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TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

Superconducting D/A Converters Generating Quantum Voltage Defined by the Reference Microwave Frequency and the AC Josephson Effect

Yoshinao Mizugaki

Published Mar 19, 2024• 0 citations
Publisher

Abstract

Superconducting Josephson devices have been developed for various electronics fields such as highly sensitive magnetometers, voltage standards, and high-speed digital signal processing. In this paper, the author describes two kinds of superconducting digital-to-analog （D/A） converters that generate quantum voltages. One is a balanced-ternary D/A converter comprising asymmetric superconducting quantum interference devices, in which the generation of either a positive or a negative zero-crossing Shapiro step is controlled by an external magnetic flux. The other is a single-flux-quantum （SFQ） D/A converter, in which SFQ pulse-frequency modulation is employed. They generate quantum voltage defined by the reference microwave frequency and the AC Josephson effect.

DOI
10.2221/jcsj.59.80

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Superconducting Josephson devices have been developed for various electronics fields such as highly sensitive magnetometers, voltage standards, and high-speed digital signal processing. In this paper, the author describes two kinds of superconducting digital-to-analog （D/A） converters that generate quantum voltages. One is a balanced-ternary D/A converter comprising asymmetric superconducting quantum interference devices, in which the generation of either a positive or a negative zero-crossing Shapiro step is controlled by an external magnetic flux. The other is a single-flux-quantum （SFQ） D/A converter, in which SFQ pulse-frequency modulation is employed. They generate quantum voltage defined by the reference microwave frequency and the AC Josephson effect.
Recently we designed and successfully tested a prototype of a superconducting digital-to-analog converter (DAC) based on processing single flux quantum (SFQ) pulses. Two unique features make the device very attractive for metrology applications: (i) The generation of output voltage is based on the fundamentally accurate Josephson frequency-to-voltage relationship. (ii) The device does not require any external signals with frequencies higher than 100 MHz. It is expected that the DAC will be used as a key component of novel standards and calibrators of ac voltage with frequency up to 1 MHz. The implemented prototype has low magnitude of output voltage (about 10 mV) which is far lower than the 1 to 10 V target magnitude.
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 design and operation of a 9 bit superconductive single‐flux‐quantum (SFQ) digital‐to‐analogue converter (DAC) are presented. The DAC comprises a 9 bit variable SFQ pulse number multiplier and a 100‐fold voltage multiplier. An SFQ pulse‐frequency modulation technique is employed for coding the analogue output voltage. The DAC is fabricated using an Nb Josephson integration technology. Its output voltage waveform, the amplitude of which is up to 2.54 m V pp , is controlled by digital input signals from a room‐temperature data generator.
Precise, accurate D/A conversion becomes increasingly difficult at high frequencies, especially for low-power devices. The authors have developed a superconducting D/A converter based on the Josephson effect. Following a design of Hamilton et al. (1995), they have added magnetic coupling of the digital input lines to simplify the input interface. In addition, they have converted to a balanced two superconducting quantum interference device (SQUID) unit cell, which allows input-to-output isolation and isolation between input lines white doubling output voltages. Their initial demonstration consists of a 2-bit NbN D/A pumped at 12 GHz, producing output levels of 0, 50, 100, and 150 microvolts into a 50-ohm load with low-speed digital input.
We present a single-flux-quantum (SFQ)-based digital-to-analog converter (DAC) generating bipolar output voltages, in which the key component is a polarity-switchable double-flux-quantum amplifier (PS-DFQA). The DAC comprised a dc/SFQ converter, an 8-bit variable pulse-number multiplier (PNM), and a ±8-fold PS-DFQA integrated on a single chip. SFQ pulse-frequency modulation was employed to realize variable output voltage amplitude, for which the multiplication factor of the variable-PNM was controlled by a commercial data generator situated at room temperature. The variable-PNM realized 8-bit resolution with a multiplication factor between 0 and 255. Bias currents fed to the ±8-fold PS-DFQA were polarity switched in synchronization with the digital code for the variable-PNM. The whole circuits including I/O elements were designed using SFQ cell libraries, and fabricated using a niobium integration process. Sinusoidal bipolar voltage waveform of 0.38 mVpp was demonstrated using a reference signal source of 43.94 MHz.