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Research on Superconductive Signal-Processing Devices.

Richard W. Ralston

Published Nov 30, 1984• 0 citations
OpenAlex

Abstract

A new technology utilizing superconductive components to realize convolvers and correlators that are capable of processing analog signals having bandwidths from 2-20 GHz is being developed. The technology has two key features: (1) low-loss and low-dispersion electromagnetic striplines provide tapped delay on compact substrates; and (2) superconductive tunnel junctions provide efficient low-noise mixing and high-speed sampling circuits. A convolver with a time-bandwidth product of 28 has been demonstrated. Projections indicate that superconductive technology should support analog devices with time-bandwidth products of 1000 or greater. Originator-Supplied keywords include: convolver, time integrating correlator, ring correlator, convolution, and supercondcutive signal processing.

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IEEE Transactions on Magnetics•Mar 1, 1991•1 citation
A new technology utilizing superconductive components to realize convolvers and correlators that are capable of processing analog signals having bandwidths from 2-20 GHz is being developed. The technology has two key features: (1) low-loss and low-dispersion electromagnetic striplines provide tapped delay on compact substrates; and (2) superconductive tunnel junctions provide efficient low-noise mixing and high-speed sampling circuits. A convolver with a time-bandwidth product of 28 has been demonstrated. Projections indicate that superconductive technology should support analog devices with time-bandwidth products of 1000 or greater. Originator-Supplied keywords include: convolver, time integrating correlator, ring correlator, convolution, and supercondcutive signal processing.
Concepts for realizing superconductive convolvers capable of processing analog signals having bandwidths up to 10 GHz have been developed and are being implemented. A preliminary convolver structure with 19 taps and a 14-ns interaction length has recently been demonstrated. This device employs a superconductive transmission line as a low-loss delay element and superconductive tunnel junctions as nonlinear mixing elements. The niobium delay line provides the relative shifting of signal and reference waveforms, niobium/niobium-oxide/lead mixers with proximity taps provide local multiplication of the two waveforms, and a short output transmission line provides coherent summation of the local products. The principles of device operation, design tradeoffs and experimental results will be presented. Convolvers can provide the essential programmable matched-filter component for extremely wide-bandwidth spread-spectrum communication systems. It appears feasible to produce a convolver with an interaction time of about 100 ns and signal processing gains of up to 1000. This superconductive analog device offers the potential of providing a real-time programmable signal processing function with the digital equivalent of about 1012operations/sec.
A superconductive convolver with tunnel-junction ring mixers has been developed and demonstrated as a programmable matched filter for near 1-GHz-bandwidth chirped waveforms. A low-loss, 14-ns-long superconductive stripline circuit provides temporary storage and relative shifting of signal and reference waveforms. These waveforms are sampled by 25 proximity tap pairs and local multiplication is performed by 25 junction ring mixers. Two short transmission lines coherently sum the local products and deliver the convolution output. The output power level of the convolver has been increased 18 dB by the incorporation of ring mixers and other output circuit improvements. These mixers employ series arrays of niobium/niobium oxide/lead junctions driven by delay-line taps in a quasi-balanced manner. The ring mixer provides higher output power levels (to -58 dBm), improved suppression of undesired mixing products and higher rf impedances than did the single-junction mixers used in the previous device. Convolvers can provide the essential programmable matched-filter component for extremely wide-bandwidth spectral analysis or spread-spectrum communication systems. The current device has a 2-GHz design bandwidth and a time-bandwidth (TB) product of 28. It produced compressed pulses with -7 dB peak-to-side-lobe levels. Design improvements to be discussed include increasing the TB product to 100 and reducing spurious side-lobe levels.
Real-time signal processing is important in many emerging applications in areas such as pulse-compression radar, spread-spectrum communications, and electronic warfare. The required digitally equivalent computational rate for these applications is on the order of 10/sup 2/ arithmetic operations per second and the required instantaneous bandwidths could approach 10 GHz. These exceed by nearly three orders of magnitude the capabilities projected for digital systems in the near future, and even exceed that of recently developed analog technologies such as surface-acoustic-wave (SAW) signal-processing devices. To meet anticipated future system needs, superconductive analog signal-processing components with bandwidths of 2 GHz have recently been realized, and the technology is being developed for the realization of 10-GHz bandwidths. Issues of fabrication technology and subsystem integration are examined for superconductive analog signal-processing devices.>
Analog superconductive components have been integrated to form a device capable of cross-correlation between wideband analog input signals. The device contains a tapped niobium delay line, tunnel-junction mixers, a lumped-element L-C resonator, and a tunnel-junction comparator. The tapped delay line is realized by a niobium stripline folded in a meander pattern on a rectangular silicon substrate. An array of Nb/Nb2O5/Pb tunnel junctions acts as a mixer to form the product of delayed samples of two carrier-offset analog signals counterpropagating along the transmission line. The resultant mixer products from the junction arrays are integrated and stored in a high-Q (≈ 600) resonator consisting of a lumped-element L-C network, tuned to the offset frequency. The low-leakage capacitor dielectric is formed from electrolytically anodized niobium. A superconductive tunnel junction imbedded in the resonator circuit is operated as a variable-threshold comparator to detect the time-integrated current stored in the resonator. Performance results from such a time-integrating correlator are presented, along with a discussion of the important design issues as they relate to analog signal processing.
Prime motivators in the evolution of increasingly sophisticated communication and detection systems are the needs for handling ever wider signal bandwidths and higher data processing speeds. These same needs drive the development of electronic device technology. Until recently the superconductive community has been tightly focused on digital devices for high speed computers. The purpose of this paper is to describe opportunities and challenges which exist for both analog and digital devices in a less familiar area, that of wideband signal processing. The function and purpose of analog signal-processing components, including matched filters, correlators and Fourier transformers, will be described and examples of superconductive implementations given. A canonic signal-processing system is then configured using these components in combination with analog/digital converters and digital output circuits to highlight the important issues of dynamic range, accuracy and equivalent computation rate. Superconductive circuits hold promise for processing signals of 10-GHz bandwidth. Signal processing systems, however, can be properly designed and implemented only through a synergistic combination of the talents of device physicists, circuit designers, algorithm architects and system engineers. An immediate challenge to the applied superconductivity community is to begin sharing ideas with these other researchers.
A superconductive tapped delay line, circuits comprising superconductor-insulator-superconductor (SIS) mixers, an L-C integrator, and a tunnel junction comparator, and superconductive digital address encoders have been integrated into a 14-channel analog signal correlator. This 25*38 mm circuit was fabricated using a Nb/Nb/sub 2/O/sub 3//Pb tunnel junction process. Preliminary testing of this device indicates the ability to process long duration, wideband waveforms with time-bandwidth products equal to 6000.