The high speed and low power needed for Spread Spectrum communications leads to a desirable application of digital superconductivity. Low temperature Nb/AlOx/Nb digital Single Flux Quantum (SFQ) circuits have been designed and fabricated in our facility, which compose critical subcircuits of the superconductive Spread Spectrum Modem. Four- and 6-bit Pseudo Random Binary Sequence (PRES) generators with output latching amplifiers have been produced and operated at up to 5.4 GHz. On chip modulation and de-modulation of these PRES codes have been performed with time resolution of less than one chip of the code. High-speed off-chip data was XORed with the on-chip PRES at up to 3.7 GHz to produce the modulated pattern. Rapid synchronization allows the code to start within one clock cycle of being fully reset, and the latching output amplifier gives enough power to do real time room temperature measurements of high-speed SFQ circuits.
The high speed and low power needed for Spread Spectrum communications leads to a desirable application of digital superconductivity. Low temperature Nb/AlOx/Nb digital Single Flux Quantum (SFQ) circuits have been designed and fabricated in our facility, which compose critical subcircuits of the superconductive Spread Spectrum Modem. Four- and 6-bit Pseudo Random Binary Sequence (PRES) generators with output latching amplifiers have been produced and operated at up to 5.4 GHz. On chip modulation and de-modulation of these PRES codes have been performed with time resolution of less than one chip of the code. High-speed off-chip data was XORed with the on-chip PRES at up to 3.7 GHz to produce the modulated pattern. Rapid synchronization allows the code to start within one clock cycle of being fully reset, and the latching output amplifier gives enough power to do real time room temperature measurements of high-speed SFQ circuits.
Superconductive electronics can provide low power, GHz speed spread spectrum satellite communications. The key components to construct the spread spectrum modem are an analog filter and a digital pseudorandom bit sequence generator, Various SFQ logic circuits are required to construct a code generator. These circuits include a shift register, a reset switch, and an XOR or XNOR. To make a 4 bit 15 sequence code generator, a four stage shift register with a fan-out of two on each stage was designed. These circuits, fabricated with eight level Nb/AlO/sub x//Nb Josephson junction integrated circuit process, showed correct operation at low speed digital tests. The low speed digital test was done by using a "catcher", the combination of a read SQUID and a Josephson transmission line (JTL) which were magnetically coupled to each other. High speed testing of the shift register used a latch amplifier, which amplified an SFQ pulse to a latching output voltage of about 2 mV. This test showed correct operation of the shift register at speeds of up to 4.6 GHz.>
Spread spectrum data modulation, transmission, and demodulation has been demonstrated between SFQ chips in separate dewars. The baseband demonstration consisted of modulating (encoding) data with a spreading code, transmitting the coded data to a Receiver and demodulating (decoding) the data using an identical spreading code. The Transmitter code was produced by a 2 GHz, 4-bit SFQ pseudorandom sequence generator creating a 15-chip spreading code, which modulated a /spl sim/133 MHz data source. This data was output by a 10/spl times/ superconducting latch providing /spl sim/8 mV of AC drive. This signal was fed through 50-/spl Omega/ coaxial cable to a SFQ Receiver chip in a separate dewar. No amplification of the AC signal between the dewars was needed, however, a slight DC bias was added to the signal as a flux bias for the input SQUID on the Receiver. The Receiver chip consisted of an identical SFQ pseudorandom sequence generator and a data demodulation gate. Demodulating the received data with the code generator produced a replica of the data signal in RZ form. Both time forward and time reversed codes for the spreading/despreading sequence were created.
Maximum-length sequences (M-sequences) are known as pseudo-random numbers generated with relatively simple digital circuits. In this paper, we present a single-flux-quantum digital circuit that generated three output bits of a 4-bit M-sequence in one clock operation. The circuit comprised three clocked exclusive-or gates, one delay flip-flop, three dc/SFQ converters (for set, reset, and low-frequency clock signals), and three SFQ/dc converters (for three parallel outputs), all of which were integrated on a superconducting Nb chip. A 10-GHz clock generator was also implemented for high-speed testing. The total number of Nb/AlOx/Nb Josephson junctions was 568. Verilog simulation demonstrated correct circuit operation with a 20-GHz clock, which corresponded to the generation rate of 60 Gb/s. The measurement was carried out in a liquid helium bath. Low-speed testing with a 1-kHz clock demonstrated periodic signals of three parallelized 4-bit M-sequence. We also confirmed that the outputs were correctly shifted when 13 clock signals of 10 GHz were fed between two 1-kHz clock signals.
Superconductor single-flux-quantum (SFQ) integrated circuits can provide classical, general-purpose digital processing at cryogenic temperature near quantum bits. We report high-frequency operation of low-power SFQ circuits using a lowered critical current density fabrication process and low-voltage design. We are developing a standard-cell library with integrated devices featuring four planarized Nb layers and a Pd resistor layer. SFQ shift registers successfully operated at up to 11 GHz at 0.3 K. The measured operating frequency is sufficient for SFQ-based digital controlling of quantum bits using impulse-shaped voltage pulses.
Superconductive Rapid Single Flux Quantum (RSFQ) logic and memory, in which ones and zeros are represented by the presence or absence within a timing window of quantized picosecond voltage pulse (/spl int/v(t)dt=h/2e=2.07 mV/spl middot/ps), corresponding to one SFQ, can be integrated into a digital computing system with an operating rate of several tens of GHz, based on the present Nb Josephson junction integrated circuit technology. It is the most promising technology beyond semiconductor transistors for low-power high-end computation. However, as the operating speed of circuits and systems increase, timing uncertainty from fabrication process variations makes global synchronization very hard. In this paper, we present a globally asynchronous, locally synchronous timing methodology for RSFQ digital design, which can solve the global synchronization problem. We also demonstrate the recent experimental results of some asynchronous circuits and systems implemented in RSFQ technology. Several key components such as a self-timed shift register, a self-timed demultiplexor, a Muller-C element, a completion detector, and a clock generator have been designed and tested. High speed operation has been confirmed up to 20 Gb/s for a prototype data buffer system, which consists two self-timed shift registers and an on-chip 5-38 GHz clock generator.
The authors argue that superconducting digital electronics is the future trend for high-speed and low-power applications. Low-T/sub c/ technology based on stable Nb-AlO/sub x/-Nb tunneling junctions has been successful in constructing most of the digital circuit logic elements. A four-stage shift register constructed with latching logic operated up to 4 GHz with a power consumption of about 60 mu W per gate. A 12-b analog-digital coverter constructed with a single flux quantum (SFQ) logic showed proper operation with the total power dissipated being only about 100 mu W. Fabrication of Josephson tunneling junctions with high-T/sub c/ materials has been tougher than with low-T/sub c/ materials. However, recently developed very high speed rapid single flux quantum (RSFQ) logic circuits can be applied to weak link type junctions fabricated with YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Simulations of the various circuits with weak link type junctions, e.g., shift registers, analog-digital converters, etc., show fast operation and reasonable margins. Applications of similar concepts can be applied to NbN circuits operating at above 10 K.>