Bruce J. Dalrymple

Single flux quantum crossbar switch

A crossbar switch has been designed using Single Flux Quantum (SFQ) gates exclusively for all internal functions. A 4/spl times/4 prototype has been fabricated in our Nb process foundry with J/sub c/ of 2000 A/cm/sup 2/. We report on the design and performance of the switch and of an individual crosspoint element at high data rates (/spl ges/1 Gbps). A novel design of a double-edge-triggered dc/SFQ converter is discussed. The requirements for output amplification and on-chip versus off-chip amplifier issues will be presented.

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.

10 K NbN DSP module for IR sensor applications

The authors report significant progress on infrared (IR) focal plane array (FPA) imaging signal processing circuits, built in NbN and operating at 10 K. The improvements to our NbN process are highlighted by the introduction of directly grounded junctions (DGJ). These DGJs substantially reduce parasitic inductance thereby compensating for the high sheet inductance of NbN films. The circuits being developed include a 16-bit SFQ counting ADC and several digital signal processing (DSP) units. We report test results of greatly improved ADC performance, which is the result of both improved designs and fabrication techniques. Signal processing units on individual chips have been designed, fabricated, and tested. They perform functions such as background subtraction, gain and responsivity correction, and data reduction. We report test results of the DSP chips performing these functions. Ultimately, these chips will be integrated on a multi-chip-module (MCM) with high bandwidth, low impedance interconnects and integrated with an IR focal plane array sensor.

Multi-Gb/s operation of flipped chip MVTL circuits

Development of a reliable flipped chip mounting technique enables demonstration of high speed, complex digital circuits. Flip chip mounting has greatly reduced parasitic inductance compared to conventional wire bonding, and permits remounting of known good die onto multi-chip modules. Superconductive digital circuits have operated to 4.3 Gb/s in our custom test station. The circuit and carrier are fabricated using TRWs foundry process. The chips are flipped onto a superconducting coplanar carrier using a low temperature solder reflow process reported on at this conference. Testing is performed in a multi-GHz, flip contact, variable temperature probe. This test facility is capable of testing circuits to 12 Gb/s. We will describe the operation and performance of our circuits at high bit rates, and design improvements intended to facilitate operation at higher bit rates with improved yield. In addition, we will discuss the use of a logic simulation tool to analyze the output words, and pinpoint the gate or gates that failed to operate properly.

An SFQ digital to analog converter

We have developed and demonstrated a digital to analog converter DAC which uses an SFQ counter to precisely divide an input reference oscillator to produce a set of binary frequencies/voltages. The binary input gates the output SFQ pulses of the counter flip flops to a passive summing network, producing an analog output current. The DAC is asynchronous (no clock to reset latching circuits), low power, and requires only N equal matched resistors. We have built and tested a 4-bit Nb DAC with data clocking rates up to 1 GHz. Using the DAC we have generated arbitrary wave forms including ramps and sine waves. By measuring the harmonic content of sine wave outputs, we deduce values for the DAC linearity.

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.

NbN and Nb SFQ device performance

The static frequency divider is commonly used as a performance benchmark for both superconductor and semiconductor digital device technologies. We present results of a static divide-by-two circuit, an NbN (1 kA/cm/sup 2/) SFQ T-flip-flop (TFF) operating to 97 GHz. Details of the measurement and operating criterion are discussed. Measurements of junction capacitance, a particularly important factor effecting device performance, are presented for TRWs NbN process. Simulations of expected device performance are shown to explain measured performance reasonably well. NbN results are presented alongside a those of a recent 8kA/cm/sup 2/ Nb divider operating at 300 GHz, as well as published Nb TFF results.

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.