超声声场的三维模拟及可视化研究

对超声换能器声场进行三维模拟及可视化的研究,基于MATLAB强大的图形图像可视化功能显示出超声声场的三维图形,通过改变相应参数更直观的观察声场的变化了解影响声场的变化的因素;为超声成像以及超声检测等工程实际问题中超声波的研究以及超声探头的选取提供相应的依据。     

基于时空变换的CT界面快速定位方法研究

相控阵超声CT成像技术现在已经逐渐应用于工业检测中,本文在通过研究相控阵理论的基础上,利用了时空变换法把检测到的三维时间信号族变换到二维空间灰度图,并且通过研究灰度图特征,达到快速对检测界面定位的目的。     

高反射性电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率

由SiO₂/TiO₂分布布拉格反射镜(DBR)和Al镜组成的混合式反射电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率。混合式反射电流阻挡层不仅增强了电流扩展效应而且有效的将射向p金属电极的光子反射防止其对p电极焊点附近光子的吸收。实验结果表明,淀积在p-GaN上1.5个周期的SiO₂/TiO₂DBR和Al镜在455nm垂直入射时的反射率高达97.8%。在20mA的工作电流下,与没有电流阻挡层的发光二极管相比,生长1.5对SiO₂/TiO₂ DBR和Al镜作为电流阻挡层的发光二极管的光输出功率提高了12.5%,且光输出功率的分布更加均匀。     

激光驻波场中铬原子的三维沉积特性分析

利用近共振激光驻波场操纵中性原子实现纳米级条纹沉积是一种新型的研制纳米结构长度标准传递方法,但仅通过一维和二维形式的仿真不能给出激光驻波场作用下中性原子沉积纳米光栅的全部信息。利用半经典模型,从铬原子在高斯激光驻波场中的运动方程出发,通过四阶Rungo-Kutta法模拟了铬原子在高斯激光驻波场中的三维运动轨迹以及三维沉积条纹结构,并分析了原子束发散、色差和球差等因素对三维运动轨迹及沉积条纹结构的影响。结果表明,利用三维仿真形式模拟高斯激光驻波场中铬原子的运动得到的结果与一维和二维形式下相比可以直观地表现出其较为详细的本质。     

CMOS低噪声放大器的噪声系数优化研究

由于CMOS晶体管的特征尺寸急速的缩放,CMOS晶体管的参数不断改进,使得最新CMOS晶体管获得的噪声系数足够低到足以应用到无线电天文学,因此,在本研究课题中选用CMOS晶体管。目前的低噪声放大器的最小噪声系数是在室温环境下,通过宽带CMOS低噪声放大器的功率匹配来获得。在本研究课题中,CMOS低噪声放大器以共源共栅极结构为基本拓扑结构,主要研究LNA的几种常用的噪声系数优化方法。通过建立小信号模型,对LNA的噪声系数进行分析,得出相应的表达式。     

GB20600标准的LDPC码的改进

尽管LDPC码已经被GB20600标准采纳作为信道编码,与其它LDPC码相比,在同样码长和码率的情况下,GB20600 LDPC码误码率性能并非最佳;GB20600标准的LDPC码的码长达7493,存在编码复杂性问题,但是GB20600 LDPC码未采用基于校验矩阵的快速算法,这给GB20600 LDPC编解码器的硬件实现带来较大的困难。本文在现有GB20600 LDPC码的设计框架下,对GB20600中LDPC码的校验矩阵进行了修改,在此基础上提出一种有效的LDPC码的快速迭代算法,使编解码器的硬件易于实现。改进后的LDPC码的编码算法具有较低的实现复杂度。仿真结果表明,改进后的LDPC码的误包率性能优于现GB20600中LDPC码的误包率性能。      

Simulation and research of percolation phenomenon in T-ZnOw resin matrix composite

A novel three-dimensional lattice model of tetrapod-like zinc oxide whisker (T-ZnOw) resin matrix composite with a coordination number of 12 is constructed based on the special structure of T-ZnOw; the percolation phenomenon of the system is simulated by the Monte Carlo method,and the percolation threshold is obtained at 23.2%.The critical mixing ratio of T-ZnOw is calculated by considering the practical factors,and the result basically agrees with the reported one.Theoretical calculation shows that the critical mixing ratio mainly depends on the L/D ratio of T-ZnOw,and is also related to the size of T-ZnOw as well as the preparation method of the composite.The microwave absorbing mechanism of T-ZnOw composite is discussed,and conductivity loss and point discharge caused by the polarization effect are regarded to be two important means of energy dissipation.     

Design of an LDO with capacitor multiplier

This paper presents a low quiescent current,highly stable low-drop out(LDO) regulator.In order to reduce capacitor value and control frequency response peak,capacitor multipliers are adopted in the compensation circuit with mathematic calculations.The phase margin is adequate when the load current is 0.1 or 150 mA.Fabricated in an XFAB 0.6μm CMOS process,the LDO produces 12.2 mV(0.7%) overshoot voltage while the current changes at 770 mA/100μs with a capacitor load of 10μF.     

Service Scheduling of Vehicle-Roadside Data Access

As vehicular networks become popular, more and more people want to access data from their vehicles. When many vehicles want to access data through a roadside unit, service scheduling becomes an important issue. In this paper, we identify some challenges in vehicle-roadside data access. As vehicles move pretty fast, the requests should be served quickly. Also, vehicles may upload data to the roadside unit, and hence the download and upload requests compete for the same bandwidth. To address these challenges, we propose several service scheduling schemes. We first propose a basic scheduling scheme called $\mathcal{D*S}$ to consider both service deadline and data size. We then enhance it by using a single broadcast to serve multiple requests. Finally, we identify the effects of upload requests on data quality, and propose a Two-Step scheduling scheme to provide a balance between serving download and update requests. Simulation results show that the Two-Step scheduling scheme outperforms other scheduling schemes.     

高功率激光光纤耦合特性研究

实现高功率激光的光纤耦合对高功率激光应用有着重要的作用.文中简要介绍了高功率激光耦合的关键技术,包括激光光纤耦合条件、耦合效率、光纤的选择和端面处理、光纤损伤机理及其阈值条件等方面.