Golay pulse encoding for microbubble contrast imaging in ultrasound

We present a technique that uses Golay phase encoding, pulse inversion, and amplitude modulation (GPIAM) for microbubble contrast agent imaging with ultrasound. This technique improves the contrast-to-tissue ratio (CTR) by increasing the time-bandwidth product of the insonating waveforms. A nonlinear pulse compression algorithm is used to compress the signal energy upon receive. A 6.5-dB improvement in CTR was observed using an 8-chip GPIAM sequence compared to a conventional pulse-inversion amplitude-modulation sequence. The CTR improvement comes at the cost of a reduction in frame rate: GPIAM coding uses four input pulses whereas most contrast imaging sequences require two or three pulses. Our results showed that the microbubble response can be phase encoded and subsequently compressed using a nonlinear matched-filtering algorithm, in order to enhance the signal from the contrast agent, while maintaining resolution and suppressing the tissue signal.     

Chirp signal matching and signal power optimization in pulse-echomode ultrasonic nondestructive testing

Chirp pulse compression is a signal correlation technique that uses frequency modulated pulses as transmitted signals. Usually, signals with linear frequency modulation are applied. They can be generated rather easily, but their spectra are not totally matched to the transfer function of ultrasonic systems. In pulse-echo mode operation, with signal duration and consequently the time-bandwidth product being critical parameters, waveforms should be applied which make full use of the available power and bandwidth resources. We report here two methods to improve the overall efficiency of an ultrasonic pulse-echo system. Transmitter signals with constant amplitude level and nonlinear frequency modulation can be generated in such a way that they are spectrally matched to the system. A formula for the calculation of such a matched nonlinear chirp signal is presented. This modulation scheme also leads to a side-lobe level reduction of the compressed pulses. The application of square wave chirps derived from sine type chirps yields an additional gain of echo signal amplitude. Moreover, the complexity of the signal generation hardware is reduced. The methods are illustrated by an example     

高分辨超短光脉冲相关测量系统

本文提出利用脉冲干涉相关信号测量飞秒脉宽的新原理。对两个可能实现的途径进行了论证。一是提出等值宽度法,给出了等值宽度在五种光脉冲形状干涉相关曲线上包络的位置;二是建立了用微机处理干涉相关信息的数学模型,并给出处理的结果。研制了适合飞秒测量的低色散光学相关器。理论分析和实验结果均表明,本方法和相关测量系统可将超短光脉冲测量的时间分辨率提高一个量级以上。这对当今世界飞秒级(10~(-15)S)光脉冲的测量是有利的。     

基于FPGA的多通道动态光散射自相关器的研制

为提高多角度动态光散射(MDLS)实验的测试效率与测量精度,开发了一种基于现场可编程门阵列(FPGA)的多通道动态光散射自相关器。该自相关器利用FPGA同时采集4个散射角度的光子脉冲信号,实现高频脉冲计数和自相关计算;设计双计数器模块保证高频无丢失计数,结合片内环形寄存区与片外DDR3芯片实现动态存储数据;提出multi-tau相关器结构设计自相关器的相关计算模块,通过USB通信模块传输自相关计算结果。该自相关器利用有限的FPGA内部硬件资源,实现了多通道自相关运算,解决了大量计数数据的存储问题,提高了MDLS实验的测试效率。利用自相关器对50～200 nm的聚苯乙烯颗粒进行MDLS实验,实验结果与样品的标称粒径表现出良好的一致性,表明该设计能有效地表征颗粒粒径信息。     

Practical spread spectrum pulse compression for ultrasonic tissue imaging

Spread spectrum pulse compression is a signal processing algorithm that enhances critical system performance parameters such as signal-to-noise ratio, peak power requirements, minimum detectable signal, and total dynamic range. For this research, a digital, real-time, Barker coded, bi-phase modulator was designed and constructed, as well as a simple ultrasonic test tank containing both synthetic targets and excised goat's liver. Upon reception and demodulation of the spread spectrum ultrasonic echo, cross-correlation with a sidelobe suppression filter was performed. Due to limitations such as narrow bandwidth, and very short minimum ranges, a practical ultrasonic pulse compression system must be restricted to short code lengths. For 13 bit Barker code compression, the expected increase in signal-to-noise ratio of 11 dB was realized; at the same time greater than 30 dB of instantaneous dynamic range was maintained.     

Mode-locked ring laser with output pulse width of 0.4 ps

The output pulse width of a mode-locked ring laser composed of an erbium-doped fiber amplifier, Mach-Zehnder optical modulator, and optical band-pass filter depends largely on the repetition frequency and the wavelength characteristics of these optical circuit elements. In previous experiments, the output pulse width was in the order of 5 ps at a repetition frequency of 5 GHz. The principal reason was that the narrow passage band of the optical circuit elements made it extremely difficult to generate an ultra-short optical pulse. Consequently, we examined how to narrow the optical pulse width by flattening the wavelength characteristics of these optical circuit elements. Furthermore, we drove the optical modulator in the cavity using a frequency multiplier to operate at an effectively higher frequency By widening the wavelength passage band of all the devices in the optical circuit, we achieved an output pulse width of 0.4 ps at a repetition frequency of 5 GHz; the pulse peak power was more than +23 dBm, and the time-bandwidth product was 0.34. We successfully tested an ultra-short optical pulse source with an output pulse width of 0.4 ps with no external pulse compression using a mode-locked ring laser     

In-line interferometric time-integrating acousto-optic correlator

A novel, efficient, stable, in-line interferometric, two Bragg cell, acousto-optic architecture is introduced for signal correlation and spectrum analysis. The processor employs an image-inversion technique to produce a correlation output that does not have to be generated on a fast spatial carrier, thus reducing the burden on the required space-bandwidth product on the output time-integrating detector array. The system is particularly suited for wide-instantaneous-bandwidth signal processing and is demonstrated as an autocorrelator for dc-to-10-MHz AM signals on a 100-MHz carrier. A fixed-spatial-carrier outputbased correlator and spectrum-analyzer architecture that is particularly limited in the wideband mode is demonstrated for comparison purposes with the proposed flexible-spatial-carrier-design-based in-line acousto-optic correlator.     

基于双正交小波信道估计改进的TR-UWB系统

针对超宽带（UWB）信号因脉冲持续时间短、时域分辨率高、在传播中出现密集多径现象的特点,利用小波函数和超宽带脉冲之间的相似特性,提出了一种基于双正交小波基的高效估计多径衰落幅度和时延等信道参数的信道估计算法,以便接收机能捕获足够的信号能量。通过滑动相关实现了同尺度上信号与小波的内积运算,避免了峰值搜索过程,简化了估计器的结构。同时,基于由信道冲激响应估计值合成的本地相关模板信号,提出了改进的发射-参考超宽带（TR—UWB）系统。Monte—Carlo仿真结果表明,该算法的归一化均方误差较小,改进的TR—UWB系统能有效抑制参考符号中噪声的影响,系统性能优于传统TR接收机。     

Accuracy of velocity estimation by Reichardt correlators

Although a great deal of experimental evidence supports the notion of a Reichardt correlator as a mechanism for biological motion detection, the correlator does not signal true image velocity. This study examines the accuracy with which realistic Reichardt correlators can provide velocity estimates in an organism's natural visual environment. The predictable statistics of natural images imply a consistent correspondence between mean correlator response and velocity, allowing the otherwise ambiguous Reichardt correlator to act as a practical velocity estimator. Analysis and simulations suggest that processes commonly found in visual systems, such as prefiltering, response compression, integration, and adaptation, improve the reliability of velocity estimation and expand the range of velocities coded. Experimental recordings confirm our predictions of correlator response to broadband images.     

Wavelet transform-based strain estimator for elastography

A new signal processing algorithm based on a wavelet transform (WT) is proposed for instantaneous strain estimation in acoustic elastography. The proposed estimator locally weighs ultrasonic echo signals acquired before tissue compression by a Gaussian window function and uses the resulting waveform as a mother wavelet to calculate the WT of the postcompression signal. From the location of the WT peak, strain is estimated in the time-frequency domain. Because of the additive noise in signals and the discrete sampling, errors are commonly made in estimating the strain. Statistics of these errors are analyzed theoretically to evaluate the performance of the proposed estimator. The strain estimates are found to be unbiased, but error variances depend on the signal properties (echo signal-to-noise ratio and bandwidth), signal processing parameter (time-bandwidth product), and the applied strain. The results are compared with those obtained from the conventional strain estimator based on time-delay estimates. The proposed estimator is shown to offer strain estimates with greater precision and potentially higher spatial resolution, dynamic range, and sensitivity at the expense of increased computation time.     

SAW device implementation of a weighted stepped chirp code signal for direct sequence spread spectrum communications systems

This paper introduces a new weighted stepped chirp code signal for direct sequence spread spectrum (DS/SS) communications systems. This code signal uses the truncated cosine series functions as the chip functions, and it is the result of discretizing a continuous wave (CW) chirp that results in enhanced performance versus a pseudonoise (PN) code and equivalent performance and easier implementation than a CW chirp. This code signal will be shown to have improved compression ratio (CR) and peak sidelobe level (PSL) versus a PN code with identical code length and chip length. It also will be shown to have a similar CR and PSL compared to a CW chirp with identical pulse length and frequency deviation. The code signal is implemented on surface acoustic wave (SAW) devices that will be used as the code generator at the transmitter and the correlator at the receiver. The design considerations for the SAW device implementation of the code signal are discussed, including the effects of intersymbol interference. Experimental data is presented and compared to the predicted results for 8 different SAW devices examining the effects of code length (9 or 13 chips), weighting (uniform, cosine-squared, and Hamming), and sampling on the performance of the code signal.     

连续波声呐中的调频信号设计方法及性能分析

相对传统的短时脉冲波主动声呐而言,连续波主动声呐是一种新型体制的声呐设备,允许在扫描周期内发射高占空比的信号,并且在发射信号的同时进行侦听,由此可以对水下目标实现连续照射,消除距离盲区。由于发射和接收机制的不同,连续波主动声呐对发射信号的波形和处理方法也各有差异,一是要考虑到"直达波"抑制问题,二是要在时间带宽积和对目标的照射时间间隔两者之间进行折中。针对上述两个问题,设计了一种在连续波主动声呐中发射的新型脉冲串信号,该类信号由多个相互正交的广义正弦调频信号串组成,以此在频域上消除回波与拷贝信号的相关性;后置处理中对接收回波提供了三种不同的方案,在时间带宽积和照射时间间隔两者之间择优选择最佳检测效果。计算机仿真结果表明:该类信号波形以及相应的处理方法可以有效地抑制直达波干扰并给出目标的速度-距离信息。     

Coded excitation for synthetic aperture ultrasound imaging

Peak acoustic power limits the signal-to-noise ratio (SNR) of real-time ultrasound images. For most conventional scan formats, however, the average power is well below heating limits. This means the SNR can be significantly increased using coded excitation. A coded system transmits a broadband, temporally elongated excitation pulse with a finite time-bandwidth product. The received signal must be decoded to produce an imaging pulse with improved SNR resulting from the higher average power in the elongated excitation. Decoding can produce significant range side lobes, however, greatly reducing image quality. All practical coding designs, therefore, represent a trade-off between SNR gain and range side lobes. A specific coding scheme appropriate for synthetic aperture imaging is presented. A 14.5 dB SNR improvement with acceptable range side lobes is demonstrated on a forward-looking imaging system appropriate for intravascular applications.     

Bi-parental product algorithm for coded waveform design in radar

The problem of obtaining long sequences with finite alphabet and peaky aperiodic auto-correlation is important in the context of radar, sonar and system identification and is called the coded waveform design problem, or simply the signal design problem in this limited context. It is good to remember that there are other signal design problems in coding theory and digital communication. It is viewed as a problem of optimization. An algorithm based on two operational ideas is developed. From the earlier experience of using the eugenic algorithm for the problem of waveform design, it was realised that rather than random but multiple mutations, all the first-order mutations should be examined to pick up the best one. This is called Hamming scan, which has the advantage of being locally complete, rather than random. The conventional genetic algorithm for non-local optimization leaves out the anabolic role of chemistry of allowing quick growth of complexity. Here, the Hamming scan is made to operate on the Kronecker or Chinese product of two sequences with best-known discrimination values, so that one can go to large lengths and yet get good results in affordable time. The details of the ternary pulse compression sequences obtained are given. They suggest the superiority of the ternary sequences.     

Proposal for optical implementation of the wigner distribution function

The Wigner distribution function (WDF) offers comprehensive insight into a signal, for it employs both space (or time) and frequency simultaneously. Whenever optical signals are involved, the importance of the WDF is significantly higher because of the diffraction (or dispersion) behavior of optical signals. Novel optical implementations of the WDF and of the inverse Wigner transform are proposed. Both implementations are based on bulk optics elements incorporating joint transform correlator architecture. A similar implementation is derived for the ambiguity function, which is related to the WDF through Fourier transformation.     

水声脉冲信号检测的新方法

水声脉冲信号是由一个运动平台上的发射器发射的。信号的调制方式、载频、幅度、脉宽以及周期均为未知。该文介绍了一种水声脉冲信号检测的新方法。水声脉冲信号接收机输出的短时谱重心是不断起伏的。起伏在高信噪比时会变得很小,而在低信噪比(或无信号)时会变得很大。由于,起伏的绝对偏差移动平均可用来度量起伏的大小,因此,它可以被用来检测水声脉冲信号。还介绍了新检测方法的原理、算法以及仿真结果。在海洋噪声、运动载体的辐射噪声以及小的多途干扰背景中,该方法能可靠地检测水声脉冲信号,并能同时对信号的一些参数作出估计。     

Robustification of noise radar detection

A study of robust detection scheme for continuous wave noise radars is presented. The probability density function (PDF) of the noise at the input of the radar is not usually Gaussian and has heavy tails generated by impulse interferences. Although the PDF of interferences at the output of the noise radar correlator is Gaussian, impulse interferences increase the processing floor, and thus decrease the overall radar sensitivity. The proposed robustification applied to the correlator?s input signal increases the radar sensitivity in the presence of impulse interferences, and does not introduce any significant losses if the input noise is purely Gaussian.     

Radiometers, autocorrelators, and matched-filter receivers by means of acousto-optic spectrum analysis

Acousto-optic spectrum analyzers provide a convenient means of separating wide-bandwidth signals into their frequency components. By a change in the rf input signal into the spectrum analyzer and by the provision of additional digital postprocessing, it is possible to perform radiometry, signal autocorrelation, and matched-filter reception. Although the acousto-optic device has a space-integrating architecture, the matched-filter receiver can be implemented for signals having time durations much longer than the acousto-optic cell. The resulting signal-to-noise ratio improvements from the receiver are consistent with the time-bandwidth product of the waveform, rather than the time-bandwidth product of the acousto-optic device. A mathematical foundation of the processor is presented along with specific receiver implementations. Computer-simulation and experimental results demonstrate key findings. In one experimental example, a linear-frequency-modulated waveform is matched-filter processed to recover a signal that is -24 dB with respect to the input noise floor.     

Pulsed energy conversion with a dc superconducting magnet

A generator system for pulsed power is described which employs a dc superconducting magnet in a magnetic flux compression scheme. Experience with a small-scale generator together with projections of numerical models indicate potential applications to fusion research and commercial power generation. When the system is large enough pulse energy can exceed that stored in the magnet and pulse rise time can range from several microseconds to tens of milliseconds.     

Chirp excitation of ultrasonic probes and algorithm for filtering transit times in high-rangeability gas flow metering

The signal processing used in an ultrasonic high-rangeability gas flow meter using times of flight is presented. The flow meter under discussion uses a combination of continuous wave and chirp signals to measure at low flow velocities, below 20 m/s, and chirp signals alone to measure high flow velocities, above 20 m/s. Because of the need for a pulse compression technique in the signal waveform design the technique of pulse compression and the choice of signal waveforms are discussed. The advantages and disadvantages of amplitude weighting vis-a-vis frequency domain manipulations of the waveforms are also discussed. To eliminate spurious times of flight, a special filtering technique is used, based on assessing the gradient of ascendingly ordered time series of time-of-flight measurements. A summary of user experience with high-rangeability gas flow meters in use on different offshore platforms and in refineries is given. Long-term tests that examined the accuracy of the high-rangeability flow meter are also described.