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IEEE Transactions on Applied Superconductivity

Investigation of a 5-bit Flash-Type SFQ A/D Converter Using 10 $\hbox{kA/cm}^{2}$ Niobium Process and Locally Isolated Grounds

Hideo Suzuki, M. Oikawa, K. Nishii, M. Hidaka

Published Jan 25, 2013• 8 citations
Publisher

Abstract

We have investigated technologies for high-frequency and stable operation in a 5-bit flash-type single-flux quantum analog-to-digital (A/D) converter. We designed and evaluated several comparators with different designs of the transformer and found that the length of the primary line was the principal factor to determine the bandwidth, which was improved from 7 to 13 GHz by reducing the length from 860 to 290 μm. We have also investigated methods to avoid incorrect operation caused by ground return current of the bias current: one is locally extracting the return current around the component circuits, and the other is to use locally isolated ground for each component circuit. An integrated 5-bit A/D converter with an error correction circuit was designed using the locally isolated ground structure, which can avoid ground return current inflow from other component circuits. The A/D converter was operated without extracting ground return current and 10 GHz operation was carried out by means of the beat frequency method in our cryocooling system. Operation for an optical input signal was also observed. In addition, 100 GHz sampling clock operation with 1 kHz input signal was confirmed.

DOI
10.1109/tasc.2013.2240351

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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.>

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Investigation of a 5-bit Flash-Type SFQ A/D Converter Using 10 kA/cm2\hbox{kA/cm}^{2}kA/cm2 Niobium Process and Locally Isolated Grounds
We have investigated technologies for high-frequency and stable operation in a 5-bit flash-type single-flux quantum analog-to-digital (A/D) converter. We designed and evaluated several comparators with different designs of the transformer and found that the length of the primary line was the principal factor to determine the bandwidth, which was improved from 7 to 13 GHz by reducing the length from 860 to 290 μm. We have also investigated methods to avoid incorrect operation caused by ground return current of the bias current: one is locally extracting the return current around the component circuits, and the other is to use locally isolated ground for each component circuit. An integrated 5-bit A/D converter with an error correction circuit was designed using the locally isolated ground structure, which can avoid ground return current inflow from other component circuits. The A/D converter was operated without extracting ground return current and 10 GHz operation was carried out by means of the beat frequency method in our cryocooling system. Operation for an optical input signal was also observed. In addition, 100 GHz sampling clock operation with 1 kHz input signal was confirmed.
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.
To improve the accuracy of astronomical data analysis, an increase in the number of output bits from the A/D converter is effective. We investigated a single-flux-quantum (SFQ) autocorrelator and an integrator that supports multiple-bit signal output from an SFQ A/D converter for astronomical data processing. The SFQ binary counter was used as the integrator that decimates the output from the SFQ autocorrelator for communication with equipment at room temperature. We designed the test circuits composed of 2-bit inputs, an autocorrelator, and an integrator that contains 5631 Josephson junctions using the 10 kA/cm2Nb process. We confirmed the correct operation of the test circuit with a normalized bias voltage margin of 102%-114% by the low-speed test. We obtained its correct high-speed operation up to a 55.1 GHz clock frequency. In terms of circuit design, scaling up the test circuit is easy. These results indicate the effectiveness of a high-precision and large-bandwidth radio astronomical observation using SFQ circuit technology.
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 developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-Tc single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating rampedge Josephson junctions with IcRn products above 1mV at 40K and small Ic spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the Jc spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200GHz at 40K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-Δ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50GHz.
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.>