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Superconductor Science and Technology

Characteristics of a voltage multiplier for a RSFQ digital-to-analog converter

Fuminori Hirayama, Masaaki Maezawa, Shogo Kiryu, Hitoshi Sasaki, Akira Shoji

Published Feb 19, 2002• 7 citations
Publisher

Abstract

We are developing a digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) logic circuits. Since the RSFQ D/A converter operates according to the Josephson frequency–voltage relationship, arbitrary waveforms can be synthesized with an accuracy equivalent to that of the Josephson dc voltage standard. We have examined the characteristics of magnetically coupled voltage multipliers (VMs). The VM is a key component in the RSFQ D/A converter for use in metrological applications.

DOI
10.1088/0953-2048/15/4/303

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We are developing a digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) logic circuits. Since the RSFQ D/A converter operates according to the Josephson frequency–voltage relationship, arbitrary waveforms can be synthesized with an accuracy equivalent to that of the Josephson dc voltage standard. We have examined the characteristics of magnetically coupled voltage multipliers (VMs). The VM is a key component in the RSFQ D/A converter for use in metrological applications.
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 voltage multiplier (VM) is a principal component in high-precision digital-to-analog (D/A) converters based on rapid single flux quantum (RSFQ) circuits. Experimental confirmation of correct operation of the VM is important to guarantee the accuracy of the D/A converter. In this study, a circuit for direct comparison of the output voltages of VMs was designed and demonstrated. An on-chip SQUID sensor was utilized to detect the VM errors. Bit error rates of a single-stage VM were successfully measured down to 10/sup -12/ in the constant-voltage regions. A method of detecting errors during transients was also proposed and carried out.
Digital-to-analog converters based on rapid single flux quantum circuits (RSFQ-D/A converters) are candidates for the next generation of ac voltage standard. We re-optimized the device parameters of a voltage multiplier (VM) cell, one of the important components in the RSFQ-D/A converter, to implement a new voltage doubler (VD) cell. The output voltage of the VD cell was twice as high as that of the conventional VM cell. We fabricated VD cells using a Nb technology. We confirmed correct operation of the VD cells at input frequencies up to 30 GHz.
Rapid-single-flux-quantum digital-to-analogue converters (RSFQ-DACs) are now under development for ac voltage standard applications. The voltage multiplier (VM), which precisely amplifies the input voltage, is a key component for RSFQ-DACs. Because the amount of bias current for a conventional VM increases in proportion to its multiplication factor, we have been looking for a VM device which is operated on a different principle. In this paper, we report our design of a 1000-fold VM comprising double flux quantum amplifiers (DFQAs) of which the amount of bias current is independent of its multiplication factor. Test circuits were fabricated using a 2.5 kA/cm2 Nb process. We confirm that the experimental results demonstrate the 1000-fold operation up to 13.2 GHz input SFQ pulse repetition frequency.
The output voltage of a rapid-single-flux-quantum digital-to-analog converter (RSFQ-DAC) is generated with a voltage multiplier (VM) that comprises multiple voltage mirror cells. We have demonstrated a new operation mode of a voltage mirror cell. It is a second-mode operation that doubles the input voltage, like the second-order Shapiro step. That is, a single cell works as a voltage doubler (VD). In this work, we propose a new parameter set of the VD (VD revision 1: VDr1) that improves the tolerance for multi-stage operation. Experimental results of a 16-stage VDr1 circuit are presented.
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.>