快速跳频性能特征及其测试的研究

跳频转换时间、驻留时间、跳频速率是快速跳频的典型性能特征。文中介绍了这些性能特征的概念,从快速跳频系统的设计层面重点讲述了跳频转换时间、驻留时间、跳频速率之间的关系以及他们和硬件电路之间的联系,分析了影响快速跳频性能的主要因素,提出了提高快速跳频性能的途径。并对跳频转换时间等快速跳频性能特征的测试方法进行了研究,给出了使用下变频以及实时频谱分析仪的测试实例。     

基于TDMA的Ad Hoc网络跳频组网的实现

跳频组网的核心技术之一是跳频同步问题。对现有的跳频同步方案做了一些改进，分析了时间信息TOD与同步头频率使用原理、同步头频率捕获过程、跳频同步与跟踪过程。时分多址接入（TDMA）的前提条件需要跳频同步的完成，结合时隙同步方法和跳频同步技术阐述了基于TDMA的AdHoc网络跳频组网的实现过程。     

用于跳频多址通信的混沌跳频码

跳频码的性能对跳频多址通信起着关键性作用。设计具有良好汉明相关和随机性且数量多的跳频码是非常重要的。本文提出一种利用由混沌映射描述的离散混沌系统设计跳频码的新方法。首先由混沌映射产生混沌时间序列，然后利用量化技术对混沌时间序列进行量化，最后由量化后的混沌时间序列产生伪随机跳频码。为了增大混沌跳频码的周期和非线性复杂度，我们利用多个不同混沌映射组成一个级联混沌映射，并用来设计跳频码。实验表明，混沌跳频码具有非常好的性能。     

基于短时傅立叶变换的跳频序列瞬时频率估计

采用了G函数的方法来模拟跳频序列，然后利用短时傅立叶变换对它进行分析与瞬时频率的估计。该方法能够有效地描绘出跳频信号的频率随时间跳变的规律（跳频图案），具有很高的时间一频率分辨率。计算机仿真验证了应用短时傅立叶变换分析跳频序列的有效性和实用性。     

低速短波非正交跳频网台信号分选技术研究

介绍了利用改进型的时间相关算法进行短波非正交跳频网台信号分选的方法。由于短波信号环境的复杂性，影响了对短波领域跳频信号的检测，使得跳频信号的持续时间这一关键技术参数无法正确获取，因此采取提高跳频信号检测的准确性和找回因“频率碰撞”而丢失的跳频频点等方法对时间相关分选算法进行改进，以提高网台信号分选的正确概率。通过实验分析，结果表明，在适当的信噪比条件下，改进型时间相关分选算法能够完成四部非正交跳频网台信号的分选，分选的正确率大大提高。     

快速跳频电台频率合成器的研究

跳频通信具有很强的抗干扰能力，是未来战场通信扣主要通信手段。跳频速率的快慢很大程度上决定了跳频电台抗干扰能力。实现快速跳通信的关键之一是要求频率转换时间极短的频率合成器，常规的频率合成技术已难以满足要求。     

跳频跟踪干扰引导算法研究及其仿真实现

本文分析了带载波信号的跳频网的特点，结合基于频率集门限的跳频网跟踪技术和开跳变时间窗方法，提出了在一定信噪比下对跳频信号进行跟踪引导的一种方案，并给出了相应的仿真结果。     

信号侦察中的跳频信号识别方法

为了解决跳频设备已经大量装备而对跳频信号的侦察识别仍然困难的问题,作者提出了两种跳频信号识别方法,包括最大相关处理法和时间相关统计法.在对跳带内的信号进行数字信道化处理后,再经过最大相关处理,时-频图上的跳频信号的分布情况将会变得非常清晰;时间相关统计法是充分利用跳频信号在时间上的断续性和每跳间的连续性来识别跳频信号.经过实验验证,上述两种识别方法的使用,在跳频信号的侦察识别上效果非常明显.     

跳频序列设计理论的研究进展

跳频技术具有抗干扰和多址组网性能，在军事无线电通信、民用移动通信、现代雷达和声纳等电子系统中具有重要的应用。跳频序列用于控制载波频率随机时间的变化规律，其性能对跳频系统的性能有重大影响。本文综述跳频序列设计理论的研究成果，并指出今后的发展方向。     

跳频通信系统中同步技术研究

同步技术是跳频通信系统关键技术之一。针对跳频通信系统中同步的要求,采用同步字头与时间信息相结合的方法实现跳频同步。首先研究了跳频同步方法、同步信息格式和初始同步等问题,最后对同步性能进行了分析。结果表明,该跳频通信系统的同步时间短、捕获概率高、虚警概率低。     

Growth of III-V semiconductor nanowires by molecular beam epitaxy

We present here the growth of GaAs, InAs and InGaAs nanowires by molecular beam epitaxy. The nanowires have been grown on different substrates [GaAs(0 0 1), GaAs(1 1 1), SiO₂ and Si(1 1 1)] using gold as the growth catalyst. We show how the different substrates affect the results in terms of nanowire density and morphology. We also show that the growth temperature for the InGaAs nanowires has to be carefully chosen to obtain homogeneous alloys.     

Influence of AlN buffer layer thickness on the properties of GaN epilayer on Si(1 1 1) by MOCVD

The effect of thickness of the high-temperature (HT) AlN buffer layer on the properties of GaN grown on Si(1 1 1) has been investigated. Optical microscopy (OM), atomic force microscopy (AFM) and X-ray diffraction (XRD) are employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness of HT AlN buffer layer, and the optimized thickness of the HT AlN buffer layer is about 110 nm. Together with the low-temperature (LT) AlN interlayer, high-quality GaN epilayer with low crack density can be obtained.     

Finite elements modeling of multilayers of Cr/CrN

One of the most used methods for modeling different materials and their properties has been finite elements. In this work, mechanical properties of Cr/CrN multilayer coatings have been modeled by using finite elements, varying the period of layers (1, 5, 10 and 20 bilayers) and the thickness of the films between 0.5 and , in order to determine the behavior of the system. For this model, the software ANSYS was used to carry out simulation of the indentation process. For the analysis, a conical Berkovich indenter was built. The simulation consists in generating the stress-strain curves in the charge mode for obtaining Young's modulus of the total system, including the substrate, which is made by stainless steel 304. The curves showed a tendency of increasing of Young's modulus as a function of number of layers and thickness, which means an increasing in hardness.     

Electrical passivation of the silicon surface by organic monolayers of 1-octadecene

The electrical properties of structures consisting of a monolayer of 1-octadecene deposited on the Si surface are investigated depending on the method of passivation of the surface prior to the deposition of the film (hydrogen and ion passivation) and the intensity of illumination which activates the addition reaction of molecules of 1-octadecene to the Si atoms. The monolayer of 1-octadecene on the Si surface is stable and provides the chemical passivation of the surface. Two types of traps are found, namely, traps for holes and electrons, whose density can be varied during deposition of the monolayer by the choice of intensity of illumination and by the method of passivation of the surface. In the case of a low level of illumination and/or the use of the iodine passivation of the surface, the electron traps prevail, and, in the case of high intensity of illumination and/or hydrogen passivation of the surface, the hole traps prevail. It is shown that the use of these films provides conductivity in thin near-surface layers of Si due to providing the mode of flat bands or accumulation of carriers near the surface.     

Surface characterization of epitaxial lateral overgrowth of InP on InP/GaAs substrate by MOCVD

Epitaxial lateral overgrowth (ELO) of InP on InP/GaAs substrates by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) was investigated. The lateral overgrowth InP layers were obtained on the SiO₂ masked InP seed layer, which was deposited on the (1 0 0) GaAs substrate by the two-step method. The surface characterization of overgrowth InP was dependent on the V/III ratio, the mask width and the growth time. When decreasing the V/III ratio or reducing the mask width respectively, the sidewalls “competition effect” was obviously observed. After a longer time, new (1 0 0)-like top surfaces were formatted because of the precursors migrating from the sidewall facets to the (1 0 0) top surfaces. The experimental findings will be explained by growth kinetics in conjunction with the different dominant source supply mechanism.     

Porous silicon thin film as CO sensor

This work presents the electric behavior of porous silicon (PS) thin films when the material's surface is exposed to carbon monoxide. PS thin films were fabricated by the electrochemical anodization method of Si-c (1 0 0) substrates with resistivity . The samples were prepared at 20 min anodization time and anodization current. Aluminum electrodes were deposited on the surface of the material by high vacuum evaporation, such that the electric conduction was parallel to the substrate's surface. The detector was placed in vacuum during 1 h and then CO was allowed into the vacuum chamber. Measurements of the I-V characteristic were carried out at atmospheric pressure, in vacuum and with CO. Changes in the resistance of the material, of about MΩ, were observed in the different samples, indicating that the material is sensitive to the presence of CO and therefore suitable as gas sensor.     

Bridgman growth of Cdx Hg1-xTe using acrt

The Accelerated Crucible Rotation Technique (A.C.R.T.) has been applied to the Bridgman growth of the mixed semiconductor Cd_xHg_1-xTe. An increase in the level of melt mixing near to the interface has been achieved and this has resulted in a much greater degree of axial and radial composition homogeneity than hitherto possible. Effects of changes in A.C.R.T. rotational parameters and crystal start composition have been assessed. An improvement in crystallinity has also been obtained which is believed to be due to transient Couette flows at the crucible walls. It is shown that an empirical approach to optimising A.C.R.T. conditions in a particular system is necessary although a theoretical study can provide the starting point. Present address: Philips Research Laboratories, Redhill, Surrey, RH1 5HA, U.K.     

Numerical optimization of an extracted HgCdTe IR-photodiodes for 10.6-μm spectral region operating at room temperature

The electrical and photoelectrical properties of long wavelength Hg_1−xCd_xTe structures have been optimized by using an exact numerical analysis. In this analysis we have been taking into account the degeneracy, non-parabolicty, deviation from thermodynamical equilibrium and graded interfaces. The band diagram, electrical field, carrier mobility, photoelectrical gain, responsivity, noise and detectivity have been calculated and optimized as a function of different variable such as alloy composition, doping concentration, thickness, and applied voltage to obtain optimized performance at room temperature. This numerical simulation can be used to optimize the mentioned parameters for other structures such as , operating in photodiode, or photovoltaic mode.     

E1通道无损伤切换方案的设计与实现

文中在分析现有E1通道切换机制的基础上,提出了一种新的无损伤切换机制,保证了E1通道上重要信息的可靠传输.提出了基于FPGA的E1通道无损伤切换的实现方案,并重点介绍了实现中采用的数据缓存、相位调整和自动切换控制等关键技术.     

Study of the long-wavelength optic phonons in AlGaInP and AlGaInAs

The investigation of the lattice dynamics of (Al_xGa_1−x)_yIn_1−yP quaternary semiconductor alloys lattice matched to GaAs has been made by Raman scattering. The Raman spectra exhibit three-mode behavior depending on the composition. A modified random element isodisplacement (MREI) model is generalized to the III-V (A_xB_1−x)_1−yC_yD-type quaternary alloys, describing the behavior of the optical phonons. The calculated result of two quaternary mixed crystals, (Al_xGa_1−x)_yIn_1−yP and (Al_xGa_1−x)_yIn_1−yAs, is in good agreement to the experimental data.