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Proceedings of the IEEE

Superconductor analog-to-digital converters

Oleg A. Mukhanov, Deepnarayan Gupta, Alan M. Kadin, Vasili K. Semenov

Published Sep 20, 2004• 185 citations
Publisher

Abstract

Ultrafast switching speed, low power, natural quantization of magnetic flux, quantum accuracy, and low noise of cryogenic superconductor circuits enable fast and accurate data conversion between the analog and digital domains. Based on rapid single-flux quantum (RSFQ) logic, these integrated circuits are capable of achieving performance levels unattainable by any other technology. Two major classes of superconductor analog-to-digital converters (ADCs) are being developed - Nyquist sampling and oversampling converters. Complete systems with digital sampling at rates of /spl sim/20 GHz and above have been demonstrated using low-temperature superconductor device technology. Some ADC components have also been implemented using high-temperature superconductors. Superconductor ADCs have unique applications in true digital-RF communications, broadband instrumentation, and digital sensor readout. Their designs, test results, and future development trends are reviewed.

DOI
10.1109/jproc.2004.833660

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Ultrafast switching speed, low power, natural quantization of magnetic flux, quantum accuracy, and low noise of cryogenic superconductor circuits enable fast and accurate data conversion between the analog and digital domains. Based on rapid single-flux quantum (RSFQ) logic, these integrated circuits are capable of achieving performance levels unattainable by any other technology. Two major classes of superconductor analog-to-digital converters (ADCs) are being developed - Nyquist sampling and oversampling converters. Complete systems with digital sampling at rates of /spl sim/20 GHz and above have been demonstrated using low-temperature superconductor device technology. Some ADC components have also been implemented using high-temperature superconductors. Superconductor ADCs have unique applications in true digital-RF communications, broadband instrumentation, and digital sensor readout. Their designs, test results, and future development trends are reviewed.
A wide variety of applications, ranging from radio-frequency (RF) receivers for broadband communications and signal detection, on earth and in space, would benefit from high-performance superconductor analog-to-digital converters (ADCs), based on magnetic flux quantization and fast switching Josephson junctions. Superconductor ADCs are capable of very high sample rates (100 GHz or more), very high linearity, and high sensitivity. Nyquist-rate ADCs use the high sample rate to digitize a wide instantaneous bandwidth (tens of GHz) and are useful for wideband spectrum monitoring as well as high-end scientific instrumentation. Delta and delta-sigma oversampling ADCs use the high linearity to achieve programmable trade-off between dynamic range and instantaneous bandwidth. These lowpass and bandpass ADCs have been used for direct digitization of narrower (tens to hundreds of MHz) RF bands in the 1-20 GHz range for a variety of communications, intelligence, electronic warfare, and radar applications. Another application area of these cryogenic ADCs is for outputs of cryogenic sensor arrays and terahertz mixers. Recent advances in various classes of ADCs for different applications are described.
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Superconductive technology offers unparalleled digital and mixed-signal circuit performance, advancing digital processing from baseband to RF stages. This allows elimination of analog filters and mixers with generally poorly controlled phase performance, causing intersymbol interference limiting an overall communication system performance. HYPRES has been developing superconducting circuit components for application to digital RF communication transceivers. The components include low-pass and band-pass analog-to-digital converters, digital decimation and correlation-based filters, digital up/down-converters, and low-time-jitter clock sources. All of these are based on ultra-fast Rapid Single Flux Quantum (RSFQ) digital circuits, with clock rates up to 20 GHz demonstrated with 3-micron technology, and being scaled to 40 GHz and above. In this paper, we will discuss design and performance of some of these components. 1.
Superconductor analog-to-digital converters (ADC) offer high sensitivity and large dynamic range. We report recent advances in the low-pass phase-modulation-demodulation (PMD) oversampled analog-to-digital converter (ADC). The oversampled data from the PMD front-end are passed along with sampling clock to an on-chip digital filter. In the past, the maximum frequency of operation of such chips were limited by that of the digital decimation filter (DDF) to about 30 GHz. Recently, we have designed a chip that allows the PMD front-end to be clocked twice as fast as the DDF, resulting in sampling rates of 46.08 GHz and 29.44 GHz for chips with critical current densities of 4.5 and 1 kA/cm2respectively. We have also designed a new PMD front-end, with a quarter-rate quantizer (QRQ) that allows 18-20 dB extension of dynamic range. We are also pursuing a subranging ADC design with two PMD front-ends connected with fluxon amplifier and subtractor circuitry that promises a dynamic range extension by 25-30 dB. We report signal-to-noise ratio and spur-free dynamic range measurement results for various ADC chips, and project performance for future improvements.
Arrays of cryogenic microbolometers and other cryogenic detectors are being developed for infrared imaging. If the signal from each sensor is amplified, multiplexed, and digitized using superconducting electronics, then this data can be efficiently read out to ambient temperature with a minimum of noise and thermal load. HYPRES is developing an integrated system based on SQUID amplifiers, a high-resolution analog-to-digital converter (ADC) based on RSFQ (rapid single flux quantum) logic, and a clocked RSFQ multiplexer. The ADC and SQUIDs have already been demonstrated for other projects, so this paper will focus on new results of a digital multiplexer. Several test circuits have been fabricated using Nb Josephson technology and are about to be tested at T = 4.2 K, with a more complete prototype in preparation.