Realization of a square-wave voltage with externally-shunted SIS Josephson junction arrays for a quantum AC voltage standard

A quantum-based ac voltage standard operating from about 1 Hz up to 10 kHz at 1 V is being developed. The standard is based on relating the output of a stable sine generator to a square wave obtained by biasing a nonhysteretic Josephson junction array alternately at steps n = -2 and n = +2. We have constructed a bias current source and a buffer amplifier connected to the array which enable fast switching between steps. In this paper, we describe the system used for generation of the square wave and present experimental results which show that the accuracy of the fundamental frequency component amplitude of the square wave is sufficient for realization of a standard with accuracy better than 1 × 10^-6.