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IEEE Transactions on Applied Superconductivity

Reduced Probability of Noise Introduced Malfunction in RSFQ Circuits by Implementing Intrinsic $\pi$-Phaseshifter

Olaf Mielke, Thomas Ortlepp, Pascal Febvre, Friedrich Uhlmann

Published Jun 1, 2009• 8 citations
Publisher

Abstract

The rapid single flux quantum electronics is characterized by a low switching energy, which makes it susceptible to noise introduced bit-errors. For industrial applications a certain noise immunity is required which is still a challenge especially for circuits of higher complexity. We analysed the influence of each individual resistor in a toggle flip-flop circuit to the overall bit-error rate. The shunt resistors of comparator structures are the dominant noise sources and bias resistors have almost no influence.

DOI
10.1109/tasc.2009.2018145

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The influence of thermal noise gives a major restriction in the development of Rapid Single Flux Quantum (RSFQ) logic circuits with high-T/sub c/ superconductors. In the past, we have demonstrated a general way for determining the digital bit error rate and for predicting operation ranges with good noise immunity. The method is based on a single Fokker-Planck equation which includes the full system dynamics. However, the complexity of this equation increases dramatically with the number of Josephson junctions. Our new approach describes a method to divide an RSFQ logic cell into several basic primitives. They can be characterized quickly in terms of switching probability for a lot of different parameters. From these analyses, look-up tables are built-up. We describe how to combine these data in order to achieve the bit-error rate for the whole cell. While saving more than 90% of computation time, the comparison with our previous results for full system analyses shows less than 4% difference. This approach provides for a powerful tool to include the influence of thermal noise into the optimization process of RSFQ logic circuits as well.

IEEE Transactions on Applied Superconductivity•Jun 1, 2003•3 citations
Reduced Probability of Noise Introduced Malfunction in RSFQ Circuits by Implementing Intrinsic π\piπ-Phaseshifter
The rapid single flux quantum electronics is characterized by a low switching energy, which makes it susceptible to noise introduced bit-errors. For industrial applications a certain noise immunity is required which is still a challenge especially for circuits of higher complexity. We analysed the influence of each individual resistor in a toggle flip-flop circuit to the overall bit-error rate. The shunt resistors of comparator structures are the dominant noise sources and bias resistors have almost no influence.
The Rapid Single Flux Quantum electronics is a superconducting, naturally digital circuit family which is currently close to getting commercially applied.RSFQ is outstanding by its very low switching energy resulting in a very low power consumption which is the reason for a significant influence on thermal noise.For industrial applications a certain noise immunity is required which is still a challenge especially for circuits of higher complexity.In superconducting electronics resistors are the only on-chip noise sources.We analyzed the influence of each individual resistor in a toggle Flip-Flop circuit to the overall bit-error rate.The shunt resistors of comparator structures are the dominant noise sources and bias resistors have almost no influence.We analyzed in detail the types of thermal introduced switching errors of an toggle Flip-Flop which was optimized with respect to the fabrication yield.Simulation studies were accomplished to investigate the influence of circuit parameters on quality and on quantity of switching errors.By identifying and manipulating the parameters with the most significant influence on the noise susceptibility an enhanced noise immunity of the device can be achieved.The goal is to improve the robustness of RSFQ circuits against the influence of thermal noise for an specific topology by changing the circuit parameters of the device.
We address the problem of reducing the noise of an RSFQ circuit, which may significantly decohere a Josephson qubit in the case of integration of these circuits. The main sources of such noise in traditional RSFQ circuits are low-ohmic shunting resistors ensuring sufficiently high damping. We developed the toggle flip-flop (TFF) cells with weakly-damped individual junctions due to high-ohmic shunts. The dominant part of necessary damping in this circuit was realized by applying an additional, so-called 'perpendicular', low-ohmic resistor connecting the centre of the storing TFF inductance to ground. The TFF circuits with junctions shunted by 50 Ω resistors and perpendicular resistor of 1.5 Ω, ensuring suppression of noise by more than one order, have been fabricated and demonstrated an operation range of ±20%. In order to make the circuit compact and, therefore, less sensitive to external noise, the passive frozen-flux-based phase shifting elements, replacing large quantizing inductances, were also included. The circuits were fabricated using PTB Nb trilayer process with critical current density j c = 100 A/cm 2 .
A major restriction in the development of a working Rapid Single Flux Quantum (RSFQ) logic circuit with high-Tc superconductors is given by the influence of thermal noise. This gives reason to ask for a general determination of the digital bit error rate. As other approaches, our method of calculating the switching probability is based on the Fokker-Planck equation. In the past few years the bit error rates for a single Josephson junction, SQUIDs and the comparator were calculated by using this theory. We demonstrate numerical solution of the multidimensional Fokker-Planck equation to calculate bit error rates due to thermal noise for a Toggle Flip Flop circuit. In the present work, we combine thermal noise analysis with the effects of process variations in order to derive rules for designing high-Tc RSFQ logic circuits.
We present a new design technique of rapid single-flux-quantum (RSFQ) logic gates for low-noise, high-speed operation. In this study, we propose the use of a damping resistor shared with a junction pair composing a comparator, in addition to their individual shunt resistors increased from the standard values. We analyzed timing characteristics and bit error rates (BERs) of several RSFQ flip-flops composed of the proposed comparators using numerical simulation. The proposed comparator showed reduced timing jitter by ~5% in association with small delay time, sharpened BER curves, and improvement in operating margins by 2-3% compared to the standard design. We fabricated 2-bit shift registers using the noise reduction technique. We obtained sharp BER curves from the measurement. The proposed method indicated that it gave wide margins.
The rapid single-flux quantum electronics (RSFQ) is a superconducting, naturally digital circuit family which is currently close to being commercially applied. RSFQ is outstanding because of its very low switching energy resulting in very low power consumption. This advantage causes, however, a significant influence of thermal noise. For industrial applications, a certain noise immunity is required which is still a challenge, especially for circuits of higher complexity. Integrating phase-shifting elements is a new concept for further improvements concerning stability against the influence of thermal noise. We have already shown that the implementation of phase-shifting elements significantly reduces the influence of thermal noise on circuit behavior by experimentally analyzing the bit-error rate (Mielke et al 2009 IEEE Trans. Appl. Supercond. 19 621–5). Concepts which are easily implementable in standard niobium technology are especially promising. The π-phaseshifter consists of a superconducting loop which is able to store a single flux quantum. The loop current related to the stored flux creates a well-defined phase shift. To achieve the correct functionality of complex circuits it is essential to store exactly one flux quantum in each π-phaseshifter during the cooling down of the chip. Thus, for studying the feasibility of this new approach, the initialization reliability of the π-phaseshifter needs to be verified. We present an experimental investigation of this reliability to obtain a general assessment for the application of the π-phaseshifter in niobium technology. Furthermore, we compare the configuration shielded by a solid ground plane with a configuration with a ground-plane hole below the π-phaseshifter. Justified by the experimental results we suggest programmable RSFQ circuits based on π-phaseshifters. The characteristics of these devices can be influenced by a controlled initialization of the π-phaseshifter. The fabrication was performed by FLUXONICS Foundry.
The influence of thermal noise gives a major restriction in the development of Rapid Single Flux Quantum (RSFQ) logic circuits with high-T/sub c/ superconductors. In the past, we have demonstrated a general way for determining the digital bit error rate and for predicting operation ranges with good noise immunity. The method is based on a single Fokker-Planck equation which includes the full system dynamics. However, the complexity of this equation increases dramatically with the number of Josephson junctions. Our new approach describes a method to divide an RSFQ logic cell into several basic primitives. They can be characterized quickly in terms of switching probability for a lot of different parameters. From these analyses, look-up tables are built-up. We describe how to combine these data in order to achieve the bit-error rate for the whole cell. While saving more than 90% of computation time, the comparison with our previous results for full system analyses shows less than 4% difference. This approach provides for a powerful tool to include the influence of thermal noise into the optimization process of RSFQ logic circuits as well.