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Generation of AC waveforms using a NbN-based programmable Josephson voltage standard system with a 10-K cooler

Michitaka Maruyama, Tatsuji Yamada, Hajime SASAKI, Hirotake Yamamori, Chiharu Urano, Nobu‐Hisa Kaneko

Published Jun 1, 2010• 9 citations
Publisher

Abstract

We are developing a system for precise AC waveform synthesis in a frequency range below kilo-hertz using a programmable Josephson voltage standard (PJVS). Our PJVS circuit is composed of 65 536 NbN-based over-damped Josephson junctions and operated at a temperature of 9-10 K with a mechanical cooler. We succeeded in generating stepwise-approximated sinusoidal waveforms with peek voltage amplitude of 2 V using a 10-bit PJVS circuit and a bias source with twelve 12-bit digital-to-analog converters. Rough estimations suggest the transient time between each step in the obtained waveforms is less than 5 μs, sufficient for sampling measurements.

DOI
10.1109/cpem.2010.5544216

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IEICE Transactions on Electronics•Jan 1, 2012•24 citations
We are developing a system for precise AC waveform synthesis in a frequency range below kilo-hertz using a programmable Josephson voltage standard (PJVS). Our PJVS circuit is composed of 65 536 NbN-based over-damped Josephson junctions and operated at a temperature of 9-10 K with a mechanical cooler. We succeeded in generating stepwise-approximated sinusoidal waveforms with peek voltage amplitude of 2 V using a 10-bit PJVS circuit and a bias source with twelve 12-bit digital-to-analog converters. Rough estimations suggest the transient time between each step in the obtained waveforms is less than 5 μs, sufficient for sampling measurements.
We have succeeded in synthesizing AC waveforms with the rms voltage of greater than 10 V using an AC-programmable Josephson voltage standard (AC-PJVS) system. The system has a NbN-based junction array, which is cooled with a compact cryocooler. The array is composed of total 524 288 junctions, enabling to generate arbitrary signals with the maximum output level of about 17 V for 15.9 GHz microwave input. In this experiment, we used a modified bias circuit with 6 parts of floating channels for eliminating interferences and also saturation in the output level. We will also discuss about heating effect of the 10 Vrms chip in the situation of using a mechanical cooler.
We propose a new method for transient-error-free AC voltage waveform synthesis using a programmable Josephson voltage standard (PJVS) array. Stepwise-approximated sinusoidal waveforms generated using PJVS can be potential tools for AC voltage calibration or related applications. However, the transient error caused by the transitions between quantized voltage steps is a crucial problem for some applications. By driving a PJVS array using high-speed periodical pulse packets and conventional DC biases, the influence of the transitions can essentially be eliminated. This paper describes the basic idea of our new method for driving a PJVS array.
An alternating programmable Josephson voltage standard (AC-PJVS) configured as a multibit digital-to-analog converter has been developed to explore the staircase-approximated waveforms synthesis at frequencies up to the acoustic range. The AC-PJVS setup is based on state-of-the-art modular electronics able to bias individually the subsections of a binary-divided array consisting of 8192 intrinsically shunted SNIS junctions. High order Shapiro steps have been employed for waveform synthesis having amplitude beyond 1 V by finely trimming the SNIS temperature using a two stage cryocooler. Alongside the experiments on the synthesis, the experimental setup was improved for direct comparisons between quantum staircase waves and conventional sine waves based on the use of a room temperature-operating phase sensitive detector and on a differential precision sampling system. Both methods maximize the accuracy obtained from the AC-PJVS synthesizer, allowing, in principle, its use for primary LF AC voltage metrology.
A waveform synthesizer adopting a superconductor–normal metal–superconductor junction array has been developed, which can generate arbitrary stepwise waveforms with a number of quantum-voltage steps up to 1 V level amplitude. As an application of the synthesizer, we have built a sampling voltmeter that measures the differential voltages between a sinusoidal waveform produced by a semiconductor-based ac source and the Josephson waveforms. We carried out extensive sampling measurements for a 50 Hz sine wave with 1 V amplitude, applying sampling apertures in the range of 55 µs ⩽ t a ⩽ 130 µs and using Josephson waveforms with 32, 60, 80 and 100 quantum steps. From the measurements, the amplitude of the ac waveform was determined with a type A uncertainty ( k = 2) of 0.15 µV. Also, we elucidated how the phase jitter in the ac waveform is reflected in the overall uncertainty for the measurements. The type B uncertainty due to the jitter is at least one order of magnitude smaller than the type A uncertainty.
Programmable Josephson voltage standard (PJVS) circuits using arrays of NbN/TiN/NbN/TiN/NbN double-junction stacks were fabricated and operated at a temperature of 10 K. Four arrays each with 32 768 junctions produced expected voltage outputs when driven at (10, 15, 17, and 20) GHz with an operating current range of 0.2-1.0 mA. Another circuit showed constant-voltage steps at /spl plusmn/6 V when driven with an 11 GHz microwave bias, indicating that over 260 000 junctions were operating properly. This circuit did not show constant-voltage steps when driven with a 19 GHz microwave bias, which was the design frequency to achieve /spl plusmn/10 V output voltage.
524,288 NbN-based Josephson junctions were integrated to produce a programmable Josephson voltage standard (PJVS) on a die of 15mm × 15mm, and the PJVS circuit was cooled to 10K using a cryocooler and operated with a current margin of about 1.0mA. Although an output voltage of 10V was required for a voltage standard, the circuit was designed to generate the maximum output voltage of 17V because it was difficult to avoid a reduction of the output voltage due to defects. Although a perfect chip without any defect was rarely fabricated, the high voltage chip that generated at least 10V was fabricated with the fabrication yield of larger than 30%. The fabrication yield was also improved by optimizing the film growth conditions to reduce the film stress and the number of particles. Applications for a secondary voltage standard and an ac Josephson voltage standard are also described.