Towards a 1 V Josephson Arbitrary Waveform Synthesizer

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 Nb_xSi_1-x as 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 V_RMS = 1006 mV(V_PP = 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.

I. Introduction

AFTER many years since the first realization of a pulse-driven AC Josephson voltage standard [1], recent developments in AC voltage standards for metrological applications show that the use of pulse-driven Josephson junction series arrays is a very promising approach. This AC Josephson voltage standard is often called “Josephson Arbitrary Waveform Synthesizer” (JAWS) [2] and it is already used in several NMI's for metrology applications [3]– [7]. For the application in JAWS the junctions are operated by short current pulses which affect the transfer of flux quanta across the Josephson junctions. According to the Josephson equation, a time-dependent voltage, which is quantized at all times, is generated at the junction series array output leads:

$$V(t)=n\cdot m\cdot{\mmb \Phi}_{0}\cdot f_{p}(t).\eqno{\hbox{(1)}}$$