Modeling of a quasi-optical Josephson oscillator

The authors have developed a computer model of a Josephson tunnel junction embedded in a general circuit with frequency-dependent impedance using the harmonic balance method. This model has been applied to the analysis of a two-dimensional Josephson junction array with integrated coupling structures, called a quasi-optical Josephson oscillator. Simulations are done for a junction with dipole, slotline, and bow-tie antennas. The results show that the junction with a bow-tie antenna gives the best performance, and the output power from an array of 4000 junctions can reach 25.7 μW at a frequency as high as 1091 GHz for niobium junctions deposited on a 0.207-mm-thick quartz substrate     

约瑟夫森混合器等效电路参数对三波混频强度的影响研究

约瑟夫森混合器是一种能够生成纠缠量子微波信号的电路。建立了约瑟夫森混合器的等效电路模型,对三波混频哈密顿量进行量子化,研究了约瑟夫森结的临界电流、分路线性电感、谐振器传输线等效电感以及外加穿过环路磁通对三波混频强度的影响。仿真结果表明,约瑟夫森结的临界电流决定了选择的线性电感最大值,而对三波混频强度影响不大;线性电感的大小决定了外加穿过环路磁通的最优值以及三波混频强度的最大值;谐振器传输线等效电感在允许范围内越小越好。研究对于有效选择电路元器件参数,提高纠缠量子微波生成能力有重要价值。     

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     

The microwave circuit model of Josephson junction at liquid nitrogen temperature

We have measured the microwave characteristics of Josephson junction at liquid nitrogen temperature by using microwave network analyser. Based upon these experimental results we present a microwave circuit model of Josephson junction at liquid nitrogen temperature and calculate the parameters of microwave circuit model of Josephson junction.

     

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。     

具有自动增益控制的射频振荡器稳定性分析

设计了一个具有自动增益控制(AGC)的电路来稳定射频功率振荡器的输出幅度,然而在加入AGC负反馈环路之后,该环路可能会产生自激振荡,使得振荡器输出的幅度更加不稳定。通过对整个电路系统传递函数的分析,采用调节反馈电路中三极管发射极电阻阻值的方法,使该电路工作在稳定的状态,进而达到稳定振荡器输出幅度的目的。     

DC instability of the series connection of tunneling diodes

An oscillator with a series connection of tunneling diodes produces significantly higher power than a single diode oscillator. However, a circuit with series-connected tunneling diodes biased simultaneously in the negative differential resistance (NDR) region of the I-V curve is dc unstable. This dc instability makes the series connection oscillator fundamentally different from a single diode oscillator. Associated with the dc instability are the phenomena of minimum oscillation amplitude and frequency. Due to the minimum oscillation amplitude, it is critical to provide the impedance match between the oscillator circuit and the series connection at the desired oscillation amplitude level. An in depth, comprehensive analysis of the dc instability is given here. Based on this analysis, a numerical procedure is developed to accurately predict the minimum oscillation amplitude and frequency. Time domain simulations which give further insight into series-connection oscillator behavior are discussed. The effect of increasing the number of diodes on the oscillator performance is explored as well. Based on numerical and simulation results, oscillators with several tunnel diodes connected in series were designed and tested. Experimental results that confirm the existence of the minimum oscillation amplitude are presented for oscillators with two, three, and four tunnel diodes     

Power optimization of high-efficiency microwave MESFET oscillators

Achieving high output power and efficiency in GaAs MESFET oscillators is mainly hampered by the device's parasitics, its static I-V characteristics, and the circuit embedding impedance. In this paper, the derivation of the relationship between oscillator output power and various circuit and device parameters is presented. From these analytical expressions, optimum operating conditions for maximum oscillator output power and efficiency are determined. The analysis method employed here is based upon a quasi-linear approach and an open-loop model of the oscillator. The design procedure is verified by measurements on an experimental circuit, which have demonstrated a dc/radio-frequency conversion efficiency of 54%     

Niobium-based integrated circuit technologies

Metallurgical and electrical properties of Nb and NbN films for use as Josephson junction electrodes and wiring layers are investigated. The crystallographic and superconducting properties necessary for Nb-based integrated circuit processes are clarified. Tunnel barrier structures of NbN-Nb oxide-NbN (Pb alloy) and Nb-Al oxide-Nb Josephson junctions have been analyzed and correlated with junction characteristics and critical current uniformity. It was found that the surface structure of a base electrode should be smooth to ensure that Josephson junctions have low leakage current and uniform critical current distribution. New types of Josephson junctions with artificial tunnel barriers such as amorphous Si or Mg oxide are reviewed. A variety of Josephson junction structures or processes have been developed for Nb-based Josephson integrated circuits in order to improve circuit performance. These include junction miniaturization, planarization, and stacked junction structures. These structures are mainly intended for Nb-Al oxide-Nb Josephson circuits. The Nb-Al oxide-Nb Josephson junction technology is by far the most advanced and has been used in logic and memory circuits, for example a 4-bit×4-bit parallel multiplier, a Josephson logic gate array, a 16-bit arithmetic logic unit, a 4-bit microprocessor, and 1-kb and 4-kb memory circuits     

A large signal analysis of IMPATT diodes

This paper presents results on RF power output and efficiency of IMPATT oscillators obtained from a large-signal model of these devices. The results are obtained from a closed-form solution of the nonlinear equations describing a Read-type IMPATT diode. The closed-form solution is obtained by assuming a short transit time through the drift region compared to the RF period. The solution is used to obtain the large-signal diode impedance. The analysis shows that the power output of an IMPATT diode depends strongly on the series load resistance presented to the active part of the diode and that the change in diode reactance with increasing bias current also depends on the series resistance. Plots of power output as a function of frequency, bias current, and load resistance are presented. Frequency tuning of the oscillator through current variation is also discussed. Experimental results are presented and compared with the theoretical ones wherever possible. The results lead to an improved understanding of such oscillators and are extremely useful in optimizing their performance and determining their limitations.     

Analysis and synthesis of pHEMT class-E amplifiers with shunt inductor including ON-state active-device resistance effects

In this theoretical paper, the analysis of the effect that ON-state active-device resistance has on the performance of a Class-E tuned power amplifier using a shunt inductor topology is presented. The work is focused on the relatively unexplored area of design facilitation of Class-E tuned amplifiers where intrinsically low-output-capacitance monolithic microwave integrated circuit switching devices such as pseudomorphic high electron mobility transistors are used. In the paper, the switching voltage and current waveforms in the presence of ON-resistance are analyzed in order to provide insight into circuit properties such as RF output power, drain efficiency, and power-output capability. For a given amplifier specification, a design procedure is illustrated whereby it is possible to compute optimal circuit component values which account for prescribed switch resistance loss. Furthermore, insight into how ON-resistance affects transistor selection in terms of peak switch voltage and current requirements is described. Finally, a design example is given in order to validate the theoretical analysis against numerical simulation.     

Review of microwave distributed superconducting vortex-flow transistor amplifiers

The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance r_m of a VFT is very small. Some methods for maximizing r_m and minimizing the output impedance r₀ also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-T_c superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.     

具有自动稳幅功能的软激励C类大功率射频振荡器

介绍了一种具有自动稳幅功能的软激励C类大功率射频振荡器。大功率射频振荡器已经广泛应用于电力电子、射频电源、低温等离子体、高频感应加热等领域。该大功率射频振荡器能够输出较高的输出电压和输出功率,并且通过对输出电压采样控制MOS管的静态工作点,稳定输出电压;另外,该设计电路起振时工作在AB类状态,稳定工作时在自动稳幅电路的作用下进入C类工作状态,实现了C类射频振荡器的软激励。最后通过仿真和实物电路测试了电路性能,并给出了振荡器输出电压、输出功率与MOS管工作状态关系的经验公式。     

LSA relaxation oscillations in a waveguide iris circuit

Large-signal computer simulations of GaAs LSA relaxation oscillations in an X-band waveguide iris circuit are presented. The study is focused on a particular oscillator and a realistic model of an experimental circuit is used. However, the results are typical for other LSA relaxation oscillators. Basic features of the microwave circuit, characteristic voltage and current waveshapes, frequency tuning with bias voltage, and oscillator starting transients are discussed. The RF output power is shown to build up in less than ten cycles. Circuit optimization for high dc to RF conversion efficiency is discussed and circuit data for nearly 30 percent efficiency are given. Finally, efficiency degradation is discussed when doping gradients are present, and effciencies of 15 and 10 percent appear possible for doping gradients as high as 20 and 60 percent, respectively. Hence, the LSA relaxation mode is shown to be less sensitive to doping gradients than the sinusoidal LSA mode.     

20 GHz Wavelet Generator Using a Gated Tunnel Diode

We demonstrate the use of a GaAs-AlGaAs gated tunnel diode (GTD) in an ultra-wideband (UWB) wavelet generator. An inductor is integrated to form an oscillator circuit, which is driven by the negative differential conductance property of a GTD. It is demonstrated that as the gate tunes the magnitude of the output conductance, the oscillator may be switched on and off, creating short RF pulses. The shortest pulses generated are 500 ps long, the highest output power for the free running oscillator is $-$4.1 dBm, and the highest oscillation frequency is 22 GHz. Analytical expressions based on the van der Pol equation describing the pulse length and amplitude are presented. This technique is applicable for high frequency impulse radio UWB implementations.      

Analytical study of the photo-effects on common-source and common-drain microwave oscillators using high pinch-off n-GaAs MESFETs

A new analytical study of the photo-effects on common-source and common-drain microwave oscillators using a high pinch-off n-GaAs MESFET has been presented in this paper. The gate-area of the MESFET with a transparent/semitransparent metal at Schottky junction is assumed to be illuminated with optical radiation. The changes in the gate-source and gate-drain capacitances by an induced photovoltage across the Schottky junction due to the incident illumination have been utilized to model analytically the photo-dependent output frequency characteristics of the microwave oscillators. It has been observed from the numerical results that the output frequency of the common-source oscillator is highly influenced by the gate-source capacitance whereas that of the common-drain oscillator is sensitive to the change in the gate-drain capacitance. Further, in both of the oscillators, the output frequencies are decreased with the increase in the intensity level of the incident optical illumination.     

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     

Current amplitude control circuits suitable for current-modeoscillators

A novel current amplitude control circuit suitable for current-mode oscillators is proposed. The circuit is a modified version of the well-known Gilbert gain cell. The technique obtains independent control of oscillation amplitude and small-signal current gain. As an example, the amplitude control circuit is applied to a current-mode oscillator. Simulations were carried out using HSPICE with 0.8 μm Nortel BiCMOS technology and Motorola RF transistors. Simulated results demonstrate that the nonlinear current gain control circuit behaves in a well defined manner. Low distortion and high frequency oscillations are easily obtained when the circuit is applied to a current-mode oscillator     

Large signal circuit characterization of solid-state microwave oscillator devices

A method of direct measurement of the admittance in X band seen by an encapsulated solid-state oscillator embedded in microwave hardware is described. It is applicable to many two-terminal devices in waveguide, coaxial line or microstrip circuit configurations. The criterion for oscillation in negative conductance oscillators is used in conjunction with these admittance measurements to determine the complete frequency-dependent 'device-plane' of Gunn and IMPATT devices in S4 (pill-prong) encapsulations, and the forms of negative conductance and susceptance variation with RF voltage amplitude and frequency have been extracted for each of these devices. It is found that for transferred-electron devices in particular, the frequency dependence of these parameters over X band is very marked. The use of the admittance measuring technique to characterize the microwave circuit is discussed with reference to the improvement of AM and FM noise by the controlled insertion of perturbations in the admittance locus, and an example is given of its use in fault-finding in a mechanically tunable waveguide oscillator.     

Simulations of electric field domain suppression in a superlattice oscillator device using a distributed circuit model

We present a method for simulating static domain formation in distributed negative differential resistance devices using a distributed circuit array model coupled with quantum transport simulations. This simulation method is applied to the case of a superlattice Bloch oscillator to ascertain the efficacy of electric field domain wall suppression by micro shunt side walls. Two independent simulation mechanisms using the same basic distributed circuit model are employed to separate simulation artifacts from true physical trends. Simulations are presented, suggesting that the presence of the micro shunt can suppress domain formation above a critical device bias voltage. The simulated dependence of this critical voltage on macroscopic device parameters is presented.