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Superconducting Analog to Digital Converters

Joel M. Schoen

Published Sep 1, 1991• 0 citations
OpenAlex

Abstract

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.

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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.
The main topic of this thesis is the evaluation of performance of super-conducting rst- and second-order low-pass sigma-delta modulators. Superconducting electronics comprise characteristics such as very high switching speed, natural quantization, high sensitivity and low noise, proper-ties that are all essential for modulators used in high performance analog-to-digital converters (ADC). Sigma-delta modulators trade speed for improved resolution using feedback mechanisms making superconducting electronics an interesting alternative technology for the implementation. With linear models and device-level simulations the performance of these modulators have been evaluated. The linear model of the rst-order sigma-delta modulator shows that the performance, for a given oversampling ratio, only depends on the cut-o frequency of the RL-lter used. This is veried by device-level simulations and the results from the two methods are in good agreement. Device-level simulations of the second-order sigma-delta modu-
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.
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.
Superconducting digital circuits are capable of producing high dynamic range analog-to-digital converters (ADCs). We present sigma-delta architectures using large (>100) oversampling ratios to give signal-to-noise ratios of greater than 100 dB in simulation. Three distinct designs, using two distinct mechanisms for feedback, are presented. All of the designs use only shunted junctions, and are therefore compatible with HTS SNS junctions. Simulations are presented for a modulator which creates feedback pulses by SQUID lobe-crossings, a modulator which uses the transresistance of a SQUID for feedback, and a modulator which uses threshold crossings in both the first and second loops for feedback.
A digital-to-analogue converter based on series arrays of high-temperature superconducting Josephson junctions can work properly when the microwave bias current distribution in large arrays is uniform. Unfortunately, the considerable microwave attenuation per overdamped junction with normal resistances RN0.1 results in a strong total attenuation of the microwave signal in a traditional layout. Also, the meander geometry of the array, necessary when using bicrystal junctions, disturbs the uniformity of the rf current, especially in the millimetre waveband. To overcome these drawbacks we coupled the meander array inductively to a parallel transmission line. The mechanism of microwave coupling in which both external and mutual frequency locking takes place was studied numerically. A circuit with a coplanar waveguide feed line was fabricated and successfully tested at an operating frequency of 30 GHz. A microstrip line with a small attenuation for pumping of junction arrays at frequencies up to 120 GHz was demonstrated.