digital- to-analog converter in 1995 [1], an important goal has been to increase the root mean square (rms) output of the quantum-accurate synthesized waveforms to 1 V. Most audio- frequency voltage calibrations that are performed with thermal voltage converters use this amplitude. This large output voltage is also important for increasing the signal-to-noise ratio of other precision measurement applications. Ten years of con- tinual research and development was required to increase the output voltage from a few microvolts for a single junction to 100 mV for dual-array circuits [2], [3]. The first practical ac Josephson voltage standard (ACJVS) system was imple- mented in the National Institute of Standards and Technology (NIST) ac voltage calibration service in 2006 [4]. During the past two years, further improvements have been made, particularly to the microwave design of the supercon- ducting Josephson circuits, which have enabled circuits with two arrays to generate rms amplitudes up to 220 mV [5], [6]. In this paper, we present a new record output voltage of Manuscript received June 6, 2008; revised August 14, 2008. First published November 21, 2008; current version published March 10, 2009. The Associate Editor coordinating the review process for this paper was Dr. Yi-hua Tang. S. P. Benz, P. D. Dresselhaus, A. Rüfenacht, and N. F. Bergren are with the National Institute of Standards and Technology, Boulder, CO 80305 USA (e-mail: samuel.benz@nist.gov). J. R. Kinard is with the National Institute of Standards and Technology, Gaithersburg, MD 20899 USA. R. P. Landim is with the National Institute of Metrology, Standardization, and Industrial Quality (Inmetro), Rio de Janeiro, RJ 20261-232, Brazil (e-mail: rplandim@inmetro.gov.br). Digital Object Identifier 10.1109/TIM.2008.2007019 275 mV rms. Although it is an incremental improvement, the result is particularly important because the circuit exceeds the threshold of 125 mV rms for a single array. This allows us to propose a practically achievable eight-array circuit that will enable the direct synthesis of 1 V rms quantum-accurate wave- forms. We describe the proposed 1 V circuit and present the measured results at 275 mV, including fast Fourier transform (FFT) measurements using a commercial digitizer at 1 kHz and 100 kHz frequencies. We also use these circuits to investigate a measurement technique, based on a resistive divider, that might be used to extend the ACJVS accuracy to higher voltages. II. HIGHER VOLTAGE ARRAYS AND CIRCUITS Because the output voltage of a single Josephson junction is only 30 μV for a typical 15 GHz bias signal, series arrays of junctions are required to achieve useful voltages in the millivolt range. Unfortunately, the voltage that can be produced by a single array is also limited because only a finite number of junctions can be placed in series before junction dissipation detrimentally attenuates the microwave bias signal along the array [7]. The maximum rms voltage of a single distributed array (with negligible capacitance) is independent of frequency and is typically around 50 mV. To further increase the voltage, two techniques have been im- plemented: 1) an ac-coupled bias technique that allows multiple arrays to be connected in series [8] and 2) improved microwave designs that counteract the dissipation and allow more junctions in a single array [5], [6]. In this paper, further improvements were made to the microwave design that allowed the number of junctions to be increased from 5120 to 6400 so that the rms voltage per array increased from 110 mV to 137.5 mV. More junctions were possible because the array transmission line impedance was tapered even further (from 50 Ω at the input to 22 Ω at the termination), whereas the previous circuits were tapered to only 32 Ω [6]. The mean critical current and the mean resistance of the junctions are 8.1 mA and 4.3 mΩ, which are similar to previous circuits. We used the ac coupling technique to double the output volt- age by summing the voltage from two arrays. A simplified cir- cuit schematic for the ac-coupling technique is shown in Fig. 1. Commercial bitstream generators typically have a second com- plementary data output (D−) that produces a bit sequence that is the ones-complement of the data output (D+), which, for our purposes, produces an analog waveform of inverted polarity. DC blocking capacitors (acting as broadband high- pass filters and audio-frequency stop-band attenuators) remove 0018-9456/$25.00 © 2008 IEEE