约瑟夫森结的电路模型及其在超导电子学中的应用

为方便约瑟夫森结及其相关电路的仿真研究,根据约瑟夫森方程首次提出了约瑟夫森结的电路模型,给出了具体的电路原理图,并进行了封装.利用这个模型可以对约瑟夫森结的相关特性进行深入系统的仿真研究,这样不必自己编写程序对系统的微分方程进行数值求解,大大提高了工作效率.利用这个模型,对约瑟夫森结的混沌行为、相位锁定特性、RSFQ电路和SQUID进行了研究,结果说明了模型的正确性和实用性,模型的建立对于促进超导器件的相关研究具有一定意义.     

Multisim在超导器件研究中的应用

 为了对约瑟夫森结及其相关电路进行仿真研究,首次在Multisim中建立了约瑟夫森结的模型.利用Multisim对直流约瑟夫森效应,交流约瑟夫森效应进行了仿真验证.随后利用在Multisim中建立的模型对电阻电容电感并联约瑟夫森结(Resistively-Capacitively-Inductively Shunted Junction,RCLSJ)模型中的混沌行为,微波感应台阶,热噪声对台阶的影响以及RSFQ (Rapid Single Flux Quantum)电路进行了仿真研究.仿真结果证明了在Multisim中建立的模型对超导器件进行仿真研究是合理可行的,这对超导器件的分析和设计具有重要意义.     

基于高阶时域有限差分法与改进节点分析法混合求解复杂传输线网络瞬态响应

该文提出一种用于求解复杂传输线网络瞬态响应的新型混合算法.通过构建混合单端口网络模型将传输线分布参数系统与集总电路分开,分别采用高阶FDTD(2,4)与改进节点电压分析法(MNA)分析传输线与端口电路瞬态响应.与以往暂态分析方法相比,高阶FDTD(2,4)的低数值色散特性,使得求解传输线时可采用粗网格离散,能方便处理电长度较长的传输线.同时直接采用电路分析方法求解端口电路,能够获取电路中各节点的电压电流波过程.通过几组数值实例验证了该方法的有效性及准确性.     

超导数字快单磁通量子电路应用初探

本文对当前正引起极大关注的新技术－超导数字电路，特别是约瑟夫森效应基础上的RSFQ逻辑电路工作机理给予了详细说明，并举例研究RSFQ电路“与”门的工作方式，对超导数字电路的应用前景进行了展望。     

传输线端接复杂电路的电磁耦合时域分析方法

该文基于时域有限差分(FDTD)方法和传输线方程,结合Ngspice软件,提出一种高效的时域混合算法,能够快速模拟空间电磁场作用传输线端接复杂电路的电磁耦合问题.该算法的优势在于实现了空间电磁场辐射与端接复杂电路瞬态响应的协同计算,且避免了对传输线和复杂电路结构的直接建模.首先,将复杂电路通过传输线的特性阻抗进行等效,采用FDTD方法结合传输线方程,求解得到特性阻抗上的入射电流响应.然后,在每个时间步上,将该电流引入复杂电路作为激励源,联合电路模型建立网表文件.最后,使用Ngspice软件读取网表文件,并仿真得到电路各元件上的瞬态响应.通过相应计算实例的数值模拟,与电磁场仿真软件CST的计算结果以及耗用内存和时间进行对比,验证了算法的正确性和高效性.     

传输线端接复杂电路的电磁耦合时域分析方法

该文基于时域有限差分(FDTD)方法和传输线方程,结合Ngspice软件,提出一种高效的时域混合算法,能够快速模拟空间电磁场作用传输线端接复杂电路的电磁耦合问题。该算法的优势在于实现了空间电磁场辐射与端接复杂电路瞬态响应的协同计算,且避免了对传输线和复杂电路结构的直接建模。首先,将复杂电路通过传输线的特性阻抗进行等效,采用FDTD方法结合传输线方程,求解得到特性阻抗上的入射电流响应。然后,在每个时间步上,将该电流引入复杂电路作为激励源,联合电路模型建立网表文件。最后,使用Ngspice软件读取网表文件,并仿真得到电路各元件上的瞬态响应。通过相应计算实例的数值模拟,与电磁场仿真软件CST的计算结果以及耗用内存和时间进行对比,验证了算法的正确性和高效性。     

含约瑟夫森结电路的特性分析和研究

提出了超导约瑟夫森结模型(并联电阻—电容结模型)，给出了约瑟夫森效应的一些重要结果。在分析约瑟夫森结电路方程时，研究了其中出现的混沌特性和子台阶现象。计算获得的结果对混沌数字通信和超导电子学的应用将起到重要作用。     

约瑟夫森阵列振荡器的相位锁定分析

系统地研究了超导亚毫米波阵列振荡器的相位锁定问题。为使超导振荡器达到高工作频率、窄线宽和高稳定的性能，约瑟夫森结与结之间的相位必须相互锁定。相位锁定可以通过结与结之间的耦合电路得以实现。通过对振荡器的各种耦合电路的比较表明，蝴蝶领结天线结构是一个比较适合约瑟夫森振荡器相位锁定的耦合电路。本文提出了一种超导亚毫米波阵列振荡器模型并对其进行了模拟计算与分析，仿真得出了振荡器各项参数值，并给出了相位锁定的条件。     

超导RSFQ RS触发器基本原理与时域仿真

在超导计算机体系研究中，超导RSFQ逻辑单元设计与仿真是其中重要的环节，但理论分析得到的结果往往不能直观地展现电路的工作过程。超导RSFQRS触发器是构成超导计算机体系的重要逻辑单元，其功能和性能对RSFQ超导计算机体系至关重要，本文在超导RSFQRS触发器理论分析基础上，着重对电路进行时域瞬态仿真，仿真结果直观地展现了电路各种状态和工作过程，并很好地验证了理论分析结果，这一工作对于超导RSFQ计算机体系的深入研究具有重要的价值。     

外场作用双导线的电磁耦合时域分析方法

基于时域有限差分（finite-difference time-domain，FDTD）法和传输线方程，并结合插值技术，研究了一种高效的时域混合算法，能够快速模拟电磁波照射自由空间和屏蔽腔内双导体传输线的电磁耦合，并实现空间电磁场与双导线瞬态响应的同步计算.该算法先采用FDTD方法模拟双导线周围空间的电磁场分布，结合插值技术构建适用于双导线电磁耦合的传输线方程，再采用FDTD的中心差分格式进行离散，从而求解得到传输线和端接负载上的瞬态响应.同时，分析双导线间距对其电磁耦合的影响，掌握其耦合规律.通过相应数值算例的模拟，并与FDTD方法进行对比，验证了该时域混合算法的正确性和高效性.     

Analysis of Series-Connected Discrete Josephson Transmission Line

Employing a generalized resistive–capacitive shunted junction model for Josephson junctions (JJs), the nonlinear wave propagation in the series-connected discrete Josephson transmission line (DJTL) is investigated. A DJTL consists of a finite number of unit cells, each including a segment of superconducting transmission line with a single array stack, or generally a block including an $N$ identical lumped JJ element. As the governing nonlinear wave propagation is a system of nonlinear partial differential equations with mixed boundary conditions, the method of the finite difference time domain is used to solve the equations. By this numerical technique, the behavior of wave propagation along the DJTL can be monitored in time and space domains. Cutoff propagation, dispersive behavior, and shock-wave formation through the DJTL is addressed in this paper.      

Linewidth measurements and phase locking of Josephson oscillatorsusing RSFQ circuits

We present a technique for linewidth measurement and phase-locking of Josephson oscillators using digital rapid single-flux-quantum (RSFQ) circuits. The oscillator consists of a resistively shunted 6 μm×6 μm Nb/AlO_x/Nb Josephson tunnel junction that is integrated with RSFQ input and output circuits. A cascade of RSFQ T flip-flops is used to directly monitor the output of the Josephson oscillator. Spectral characteristics have been measured directly for oscillator frequencies ranging from 10-50 GHz. The linewidth can be reduced by over 100 times by phase-locking the oscillator to an RSFQ pulse train generated by an external sinusoidal signal. These Josephson oscillators can be used as on-chip stable high frequency clocks for RSFQ circuits     

Superconductor ICs: the 100-GHz second generation

In the shadow of high-temperature superconductivity, 35 years of research in low-temperature superconductor ICs quietly comes to fruition, yielding the world's fastest circuits. They are made with a superconducting metal, niobium, rather than a compound semiconductor. Their exotic technology is based on Josephson junction devices and the transmission of single quanta of magnetic flux along superconductor interconnects. This paper describes these ICs and also a logic family called rapid single flux quantum (RSFQ) logic, which is based on the storage and transmission of quanta of magnetic flux.     

Picosecond nodal testing of centimeter-size superconducting Nb microstrip interconnects

We have implemented a low-temperature electro-optic sampling system for non-invasive, nodal testing of superconducting Nb integrated circuits. With submillivolt sensitivity and a subpicosecond temporal response, this system has been used to perform nodal analysis on rapid-single-flux quantum (RSFQ) devices and superconducting microstrip interconnects. Here we demonstrate that by measuring the propagation of 6-ps-wide pulses at various test nodes, we are able to fully characterize a superconducting microstrip waveguide the size of an entire chip. The transmission line was selected not only to perform the first complete characterization of a superconducting microstrip, but also to demonstrate full nodal testing of a foundry-fabricated RSFQ integrated circuit. Finally, our results provided much-needed feedback for improving computer simulations of superconducting digital circuits.     

The finite-difference time-domain solution of lossy MTL networkswith nonlinear junctions

We describe a numerical technique to solve lossy multiconductor transmission line (MTL) networks, also known as tube/junction networks, which contain nonlinear lumped circuits in the junctions. The method is based on using a finite-difference technique to solve the time-domain MTL equations on the tubes, as well as the modified nodal analysis (MNA) formulation of the nonlinear lumped circuits in the junctions. The important consideration is the interface between the MTL and MNA regimes. This interface is accomplished via the first and last finite-difference current/voltage pair on each MTL of the network and, except for this, the two regimes are solved independently of each other. The advantage of the FDTD method is that the MTL equations may contain distributed source terms representing the coupling with an external field. We apply the method to previously published examples of multiconductor networks solved by other numerical methods, and the results agree exceptionally well. The case of an externally coupled field is also considered     

PCB上微带线的电磁耦合时域建模分析方法

基于时域有限差分方法和传输线方程,结合高效网格建模技术,文中提出了一种高效的时域建模算 法,它能有效解决微带线的电磁耦合建模问题,实现空间电磁场与微带线瞬态响应的同步计算。首先,结合经验公 式,计算得到微带线的单位长度分布参数,构建适用于微带线电磁耦合分析的传输线方程。然后,采用时域有限差 分(Finite-Difference Time-Domain, FDTD)方法,结合非均匀网格技术和自动网格生成技术,仿真得到微带线激励场, 并在每个时间步进上引入传输线方程获得等效分布源项。最后,对传输线方程使用FDTD 的中心差分格式进行离 散,实现微带线及其端接电路上瞬态响应的迭代求解。为了验证时域建模算法的正确性和高效性,通过自由空间和 屏蔽腔内PCB 上微带线电磁耦合的数值模拟,从计算精度和耗时两方面与传统FDTD 方法的计算结果进行了对比。     

Shock-wave generation and pulse sharpening on a series arrayJosephson junction transmission line

A nonlinear transmission line can generate a shock wave. However, dispersion and granularity of the line offset the theoretical discontinuity associated with the shock wave and force the wave to a stable form. Such stable waveforms are examined in the context of superconducting technology. In particular, the characteristics of a tranmission line composed of a series of Josephson junctions are studied. Results indicate that by using available technology, such a transmission line can be used to generate 5-mV edges as sharp as 2 ps on lines with impedances of 50 Ω     

A novel approach for the incorporation of arbitrary linear lumped network into FDTD method

A novel approach is proposed to incorporate arbitrary linear network into one grid-cell of finite-difference time-domain (FDTD) method in this work. By correlating the node voltage and current with the impedance (or admittance) of the network in the integral form, an efficient recursive updating algorithm is proposed to fit the lumped networks to the FDTD Yee's equations. This approach has good numerical accuracy and numerical stability. An example of a parallel transmission line terminated with a lumped network is used to demonstrate the validity of this approach.     

Resonance-free low-pass filters for the AC Josephson voltage standard

We have designed and characterized superconducting integrated circuits for the ac Josephson voltage standard that demonstrate significantly improved performance. The typical circuit consists of an array of superconductor-normal metal-superconductor Josephson junctions, which are placed in a transmission line and biased with a broad-band (dc-to-15 GHz) pulse-drive waveform. Additional low-speed (dc and audio frequency) bias and output leads contain on-chip inductors that act as low-pass filters. The operating margins of the array were improved by adding resistive shunts across these inductors to damp their intrinsic resonances. These resonances had previously degraded the integrity of the broad-band signal driving the array. We present simulations and measurements of these improved circuits that demonstrate no resonances in the range of 0.1-20 GHz. Moreover, the operating margins of the ac Josephson voltage standard were improved.     

Lumped 50-Ω Arrays of SNS Josephson Junctions

For the first time, lumped series arrays of Josephson junctions have been fabricated with a transmission line-matched 50-Omega resistance. These arrays also have the thousands of junctions necessary to produce a metrologically significant voltage. This approach is expected to increase the output voltage per array and to optimize their performance for Josephson voltage standards. Traditional Josephson arrays for voltage standards have used distributed microwave structures, where array lengths are several multiples of the driving wavelength. The lumped arrays in this work have physical lengths shorter than a quarter of the microwave drive wavelength and total normal-state resistances nearly equal to the transmission line impedance. Fabrication of these arrays was made possible by use of a newly developed Nb-(MoSi₂-Nb)_n stacked junction technology. We present measurements of the microwave response of lumped arrays with total normal resistances up to 54 Omega and with various termination resistances. A simple numerical model is presented that accounts for the spatial distribution of the microwave current and for the nonuniformity of the junction critical currents. The resulting simulations agree well with experimental results