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Conference on Precision Electromagnetic Measurements

Generation of 10 Vrms waveforms using AC-programmable Josephson voltage standard system with 10 K cooler

Michitaka Maruyama, Yasutaka Amagai, Hirotake Yamamori, Shih‐Fang Chen, Chiharu Urano, Nobu‐Hisa Kaneko

Published Aug 1, 2014• 7 citations
Publisher

Abstract

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.

DOI
10.1109/cpem.2014.6898436

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IEEE Transactions on Instrumentation and Measurement•Mar 21, 2005•11 citations
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 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.
A 10-V programmable Josephson voltage standard (PJVS) circuit including 524 288 vertically stacked NbN/TiN/NbN Josephson junctions was successfully fabricated without any defects. The chip was cooled with a compact cryocooler at a temperature of 9.8 K and generated an output voltage of 17.3 V with the first Shapiro step height of 1.0 mA under microwave irradiation at 16 GHz. Furthermore, the circuit design having the high output voltage increased the number of available chips for the 10-V PJVS. The fabrication yield for the chip, which generates greater than 10 V, was 36%, which is ten times greater than that for a perfect chip without any defects. This method was not effective for the chip that had serious defects, such as a disconnection across the array or short between the array and ground; thus, efforts to reduce such defects, e.g., frequent maintenance inside vacuum chambers for film preparation and etching, were found to be essential.
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.
The establishment at PTB of an AC Josephson voltage standard (Josephson Arbitrary Waveform Synthesizer- JAWS) based on pulse-driven Josephson arrays is focused on achieving an output voltage of at least 1 Vrms which is required for many metrology applications. In this paper, we approached this goal by increasing the number of active junctions in two ways. Firstly, we fabricated arrays containing triple-stacked SNS-type Josephson junctions with NbxSi1-xas barrier material. We obtained current operation margins of about 0.2 mA with arrays of up to 9000 Josephson junctions at an rms voltage of 355 mV. Secondly, we used a new 8-channel ternary pulse pattern generator (PPG) to bias up to 8 arrays connected in series. An output voltage of VRMS= 1006 mV(VPP= 2.845 V) was achieved by using 8 arrays (arranged on 4 separate chips) with 63000 junctions in total. Higher harmonics are suppressed by at least -116 dBc. The fabrication process and the experimental setup will be described, as well as experimental results that are leading towards our 1 V goal.
A compact Programmable Josephson Voltage Standard (PJVS) system based on a cryocooler is under development at the National Institute of Advanced Industrial Science and Technology (AIST). The system uses a 2 V NbN/TiN/NbN PJVS chip operating at 12 K. Since the PJVS chip requires cooling power of less than 100 mW, it is possible to use a compact-size cryocooler, which may be mounted inside a standard 19-inch rack-mount chassis. By combining the 2 V PJVS chip with a high-precision semiconductor voltage multiplier circuit, it is possible to generate arbitrary voltages up to 20 V.
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.