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IEEE Transactions on Applied Superconductivity

Magnetically coupled Josephson D/A converter

Andrew D. Smith, D.J. Durand, Bruce J. Dalrymple

Published Mar 1, 1999• 4 citations
Publisher

Abstract

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.

DOI
10.1109/77.763256

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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.
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.
A flash-type superconducting A/D (analog/digital) converter is proposed. The basis low-frequency operation of this converter, which compromises a quantum flux parametron (QFP) and an RF SQUID (superconducting quantum interference device), has been observed. The RF SQUID consists of a Josephson junction and a load inductor which is part of the QFP. It converts an analog signal to period digital signals of one quantum flux (2.07*10/sup -5/ W-b) per cycle. The QFP works as both an amplifier and a comparator of the periodic digital signals, by adding an exciting signal. This A/D converter is expected to achieve high-speed operation of 20 GHz. Several 4-bit A/D converter circuits were fabricated, using the conventional Nb-Pb alloy process, with a junction size of 5 mu m*5 mu m and supercurrent density of 200 A/cm/sup 2/. Basic operation at 100 KHz was confirmed experimentally.>
We developed a digital-to-analog converter (DAC) having a current output and incorporating a superconducting quantum interference device (SQUID). The linearity of the device is based on the periodic dependence of the SQUID response on the magnetic flux coupling to the SQUID ring. The period is a flux quantum (Φ(o) = h/2e, where h is Planck's constant and e is the elementary charge). Because of the device's quantum accuracy, the output is essentially linear. The SQUID DAC was constructed using a commercially available rf-SQUID, the periodicity of which has been experimentally verified below 3420 Φ(o). An adjustable number of current amplitudes are dependent on maximum output (I(max)). The output current is adjustable to ~100 000 different values when I(max) ~1 mA and ~1,000,000 when I(max) ~0.1 mA. The short-term fluctuation of ~0.15 ppm was attributable to flux resolution of the SQUID. As a demonstration, measurements showing the voltage linearity of a digital multimeter were performed.
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Edge-triggered periodic comparators based on low-inductance superconducting quantum interference devices (SQUIDs) have been combined with a 5-mV step generator used as a clocked power supply to produce an analog-to-digital (A/D) converter with the widest input bandwidth of which the author is aware. The noise and linearity have been measured to be more than adequate for 6 b. Asynchronous beat frequency tests indicate that the current design is capable of about four effective bits resolution at 5-GHz input bandwidth and 3 b at 10 GHz. The aperture jitter at 6-GHz clock rates has been measured to be 100 fs rms indicating that the intrinsic noise is low enough to permit subpicosecond aperture times in faster designs. Further advantages include tolerance of large critical current variations and a sinewave clock as the only high-speed waveform necessary to drive the circuit. The amplitude and spectral purity of the sinewave are not critical as long as it reaches a threshold value, since it is regulated by the step generator.>
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.