Arnold H. Silver

The distributed Josephson inductance phase shifter

The authors report on a novel microwave phase shifter featuring rapid electronic adjustment, continuous phase control true time delay operation, high device fault tolerance, and very broadband operation. By coupling a large number of superconducting quantum interference devices (SQUIDs) to a superconducting microstrip transmission line, a variable magnetic medium in which the wave velocity is controlled electronically is created. The authors have measured 60 degrees phase shift at 10 GHz, and wideband operation from 5 to 15 GHz for an 8-cm-long Nb transmission line coupled to 1600 SQUIDs, each containing a single Nb/AlO/sub x//Nb tunnel junction. The observed phase shift corresponds to a change in wave velocity of about 1 part in 60.<<ETX>>

A new concept for ultra-low power and ultra-high clock rate circuits

Compared with semiconductors, SFQ logic is very fast and dissipates extremely low power. But it does not approach the theoretical power dissipation associated with an SFQ switching event and single gate speed in complex circuits. For large circuits and systems, e.g., petaflops computing, we must reduce on-chip dissipation, achieve faster clocked logic operation, and increase gate density. CMOS logic dissipates the energy required to switch a transistor pair and dissipates no power between switching events. We describe a new SFQ circuit concept that mimics CMOS to achieve ultra-low power dissipation and ultra-high clock rates. This results in physically compact, self-clocked, complementary logic (SCCL), in which clock distribution is frequency-independent. The basic element in this logic family is a simple two-junction comparator. Using TRWs 2 kA/cm/sup 2/ Nb design rules, we simulated basic digital components: shift register, AND, OR, and NOT at 20 GHz. We present the simulated and measured performance.

Superconductivity in electronics

Superconductivity is an emerging technology for high performance electronics. It offers unique and beneficial attributes for sensor, signal processing, and communication systems. Yet, industry faces technical hurdles and market obstacles to identifying business targets. The author presents his vision of the future of superconductor electronics in space, his perspective of the past, and his approach for achieving success. The ability to define and achieve successful paths which overcome the technical and market barriers will determine the viability of superconductivity in the electronics industry, and coincidentally of applied research in this technology.

Wide bandwidth oscillator/counter A/D converter

We present theory and performance data on oscillator/counter A/D converters fabricated in TRWs Nb process. Circuits with junction critical current density of 2 kA/cm/sup 2/ are discussed. This simple, low power A/D converter architecture is uniquely enabled by the wide bandwidth voltage-controlled oscillator (VCO) and compatible, ultra-fast superconductor single-flux-quantum (SFQ) divide-by-two flip-flops. The measured signal-to-noise ratio (SNR), frequency response, and signal distortion are discussed within the framework of a basic theory of performance as well as time-domain simulations. Noise contributions from quantization error, aperture jitter, and thermal noise are included. The measured signal-to-noise ratio (SNR) is shown to be within a few dB of theoretical performance over more than two decades of frequency. The frequency response is shown to fit well to the expected function through 1 GHz of signal frequency. Harmonic distortion is shown to be consistent with the non-linearity in the front-ends DC I-V characteristic which is subject to design improvement. This architecture extends to higher performance using a multi-junction VCO. Measured two-junction VCO SNR shows essentially the theoretical improvement over that of a single junction.

Error rate measurements of a Josephson single flux quantum binary ripple counter

Single-flux quantum devices are used in superconductive analog-to-digital converters (ADCs), shift registers, and memory cells. They have been proposed for logic applications. The authors report the performance of high-speed superconducting, single-flux quantum (SFQ) ripple counters. Both memory and logic functions of the counter are investigated. Errors in logic operation produce bit error rates (BERs) as low as 0.22 errors per million binary operations, measured while counting 100-MHz pseudorandom input pulses. Errors in memory function do not occur on the time scale of the measurements. The BER is shown to be nearly independent of input bit rate and pattern, but strongly dependent on the counter cell operating point.<<ETX>>

Superconductive digital readout for IR FPA sensors

We have built and demonstrated an all superconductive digital readout for use in an IR focal plane array sensor. High performance, ultralow power superconductive circuits perform the functions of low noise preamplification and analog to digital conversion. The superconductive readout was tested with a variety of detectors, including InSb, Si:As, and a thin film NbN superconducting detector. Light sources included a HeNe laser (0.6 micron), a CO2 laser (10 microns), and a blackbody (400 to 900 K). In each case, the detector and readout circuitry was assembled into a 2 inch diameter, 6 inch long test package cooled in a single dewar. We demonstrated the functionality of the detector/readout channel from input photons to output digital signal. The superconductive readout reported here used Nb-based circuits operating at 4 K. An NbN squid amplifier and detector have subsequently been demonstrated above 10 K. We discuss the extension of the entire digital readout to operating temperatures above 10 K.

Enhanced optical detection in a Josephson junction

Shifting of the energy gap of a Nb Josephson junction under direct optical illumination was measured. The response is linear with optical input power over more than five orders of magnitude and is nearly independent of temperature from 4 to 8 K. The rise time of this signal is faster than the 2- mu s rise time of the chopped light signal. These direct signals are 500 to 1500 times larger than those obtained when the same optical power is focused elsewhere on the substrate. This enhanced direct response is interpreted as resulting from thermal isolation of the Josephson junction from the substrate due to thermal boundary resistance.<<ETX>>

A three-terminal semiconductor-superconductor transimpedance amplifier

The authors report an improved hybrid three-terminal transimpedance amplifier (TIA) with significant current gain. The TIA consists of a semiconductor diode configured for injection of electrons into a thin base electrode (<25 nm) superconductor-insulator-superconductor junction, whose response is read out by low-impedance superconductive electronics. An input dynamic impedance greater than 10/sup 11/ Omega , an output dynamic impedance of approximately 10/sup -3/ Omega a current gain of 20, and an effective input noise current less than 10/sup -14/ A/ square root Hz were achieved. The TIA was operated in a sensor test bed with an extrinsic silicon infrared (IR) detector and superconductive analog-to-digital (A/D) converter. This device permits matching state-of-the-art semiconductor IR detectors with superconductive A/D converters, enabling a fully digital cryogenic focal plane array sensor with high sensitivity and speed but reduced power consumption.<<ETX>>

Application of superconductive electronics to LWIR sensor systems

One of the key issues in the development of IR focalplane systems is the need for low-noise and low-power read- out circuitry which is compatible with the detector technology. Superconductive circuitry offers several advantages over more conventional circuitry. These include much lower power consumption and the possibility of unique circuit topologies relevant for developing advanced monolithic detectors. On-chip signal processing through A/D conversion with digital gamma-ray suppression and digital integration appears possible. This paper reviews the progress that TRW has made in developing several of the key components of such a superconductive read-out system. We have developed a semi- conductor/Josephson-junction 3-terminal device for direct interfacing to semiconductor detectors as well as thin-film superconductive detectors. The 3-terminal device has a current gain of : 500 and provides an optimum interface to further superconductive stages. Performance when coupled to a SQUID read-out will be discussed.

Potential architectures for superconductive IR focal plane sensors

Extremely low power superconductive electronics (SCE) (low noise preamplifier, analog to digital converter, multiplexer, etc.) for very large focal plane arrays can significantly reduce the overall sensor system power, hence its weight and volume, thus reducing overall mission cost. The general architecture for a Z-plane, all-superconducting technology focal plane signal processor is presented illustrating the functional elements and their general configurations. The low noise and speed of the TRW developed SCE permits unique solutions to focal plane array signal processing issues such as in-line gamma suppression and digital signal integration.