Starting conditions for backward-wave oscillators with large loss and large space charge

Starting conditions of backward-wave oscillators are obtained both analytically and numerically for large loss and large space charge. Analytical results are obtained for zero loss and space charge up to infinity and for zero space charge and loss up to infinity. Computer results are carried out for circuit loss up to 225 dB and space charge up to QC = 25. An analytical equation is developed for computing starting conditions for large loss approaching infinite values and for large space charge approaching infinite values. Since the starting C is proportional to the loss per wavelength, large values of loss in a backward-wave oscillator can prevent oscillation from occurring unless the current can be made sufficiently large; i.e., the starting C is independent of length.     

Theory of the clinotron: A grating backward-wave oscillator withinclined electron beam

A nonlinear theory of the clinotron, which is a modification of the backward-wave oscillator (BWO), is presented. The problem of electron-wave interaction is treated in a self-consistent formulation. Theoretical results are presented on starting conditions, steady-state oscillations, and temperature regimes. Potentials of the clinotron for further upgrading of efficiency and output power are examined     

Phase Lock-In of MM/SUBMM Backward Wave Oscillators: Development, Evolution, and Applications

The development and evolution of a Phase-Lock Loop (PLL) system up to the highest frequencies of Backward Wave Oscillators (BWO) is considered starting from the first submillimeter BWO PLL in 1970. Improvements and increase of working range near to Terahertz are followed. Development of the series of commercial BWO-based millimeter wave frequency synthesizers, extension of the BWO PLL beyond Terahertz as well as recent progress in fast millimeter wave frequency synthesizers are described. Applications of BWO PLL systems for physical and technical measurements are discussed and some proposals for the next generation of BWO-based synthesizers are presented.     

Backward-wave oscillator starting conditions

Detailed calculations of the starting conditions for backward-wave oscillations were carried out in the region 0 < QC < 0.25. The coupled-mode theory is called upon to explain the complex nature of the propagation constants for backward-wave interaction.     

文氏桥式振荡电路特性及数值仿真分析

正弦波振荡电路是模拟电路课程的基本教学内容,几乎所有的教材都采用反馈的概念来分析正弦振荡电路的特性.本文通过列写文氏桥式振荡电路的电路方程,分析了该电路的振荡特性,并采用数值仿真的方法模拟起振条件,得到了与现有教材对该电路分析一致的结果.上述分析结果可供讲授电路理论及模拟电路课程的教师参考.     

Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers

A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QC) laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schro/spl uml/dinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.     

Coupled-Mode Description of Crossed-Field Interaction

The coupled-mode theory is developed for two-dimensional M-type flow, and a system of five coupled-mode equations is obtained. A fifth degree secular equation is found for the perturbed propagation constants of the system. Under weak space-charge field conditions, both the forward-wave and backward-wave interactions may be described in terms of only two coupled modes. The two-mode theory is applied to the calculation of starting conditions for the M-BWO, and to the M-FWA. The conditions for beating-wave amplification are determined, and the variation of the mode amplitudes with distance is given.     

Construction of quasi‐cyclic low‐density parity‐check codes with low encoding complexity

Low encoding complexity is very important for quasi‐cyclic low‐density parity‐check (QC‐LDPC) codes used in wireless communication systems. In this paper, a new scheme is presented to construct QC‐LDPC codes with low encoding complexity. This scheme is called two‐stage particle swarm optimization (TS‐PSO) algorithm, in which both the threshold and girth distribution of QC‐LDPC codes are considered. The proposed scheme is composed of two stages. In the first stage, we construct a binary base matrix of QC‐LDPC code with the best threshold. The matrix is constructed by combining a binary PSO algorithm and the protograph extrinsic information transfer (PEXIT) method. In the second stage, we search an exponent matrix of the QC‐LDPC code with the best girth distribution. This exponent matrix is based on the base matrix obtained in the first stage. Consequently, the parity‐check matrix of the QC‐LDPC code with the best threshold and best girth distribution are constructed. Furthermore, bit error rate performances are compared for the QC‐LDPC codes constructed by proposed scheme, the QC‐LDPC code in 802.16e standard, and the QC‐LDPC code in Tam's study. Simulation results show that the QC‐LDPC codes proposed in this study are superior to both the 802.16e code and the Tam code on the additive white Gaussian noise (AWGN) and Rayleigh channels. Moreover, proposed scheme is easily implemented, and is flexible and effective for constructing QC‐LDPC codes with low encoding complexity. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of Quantum-Cascade Lasers Using Single-Pass Transmission Spectroscopy

The study of the electron distribution and intersubband transitions in quantum cascade (QC) lasers provides fundamental insights into the laser operation. Measurements on intersubband transitions are often performed on QC lasers under nonoperating conditions, i.e., at zero electric field. In this study, a single-pass transmission technique that allows for probing of QC lasers under operating conditions was used. The measurements and analysis were done on QC lasers with vertical and diagonal transition active region designs operating at $lambdasim 10 mu{hbox {m}}$. The single-pass modal gain coefficients, extracted for the vertical transition design, were 24.04 cm/kA at 100 K, 21.36 cm/kA at 125 K, 14.25 cm/kA at 150 K, and 5.26 cm/kA at 175 K.      

回旋返波管的线性研究

回旋返波管是基于电子回旋脉塞机理,具有宽频带连续调谐特性的毫米波、准毫米波大功率辐射源。在强干扰机、定向能电磁辐射武器、高性能材料处理和微波催化等方面具有重大应用前景,国际上很重视它的理论和实验研究。在此选择合适模式的一次谐波为例,对回旋返波管进行分析计算,得到色散方程、起振频率和起振长度等相关参数。着重以2A工作电流来分析起振长度和起振频率,得到了回旋返波管的基本参数,为_T-程设计回旋返波管提供了一定参考。     

Efficiency of tapered backward-wave oscillators

The effect of a tapered circuit phase velocity on the efficiency of an O-type backward-wave-oscillator (BWO) is investigated. Circuit tapers based on a hard-kernel-bunch approximation are derived. It is shown that when such tapers are applied to realistic BWO's higher efficiencies result. An experimentalS-band BWO with a variable pitch helix designed on the basis of the hard-kernel-bunch theory was constructed. An efficiency improvement of 20 per cent to 80 per cent over a frequency range of 2 to 3.75 Gc has been obtained.     

太赫兹返波管频率稳定性的检测方法研究

提出了一种返波管频率稳定性快速检测方法。利用一个抛物面镜对返波管辐射信号进行收集和准直,然后将探测器放置在准直平行光束的后方进行检测,并使所测信号经示波器显示和读出。根据法布里-珀罗干涉原理估测返波管的单频频率输出稳定性。本文方法对于提高基于返波管的频域高分辨率太赫兹波谱系统的性能具有重要的应用价值。     

Phase Locked BWO System for Open Resonator Measurement in D-Band

The combination of a D-Band phase-locked backward-wave oscillator (BWO) with a spectrum analyzer for measurement of permittivity and low loss-tangent is presented. For measuring low loss tangent material, such as CVD diamond and high purity semi-insulating $(HPSI)$ $4H-{rm SiC}$ , at millimeter wave ranges, it is necessary to precisely measure an increase of a few kHz in a line-width of 200 kHz. We describe a phase-locked loop with frequency conversion that combines a BWO source and a microwave spectrum analyzer to obtain line-width measurements with less than 2 kHz (less than 1%) standard deviation in D-Band millimeter wave.      

Preparation of point-line Bi2WO6@TiO2 nanowires composite photocatalysts with enhanced UV/visible-light-driven photocatalytic activity

Bi₂WO₆@TiO₂ nanowires composite photocatalysts (BWO-TNWS) with point-line structures have been successfully fabricated by hydrothermalsynthesis method, and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectra (UV–vis DRS), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The effects of coupling narrow-band-gap semiconductor Bi₂WO₆ (BWO) to photocatalytic activity for degrading Rhodamine B (RhB) and Phenol under UV–vis light irradiation were investigated. The results demonstrate that the photocatalytic activities of the prepared photocatalysts are associated with the content of Bi₂WO₆ (BWO). 20% BWO-TNWS (containing 20 wt% BWO) composite exhibits the highest degradation rate for RhB and Phenol up to 78% and 33%, respectively. It can be concluded that the improved photocatalytic performance of the BWO-TNWS composite is mainly ascribed to its high photoinduced charge separation rate resulting from the effective heterojunction structure of BWO and TNWS, as well as the enlarged optical response range owing to coupling narrow-band-gap semiconductor BWO.     

矩形波导加载光栅结构的太赫兹振荡器

本文采用矩形波导加载光栅的慢波结构作为太赫兹返波管的高频结构,通过理论分析和电磁仿真研究了该慢波结构的色散特性和互作用阻抗,理论分析结果和仿真结果能很好地吻合。在理论分析的基础上,设计了一个中心频率为340GHz的返波管,经粒子模拟软件计算,在较低电流密度的情况下该返波管输出功率达100mW且可调带宽约30GHz。     

Integral equation solution for traveling–wave tube parameters

An integral equation method for evaluating the electronic behavior of traveling-wave amplifiers is given. This method is utilized to obtain the effects of a thick beam and variations in dc density and dc velocity across the beam. Curves of the space charge parameter 4QC and impedance K are obtained for thick beams, which compare favorably with those of Pierce.     

模拟电子小信号放大器实验板的设计与制作

“模拟电子技术”课程的特点是理论与实践紧密结合,属于高职高专院校电子应用类专业的核心课程。介绍了当前“模拟电子技术”实验教学的现状及存在的问题,开展项目式实验教学改革的意义。小信号放大器的知识是模拟电子教学的重点与难点,直接决定了学生对模电课程的认知水平,是模电教学成功的关键。改变传统的实验教学模式,以小信号放大器的设计与制作为例,研究了“模拟电子技术”开展项目式实验教学的的方法与步骤。通过理论教学与项目制作相结合的方式,激发了学生学习兴趣和学习动力,提高了实验效果。     

Absorbing boundary conditions for the TLM method

The numerical behavior of different absorbing boundary conditions when applied to the transmission-line modeling (TLM) method is presented. These conditions may be classified into three different groups according to the way they are derived. The first group is obtained by discretizing one-way analytical conditions derived for the analytical wave equation. The second group is a set of discrete conditions directly obtained for the discrete wave equation. The last group is based on appropriate reflection coefficients derived purely from transmission-line theory. Because of their different behaviors, the numerical study is explicitly carried out for both two- and three-dimensional free-space scattering problems     

Monte Carlo simulation of the GaAs permeable base transistor

A two-dimensional multiparticle Monte Carlo (MC) method for the solution of the Boltzmann transport equation has been implemented and the results compared with the conventional drift-diffusion equation solution obtained for both a uniformly doped and an n⁺-n-n⁺GaAs permeable base transistor structure. Improved high-frequency performance is predicted by the MC simulation. Two-dimensional boundary conditions for a "regional" MC analysis have been applied to reduce the computer time that would be spent largely in analyzing the device retarding field region and the neutral regions of the device. The dc parameters, I-V characteristics, and unity current gain-frequency (fT) are discussed. In the n⁺-n-n⁺doped structure, a cooling effect was found that significantly enhances the device frequency performance by reducing the satellite valley population of electrons.     

On Optimal Analog Repeater Systems

In this paper, the optimal analog repeater system which transmits maximum mutual information (MI-system) is discussed. The solution is obtained by conventional variational method and the functional structure of the resulting optimal system is investigated. The system consists of a whitening filter, an "equivalent cable," and a "system equalizer," the concepts of which are newly introduced and defined in the paper. For comparison, the repeater system with minimum mean-square error (ME-system) is analyzed by the analogous method. It is shown that the whitening filter and the "equivalent cable" are invariant for both systems and that the only part which varies with the optimizing criterion is the "system equalizer." Finally, these results are applied to the special case of single stage and the relation to Shannon's water pouring theorem is discussed.