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IEEE Transactions on Applied Superconductivity

A comparison of two types of single flux quantum comparators for a flash ADC with 10 GHz input bandwidth

Paul D. Bradley, Sergey V. Rylov

Published Jun 1, 1997• 16 citations
Publisher

Abstract

We compare the SQUID wheel/Quantum Flux Parametron (QFP) comparator to a new Rapid Single Flux Quantum (RSFQ) compatible design. Both have been simulated to demonstrate /spl sim/0.5 ps threshold accuracy which would permit the construction of a flash analog-to-digital converter with six effective bits of resolution at 10 GHz input bandwidth, over three times better than the best performance demonstrated with any technology, At lower input frequencies, both designs have demonstrated that a 10-bit flash ADC is possible. Although simulations of the QFP-based design are more accurate at high signal slew rates due to its symmetry, the RSFQ-based design has a better signal-to-noise ratio and a faster and more flexible clocking scheme which ultimately prove to be more important.

DOI
10.1109/77.621790

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IEEE Transactions on Applied Superconductivity•Jun 1, 2009•10 citations
We compare the SQUID wheel/Quantum Flux Parametron (QFP) comparator to a new Rapid Single Flux Quantum (RSFQ) compatible design. Both have been simulated to demonstrate /spl sim/0.5 ps threshold accuracy which would permit the construction of a flash analog-to-digital converter with six effective bits of resolution at 10 GHz input bandwidth, over three times better than the best performance demonstrated with any technology, At lower input frequencies, both designs have demonstrated that a 10-bit flash ADC is possible. Although simulations of the QFP-based design are more accurate at high signal slew rates due to its symmetry, the RSFQ-based design has a better signal-to-noise ratio and a faster and more flexible clocking scheme which ultimately prove to be more important.
Successful utilization of the quasi one-junction SQUID (QOS) as a signal comparator for analog-to-digital conversion (ADC) has been demonstrated by several groups, This idea is extended to show how similar comparators can be integrated into an all rapid single flux quantum (RSFQ) digital architecture. Comparators are interrogated by SFQ sampling pulses and in turn give output according to the RSFQ convention. One- and two-bit ADC's have been simulated, fabricated and tested at low speed. Total performance of these devices is estimated within the framework of a multiple bit parallel flash ADC architecture.>
The authors report on the high-speed operation of a superconducting comparator circuit, based on coupling the quantum flux parametron (QFP) to an RF SQUID, which can be used to build a flash-type analog-to-digital converter (ADC). Simulations of this circuit show that it is expected to achieve operation with input signal bandwidths greater than 4 GHz and with a dynamic range equal to at least 4 b of resolution. A QFP-based comparator fabricated with a process using NbN/Pb-alloy Josephson junctions of 5 mu m by 5 mu m and a current density of 100 A/cm/sup 2/ has been examined to evaluate the properties of the QFP-ADC. Analog-to-digital conversion of the comparator has been observed with a QFP activation frequency up to 18.2 GHz. By employing a sampling method, input signals with frequencies up to 5.4 GHz have also been digitized.>
We are developing an analog-to-digital converter (ADC) based on rapid single-flux quantum technology for radio-astronomic spectroscopy applications where very wide bandwidth is required and relatively small resolution is sufficient. The ADC consists of periodic-flash comparators, a demultiplexer and output drivers; and is sufficiently simple to be implemented on a single chip using a conventional 2–3 μm Nb trilayer technology. A 2-bit ADC has been designed for operation at 16 GHz of the sampling frequency. All the subsystems of the ADC have been fabricated using standard 1.6 kA cm −2 Nb/AlO x /Nb technology and have been successfully tested.
A flash-type superconducting A/D (analog/digital) converter is proposed. The basis low-frequency operation of this converter, which compromises a quantum flux parametron (QFP) and an RF SQUID (superconducting quantum interference device), has been observed. The RF SQUID consists of a Josephson junction and a load inductor which is part of the QFP. It converts an analog signal to period digital signals of one quantum flux (2.07*10/sup -5/ W-b) per cycle. The QFP works as both an amplifier and a comparator of the periodic digital signals, by adding an exciting signal. This A/D converter is expected to achieve high-speed operation of 20 GHz. Several 4-bit A/D converter circuits were fabricated, using the conventional Nb-Pb alloy process, with a junction size of 5 mu m*5 mu m and supercurrent density of 200 A/cm/sup 2/. Basic operation at 100 KHz was confirmed experimentally.>
We have designed a fully integrated 5-bit flash-type single-flux quantum (SFQ) analog-to-digital converter (ADC), in which an error correction and a bit-interleaving circuit are integrated with complementary quasi-one-junction SQUID (CQOS) comparators, and we verified its operation in experiments. Two types of look-back error correction circuits with different clock schemes were designed for the integrated ADCs to avoid the gray zone in comparators and to convert gray code to binary code. The interleaving circuit was also used in the ADC to add one extra bit. A binary 5-bit A/D converter was integrated into an IC chip. Then, input waveforms at low frequencies were successfully retrieved from the binary data of the A/D converter without any errors. We also confirmed that the A/D converter operated properly by conducting tests in our cryocooling system using a 4K-GM cryocooler after the chip was flip-chip bonded onto a multi-chip-module (MCM) carrier. We also used the beat frequency method to test and confirm that the CQOS comparator operated effectively at high frequency, i.e., 3 bits at 15 GHz in the binary code operation and 4 bits at 15 GHz in the gray code operation. Furthermore, operations at sampling frequencies of up to 32 and 50 GHz with a low-frequency analog input signal were experimentally confirmed for a 4-bit comparator circuit with a critical current density Jcof 2.5 kA/cm2and 10 kA/cm2, respectively.
We propose a possible application of a high-speed flash type single flux quantum (SFQ) analogue-to-digital converter (ADC) to optical communication systems and their measuring instruments. One of the problems in optical networks is the chromatic dispersion over a standard single mode fiber, which has to be recovered by optical dispersion compensation. Recently, there has been great interest in 100-Gbit/s Ethernet technologies as next generation optical networks in which chromatic dispersion is more severe. Among several types of architectures, digital coherent optical communication system is a promising technology. Correction of the dispersion is performed electrically after digital conversion without any optical compensation, which would ease system construction. Digital storage oscilloscopes with high-speed ADCs are crucial for investigating the architecture and for constructing 100-GbE systems. A flash-type SFQ ADC is attractive for its high-speed sampling clock capability, especially as an instrument for a digital coherent receiver. The maximum operating frequency of the comparator in ADCs was studied by simulation for various critical current densities (Jc). The simulation suggested that using Jcof 20-40 kA/cm2enables the comparator to operate at a clock frequency of more than 100 GHz. An O/E converter, a uni-traveling-carrier photodiode, operable at low-temperature, has been developed and 40-GHz operation was confirmed at 5 K. We have also designed a 1-bit slice of a superconducting quantum interference device-(SQUID) wheel-type ADC and its test circuit including a doubler circuit and two driver types.