Implementation of a likelihood ratio test for laser Doppler velocimeter burst detection

A likelihood ratio test for laser Doppler velocimeter burst detection is derived and implemented with the appropriate photon detection statistics. This detector utilizes the first point of the autocorrelation function of the laser Doppler velocimeter photodetector signal and relies on a simple identity for this point. The detector looks at the ratio of two probability functions of the data (burst and no burst) to make its decision. Because the detector does not depend on the absolute strength of the signal, detection down to very low (-10-dB) signal-to-noise levels can be achieved. Because the autocorrelation function is insensitive to the phase of the signal, the detector will operate reliably with signals containing multiple overlapping bursts.     

A new adaptive mean frequency estimator: application to constant variance color flow mapping

A class of adapted mean frequency estimators is proposed for color flow mapping. These estimators can be fitted to the specific characteristics of a given Doppler signal to optimize the compromise between the range of analysable frequencies and the variance of mean frequency estimation. A sub-optimal estimator is derived for real-time applications, and an adaptive criterion based on the Doppler signal variance is developed for color flow mapping applications. Its performance is compared to that of the usual correlation phase estimator on simulated Doppler signals and on synthetic Doppler images. An improvement in image quality is achieved, mainly for low signal-to-noise ratio Doppler signals.     

A compressive MUSIC spectral approach for identification of closely-spaced structural natural frequencies and post-earthquake damage detection

Motivated by practical needs to reduce data transmission payloads in wireless sensors for vibration-based monitoring of engineering structures, this paper proposes a novel approach for identifying resonant frequencies of white-noise excited structures using acceleration measurements acquired at rates significantly below the Nyquist rate. The approach adopts the deterministic co-prime sub-Nyquist sampling scheme, originally developed to facilitate telecommunication applications, to estimate the autocorrelation function of response acceleration time-histories of low-amplitude white-noise excited structures treated as realizations of a stationary stochastic process. Next, the standard multiple signal classification (MUSIC) spectral estimator is applied to the estimated autocorrelation function enabling the identification of structural natural frequencies with high resolution by simple peak picking in the frequency domain without posing any sparsity conditions to the signals. This is achieved by processing autocorrelation estimates without undertaking any (typically computationally expensive) signal reconstruction step in the time-domain, as required by various recently proposed in the literature sub-Nyquist compressive sensing-based approaches for structural health monitoring, while filtering out any broadband noise added during data acquisition. The accuracy and applicability of the proposed approach is first numerically assessed using computer-generated noise-corrupted acceleration time–history data obtained by a simulation-based framework examining white-noise excited structural systems with two closely-spaced modes of vibration carrying the same amount of energy, and a third isolated weakly excited vibrating mode. Further, damage detection potential of the developed method is numerically illustrated using a white-noise excited reinforced concrete 3-storey frame in a healthy and two damaged states caused by ground motions of increased intensity. The damage assessment relies on shifts in natural frequencies between the pre-earthquake and post-earthquake state. Overall, numerical results demonstrate that the considered approach can accurately identify structural resonances and detect structural damage associated with changes to natural frequencies as minor as 1% by sampling up to 78% below Nyquist rate for signal to noise ratio as low as 10dB. These results suggest that the adopted approach is robust and noise-immune while it can reduce data transmission requirements in acceleration wireless sensors for natural frequency identification and damage detection in engineering structures.     

基于深度学习的OFDM半相干水声通信方法

针对正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)水声通信中常用的相干和非相干通信分别面临的对多普勒敏感和频谱效率低的问题,提出一种高阶幅度键控调制的半相干通信技术,将OFDM符号时频帧结构中全部频点采用高阶幅度键控调制方式,并利用信号幅度信息完成半相干信道估计。通过两种基于深度学习的算法优化半相干信道估计这一非线性过程,较非相干通信有效提高了频谱效率,较一定信噪比下的相干通信提高了鲁棒性,降低了误比特率和系统复杂度,并利用元学习算法降低深度学习算法对训练数据的依赖。最后,提取海试信道数据,完成OFDM半相干水声通信系统仿真,验证了所提方法在频谱效率和系统误比特率性能方面较非相干和相干通信的优势,当信道长度改变时,基于元学习的算法依然可以获得较好的性能。     

基于DSP的频谱分析型激光多普勒信号处理器的研究

为了提高激光测速系统中信号处理速度,获取更高的测量精度和实时检测能力,本文从硬件结构、传输策略和试验结果入手,提出了利用DSP芯片作为频率分析处理器的多普勒测速系统.通过对A/D控制策略,光电数据采集压缩,DMA传输和DSP处理器中的FFT谱分析和频谱校正算法的研究和对数变频模式模拟电压信号测量试验,实现了多普勒信号处理器在显示采样频率为7MHz,速度显示间隔为0.3ms的实时检测和频率细化能力.     

Time-domain estimation of the center frequency and spread of Doppler spectra in diagnostic ultrasound

Two sets of estimators of the center frequency and mean square bandwidth of a Doppler spectrum are analyzed. The first set is based on a linear combination of the complex autocorrelation of the Doppler signal for different lags. It is shown that the argument (phase angle) of the complex autocorrelation function at unit sample lag is a close approximation to the mean Doppler shift, whereas its modulus (magnitude) gives information about the mean square bandwidth of the spectrum. Approximate expressions for the bias and variance of the estimators are derived, valid for long averaging times. The performances of the estimators are compared for signals with rectangular spectra and different signal-to-noise ratios.     

Largely extended light-emission shift of ZnSe nanostructures with temperature

ZnSe nanowires and nanobelts with zinc blende structure have been synthesized. The morphology and the growth mechanisms of the ZnSe nanostructures will be discussed. From the photoluminescence (PL) of the ZnSe nanostructures, it is interesting to note that red color emission with only a single peak at the photon energy of 2 eV at room temperature is obtained while the typical bandgap transition energy of ZnSe is 2.7 eV. When the temperature is reduced to 150 K, the peak wavelength shifts to 2.3 eV with yellowish emission and then blue emission with the peak at 2.7 eV at temperature less than 50 K. The overall wavelength shift of 700 meV is obtained as compared to the conventional ZnSe of about 100 meV (i.e., sevenfold extension). The ZnSe nanostructures with enhanced wavelength shift can potentially function as visible light temperature-indicator. The color change from red to yellowish and then to blue is large enough for the nanostructures to be used for temperature-sensing applications. The details of PL spectra of ZnSe at various temperatures are studied from (i) the spectral profile, (ii) the half-width half-maximum, and (iii) the peak photon energy of each of the emission centers. The results show that the simplified configuration coordinate model can be used to describe the emission spectra, and the frequency of the local vibrational mode of the emission centers is determined.     

LMD的LabVIEW实现及其在齿轮故障诊断中的应用

将局部均值分解(local mean decomposition,LMD)算法在LabVIEW平台上加以实现,开发出LabVIEW的LMD模块。为减小误差,采用三次样条插值法代替滑动平均法来获得局部均值函数和包络估计函数,用形态学滤波算法得到瞬时频率和瞬时幅值的平滑曲线,并通过仿真信号验证LMD算法对于多分量信号的分解能力。最后,利用开发的模块对实测齿轮磨损、断齿故障信号进行分析,成功提取出故障特征频率信息,结果表明开发的LMD模块可以有效应用于齿轮故障的诊断。     

Electro-optically modulated polarizing Fourier-transform spectrometer for plasma spectroscopy applications

A new electro-optically modulated optical solid-state (MOSS) interferometer has been constructed for measurement of quantities related to the low-order spectral moments of line emission from optically thin radiant media such as plasmas. When Doppler broadening is dominant, the spectral moments give the Radon transform of corresponding moments of the velocity distribution function of the radiating species. The instrument, which is based on the principle of the Fourier-transform spectrometer, has high etendue and is rugged and compact. When electro-optical path-length modulation techniques are employed, the spectral information is encoded in the temporal frequency domain at harmonics of the modulation frequency and can be obtained by use of a single photodetector. Specifically, for a plasma in drifting local thermodynamic equilibrium the zeroth moment (brightness) is given by the average signal level, the first moment (shift) by the interferometric phase, and the second moment (linewidth) by the fringe visibility. To illustrate the MOSS performance, I present spectroscopic measurements of the time evolution of the plasma ion temperature and flow velocity for rf-heated discharges in the H-1 heliac, a toroidal plasma magnetic confinement at the Australian National University.     

Analysis of handoff algorithm-based on Doppler effect and RSSI measurements in GSM-R network

Compared with other kinds of networks, the mobiles in GSM for railway networks move faster, so the Doppler effect is more severe. A proposed handoff algorithm based on Doppler effect and RSSI measurements is evaluated. The algorithm performs handoff when the Doppler frequency shift value is negative and the measured signal strength of the adjacent base station exceeds that of the serving base station by a hysteresis level. The hysteresis level varies with different Doppler shift values. The average number of handoffs, average delay of handoffs, and average dropped calls are adopted as criteria for performance. Numerical results are presented to demonstrate the feasibility of the Doppler-effect-based algorithm. The proposed algorithm is compared with two other handoff algorithms. One is based on absolute and relative signal strength measurements; the other on hysteresis variable. It is found that the proposed handoff algorithm performs better than the other two handoff algorithms in log-normal fading environments.     

Photon address digital detector system (PADDS): a cost-effective imaging photon detector

We describe the implementation of an imaging photon detector for the photon address digital detector system (PADDS). The concept is based on combining an image intensifier with a position-sensitive photomultiplier tube with crossed-wire anodes. Particular emphasis is placed on modularity and flexibility. A digital signal processor evaluates events in real time. The compact detector system is able to process photon events with high precision in time with only moderate computing power of the host system. Laboratory experiments show the feasibility of the approach presented for observing rapidly varying sources at low light levels.     

Experimental evaluation of intrinsic and nonstationary ultrasonic Doppler spectral broadening in steady and pulsatile flow loop models

Intrinsic and nonstationary Doppler spectral broadening, and the skewness of the spectral representation, were evaluated experimentally using porcine red cell suspensions as ultrasonic scatterers. Theoretically, the relative Doppler bandwidth, defined as the intrinsic bandwidth divided by the mean Doppler frequency shift, should be velocity independent. The relative Doppler bandwidth invariance theorem was experimentally verified with an in vitro steady laminar blood flow model. It is shown that the relative bandwidth is both independent of the flow velocity and blood hematocrit. Using a pulsatile laminar flow model, the authors demonstrated that the relative Doppler bandwidth invariance theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed during acceleration while a negative skewness was measured during the deceleration of blood. The effect of the window duration used in the Fourier spectral computation, on nonstationary broadening, is characterized.     

血栓多普勒信号的多参数提取及分类

血栓的准确检测可以用于早期脑血管疾病的诊断，超声多普勒是一种无损的血栓检测技术。文章使用三种信号处理方法：传统的声谱分析法、小波分析法、renyi信息量分析法对血栓多普勒信号进行分析，提取出相应的特征参数，然后对敏感的特征参数采用反向传输(Back-Propagation，简称BP)神经网络进行分类，建立起血栓、干扰噪声和正常血流信号的自动判别系统。通过对300例仿真多普勒信号和163例临床采集的大脑中动脉多普勒信号进行分析，结果表明：本文建立的系统对血栓的检测率高于传统的方法，有望可用于血栓多普勒信号的自动检测。     

Statistical treatment of photon/electron counting: extending the linear dynamic range from the dark count rate to saturation

An experimentally simple photon counting method is demonstrated providing 7 orders of magnitude in linear dynamic range (LDR) for a single photomultiplier tube (PMT) detector. In conventional photon/electron counting methods, the linear range is dictated by the agreement between the binomially distributed measurement of counted events and the underlying Poisson distribution of photons/electrons. By explicitly considering the log-normal probability distribution in voltage transients as a function of the number of photons present and the Poisson distribution of photons, observed counts for a given threshold can be related to the mean number of photons well beyond the conventional limit. Analytical expressions are derived relating counts and photons that extend the linear range to an average of ～11 photons arriving simultaneously with a single threshold. These expressions can be evaluated numerically for multiple thresholds extending the linear range to the saturation point of the PMT. The peak voltage distributions are experimentally shown to follow a Poisson weighted sum of log-normal distributions that can all be derived from the single photoelectron voltage peak-height distribution. The LDR that results from this method is compared to conventional single photon counting (SPC) and to signal averaging by analog to digital conversion (ADC).     

A theoretical and experimental analysis of the received signal fromdisturbed blood flow

A theoretical and experimental study of the received ultrasonic signal from calibrated stenotic flow phantoms is presented. A finite element analysis of the velocity profile for 30, 50, and 80% stenoses provides a basis for the study of the experimental results. High-resolution images of the returned signal obtained from a unique experimental system and a high volume concentration of scatterers are then presented. The authors show that in the presence of 30 and 50% stenoses, particularly for the low velocities which would be associated with diastole, the duration of the signal correlation increases in a region which is distal to the stenosis and near the vessel walls, rather than the expected decrease. This results from the decrease in the mean velocity and velocity spread within this region. In the presence of high velocities associated with systolic flow, the magnitude of the reverse flow component increases as does the peak velocity in the center of the vessel. These changes produce an increase in the radial velocity gradient, a shift in the gradient peak, and a decrease in the correlated signal interval in comparison with laminar flow. Thus, the spatial variation in the mean velocity and velocity gradient, and spatial variation in the signal correlation can be used to detect the change in the flow profile     

Correction of pulse-height spectra for peak pileup effects using periodic and random pulse generators

Peak pileup decrease the counting efficiency and distorts the recorded spectral shape in a pulse-height spectroscopy system. To correct for counting efficiency, it is common to estimate the photon detector input count rate by adding a pulse generator peak, wth known count rate, to a spectrum. A rigorous set of equations is developed to predict the number of piled up counts when either a periodic or a random pulser is used. Using these equations and an empirical treatment of ADC dead-time, expressions for the input count rate are derived for two cases: (1) “sparse” spectra dominated by one or a small number of photon peaks, and (2) “dense” spectra in which photon events occupy most of the spectral channels. Only the first has been treated previously. We provide formulae for the second case both for the situation where the pulser peak is superimposed on the photon region, and where it is separate from the photon region. The pileup equations are incorporated in an iterative routine which corrects for spectral distortion. The analysis has been applied to spectra of ^99mTc and medical bremsstrahlung, measured with a high-purity germanium spectroscopy system.     

Optoelectronic transmitters for medical ultrasound transducers

Optoelectronics and fiber optics can be used to miniaturize and improve the flexibility of the transducer cable and transducer handle of medical diagnostic ultrasound scanners. The reduction in size has gained importance as 2-D array transducers with up to 1000 independent channels become accepted to improve diagnostic ultrasound images. The authors describe the analysis, design, fabrication and testing of a prototype silicon photoconductive semiconductor switch (PCSS). The monolithic silicon PCSS was used in combination with an infrared semiconductor diode laser with a fiber optic “pigtail” to shock excite and burst excite a 2-D array transducer element resonant at 2.5 MHz. Optically controlled voltage, current, and ultrasound pulses are compared to those from conventional electronic shock excitation and narrow band Doppler pulses. The optically triggered ultrasound pulse for single shock excitation produced 30 V spikes at the 2-D array element with a fall time of 200 nsec and a rise time of 2 μsec with a peak current through the transducer element of 34 mA. An optically produced burst of eight pulses at a frequency of 2.5 MHz produced 11 V spikes at the transducer with a fall time under 100 nsec and a rise time of approximately 300 nsec. The peak current per pulse was 25 mA through the transducer element. These results show the feasibility of applying optoelectronic technology to replace conventional electronic transmitter technology     

Pinhole-array x-ray spectrometer for laser-fusion experiments

A pinhole-array x-ray spectrometer for laser-fusion experiments is demonstrated. An array of approximately 300 pinholes is placed in front of a flat-crystal spectrometer, yielding target images at photon energies ~10 eV apart (for photon energies of ~4 to 5 keV). For wideband radiation the images are two dimensional, whereas when a single spectral line is used, the field of view in the direction of dispersion is limited. However, single spectral line images can have a field of view sufficient for imaging the compressed target core. We show the image at the Ti-Kalpha-line fluorescence from a Ti-doped shell, which we show to be excited by continuum radiation from the compressed core. The Kalpha image delineates the cold, compressed shell at peak compression, which can otherwise be obtained only through backlighting. In addition, the array provides spectra of high spectral resolution because of the reduction in the effective source size.     

光子计数动态剪切干涉仪的实时波前处理机

介绍了光学计数方法下的波前处理机的构成及工作原理，解决了弱光条件下的光子脉冲计数，并以ＴＭＳ３２０Ｃ２５高速数字信息处理器为核心，快速完成了波前相位计算，波前重构，控制信号的并行输出及波面信息的实时监测。     

Time-domain estimation of the center frequency and spread of Doppler spectra in diagnostic ultrasound

Two sets of estimators of the centre frequency and mean-square bandwidth of a Doppler spectrum are analyzed. The first set is based on a linear combination of the complex autocorrelation of the Doppler signal for different lags. It is then shown that the argument (phase angle) of the complex autocorrelation function at unit sample lag is a close approximation to the mean Doppler shift, whereas its modulus (magnitude) gives information about the mean-square bandwidth of the spectrum. Approximate expressions for the bias and variance of the estimators are derived, valid for long averaging times. The performances of the estimators are compared for signals with rectangular spectra and different signal-to-noise ratios.