A two wave theory of traveling-wave tubes and backward-wave oscillations

Starting from Pierce's small-signal TWT equations, a two-wave theory for complicated TWT's and BWO's is developed. Explicit gain expressions for TWT's with severs and velocity tapers are given. A detailed treatment is given of a BWA structure with a single-step velocity taper in the center. The BWO start oscillation current is calculated as a function of the taper strength. The effciency of a step velocity taper as a remedy for backward-wave oscillations in the higher passbands of a TWT is then analyzed. It is shown that a suitable taper can increase the start oscillation current by more than a factor of 10 without adverse effects on the TWT performance if the backward waye coupling impedance is low. A tube with a weak linear taper is analyzed by a perturbation technique and it is found that there is no significant difference between the start oscillation currents for tubes with weak linear or abrupt tapers of comparable strengths.     

Partial internal model control

Internal model control (IMC) is a well-known and effective control scheme. However, when unstable processes are concerned, the original IMC structure cannot be directly used for control system implementation. In this paper, a new scheme called partial internal model control (PIMC) is proposed, which is capable of controlling both stable and unstable processes. In PIMC, a process model is expressed as the sum of the stable and antistable parts and only the stable part of the process model is used as the internal model. The process stable part is canceled by the internal model and the remaining antistable part is stabilized and controlled with a primary controller, which is usually a PID-type regulator when the antistable part is of a low order. Various properties of a PIMC system such as internal stability and robust stability are analyzed. The design of PIMC is discussed in detail. Various simulation examples are included for illustration and a real-time implementation on a motor system is presented     

High-Chip-Count UWB Biphase Coding for Multiuser UWB-Over-Fiber System

In this paper, we propose and demonstrate a novel technique to optically generate high-chip-count, phase-coded direct-sequence (DS) ultrawideband (UWB) signals for multiple-access UWB communications. In the proposed system, a lightwave from a laser source is phase-modulated by a Gaussian pulse train. The phase-modulated lightwave is then sent to a polarization modulator, to modulate the polarization state of the lightwave by a code pattern. The polarization-coded optical signal is then converted into a biphase-coded DS-UWB signal by a polarization-dependent frequency discriminator. The key device in the proposed system is the frequency discriminator, which is implemented using a length of polarization maintaining fiber (PMF) and a polarizer. A 127-chip, biphase-coding DS-UWB that has a data rate of 26.46 Mb/s and a chip rate of 3.36 Gb/s is experimentally generated. A multiuser UWB-over-fiber system is then proposed and a two-user system is demonstrated, in which the encoding is performed experimentally and the decoding is performed by numerically calculating the correlation between the coded UWB signal and the signature sequence. The signal of each user is well recognized. An effective two-user UWB-over-fiber system based on the DS-UWB technology is thus demonstrated.      

Firewalls fend off invasions from the Net

Whether a computer is in a corporation, government agency, university, small business, or at home, if it is linked to a network, hackers are a risk. The first line of defense against them is a firewall, provided it is set up correctly. Firewall technology is a set of mechanisms that collectively enforce a security policy on communication traffic entering or leaving a guarded network domain. The security policy is the overall plan for protecting the domain. Embodied in hardware, software, or both, a firewall guards and isolates the domain. Broadly, firewalls attempt to maintain privacy and ensure the authenticity of data communications that pass through their domain's boundaries. Whether data is entering or leaving a domain, it is protected from eavesdropping (passive wiretapping) and change (active wiretapping). But only communication traffic entering or leaving a domain comes under the influence of firewall technology     

GaAs dynamic memory design

Subthreshold leakage loss is a serious problem for GaAs dynamic memory. Since the leakage current in a MESFET is several orders of magnitude higher than that in a MOSFET, it is difficult to retain the charge at dynamic nodes resulting in data storage errors, In order to solve this problem, a novel DRAM architecture is proposed. The design is based on a cell consisting of a MESFET switch and a metal-insulator-metal (MIM) planar capacitor as the storage element. The leakage current is reduced by a level-shift technique and a self-biased transistor is used to maintain the dynamic charge during the sense period. A high performance sense amplifier is used to detect small bit line voltage changes and refresh the stored data. A 1 Kb prototype, fabricated in a 1 μm nonself-aligned GaAs MESFET technology, exhibited a total read/write access time of the order of 3 ns     

Diffraction and Reflection of a Plane Electromagnetic Wave by a Right-Angled Impedance Wedge

The interior of a right-angled impedance wedge is a natural model of a corner reflector and is of interest in the development of wireless propagation models. Using a previously-developed solution for the diffraction of a plane wave by a wedge of arbitrary angle, the geometrical optics field is determined and the diffracted field is computed. If an impedance compatibility condition is not met, the geometrical optics field is discontinuous across a plane specified by the edge of the wedge and the incident field direction. The diffracted field is required to compensate for this and its magnitude is proportional to the discontinuity. The field is computed for a variety of impedances and incident angles. Simple analytical approximations are also provided and their accuracy quantified.      

A Study on Straight-Line Tracking and Posture Control in Electric Bicycle

The development of automatic control for driving a bicycle is a challenging theme and is expected to be a human assist system. Previously, an acceleration-based method for stabilizing bicycle posture was proposed by the authors. In the experiments with this controller, the posture of the bicycle might be stabilized, but it is impossible to run on the desired trajectory, because there is no consideration with respect to a trajectory control. For the sake of expanding this system into more sophisticated equipment, a realization of the trajectory control for the bicycle is important. From the viewpoint of an assist system for human motion, a unified control of posture and trajectory brings a sophisticated function to a bicycle, and a high-performance bicycle is expected to be a convenient vehicle, similar to a small car. This paper proposes two strategies to stabilize bicycle posture and trajectory control that realizes a straight-line tracking: one is a lateral velocity controller, and the other is a steering function controller. The validity of the proposed approaches is evaluated by simulations and experiments.     

EMD分解区域的数据研究

本文介绍了一种新的非线性、非平稳信号的处理方法—HHT方法,并运用此方法对地震波进行了理论分析,详细研究了其中的EMD方法及其实现过程;通过大量的数据分析,总结了这一方法的优越性,指出其具有良好筛分及不可良好筛分的适用数值范围,并总结了其中蕴涵的规律。当组合简谐波的频率固定,A2/A1很小,超过某一限值时;或者A2/A1很大,超过某一限值时,数据值不能用EMD方法进行筛分。当组合简谐波的幅值比A2/A1固定不变,而频率变化时,当f2/f1很小时,如超过某一限值;或者f2很接近f1,f2/f1超过某一限值时,数据y值不能用EMD方法进行筛分。      

激光诱导放电毛化技术中放电坑的表面形貌研究

利用高重频YAG激光作用在固体表面所产生的等离子体使工件和电极之间在电压远低于击穿阈值的条件下产生放电。实验结果显示 ,放电坑基本上呈火山坑形 ,既有单坑结构 ,也有多坑结构 ,其形貌受到放电波形、电源极性、放电介质等因素的影响。放电坑表面形貌的规律是 :①除了涂油时的阳极放电坑是单坑结构以外 ,其他条件下的放电坑都是复合多坑结构 ;②单坑结构呈火山坑形 ,坑底为圆弧形 ,熔凝物堆积在坑的边缘 ,多坑结构则是一个大的放电坑中有多个凸起尖峰     

On the Capacity of Randomly Used Shared Channels

The channel capacity of a communication channel made available to a user on a shared basis with other users is here considered. An overall system bandwidth B is divided into segments and parceled out to subscribers in an overlapping manner. Viewing emissions by the sharing user as Gaussian noise, the channel capacity is treated as a random variable, and its probability function, mean, and variance are studied. For a fixed power to each user, it is found that the average capacity of a user is maximum when the number of multiple assignments of a frequency band is approximately the inverse of the probability of use. The increase in channel capacity over exclusive assignment is also determined. The probability function of the capacity is worked out for some specific case to show its dependence on the frequency assignment policy, and a general procedure for determining the probability function is given.     

A tunable polarization-independent liquid-crystal Fabry-Perotinterferometer filter

The authors propose a simple structure for a tunable polarization-independent wavelength-selective filter. This successfully fabricated filter consists of a liquid crystal Fabry-Perot interferometer, a γ/4 plate, and a mirror. The input light is divided into a transmitted light and a reflected light. The polarization of this reflected light is rotated 90° by the γ/4 plate and the mirror and then this light is input again into the filter. The bandwidth is 0.48 nm and the tuning range is 50 nm. Fiber-to-fiber loss is approximately 3.9 dB and polarization dependence of the loss is approximately 0.3 dB     

Optical pulse sequence transmission through single-mode fibers:interference signal analysis

The transmission of a pulse sequence through a single-mode fiber is considered under first- and second-order dispersion. An expression is given for the output power signal envelope in terms of the Fourier transform of a spectral function. The output envelope is composed of two terms: a linear one which is a distorted replica of the information sequence, and a nonlinear interference term. The latter is expressed in a more meaningful way and analyzed. The results allow a characterization and a classification of the interference pulses in terms of physically intuitive parameters     

Filter-embedded design and its applications to passive components

A design in which a filter is embedded in a fiber fixed on a substrate is proposed. It was successfully applied to the fabrication of low loss fiber optic components for a single-mode fiber transmission system. These components have a wide operational temperature range. The design is easily adapted to a multifilter or multifiber component. Filter embedding is achieved in two ways. One is to insert a thin filter chip in a slit which is machined into a fiber fixed on a substrate by a dicing saw (filter inserted type). The other is to evaporate a filter film directly on a fiber end face and butt this fiber to another fiber on a guide groove on a substrate (filter evaporated type)     

One-to-All Broadcasting in Petersen-Torus Networks for SLA and MLA Models

In a network, broadcasting is the dissemination of a message from a source node holding a message to all the remaining nodes through a call. This letter proposes a one-to-all broadcasting algorithm in the Petersentorus network PT(n, n) for the single-link-available and multiple-link-available models. A PT(n, n) is a regular network whose degree is 4 and number of nodes is 10n², where the Petersen graph is set as a basic module, and the basic module is connected in the form of a torus. A broadcasting algorithm is developed using a divide-and-conquer technique, and the time complexity of the proposed algorithm approximates n+4, the diameter of PT(n, n), which is the lower bound of the time complexity of broadcasting.     

A dual planar reflectarray with synthesized phase and amplitude distribution

A quasi-planar reflector arrangement for generating an arbitrary phase and amplitude distribution in the antenna aperture and thus a wide range of far field patterns is presented. A parallel pair of reflectarrays is used. One is implemented as a standard reflectarray using rectangular patches on a microwave substrate with metallized backside. The other reflectarray is provided with a polarizing grid, which acts as a ground plane for one polarization and is transparent for the orthogonal polarization. An offset rectangular feed horn is embedded in the lower reflectarray to illuminate the upper reflector. A design procedure for a linear polarized antenna with a sector beam in the azimuth and a narrow beam width in the elevation is presented and is verified with measurement results.     

基于ZEMAX的半导体激光准直镜设计方法研究

针对目前采用单个球面镜压缩激光时存在发散角效果差或者球面镜片数过多的问题,通过ABCD矩阵法从理论上对高斯光束经过单透镜时的传输特性进行了分析,得出了单透镜无法实现半导体激光光束理想准直的结论。提出了一种基于ZEMAX的半导体光源准直镜的设计方案,并给出了设计与优化方法。结合工程中常用的808nm半导体激光器,设计了双片型808nm半导体激光准直镜,并在ZEMAX中使用合适的优化函数和权重对像差进行了校正。通过采用非球面镜获得了较好的准直效果,发散角达到了0．032mrad。该设计使用的镜片数较少,结构简单。     

基于MutiSIM的蔡氏混沌序列仿真研究

混沌序列是非线性方程描述的确定系统所产生的介于周期振荡与噪声之间的一种复杂振荡序列.蔡氏电路是一个经典,且简单的三阶自治系统,能够产生丰富的混沌现象。在对蔡氏电路进行数学建模的基础上,采用MultiSIM仿真软件对其进行仿真,并分析其产生的混沌信号的功率谱,从而找出一个最佳的电路参数,得到一个最好的,可用的混沌序列。     

Robust detection of a known signal in nearly Gaussian noise

A detector that is not nonparametric, but that nevertheless performs well over a broad class of noise distributions is termed a robust detector. One possible way to obtain a certain degree of robustness or stability is to look for a min-max solution. For the problem of detecting a signal of known form in additive, nearly Gaussian noise, the solution to the min-max problem is obtained when the signal amplitude is known and the nearly Gaussian noise is specified by a mixture model. The solution takes the form of a correlator-limiter detector. For a constant signal, the correlator-limiter detector reduces to a limiter detector, which is shown to be robust in terms of power and false alarm. By adding a symmetry constraint to the nearly normal noise and formulating the problem as one of local detection, the limiter-correlator is obtained as the local min-max solution. The limiter-correlator is shown to be robust in terms of asymptotic relative efficiency (ARE). For a pulse train of unknown phase, a limiter-envelope sum detector is also shown to be robust in terms of ARE.     

Complexity reduction of digital filters using shift inclusive differential coefficients

We present a graph theoretical methodology that reduces the implementation complexity of the multiplication of a constant vector and a scalar. The complexity of implementation is defined as the required amount of computations like additions. The proposed approach is called minimally redundant parallel (MRP) optimization and is mainly presented in a finite impulse response (FIR) filtering framework to obtain a low-complexity multiplierless implementation. The key idea is to expand the design space using shift inclusive differential coefficients (SIDCs) together with computation reordering using a graph theoretic approach to obtain maximal computation sharing. The problem is formulated using a graph in which vertices and edges represent coefficients and computational cost (number of resources). The multiplierless solution is obtained by solving a set cover problem on the vertices in the graph. A simple polynomial run time algorithm based on a greedy approach is presented. The proposed approach is compared with common-subexpression elimination to show slightly better results and is combined with it for further reduction of complexity. Simulation results show that 50-60% complexity reduction is achieved by only applying the MRP algorithm, and 70% complexity reduction is obtainable by combining it with common-subexpression elimination under a delay constraint of two or three.     

Human cooperative wheelchair for haptic interaction based on dual compliance control

A human and a robot will carry out a task which is not attainable by themselves. In particular, a human recognizes environment and plans his trajectory without collision with obstacles. On the other hand, a robot generates a controlled force more than a human. In this paper, the best combination of human ability and robot capacity is considered. Based on force commands from a human, a robot supports it A reaction torque observer is implemented in a robot to observe an environmental disturbance. Environmental disturbance is classified into translational and rotational direction modes. As a result, adaptive force control in each mode is attained. Dual compliance control is applied to a wheelchair. A wheelchair that has the abilities of power-assist and relaxation of contact force is developed in this paper. As a result, operationality and stability are improved. The numerical and experimental results show the viability of the proposed method.