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IEEE Transactions on Applied Superconductivity

Josephson voltage standard based on single-flux-quantum voltage multipliers

C.A. Hamilton

Published Jan 1, 1992• 46 citations
Publisher

Abstract

A Josephson single-flux-quantum voltage/frequency multiplier circuit is proposed as the basic building block for a new generation of voltage standards. The circuit uses magnetic coupling to synchronize a series array of independent junction oscillators to the flux flow in a Josephson transmission line. A cascade of these circuits can multiply an arbitrarily low input frequency up to the frequency limit of the circuit ( approximately=250 GHz) and then add the voltages across approximately 30000 oscillators to generate precise voltages up to 10 V. Because the oscillators can be switched on and off with a bias current, the output voltage is rapidly programmable. A complete design for a voltage standard programmable in 1.2- mu V increments to a maximum of 10 V is proposed. Using existing fabrication technology, the circuit would cover a substrate area of about 1 cm/sup 2/ and use 67210 junctions.>

DOI
10.1109/77.160152

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IEEE Transactions on Instrumentation and Measurement•Apr 1, 1997•1 citation
A Josephson single-flux-quantum voltage/frequency multiplier circuit is proposed as the basic building block for a new generation of voltage standards. The circuit uses magnetic coupling to synchronize a series array of independent junction oscillators to the flux flow in a Josephson transmission line. A cascade of these circuits can multiply an arbitrarily low input frequency up to the frequency limit of the circuit ( approximately=250 GHz) and then add the voltages across approximately 30000 oscillators to generate precise voltages up to 10 V. Because the oscillators can be switched on and off with a bias current, the output voltage is rapidly programmable. A complete design for a voltage standard programmable in 1.2- mu V increments to a maximum of 10 V is proposed. Using existing fabrication technology, the circuit would cover a substrate area of about 1 cm/sup 2/ and use 67210 junctions.>
New types of followers and follower/doublers of DC voltage are proposed and analyzed. Operation of the devices is based on synchronization of the single-flux quantum (SFQ) pulses generated by overdamped Josephson junctions coupled by means of passive capacitive or inductive circuits. Two ways of combining the followers and doublers to get DC voltage multipliers and standards are described. Possible applications of these devices in A/D and D/A converters and RSFQ (rapid SFQ) logic circuits are also discussed. It is believed that the proposed multipliers and standards could produce stable output voltages in the 1-10 V range with output currents in the 0.1-1 mA range.>
Precise voltage generation is a unique feature of single-flux-quantum (SFQ) circuits, in addition to their high-speed digital signal processing with low power consumption. We investigated SFQ pulse-frequency modulation D/A converters for metrological applications. In our SFQ-based D/A converters, the maximum output voltage is determined by the maximum SFQ pulse-frequency at the pulse number multiplier, and by the voltage multiplication factor at the voltage multiplier. In this study, we present our new design for a double-flux-quantum amplifier (DFQA) that works as a quantum voltage multiplier. In the new parameter set, we tuned the damping parameters of the Josephson junctions to realize proper propagation of SFQ pulses. A 1000-fold DFQA designed with the new parameter set was fabricated using a 25-μ A/μ m2Nb/AlOx/Nb integration technology. A 1000-fold voltage multiplication was confirmed for the input voltage up to 43 μV, with a corresponding SFQ repetition frequency of 21 GHz. That is, the output voltage reached 43 mV.
Abstract We demonstrate the frequency synchronization of multiple single-flux quantum (SFQ) oscillators with different oscillation frequencies. To synchronize these SFQ oscillators, a common constant bias current is supplied to the SFQ oscillators without any bias resistors. When an SFQ oscillator oscillates at a frequency of f, the average voltage across the Josephson junction comprising the SFQ oscillator is f Φ 0 , where Φ 0 is the flux quantum in the superconductor. The bias currents supplied to the SFQ oscillators are redistributed to eliminate the average voltage difference output from the SFQ oscillators. As a result, the oscillation frequencies of all the SFQ oscillators are synchronized. Simulation results indicate that SFQ oscillators with an oscillation frequency difference of more than 50 GHz can be synchronized. We experimentally demonstrate the frequency synchronization of two SFQ oscillators composed of circular Josephson transmission lines. Frequency synchronization is expected to contribute toward the development of a low-power stable clock source stabilizing SFQ circuit operation.
Digital to analog converters based on the Josephson effect are promising for voltage standards, because they produce voltage steps with ultimate precision and stability. In this paper, we describe a project to develop a Josephson D/A converter designed for synthesizing a sinusoidal waveform with metrological accuracy. The D/A converter is based on RSFQ (Rapid Single Flux Quantum) logic circuits, and consists of a frequency multiplier (FM), a pulse distributor (PD), and a number of voltage multipliers (VMs). Each VM circuit, corresponding to the n-th bit digital code, multiplies the number of SFQ pulses by a factor of 2/sup n/. By gating the input SFQ pulses from the FM to the VMs using the PD circuits, a programmable output voltage is obtained. Possible sources of uncertainties in the measurement of the rms value of the synthesized sine wave are discussed.
The DC voltage across a Josephson junction is exactly determined by the number of single flux quanta Φ0passing the junction per second. By locking this flux train to an external sinusoidal microwave with the frequency f or a pulse train with the pulse rate f in such a way that one flux quantum is transferred per microwave period or per pulse respectively, the junction DC voltage becomes V=Φ0f in the simplest case. The precision of V is only determined by the uncertainty of the frequency control. As the output voltage is relatively small (V=140μV for f=70GHz), large series arrays of n Josephson junctions must be used to increase the voltage to practical levels V=nΦ0f. With a sinusoidal 70 GHz microwave drive and series arrays of up to 14000 SIS tunnel junctions modern DC voltage standards were developed in the past which may reach a reproducibility of a few parts in 10-10. These standards are commercially available and form the basis of the legal voltage metrology of most of the national laboratories worldwide. Because the reference voltages of these standards are widely overlapping it is difficult to rapidly adjust the desired voltage. It was therefore impossible to use these quantum voltmeters for the precise analysis of AC voltages. For this purpose the arrays had to be modified in many respects. The direct way to generate arbitrary Josephson waveforms V(t) is to rapidly alter the frequency f(t) with respect to the desired waveform V(t)=nf(t)Φ0. It requires a precise flux transfer control over a very wide frequency range.
A computer-controlled Josephson voltage standard has been developed comprising the hybrid integration of a planar oscillator and a Josephson series array. A 10 GHz voltage controlled oscillator has been frequency-stabilized to a reference frequency by a phase-locked loop. A series array has been designed and fabricated in Nb/Al/sub 2/O/sub 3//Nb technology. To obtain quantized voltage steps with a drive frequency of 10 GHz, the critical current density of the Junctions is reduced to below 2 A/cm/sup 2/ by variation of the oxidation parameters. The hybrid integration of a Josephson circuit with a planar MESFET oscillator produced quantized voltage steps up to 980 mV with a step width of 2 /spl mu/A.