自旋条件下弹载PD雷达回波特性研究

为了精确研究自旋条件下弹载PD雷达的回波信号,本文建立弹头再入自旋运动模型,得到了点波束回波的延时和多普勒频率公式。通过仿真,验证了模型和公式的正确性。本文所得的结论为宽波束雷达回波精确模拟奠定了坚实的基础,具有很强的借鉴意义。     

伪回声测距

针对无线传感器网络的特点,提出了基于测量经伪随机码相移键控调制的声波信号在无线传感器网络的节点之间往返传播时间的测距方法.发起测距过程的主动节点向目标节点发送调制声波信号,目标节点收到调制声波信号后,根据声波信号所携带的数字信息重建该信号,并回传给主动节点.主动节点测量发出的声波信号和从目标节点返回声波信号之间的时延,同时考虑到对调制声信号进行重建造成的信号延迟,从而估算出节点之间的距离.参与测距的节点之间不需要进行时间同步,也不需要预先知道对方使用的伪随机序列.利用声波信号携带的伪随机码序列确定声波信号时延的范围,然后再从确定的范围内求出声波信号的时延,从而极大地减小了算法的运算量.     

一种改进的声回声抵消算法

NL MS(normalized least mean square)算法是是自适应信号处理中应用最广泛的算法之一 ,它的最大缺点是收敛速度慢 ,对非平稳信号自适应能力差。为提高声回声抵消算法的收敛速度 ,采取了将语音分析中常用的线性预测作为前置滤波器的方法来减少参考信号的相关性 ,采用步长控制的方法来提高收敛速度 ,同时保持了 NL MS算法稳定性好和算法简单的优点。最后在模拟环境下对算法进行了测试 ,证明了比 NL MS算法具有更高的收敛速度和回声抵消量     

Review on echo state networks

回声状态网络是近年来新兴的一种递归神经网络,独特而简单的训练方式以及高精度的训练结果已使其成为当前研究的热点之一.在该网络中,引入了储备池计算模式这一新的神经网络的建设方案,克服了之前网络模型基于梯度下降的学习算法所难以避免的收敛慢和容易陷入局部极小等问题.围绕这种新型网络结构,国内外许多学者开展了多样的研究.本文全面深入介绍了回声状态网络这一新兴技术,讨论了回声状态网络的优缺点,并综合近年的研究现状,总结了回声状态网络的主要研究工作进展和未来的研究方向.     

回声状态网络的研究进展

回声状态网络是近年来新兴的一种递归神经网络,独特而简单的训练方式以及高精度的训练结果已使其成为当前研究的热点之一.在该网络中,引入了储备池计算模式这一新的神经网络的建设方案,克服了之前网络模型基于梯度下降的学习算法所难以避免的收敛慢和容易陷入局部极小等问题.围绕这种新型网络结构,国内外许多学者开展了多样的研究.本文全面深入介绍了回声状态网络这一新兴技术,讨论了回声状态网络的优缺点,并综合近年的研究现状,总结了回声状态网络的主要研究工作进展和未来的研究方向.     

基于高斯模型的多重超声回波信号重数估计

精确估计多层材料超声回波信号的重数在超声检测上有着要意义。将小波变换方法用于多层材料超声回波参数估计中,根据高斯模型以超声回波信号的小波变换为基础、利用智能人工蜂群算法,估计出多重超声回波信号的各个参数。采用Akaike Information Criterion(AIC)准则,对叠加的两重和三重超声回波信号的重数进行估计。仿真结果表明,本算法可以实现多重超声回波信号重数的有效估计。用实验测试获得的回波对算法的性能进行了验证,结果证明了该算法的可行性和实用性。     

光子静止质量与雷达回波延迟

讨论了弯曲时空下,当光子静止质量不为零时引力的时间延迟效应,即光子静止质量对在地球上发射再从行星上反射回来的掠过太阳的雷达回波延迟的影响.计算了当光子静止质量不为零时对原有电磁波运动方程的影响,并将实验所得数据与理论计算数据之间的误差归结为光子静止质量引起,由火星回波延迟的实验数据给出光子静止质量上限为2.2×10-43g.同时讨论了实验中误差的来源,并提出了进一步提高实验精度和获得光子静止质量更高上限的途径.     

Electronic spin transport and spin precession in single graphene layers at room temperature

Electronic transport in single or a few layers of graphene is the subject of intense interest at present. The specific band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states, has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, absence of weak localization and the existence of a minimum conductivity. In addition to dissipative transport, supercurrent transport has also been observed. Graphene might also be a promising material for spintronics and related applications, such as the realization of spin qubits, owing to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. Here we report the observation of spin transport, as well as Larmor spin precession, over micrometre-scale distances in single graphene layers. The 'non-local' spin valve geometry was used in these experiments, employing four-terminal contact geometries with ferromagnetic cobalt electrodes making contact with the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals that reflect the magnetization direction of all four electrodes, indicating that spin coherence extends underneath all of the contacts. No significant changes in the spin signals occur between 4.2 K, 77 K and room temperature. We extract a spin relaxation length between 1.5 and 2 mum at room temperature, only weakly dependent on charge density. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around ten per cent.     

带双端发声检测的回声消除改进算法

该文首先对传统的回声消除算法中,双端发声的检测机制进行了深入分析,指出这些检测算法都存在一定的延时,这些延时导致了在检测到双端发声之前滤波器系数已经一定程度的发散,从而影响到双端发声段中的回声消除效果.针对于此,该文提出了一种新的带有滤波器缓存的回声消除算法.该算法在自适应滤波器收敛到较好时将其系数存入到缓存,而在检测到双端发声时将缓存中的系数重新取回以重置滤波器.试验结果表明,该算法与传统算法相比,在双端发声期间效果得到了明显改善,而且运算量相当.     

Spectroscopy of spontaneous spin noise as a probe of spin dynamics and magnetic resonance

Not all noise in experimental measurements is unwelcome. Certain fundamental noise sources contain valuable information about the system itself-a notable example being the inherent voltage fluctuations (Johnson noise) that exist across any resistor, which allow the temperature to be determined. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations. For example, statistical fluctuations of N paramagnetic spins should generate measurable noise of order N spins, even in zero magnetic field. Here we exploit this effect to perform perturbation-free magnetic resonance. We use off-resonant Faraday rotation to passively detect the magnetization noise in an equilibrium ensemble of paramagnetic alkali atoms; the random fluctuations generate spontaneous spin coherences that precess and decay with the same characteristic energy and timescales as the macroscopic magnetization of an intentionally polarized or driven ensemble. Correlation spectra of the measured spin noise reveal g-factors, nuclear spin, isotope abundance ratios, hyperfine splittings, nuclear moments and spin coherence lifetimes-without having to excite, optically pump or otherwise drive the system away from thermal equilibrium. These noise signatures scale inversely with interaction volume, suggesting a possible route towards non-perturbative, sourceless magnetic resonance of small systems.     

SAR雷达回波数据的实时采集与存储

随着合成孔径雷达(SAR)系统的发展,高精度的雷达信号回波采集与实时存储成为系统的必然要求。提出了一种基于PCI总线数据采集与实时硬盘存储方法,借助于TS201的FLYBY传输模式向PCI总线传输数据,编写内存驱动程序,建立物理地址和虚拟内存地址的映射。CPU可以直接从内存中读取数据并存盘,减少了CPU和DSP之间的信息交换,大大加快了数据存盘的速度。     

Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve

Finding a means to generate, control and use spin-polarized currents represents an important challenge for spin-based electronics, or 'spintronics'. Spin currents and the associated phenomenon of spin accumulation can be realized by driving a current from a ferromagnetic electrode into a non-magnetic metal or semiconductor. This was first demonstrated over 15 years ago in a spin injection experiment on a single crystal aluminium bar at temperatures below 77 K. Recent experiments have demonstrated successful optical detection of spin injection in semiconductors, using either optical injection by circularly polarized light or electrical injection from a magnetic semiconductor. However, it has not been possible to achieve fully electrical spin injection and detection at room temperature. Here we report room-temperature electrical injection and detection of spin currents and observe spin accumulation in an all-metal lateral mesoscopic spin valve, where ferromagnetic electrodes are used to drive a spin-polarized current into crossed copper strips. We anticipate that larger signals should be obtainable by optimizing the choice of materials and device geometry.     

Electrical detection of spin precession in a metallic mesoscopic spin valve

To study and control the behaviour of the spins of electrons that are moving through a metal or semiconductor is an outstanding challenge in the field of 'spintronics', where possibilities for new electronic applications based on the spin degree of freedom are currently being explored. Recently, electrical control of spin coherence and coherent spin precession during transport was studied by optical techniques in semiconductors. Here we report controlled spin precession of electrically injected and detected electrons in a diffusive metallic conductor, using tunnel barriers in combination with metallic ferromagnetic electrodes as spin injector and detector. The output voltage of our device is sensitive to the spin degree of freedom only, and its sign can be switched from positive to negative, depending on the relative magnetization of the ferromagnetic electrodes. We show that the spin direction can be controlled by inducing a coherent spin precession caused by an applied perpendicular magnetic field. By inducing an average precession angle of 180 degrees, we are able to reverse the sign of the output voltage.     

声学回声消除算法研究

在回声消除中,自适应滤波是核心,而滤波器系数的更新又是自适应滤波的核心.在声学回声消除领域,滤波器系数的更新最常使用的算法是LMS算法,目前针对LMS算法有两种改进算法,即NLMS和NBLMS算法.基于TI公司的DSP芯片的硬件资源特点来比较NLMS(Normalized LMS)和NBLMS(Normalized Block Update LMS)算法,为消除声学回声提供选择依据.     

炮瞄雷达回波模拟器的设计

介绍利用复杂可编程逻辑器件CPLD和直接数字合成专用电路DDS,设计雷达信号回波模拟器的硬件系统.该系统能够模拟雷达的回波信号,代替实际目标来检测雷达的性能,减少了雷达系统的设计周期.AD9858芯片的利用,进一步使该解决方案的速度加快,满足了低相位噪音、快速频率切换等要求.     

基于经验模态分解的水下目标回波提取

为在噪声中有效提取水下目标回波,提出一种基于经验模态分解的目标回波提取方法,分别对目标回波与噪声进行经验模态分解,得到二者各阶的固有模态信号分量;计算二者各阶的固有模态分量的能量,求取对应各阶的能量差,并与门限相比较,选取大于门限的固有模态分量重构目标回波信号.分析了经验模态分解阶数和选择门限等因素对重构信号精度的影响.仿真结果表明,该方法可精确提取目标回波.采用该方法提取信号的均方误差均低于自适应滤波方法,尤其在低信噪比时.