共查询到18条相似文献,搜索用时 187 毫秒
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介绍了高场磁共振成像(MRI)多源发射技术的原理,提出了一种用于高场MRI系统的多源射频信号发射机.它能并行输出多路频率、相位、幅度,可快速独立调节的射频脉冲信号.该射频发射机的实现基于单片现场可编程门阵列(FPGA)和多通道数模转换器(DAC)芯片,FPGA读取预存于双端口随机存取存储器(RAM)中的射频信号参数,并利用读取的参数分别实现每路信号的直接数字频率合成(DDS)和信号调制等核心功能,获得多路数字射频信号;FPGA输出的数字信号经过高性能DAC转化为模拟信号,即所需要的射频信号.该射频发射机在设计中大量采用软件无线电技术,即利用Xilinx提供的IP核实现DDS和信号调制等主要功能,具有集成度高、体积小、灵活度高的优点,同时,该设计可以大大缩短开发时间,有效降低实现的难度和成本,为高场MRI谱仪的多源射频发射机的设计研制提供了一种低成本、高效、高性价比的方案. 相似文献
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梯度是磁共振成像(MRI)的关键环节.通过采集谱仪梯度波形信号并进行分析,提取出各通道波形的特征点,从而有助于快速准确地判断谱仪梯度硬件电路或脉冲序列编写是否存在问题.采用虚拟仪器LabVIEW软件控制高速采集卡DAQ-2005设计实现多路采集系统,对谱仪的梯度输出进行采集.通过对波形数据进行直方图统计、滤波、差分计算等分析,提取出波形的特征点,这些特征点包含时间与幅度信息.使用实验室自主研发的谱仪进行了多次实验,对该方法进行验证,证明了该方法的有效性,也为谱仪研制和脉冲序列开发提供了一种辅助测试方法. 相似文献
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提出一种用于单载波数字相干光通信系统的自适应色散补偿方法.在发射机端,利用强度光调制器在光信号两侧加入脉冲幅度调制导频光信号.搜索信号谱的峰值确定光信号两侧的PAM-PTs线状谱.在色散估计之前,通过其中一个PAM-PTs的频率漂移来估计激光器发射频偏,并将信号谱线沿着频率轴反向平移,补偿发射频偏,实现信号谱频域均衡.在接收机端,利用幅度脉冲调制导频信号脉冲时延估计色散大小,在色散补偿之前通过幅度脉冲调制导频信号频移补偿,并消除接收机本振频偏对频域色散补偿的影响.最终,幅度脉冲调制导频的光场信息被充分用来精确进行色散估计和色散补偿.数值仿真表明:每次色散估计的误差小于±65ps/nm,而且距离从200km到1 000km之间的平均色散估计误差小于±10ps/nm.该方法色散补偿精度高,计算量小. 相似文献
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Chaotic on off keying for secure communications 总被引:4,自引:0,他引:4
We experimentally demonstrate a chaotic on-off keying method for secure communications by using chaos synchronization in two microchip lasers. The output of the microchip laser in the transmitter is externally modulated with an acousto-optic modulator at ~4 MHz . One encodes a digital message in the chaotic carrier by turning the modulation on and off at 100 kHz. Because the accuracy of synchronization for the slave laser in the receiver tends to be degraded in the presence of external modulation in the injection laser signal, one can distinguish two binary states. The digital message can be recovered as an envelope of the chaotic oscillation when the difference between the two laser outputs of the transmitter and the receiver is calculated. 相似文献
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Chaos communication based on synchronization of discrete-time chaotic systems 总被引:2,自引:0,他引:2 下载免费PDF全文
A novel chaos communication method is proposed based on synchronization of discrete-time chaotic systems. This method uses a full-order state observer to achieve synchronization and secure communication between the transmitter and the receiver. Further, we present a multiple-access chaotic digital communication method by combining the observer with the on-line least square method. Simulation results are also given for illustration. 相似文献
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A spectrometer allowing the measurement of signals with spin-spin relaxation times as short as 0.5 micros has been developed. It consists of a transmitter based on a commercial spectrometer and a homemade receiver. The acquired signals are imported in the software of the commercial spectrometer. This is a simple and rather cheap way to transform a commercial rig into a spectrometer allowing measurements on systems with short T2 such as found in magnetic systems. 相似文献
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提出了基于现场可编程门阵列 (FPGA) 技术的混沌直接序列扩频信号盲解调的硬件电路实现方法. 设计了混沌直接序列扩频信号发射机与接收机. 发射机可产生10种不同的混沌直接序列扩频信号. 为方便接收机的硬件电路实现, 对无先导卡尔曼滤波混沌拟合盲解调算法进行了简化, 在简化模型的基础上设计了接收机硬件结构. 提出了一种动态调整偏移因子的新方法, 使接收机能实时适应混沌映射的变化. 通过高斯白噪声信道及多径信道条件下的盲解调实验, 验证了盲解调算法硬件实现的抗噪声与抗多径性能, 以及对10种不同的混沌直接序列扩频信号的自适应破译效果.
关键词:
FPGA
混沌直接序列扩频通信
盲解调 相似文献
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基于ARM和CPLD的NMR谱仪前放控制设计 总被引:1,自引:1,他引:0
提出了一种基于ARM(Advanced RISC Machine,先进精简指令计算机)和CPLD(Complex Programmable Logic Device, 复杂可编程逻辑器件)的NMR(Nuclear Magnetic Resonance,核磁共振)谱仪前放控制系统设计方案. 该控制系统以ARM为控制核心、 CPLD为控制命令执行单元, 控制前置放大器和混频器的工作状态. 本电路包括以下主要功能:1) 前置放大器收发切换的门控信号电平转换;2) 氘梯度匀场时身频通道自动切换;3) 探头调谐的信息显示;4) 锁前放和混频器的配置. 该设计方案具有系统运行速度高、 控制灵活和成本低的优点,并且可以有效减小系统噪声,降低控制部件对前置放大器和混频器的干扰. 相似文献
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Yoh Imai Satoshi Yamauchi Hirohisa Yokota Takehito Suzuki Kansuke Tsuji 《Optics Communications》2009,282(20):4141-4145
We numerically investigated the dependence of the decryption characteristics in a message modulation type chaos secure communication system using optical fiber ring resonators (OFRRs) on the coherence condition of carrier light. Since the output dynamics of OFRRs were formed on the basis of interference phenomenon among the circulated components in OFRRs, the coherence condition of input light affected inherently the OFRR output dynamics, resulting in the decryption characteristics. It was found that the decryption characteristics degraded as coherence length increased, i.e., as the spectral width of the input light decreased. The decryption characteristics also grew worse as the parameter differences between the transmitter and the receiver became larger. 相似文献
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We introduce a new method for asymmetric (public key/private key) encryption exploiting properties of nonlinear dynamical systems. A high-dimensional dissipative nonlinear dynamical system is distributed between transmitter and receiver, so we call the method distributed dynamics encryption (DDE). The transmitter dynamics is public, and the receiver dynamics is hidden. A message is encoded by modulation of parameters of the transmitter, and this results in a shift of the overall system attractor. An unauthorized receiver does not know the hidden dynamics in the receiver and cannot decode the message. We present an example of DDE using a coupled map lattice. 相似文献