大气折射引起的卫星测控系统速度误差分析

由于大气各高度层介电特性的不均匀性,无线电波沿地空链路传播时会产生折射效应,为卫星的定轨测控系统引来不同程度的折射误差. 针对单频单站卫星系统的定轨方式,给出了基于多普勒频移测速原理的高精度速度误差修正算法,并与基于距离误差变化率原理的测速算法和实测数据进行了比较,验证了算法的准确性. 利用修正算法分别仿真计算了大气折射时延效应和弯曲效应导致的速度误差;基于不同频段、不同轨道高度的卫星系统参数,比较分析了对流层、电离层折射效应造成的速度误差变化特性. 分析结果可为当前地空链路的卫星测控系统提供准确的速度误差修正量参考,进而合理准确地评估大气折射对星载单频接收机测速误差的影响.     

短沟道CaAs MESFET中的速度过冲效应

已经发现短沟道CaAs MESFET(栅长短于1μm)的器件参数与栅长有关,随着栅长的缩短,出现速度峰的电场E_p和速度峰V_p的数值要单调增加,这是由于短沟道器件中出现非平衡速度过冲效应所致.文中给出了各种栅长下的E_p和V_p,还给出了考虑到非平衡速度过冲效应后GaAs的速-场近似关系.     

短沟道GaAs MESFET中的速度过冲效应

已经发现短沟道CaAs MESFET(栅长短于1μm)的器件参数与栅长有关,随着栅长的缩短,出现速度峰的电场E_p和速度峰V_p的数值要单调增加,这是由于短沟道器件中出现非平衡速度过冲效应所致.文中给出了各种栅长下的E_p和V_p,还给出了考虑到非平衡速度过冲效应后GaAs的速-场近似关系.     

硅太阳电池发射区复合的分析

本文全面考虑了太阳电池发射区的重掺杂效应,计算了高斯分布下,表面杂质浓度和表面复合速度对内部复合速度及发射区暗电流的影响.     

考虑速度过冲的单载流子光探测器特性

通过在器件模拟中引入考虑了速度过冲效应的水动力学模型,对单载流子光探测器(UTC-PD)的传统漂移扩散模型进行了改进.结果表明,电子的速度过冲有效地减小了空间电荷效应,从而提高了器件的带宽.同时,通过对器件的直流和交流特性分析,研究了吸收层和收集层参数对器件性能的影响.     

浅析电磁偏转效应在日常生活中的应用

电磁偏转效应在生活中有很多应用,本文从黑白电视机显像管,速度选择器,磁流体发电机三个方面对电磁偏转效应在日常生活中的运用进行了探讨.     

在小间隙放电中用Bernoulli定理分析电极移动速度效应

小间隙静电放电中电极移动速度对放电参数影响的内部因素,是受到很多研究者关心的问题.通过对实验数据的系统分析,应用流体力学中Bernoulli定律,对放电参数电极速度效应的内部原因,提出了较好的理论解释.电极的快速运动速度造成了放电间隙气体压强的下降,从而加速了载流子从电极到靶的运动过程,进而明显改变了放电参数的数值.初步的数值计算表明,表面过程因电极快速移动得到了加强.这对于电极移动速度效应进一步的研究,给出了一种较为明确的方向.     

场效应器件中电流管或细颈的收缩现象

本文的分析可以用来研究处在载流子速度极限情况下的场效应器件的物理量的饱和效应。为此,采用了由两个平行的圆柱体栅限定的短沟道结构。在源和漏之间加上高的漏电压后,建立起有限截面的导电细颈(goulet),在此截面内载流子速度接近于极限值。从而,本文推广了最初由 J.R.霍塞提出的关于载流子速度极限效应变为显著的情况下的电流沟道结构的分析。     

激光陀螺捷联惯导尺寸效应误差分析与补偿

在高动态条件下,加速度计尺寸效应已成为影响激光陀螺捷联惯导系统精度的重要误差源.文中从理论上分析了尺寸效应的产生机理,认为尺寸效应的产生是由于加速度计测量点不一致而引起,分析了激光陀螺机械抖动引起的尺寸效应误差.对加速度计组件在一般安装关系下的尺寸效应误差模型进行了推导.对于加速度计非正交安装情形,在常规静态标定模型基础上,推导了考虑尺寸效应后的动态标定模型.以导航速度为观测量,建立了加计组件尺寸效应误差补偿的一般模型方程.一系列的试验证明,尺寸效应补偿有效地提高了导航精度.     

回旋管双阳极磁控注入电子枪分析与数值计算

根据回旋管对电子枪的要求,考虑相对论效应和空间电荷效应,编制了一套大型的计算程序.利用计算机辅助设计,计算出了电极形状、电子轨迹与电子注参量.计算表明,这种电子枪具有速度零散小、层流性较好、体积小等优点.     

极化雷达导引头对多路径干扰的检测识别新方法

多路径干扰是对抗机载火控雷达或雷达导引头的有效自卫干扰方式之一,兼具距离、速度、角度维的欺骗干扰效果,目前对该干扰样式难以进行有效识别和抑制。该文提出采用极化雷达导引头工作体制,分析了多路径干扰对雷达的作用机理,建立了多路径干扰和直达干扰信号的回波信号正交极化信号模型,提出了基于信号极化相位统计特性差异的多路径干扰检测方法。该方法物理内涵清晰,实现简单,不仅可以判断干扰信号的存在和类型,还能够鉴别干扰的极化。通过仿真试验,验证了方法的可行性,为有效对抗多路径干扰提供了有益参考。      

On experimental determination of carrier velocity in deeply scaledNMOS: how close to the thermal limit?

Continued success in scaling bulk MOSFETs has brought increasing focus on fundamental performance limits. It has been proposed that drain current is ultimately limited by the rate at which carriers can be thermally injected from the source into the channel. In this work, we show that commonly used techniques for experimentally determining carrier velocity are insufficient to determine how close modern MOSFETs operate to the ballistic or “thermal limit.” We propose a new technique and show that an advanced 1 V NMOS technology with L_eff<50 nm operates at no more than ~40% of the limiting thermal velocity     

Characterization of Generation Lifetime and Surface Generation Velocity of Semiconductor Wafers by a Contactless Zerbst Method

A contactless Zerbst method has been developed to characterize the generation lifetime and the surface generation velocity of a semiconductor wafer. This characterization is unaffected by the gate leakage current or the device fabrication process. In this study, this contactless Zerbst method was used to characterize the generation lifetime and the surface generation velocity of a partially Au-doped Si wafer. The results demonstrate that the contactless Zerbst method is a powerful technique for characterizing the generation lifetimes and the surface recombination velocities of semiconductor wafers.     

Interface recombination velocity measurement in SiO2/Si

The photothermal technique has been used in its orthogonal configuration in order to determine the interface recombination velocity between SiO₂ ultra-thin film and Si substrate. This investigation has been performed by studying the variation of the photothermal signal according to the square root modulation frequency of the pump light beam. A general one-dimensional theoretical model taking into consideration the nonradiative recombination process has been developed. The interface recombination velocity has been evaluated by fitting the experimental curves of the phase and normalized amplitude of the photo-thermal signal with the corresponding theoretical ones.     

Restoration of the velocity field of the heart from two-dimensional echocardiograms

A method to quantify the motion of the heart from digitized sequences of two-dimensional echocardiograms (2-D) echos was recently proposed. This method computes on every point of the 2-D echoes, the 2-D apparent velocity vector (or optical flow) which characterizes its interframe motion. However, further analysis is required to determine what part of this motion is due to translation, rotation, contraction, and deformation of the myocardium. A method to locally obtain this information is presented. The proposed method assumes that the interframe velocity field U(xy), V(x,y) can be locally described by linear equations in the form U(x,y)=a+Ax+By; V(x,y)=b+Cx+Dy. The additional constraint was introduced in the computation of the local velocity field by the method of projections onto convex sets. Since this constraint is only valid locally, the myocardium must be first divided into sectors and the velocity fields computed independently for each sector.     

Surface Current Density Mapping for Identification of Gastric Slow Wave Propagation

The magnetogastrogram (MGG) records clinically relevant parameters of the electrical slow wave of the stomach noninvasively. Besides slow wave frequency, gastric slow wave propagation velocity is a potentially useful clinical indicator of the state of health of gastric tissue, but it is a difficult parameter to determine from noninvasive bioelectric or biomagnetic measurements. We present a method for computing the surface current density from multichannel MGG recordings that allows computation of the propagation velocity of the gastric slow wave. A moving dipole source model with hypothetical as well as realistic biomagnetometer parameters demonstrates that while a relatively sparse array of magnetometer sensors is sufficient to compute a single average propagation velocity, more detailed information about spatial variations in propagation velocity requires higher density magnetometer arrays. Finally, the method is validated with simultaneous MGG and serosal electromyography measurements in a porcine subject.     

激光全息干涉法测量液相扩散系数

在传递现象的研究中 ,全息干涉技术已被公认为一种最直观有效的方法。由于其具有精度及灵敏度高 ,信息量大 ,无干扰 ,可进行瞬态测量和整场观察等优点 ,已被广泛应用于流场中诸如速度 ,温度 ,浓度和密度等许多物性参数的定性或定量测量。液相分子扩散系数是描述质量传递的最重要的物性参数之一 ,由于缺乏精确的理论预测方法 ,迫切需要开发一种精密测试技术以满足实际应用和理论验证的需要。为此 ,激光全息方法受到了广泛重视。本文介绍了激光全息干涉技术及其在液相扩散系数测定中的应用进展 ,并进行了评述。     

对目标速度跟踪的ZFFT方法

正 根据闭环控制的原理,对单个目标实现速度跟踪的关键在于形成一个能时刻套住目标的速度门。使用模拟式跟踪环可以完成这一任务。一种可能的方法是在距离门中放后面用鉴频器和自动频率控制(AFC)环路来实现。这种方法是以窄带中放的通带构成速度门。当目标径向速度变化时,鉴频器输出的误差信号经低通滤波器后加到压控振荡器(VCO)上,调整VCO的振荡频率去跟踪信号的多普勒频率。这相当于误差信号控制一个速度门在频率轴上连续地移动,从而构成一个闭环的速度跟踪环路。另一种可能的方法是在中放输出端接一个多普勒滤波器组,用它覆盖预期的多普勒频移范围,通过比较相     

Sensorless speed control of nonsalient permanent-magnet synchronous motor using rotor-position-tracking PI controller

This paper presents a new velocity estimation strategy of a nonsalient permanent-magnet synchronous motor (PMSM) drive without a high-frequency signal injection or special pulsewidth-modulation (PWM) pattern. This approach is based on the d-axis current regulator output voltage of the drive system that has the information of rotor position error. Rotor velocity can be estimated through a rotor-position-tracking proportional-integral (PI) controller that controls the position error to zero. For zero and low-speed operation, the PI controller gains of rotor position tracking controller have a variable structure according to the estimated rotor velocity. In order to boost the bandwidth of the PI controller around zero speed, a loop recovery technique is applied to the control system. The proposed method only requires the flux linkage of the permanent magnet and is insensitive to parameter estimation error and variation. The designers can easily determine the possible operating range with a desired bandwidth and perform vector control even at low speeds. The experimental results show the satisfactory operation of the proposed sensorless algorithm under rated load conditions.