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IEEE Transactions on Magnetics

Operation of a superconducting analog-to-digital converter at short conversion times

Richard L. Kautz, Frances L. Lloyd

Published May 1, 1983• 23 citations
Publisher

Abstract

The accuracy of a six-bit superconducting analog-to-digital converter has been tested at short conversion times. The accuracy was found to be good for conversion times down to 0.5 ns but significant errors were observed at a conversion time of 0.1 ns. These errors can be understood in terms of unwanted switching events that occur when the mode boundary separating two flux states is crossed rapidly.

DOI
10.1109/tmag.1983.1062378

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The accuracy of a six-bit superconducting analog-to-digital converter has been tested at short conversion times. The accuracy was found to be good for conversion times down to 0.5 ns but significant errors were observed at a conversion time of 0.1 ns. These errors can be understood in terms of unwanted switching events that occur when the mode boundary separating two flux states is crossed rapidly.
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 superconductive analog-to-digital converter which uses a DC SQUID as a quantizer and a flip-flop counter as a digitizer has been designed, fabricated, and tested. The circuit was fabricated using a ten-level niobium process. Tests at 4.2 K demonstrated counting to the full 12-b accuracy of the design, monotonic A/D conversion with linearity to less than 1 LSB over the more than 9-b range of conversion, read-on-the-fly operation with counter overflow, and counter operation with a gate current bus.
Contents: Superconducting Analog-to-Digital Converter Work at MITRE; Development of a Rapidly Tunable Microwave Source; Design and Evaluation of a Josephson Array Oscillator; Obtaining High-Accuracy Measurements from Low-Accuracy Measurements; Superconducting Microwave Transmission Lines; High Performance, Superconducting Analog-to-Digital Converter; Edge Sharpening with Josephson Junction; Design Evaluation of a Subranging Superconducting Analog-to-Digital Converter; Feasibility Study of a Superconducting Sigma-Delta Analog-to- Digital Converter, and VHDL Simulation Study of Superconducting Sigma Delta Modulators.
This work addresses new signal processing circuits using the special features of superconductivity. A novel flash-type analog-to-digital converter based on a comparator invented in the preceding contract period was demonstrated. The comparator was shown to be useful as a logic gate and an encoder was designed with it. A high-resolution delta-sigma analog-to-digital converter was devised with superconductive components in spite of the lack of an analog integrator in this technology. Positive theoretical results are being followed up experimentally. A simple flux-shuttle single-flux-quantum shift register was devised and several different readout schemes were studied. A six- bit-long version was successfully tested at 1 GHz. A decoder that takes in a five-bit word to select one of 32 output lines was completed. The design involved very tight limitations on current and power. The decoder was combined with a serial-to-parallel converter and operated at 2 GHz. A study of the appropriate architectures for various types of superconductive or Josephson digital technology was developed: an inductance-extraction program.
Software-Defined Radio (SDR), as explained in [1], is a promising concept for the wireless communication industry since the entire transceiver function can be implemented in software, avoiding the replacement of hardware each time the system has to be upgraded. However, the practical implementation of such a system is depended on the hardware, which consists of precise, ultra highspeed electronic devices such as Analog-to-Digital Converters (ADCs). The present state-of-the-art semiconductor ADCs cannot be used for the implementation of SDR in the near future [2]. Some of the factors that hamper the implementation are the requirements of very high speed of a few tens of GHz and the high resolution (e.g. 16 bits) at these speeds. On the other hand, using the Low-Temperature Superconducting (LTS) technology [3], which relays on the quantum mechanical property of superconductors, very high-speed electronic devices with the accuracy of a flux quantum, can be constructed. In a closed superconducting loop, the magnetic flux can exist only in discrete quantized amounts equal to the multiples of the magnetic flux quantum, φ0 = h/2e ~ 2.07x10 Wb. The theoretical speed limit of such a Rapid Single Flux Quantum (RSFQ) device is close to 1 THz [4]. These RSFQ circuits are the fastest of all devices built in any known technology. The basic building block of an RSFQ circuit is the Josephson Junction (JJ), which is formed when two superconductors are separated by an interface. Since RSFQ circuits work at extremely high speeds, testing and verification of such a device is a difficult and challenging task. Direct testing at these speeds is not possible at this moment. Furthermore, the costs of the equipment required for such tests would be extremely high. Hence methods have to be developed to reduce the requirements of external