液晶光阀的双稳态特性研究

测量了TB3639型液晶光阀在电压驱动下的光谱特性及其光学双稳态特性,并讨论其产生光学双稳态特性的非线性效应物理机制,分析了液晶光阀的光学双稳态特性不明显的原因.分析结果表明,扭曲角对液晶光阀的光学双稳态特性影响较大,由于TN型液晶光阀的扭曲角只有90°,导致其双稳态特性不是很明显,TB3639型液晶光阀在某一电压范围内存在光学双稳态特性.     

通孔氧化铝模板的制备和光学特性

采用阳极氧化法,制备了二维有序纳米通孔氧化铝模板.研究了工艺参数对通孔有序性、孔径、模板厚度等的影响,测量了通孔氧化铝模板的光透过、光吸收和光发射等光学特性.结果表明,在波长360 nm附近通孔氧化铝模板的光透过谱线和光吸收谱线发生突变,波长大于360 nm时,光透过增强;波长小于360 nm时,光吸收增强.通孔氧化铝模板的光致发光强度和峰位与激发光波长有关,光致发光谱范围在340～600 nm.     

GaN基MSM结构紫外探测器光响应特性

在求解一维电流连续性方程和传输方程的同时考虑表面态陷阱的作用,获得了GaN基MSM结构紫外探测器在稳态光照下的电流随电压变化的解析解,从而导出了其光响应特性主要参数,并解释了电流和响应度随偏压变化的原因和光增益现象.将该模型应用于具体器件,实验测得饱和临界偏压约6 V,稳态电流6×10-8A,响应率0.085 7 A/W,与理论计算较吻合.     

Feasibility study of the application of voltage contrast to printed circuit board

Voltage contrast (VC) has been a powerful tool for the failure analysis of integrated circuits and multichip module. As the packing density of printed circuit board (PCB) is increasing, conventional failure analysis methods to detect open or short circuit in PCBs are no longer adequate, and voltage contrast method could be a method for this purpose. However, unlike the cases of integrated circuits and multichip module, there are many areas in PCB that will produce serious charging effect when examine under the scanning electron microscope. One of the areas is the presence of solder mask on PCB.This work examines the feasibility of using voltage contrast for PCB failure analysis. Specially designed PCB is used for experimentation, and it is found that positive bias on one track and zero bias on another copper track provide a better image contrast as compared to negative and zero biases on the tracks. Also, the variation of the image contrast with different spacing between inter copper tracks has studied. It is found that the variation depends on the presence of solder mask and its location. The variation can be very different for negative bias case as compared to the positive bias case.Finite element analysis is also performed to explain the experimental observations. All the observations can be well explained by the charging effect of the solder masks. The charging effect of solder mask is indeed very significant in affecting the image contrast, and it could reduce the contrast to almost zero in some cases.     

Feedback Reduction for Multiuser OFDM Systems

Feedback reduction in multiuser orthogonal frequency-division multiplexing (OFDM) systems has become an important issue due to the excessive amount of feedback required to use opportunistic scheduling, particularly when the number of users and carriers is large. In this paper, we propose a novel feedback-reduction scheme for efficient downlink scheduling. In the proposed scheme, each user determines the amount of feedback based on the so-called feedback efficiency in a distributed manner. The key idea is to give more of an opportunity for feedback to users who are more often scheduled. Simulation results demonstrate that the proposed scheme can substantially decrease the feedback load while achieving almost the same scheduling performance as in the case of full feedback. In addition, the proposed scheme offers unique advantages over existing ones. First, it is not tailored to a specific scheduling policy; thus, it has adaptability to the change of the underlying scheduling policy. Second, the total feedback load can be maintained below a target level, regardless of the number of users in the system.      

Self‐Crosslink Method for a Straightforward Synthesis of Poly(Vinyl Alcohol)‐Based Aerogel Assisted by Carbon Nanotube

As a new concept, a self‐crosslink mechanism for hydrothermal synthesis of poly(vinyl alcohol) (PVA) aerogel, assisted by multiwall carbon nanotubes, is reported. PVA, working as a low‐cost and commercially available raw material, exempts the complicated synthesis process and reserves its nontoxic nature since no organic crosslinkers are used in the synthesis process. The crosslink density and many other properties of the products can be easily tuned by simply altering the concentration of PVA precursors, which is considered to be another feature of our method. Dehydration between hydroxyl groups occurs in the hydrothermal process, leading to a reverse wettability of the products from hydrophilic to hydrophobic, thus their absorbing capacity for several organic solvents, such as bean oil and crude oil, is investigated. The absorbate has 10–52 times the original weight of the aerogel. As exhibited by the cytotoxic tests, the product has neglectable toxicity, suitable for application in environmental bioengineering. Furthermore, the product can be used as a facile substrate for transformation into conductive aerogel by in situ hybridizing with polypyrrole, showing a conductivity of 0.16 S m^?1. As it is rich in hydroxyl groups, the aerogels are believed to be further functionalized by the reactions related to the hydroxyl group.     

一种斜入射多膜窄带滤光片透射特性分析

垂直入射的多膜窄带滤光片两偏振态的透射特性是相同的,而对于斜入射的多膜窄带滤光片,其两种偏振态的透射特性发生分离.本文对于所设计的155层三腔薄膜窄带滤光片,分为(无吸收)垂直入射、(无吸收)有角度(11.5°)入射、(有吸收)有角度(11.5°)入射三种情况的透射率随波长变化关系进行了分析.模拟图形结果显示了透射特性的变化趋势.     

Energy-Efficient Synchronous-Logic and Asynchronous-Logic FFT/IFFT Processors

Two 128-point 16-bit radix-2 FFT/IFFT processors based on synchronous-logic (sync) and asynchronous-logic (async) for low voltage (1.1-1.4 V) energy-critical low-speed hearing aids are described. The two processors herein are designed with the same function and similar architecture, and the emphasis is energy efficacy. The async approach, on average, features ~37% lower energy per FFT/IFFT computation than the sync approach but with ~10% larger IC area penalty and an inconsequential 1.4 times worse delay; the async design can be designed to be 0.24 times faster and with largely the same energy dissipation if the matched delay elements and the latch controllers therein are better optimized. In this low-speed application, the lower energy feature of the async design is not attributed to the absence of the clock infrastructure but instead due to the adoption of established and proposed async circuit designs, resulting in reduced redundant operations and reduced spurious/glitch switching, and to the use of latches. The prototype async FFT/IFFT processor (in a 0.35-mum CMOS process) can be operated at 1.0 V and dissipates 93 nJ.     

车载负性CLCD电光特性研究

彩色液晶显示器件(CLCD)在相同电压驱动条件下,各波长电光特性有很大差异,从而对图像显示的色彩饱和度有较大的影响。用UV-Vis8500型双光束紫外/可见分光光度计测量在不同电压驱动下负性TN-LCD,STN-LCD和VA-LCD的电光特性,分析比较其三基色的电光特性、阈值特性和陡度随波长的变化关系。结果表明,TN-LCD的Vth、Vsat和V50随波长的增大而减小,而Vth、Vsat和V50减小的快慢基本一致;STN-LCD的Vth、Vsat和V50随波长的增大而减小,而Vth、Vsat和V50减小速度在各波长上快慢不一;VA-LCD的Vth、Vsat和V50都是随着波长的增加而逐渐增大,而Vsat增大速度比V50快,V50增大速度比Vth快。对常用三种不同负性液晶显示器件驱动电压的选取进行比较分析,其结果为CLCD选取驱动电压提供一定的指导作用。     

System Throughput Analysis of Rate Adaptive TDMA System Supporting Two Class Services

In this paper, an adaptive transmission scheme is proposed to improve the spectral efficiency of TDMA systems. Services accommodated in such systems are classified as QoS services and BE services. Unlike conventional adaptive TDMA systems where the time slot duration is fixed for each user, our proposed system employs transmission slots with adaptive duration. For QoS services, rate adaptive modulation is adopted to make use of the instantaneous channel conditions of individual user. For BE services, a media access control strategy (MAC) that takes the instantaneous transmission conditions of all physical links of all BE services into consideration is employed. A framework to study the proposed system in terms of the average packet loss and the average system throughput is presented. The maximum number of QoS services that can be supported in the system while a predefined requirement on the average packet loss is still satisfied is investigated. A comparison study shows that our proposed scheme has higher system throughput over both conventional fixed and adaptive TDMA systems. Ronghong Mo received her B.Sc. and M. Sc. Degree from Zhongshan University, Guangzhou, China, both in Physics. From July 1999 to July 2003, she was a research scholar in the department of Electrical and Computer Engineering in National University of Singapore (NUS), toward her PhD degree. She is currently working as a research engineer in NUS. Her research interests include adaptive modulation, synchronization and channel estimation in wireless communications. Yong Huat Chew received the B.Eng, M.Eng and Ph.D. degrees in Electrical Engineering from National University of Singapore, Singapore. Since 1996, he has been with the Institute for Infocomm Research (formerly also known as Center for Wireless Communications and Institute for Communications Research), an institute under Agency for Science, Technology and Research, where he is presently a lead Scientist. His research interests are in wireless communications, transmission over HFC, DSL and all-optical networks.