基于单片FPGA的磁共振成像梯度计算模块

提出了一种用于磁共振成像的高集成度的数字梯度计算模块. 它可以实时计算任意层面成像所需的梯度波形,并能对X、Y、Z三个通道做波形预增强处理. 该模块基于单片FPGA器件,梯度波形数据预存于FPGA内嵌的RAM中,波形更新时间最小为1 μs. 在FPGA内部通过复用一种快速IIR滤波器算法,能在1 μs时间内实现包含6组不同时间常数和幅度的预增强运算. 实验证明该系统具有通用性好、体积小和成本低等特点,为磁共振谱仪的研制提供了一种紧凑、灵活的梯度波形发生方案.      

一种具有数字预加重的磁共振成像梯度波形发生器

提出一种用于磁共振成像的高性能,高集成度基于PCI总线的数字预加重梯度波形发生器,它具有独立的X,Y,Z三通道预加重梯度波形输出,其主要特点在于此设计采用了现场可编程门阵列技术实现. 本文所述的预加重梯度发生器,其数字梯度预加重和数字逻辑模块全部集成在单片可编程芯片中,这样不但可以产生高性能的预加重梯度波形,而且可以简化磁共振成像仪的设计. 本文详细讨论了此梯度波形发生器的设计,最后给出了此设计的测试指标和实验结果.      

磁共振成像数字谱仪的射频发射/接收通道相位相干性的研究

讨论了一种在核磁共振成像数字谱仪中保证射频发射机与接收机之间射频信号相位相干的方法,该方法在成像脉冲序列执行过程中,同步切换发射机与接收机的频率,并在脉冲序列最后,将发射和接收频率恢复到脉冲序列最初的频率值,从而实现发射与接收的信号相干,其优点是无需加入额外的"回绕延时",从而使得脉冲序列的编写得到简化,并且该方法不依赖于磁共振谱仪发射机和接收机硬件部分的具体结构,是适用于数字化谱仪的一种普遍的方法.     

基于全可编程SoC和LabVIEW的磁共振接收系统设计

本文提出了基于全可编程片上系统（System-on-a-Chip,SoC）和实验室虚拟仪器工程平台（Laboratory Virtual Instrument Engineering Workbench,LabVIEW）的磁共振信号接收系统设计.使用集成了ARM（Advanced RISC Machines）和现场可编程门阵列（Field Programmable Gate Array,FPGA）的全可编程SoC作为接收机的主芯片,利用Xilinx提供的数字信号处理（Digital Signal Processing,DSP）开发工具System Generator设计了数字下变频（Digital Down Converter,DDC）算法,并实现了接收机硬件电路.使用可视化编程平台LabVIEW设计了磁共振上位机软件,完成了磁共振信号的显示、存储和与接收机通信的工作,提高了软件开发效率.实验结果表明,本文设计的接收机能正确接收磁共振回波信号,且具有较高的信噪比.     

磁共振成像谱仪梯度波形分析

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.     

MATLAB平台下声卡任意波形发生器的设计

介绍了一种基于MATLAB GUI的虚拟仪器设计方法.系统以手绘波形为信号数据源.通过调用MATLABDAQ工具控制声卡D/A转换器,实现了声卡在音频范围内任意波形信号的输出.     

水下光通信PPM数字接收机的DSP实现

水下光通信充分利用了海水对激光的衰减窗口效应,具有隐蔽、安全、非接触和快速机动等特性,兼具无线通信和光纤通信的优点,其关键技术在于光收发端机的研制;PPM(Pulse Position Modulation)数字脉冲位置调制具有低平均功率和高峰值功率等特性,特别适合用于无线数字光通信。两者性能的融合需要高灵敏度数字光接收机的实现。利用阵列光电检测接收、高速A/D转换和信号处理等相关技术,采用DSP处理器,高灵敏度的数字光PPM接收机得以实现,实验和测试表明它大大降低了接收机对于整个系统信噪比的要求,获得了较好的性能。     

一种基于单片机无线收发系统设计

设计一种基于STC89C51单片机的无线收发系统,本设计采用频率合成技术、FM调制技术及超外差解调技术,单片机对锁相环本振频率(发射43MHZ、接收53.7MHZ)锁定并通过液晶显示,本振频率在一定的范围内可调。将音频信号、咪头信号和数字英文字母信号作为调制信号,最后实现了音频、数字、英文字母等信号的重现。     

离散空间上的最小能量框架及其在矩形脉冲信号去噪中的应用研究

最小能量(小波)框架在信号处理领域有着广泛的应用前景,但目前只能应用在连续信号上.为解决这一问题,给出了离散信号空间上的最小能量框架的定义,并证明了它所具备的一些优良性质.在实际应用中,针对通信领域中的受加性高斯白噪声污染的二进制矩形脉冲信号提出一个新的去噪算法,利用离散空间上一个最小能量框架对接收波形的抽样离散数列进行去噪工作,获得了较好的处理效果.仿真结果表明,如果利用该算法对接收波形进行去噪预处理,则接收机可以降低误码率,在信噪比4 dB 处获得了3.4 dB的性能增益. 关键词： 离散信号空间 最小能量(小波)框架 二进制矩形脉冲信号 去噪     

基于角度传感器的偏振光检测的研究

用精密角度传感器代替了原来的机械角度盘,用硅光电池来接收光强分布,通过数字显示器接收角度传感器的角度变化电信号和偏振光强分布,直观地显示出偏振光实验的光强变化波形和数字读数.并对设计的偏振光实验仪进行相关实验的测量,测量精度能达到较高的要求.     

磁共振成像信号的数字化接收系统设计

磁共振成像信号的采集是磁共振成像系统当中最重要的环节之一,设计性能良好的信号接收系统直接关系到成像质量的好坏. 本文根据磁共振信号强度弱、频率高、带宽窄的特性对接收系统进行了统一设计. 详细地对可控增益放大、高速A/D、数字解调以及抽样滤波器等几个部分进行了讨论,给出一整套设计方案,尤其对多级抽样滤波器,根据磁共振成像的性能需求和滤波器的不同特性给出了抽样系数分配和滤波器设计的一般方法. 根据该方案设计出的磁共振信号接收系统有较高的信噪比,已经应用在临床医疗诊断当中.      

Digital detection and processing of multiple quadrature harmonics for EPR spectroscopy

A quadrature digital receiver and associated signal estimation procedure are reported for L-band electron paramagnetic resonance (EPR) spectroscopy. The approach provides simultaneous acquisition and joint processing of multiple harmonics in both in-phase and out-of-phase channels. The digital receiver, based on a high-speed dual-channel analog-to-digital converter, allows direct digital down-conversion with heterodyne processing using digital capture of the microwave reference signal. Thus, the receiver avoids noise and nonlinearity associated with analog mixers. Also, the architecture allows for low-Q anti-alias filtering and does not require the sampling frequency to be time-locked to the microwave reference. A noise model applicable for arbitrary contributions of oscillator phase noise is presented, and a corresponding maximum-likelihood estimator of unknown parameters is also reported. The signal processing is applicable for Lorentzian lineshape under nonsaturating conditions. The estimation is carried out using a convergent iterative algorithm capable of jointly processing the in-phase and out-of-phase data in the presence of phase noise and unknown microwave phase. Cramér-Rao bound analysis and simulation results demonstrate a significant reduction in linewidth estimation error using quadrature detection, for both low and high values of phase noise. EPR spectroscopic data are also reported for illustration.     

Heterodyne direct-detection OOFDM system with low sampling rate

A method of multi-channel receiving for high bit rate heterodyne direct-detection optical orthogonal frequency-division-multiplexing （OOFDM） system is proposed to reduce the sampling rate demand of analog-to-digital converter （ADC）. The sampling rate of ADCs can be reduced to 1/N that of the original signal bandwidth in an N-channel receiving system. Aided by a succeeding digital signal processing （DSP） at the receiver, aliasing free signal can be recovered. A back-to-back experimental result is given for a 4-channel system, based on which, a down conversion process for heterodyne can be reduced. The signal rebuilding algorithm is given and analyzed in its complexity and noise tolerance.     

基于NI PXIe-7966R的磁共振成像接收机设计

提出了一种基于NI PXIe-7966R（National Instruments Corporation,美国）的磁共振成像（MRI）接收机设计方案,进行磁共振信号直接采样、数字下变频（DDC）和数据上传,以及磁共振图像恢复．使用NI LabVIEW FPGA（National Instruments Corporation,美国）开发平台,对NI PXIe-7966R板载现场可编程门阵列（FPGA）内构建的所有功能模块进行了设计仿真和硬件描述语言生成,使其能灵活实现DDC功能．设计的接收机的采样速度为50 Mbps、模数转换位数为16位、带宽设置范围为100 Hz~1 MHz,并具有较好的滤波效果．实验结果表明,该设计方案是一种高性能的磁共振接收机方案．     

8×8APD阵列激光三维成像接收机研制

为了实现对目标的无扫描阵列激光三维成像并研究系统参数对三维成像距离分辨率的影响,研制了8×8 pixel激光三维成像接收机。接收机采用线性模式APD阵列,设计了模拟信号放大、阈值处理将回波光信号转换为数字信号后,利用FPGA设计实现64通道高精度阵列计时系统,实现了对目标的无扫描实时三维成像功能。首先对设计完成的三维成像接收机组成及成像原理进行了介绍,对三维成像接收机中APD探测器阵列信号的模拟处理和数字处理流程和实现方式进行了说明。随后分别对三维成像的核心FPGA计时系统及探测器整体进行了电子学测试和实验测试。测试结果表明,FPGA计时子系统的时间分辨率优于140 ps,三维成像系统整体距离分辨率在0.2 m左右。最后对分辨率的误差进行了分析,结果表明,激光回波强度波动是影响此接收机距离分辨率的最主要因素。     

基于异构双核的磁共振接收机的设计

提出了基于异构双核的低场核磁共振（Low-field Nuclear Magnetic Resonance,low-field NMR）接收机的设计,具有ARM（Adanced RISC Machines）和现场可编程门阵列（Field Programmable Gate Array,FPGA）先进的异构双核结构、高速的模数转换器（Analog to Digital Converter,ADC）采集、增益的可控以及可视化显示等特性,提高了整个系统的性能指标.设计中采用Xilinx公司的系统开发工具System Generator来实现内部信号处理功能.实验结果表明,数字接收机具有结构紧凑、可重构性强、采样速率高和成本低等特点,并且增益的可控使接收机获得了较大的动态范围,提高了重建图像的信噪比（Signal-to-Noise Ratio,SNR）.     

Multi-level quantization and blind equalization based direct transmission method of digital baseband signal

The direct transmission of digital baseband signals has practical significance in the field of Ethernet terminal connection, high-speed digital communication, data transmission of various types of information peripherals. The signal amplitude gradually decays while the transmission distance increases. Also the attenuation is proportional to the signal frequency, resulting in signal distortion and receiving error. It is a common method for digital baseband signal transmission to use pre-emphasis chip and equalizer chip to improve the transmission quality with a wide range of mature applications. This paper describes a new type of digital signal transmission method, as the receiver using analog-to-digital converter, instead of equalizer chip, to achieve the multi-level quantization of receiving time-domain data waveform. The waveform of the transmitted digital high and low level signal is sampled into multi-bit values. Then, the paper realizes adaptive frequency domain equalization based on soft threshold and makes use of multi-level quantization soft information for error correction. Error correcting code is mainly used to correct the error caused by the channel bandwidth limit, external noise or interference in the process of data transmission, so as to improve the stability and reliability of the transmission. The paper uses the two-stage error correcting codec system based on both Turbo and BCH coding, to achieve the high performance of Turbo code, and good characters of respond time and complexity. The transmitter outputs 12.5 MHz pseudo-random sequence through a 199.93 meter unshielded balanced twisted pair transmission medium. And the receiver circuit using a 62.5MSPS analog-to-digital converter over-samples the waveform to 8-level quantity. The output error of a 65536 bit pseudo-random sequence is less than 8 bits, and the error correction can be further improved by 8b-10b codec. Compared with the traditional pre-emphasis and balanced interface ICs connection, the method described in this article has the advantages of longer transmission distance, better flexibility and wider scope of use.     

Chaotic on off keying for secure communications

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.     

Analogue implementation of analytic signal processing for pulse-echo systems

An alternative to rectification is proposed for detection of an ultrasonic signal. This method is especially useful in medical and non-destructive evaluation (nde) applications. With this method, the magnitude of the complex analytic signal is used to define the envelope of the ultrasonic waveform. The square of this quantity has been shown elsewhere to be equal to the true rate-of-arrival of energy. An earlier study, using digital data processing, has already demonstrated the superior resolvability of closely spaced interfaces obtained with the analytic signal magnitude, as compared to conventional rectification. Here, an analogue implementation is presented which utilizes single-sideband techniques to obtain both quadrature components of the analytic signal and its magnitude. A conventional transducer, pulser, and receiver are used.