Structural dependence of microwave dielectric properties of spinel structured Mg2(Ti1-xSnx)O4 solid solutions: Crystal structure refinement,Raman spectra study and complex chemical bond theory

Mg₂(Ti_1-xSn_x)O₄ (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg₂(Ti_1-xSn_x)O₄ average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg₂(Ti_1-xSn_x)O₄ compounds were obtained (${\epsilon }_r$= 12.18, $Q\times f$?=?170,130?GHz, ${\tau }_f$?=??53.1?ppm/°C, x?=?0.2).     

SIMPLIS环境中开关电源集成电路控制器模型的开发及应用

介绍现代开关电源行业仿真工作的意义和现状,以及专用仿真软件包SIM etrix/SIMPLIS的优点和不足。通过实例给出SIMPLIS环境中集成电路控制芯片的建模过程,并就其中的关键技术予以简单讨论。     

Blind estimation of OFDM carrier frequency offset via oversampling

Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed.     

新版《建筑抗震设计规范》的主要改进

作者通过新旧抗震设计规范几个方面的对比，简要介绍了抗震设计新规范的主要改进情况，对结构设计人员学习抗震设计新规范有一定的指导作用。     

Mining predecessor–successor rules from DAG data

Data mining extracts implicit, previously unknown, and potentially useful information from databases. Many approaches have been proposed to extract information, and one of the most important ones is finding association rules. Although a large amount of research has been devoted to this subject, none of it finds association rules from directed acyclic graph (DAG) data. Without such a mining method, the hidden knowledge, if any, cannot be discovered from the databases storing DAG data such as family genealogy profiles, product structures, XML documents, task precedence relations, and course structures. In this article, we define a new kind of association rule in DAG databases called the predecessor–successor rule, where a node x is a predecessor of another node y if we can find a path in DAG where x appears before y. The predecessor–successor rules enable us to observe how the characteristics of the predecessors influence the successors. An approach containing four stages is proposed to discover the predecessor–successor rules. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 621–637, 2006.     

身份诈骗日益猖獗主因: 当散兵游勇碰到黑道联盟

军队能够剿灭土匪,因为通常前者军纪严明,而后者为乌合之众;警察能打击歹徒,因为惩治单位比一般罪犯训练有素,如果上述情况反转过来,黑道人物组成联盟, 互通情报,而正义之师只是散兵游勇,那么必然会“道高一尺、魔高一丈”。现今商业活动高度依赖网络,而随之萌生的身份盗窃及诈骗罪行日益猖獗,上述“角色错配”是其中一个主因。     

大型往复式压缩机的配管设计

大型往复式压缩机功率大、占地面积大,管道振动问题也更为突出。笔者结合实践经验,对大型往复式压缩机的配管设计进行探讨,说明了平面布置与管道防振设计的密切关系,并介绍了管道防振设计的方法和步骤,以及相对简单的复杂管系气柱固有频率的转移矩阵计算方法。     

Data-efficient blind OFDM channel estimation using receiver diversity

We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms.     

SrTiO3基压敏电阻的研究及其在武器系统中的应用

电子设备的广泛使用带来的设备干扰及浪涌冲击等问题使人们开始关注SrTiO3基压敏电阻器件材料的开发.本文介绍了SrTiO3基压敏电阻的一般制造配方和工艺流程.和ZnO基压敏电阻相比,此材料在低压领域具有更优良的电气性能,如较大的电容量(C=10～150nF);在高于电压临界值又具有较低压敏电压;非线性系数在5～43之间可调;耐浪涌能量大;并且具有自复位特性.这些特性使得它具有高频噪声吸收、前沿快速上升型脉冲噪声吸收和浪涌吸收等功能.针对武器系统的高性能要求,本文提出了SrTiO3基压敏电阻在直流电机消噪、电源输入端、吸收电感性负载开关浪涌、旁路电容器、通信线路防护、防止电涌冲击等方面的应用.     

UHF频段卫星移动通信系统若干问题的探讨

本文根据我国陆海空移动用户的需求与特点，借鉴国外经验，提出了建设ＵＨＦ频段卫星移动通信系统的建议；对照已建成的Ｃ波段卫星固定通信系统，分析了其优缺点；最后深入探讨了应用中的若干技术问题。