Microwave Acoustic Simulation of Airborne RADAR Ground Echoes

A method is described for generating simulated microwave frequency RADAR terrain echoes. This method makes it possible for the first time to realistically duplicate in the laboratory the principal characteristics of echoes due to RADAR motion and terrain roughness. These characteristics include variation of echo delay, Doppler shift, random fine structure, and their time variation. The simulation is based on modeling of the RADAR beam propagation and diffuse reflection processes by use of microwave acoustic energy in a solid medium. Scaling relations between RADAR and acoustic model parameters are derived. An experimental program was carried out whose purpose was to develop a practical solution to the problem of varying the distance between a microwave acoustic transducer and a reflecting surface, and to determine the overall feasibility of the simulation method. Results indicating its practicability are presented and directions for further work are suggested.     

A Unified Method of Broad-Band Echo Characterization for Diagnostic Ultrasound

Blood flow can be described by a scattering function, i.e., by the expected distribution of Doppler shifts and delays that are introduced by a collection of moving point scatterers. An equivalent representation is the echo autocovariance function, measured for a sequence of echoes from different transmitted pulses. Tissue structure can be characterized by the frequency dependence of acoustic reflectivity for echoes from large acoustic discontinuities (highlight echoes), and by the "texture" of the echo from a distributed reflector. Highlight structure and frequency dependence of tissue reflectivity can be obtained from the signal-echo cross-ambiguity function, and texture differences are related to differences in echo power spectra and auto-covariance functions.     

Voltage-controlled SAW oscillator on temperature-stable quartz substrate

The frequency of a surface acoustic wave (SAW) oscillator fabricated on the temperature-stable X 33 orientation of quartz is found to vary linearily with an applied bias voltage. The fractional frequency change per unit biasing field has a value of 9.5 × 10^-12m/V. Fractional frequency changes greater than 200 ppm have been obtained.     

Microwave acoustics

During the last few years, generation and propagation of acoustic waves have been extended to frequencies high into the gigacycle region. This paper deals with microwave acoustics, with emphasis on effects and techniques utilized at cryogenic temperatures. The extremely short wavelengths require the use of single crystal propagation media, free from defect scattering, supercooled to reduce wave attenuation from phonon interactions. Interactions of acoustic phonons with other phonons, electrons or holes, and spin waves are possible, with interchanges of energy that can produce attenuation or amplification of acoustic waves. The fundamental characteristics of acoustic wave propagation in crystanine media are discussed, with a review of experimental techniques and results. Microwave acoustic waves in solids and transducers suited to generation and detection are discussed, and a basic analysis of plane wave piezoelectric coupling is presented.     

一种基于二阶盲信号分离的多路声回波抑制模型

本文提出了一个基于二阶盲信号分离的多路声回波抑制模型.该模型回避了多路声回波对消中因声回波源信号间的强互相关性所致的固有的解的非唯一性问题,而是充分利用了这种互相性来去除声回波.模型仅添加一个辅助麦克风并巧妙置位即可对各路麦克风信号中的多路声回波进行有效的分离和抑制.为了实时处理的目的,提出了一个计算复杂度低且收敛稳健的二阶频域盲信号分离算法来检验该模型.实验结果充分确认了提出模型的有效性.     

Time-varying ice crystal orientation in thunderstorms observed withmultiparameter radar

Repeated changes associated with lightning have been observed with multiparameter radar in the echoes from the tops of Florida thunderstorms. These lightning-related radar signatures are interpreted as changes in the orientation of ice crystals being preferentially aligned parallel to the in-cloud electric field. The changes occur at intervals on the order of 10 s and are easily observed in the signatures of the differential propagation phase shift and the linear depolarization ratio which are sensitive to propagation effects caused by the oriented ice crystals. The orientation of ice crystals aloft has been previously observed using circularly polarized radar while the simultaneous differential phase shift and linear depolarization measurements reported were obtained with a dual-linear polarized radar. The observations indicate crystal orientation angles greater than 450 and occasionally near vertical. In one case, the crystals were found to be oriented in a layer near radar cloud top spanning a 20-km range and 3 km in depth     

基于非恒Q衰减模型探地雷达的回波仿真

电磁波在地下介质层中的传播特性及回波仿真技术对于反演地下介质层结构具有十分重要的意义。文中研究了探地雷达信号在有耗介质层中传播的回波仿真问题,基于Debye色散模型定义介质层电磁参数,讨论电磁波在地下介质层中的非恒Q衰减特性和模拟回波信号,得到了探地雷达脉冲在层状介质中传播的回波信号模型和反射回波曲线。仿真结果与当前通用的探地雷达数据仿真软件GprMax进行对比,证明了该方法的有效性。     

Superconducting-cavity stabilized oscillators with improved frequency stability

X-band microwave oscillators stabilized with superconducting niobium cavities with loaded Q's of about 10¹⁰achieved short-term frequency stabilities as low as σy=6 × 10^-16, and typical long-term fractional frequency drifts of ± 2 × 10^-13per day.     

Amplification factor of echo signals in ferrimagnetic materials

It is known that spin echoes in ferrimagnetic materials can only be excited on the application of high-power microwave signals. The amplification factor of spin echoes is calculated for ferrimagnetic materials; long-range dipolar field, external RF-field couplings, and magnetization relaxations are included in the calculations. The inclusion of such interactions provides a mechanism by which realistic amplification of echoes may be calculable. The theoretical estimate of amplification is in reasonable agreement with previous experiments. Thus, the effects of carrier frequency, field gradient, and microwave power on echo amplification are calculated     

Polarization echoes in piezoelectric semiconductors

In many piezoelectric crystals, application of a se-quence of two or three microwave electric field pulses creates a polarization which radiates subsequent “echo” signals (analogous to spin echoes). Two distinct types of echo phenomena may be delineated. Both involve a nonlinear interaction of a forward propagating acoustic wave (ω,k) (launched by the first microwave pulse (ω,0), with the uniform microwave electric field of the second pulse (ω,0) . The nonlinearities are derivable on the basis of field induced ionization of electrons from shallow impurity states (traps). The first echo phenomenon, which we term “parametric echo”, is a backward wave parametric process. The pump being provided by the 2ω component of the conduction band electron density resulting from ionization by an ω electric field. In the second phenomenon, “holographic echo”, electrons are transferred among the traps to create a static charge distribution which is a stored hologram representing the interference pattern (with wave vectork) of the acoustic and electric fields. At low temperatures (4.2°K) in CdS, CdSe, and CdTe the pattern is stable in the dark for months. A third (or any subsequent) pulse, (ω,0) or (ω,k), produces an output echo (ω,k) or (ω,0), respectively, through electrostriction. Under illumination by low level white light the variation of echo amplitude with microwave power involves the energy distribution of the trap levels, and the latter may be obtained from the data. Results for a fully compensated CdS:S sample are presented.     

天发船收高频雷达海面目标回波建模

天发船收高频雷达系统中,由于电离层随机扰动和接收船六维摆动运动影响,目标回波空时谱发生展宽和形变,严重影响雷达探测性能。本文对天发船收高频雷达海面目标回波建模问题开展研究,首先分析了动态空间几何关系变化下目标回波的多普勒频移特性,建立了理想条件下的空时目标回波模型;在此基础上,引入电离层不规则体和六维摆动空时相位扰动影响,分别构建电离层随机扰动相位仿真器和六维摆动扰动相位模型,进而建立了天发船收高频雷达扰动目标空时回波模型,实验数据验证了模型的有效性。     

含气泡液体内的声学参数研究

基于声压与质点振动速度的边界条件,在考虑粘滞阻力、表面张力的情况下,研究了含有气泡液体内声波的传播。利用球贝塞尔及汉克尔函数,对声传播的相关参数进行了理论研究,并对声波的传播速度及衰减系数进行数值求解。结果表明,声速随气泡体积分数的增大而减小,衰减系数却增大,气泡半径对其影响较小;当声波的频率较低时,气泡的存在对声波传播的速度影响较大;而频率较高时,能量更易损失。     

温度效应对声表面波传播的影响

通过引入材料的高阶热膨胀系数和高阶热弹性系数,对半无限大石英晶体内声表面波的传播方程进行了求解,得到了不同温度下的声表面波波速.计算结果表明,温度变化对AT切和ST切石英晶体的声表面波波速有显著影响.最后分析了ST切石英晶体的声表面波的温频特性,结果表明,由温度变化引起的频率漂移很明显.     

强背景杂波下月球次表面回波的识别方法

为有效探测月球次表层结构,本文基于月球次表面天底点回波和表面非天底点回波（杂波）的相干与非相干特性,提出在月球次表面地形变化不大的情况下,由累积取平均的方法来减小表面非天底点回波从而识别次表面回波的方法。以数值模拟的月海与月陆表面雷达探测仪回波为例,验证了该方法的正确性与可行性,并讨论了累积平均数目对次表面回波提取结果的影响。本文所述方法也可以应用到火星等其他外星球次表层结构的探测中。     

Microwave solid-state delay lines

The low-loss propagation of microwave phonons at room temperature in certain single crystals makes practical the construction of microwave acoustic delay (MAD) lines. The properties of these solid-state units are discussed and compared to those of more conventional microwave delay devices. Various possible applications of solid-state lines are suggested and the present status of these units is reviewed. The dispersive properties of magnetoelastic and magnetostatic wave propagation are also outlined and practical considerations governing their use are discussed. Microwave acoustic delay devices not only offer size and weight advantages over conventional microwave delay lines, but also offer improved performance and in many cases lower cost than the conventional units.     

Features of propagation of surface acoustic waves in two-dimensional phononic crystals on the surface of a lithium niobate crystal

Two-dimensional phononic crystals are fabricated on the surface of a lithium niobate crystal. A numerical method is developed for calculating the frequency dependences of the transmission and reflection coefficients of a surface acoustic wave for a rectangular region with two-dimensional periodic structure on the surface of an elastic body. Measurements of the frequency dependences of the reflection and transmission coefficients are performed for propagation in the part of the lithium niobate crystal surface on which two-dimensional phononic crystals were fabricated. Forbidden zones are found in the transmission spectra of the waves transmitted through phononic crystals. It is shown that, if the width of the incident wave beam is less than the width of the region with the two-dimensional periodic structure (the two-dimensional phononic crystal), then a resonant increase in the transmission coefficient arises near the center of the rejection band.     

Return loss characteristics of optical fiber connectors

This paper describes the return loss characteristics for four typical contact type connectors: perpendicular and oblique endface connectors employing either physical contact or contact via index matching material. The high index layer on the polished fiber endface and the index matching material, which are important connector parameters, are investigated by measuring the return loss from one perpendicular fiber endface coated with the material and by etching the fiber endface. It is useful to apply two materials with different refractive indices to derive the refractive index and thickness of high index layers on perpendicular fiber endfaces. The index matching material we use has a refractive index of 1.467 at 20°C and a thermal coefficient of -2.9×10^-4/°C at 1.3 μm. We performed return loss calculations and experiments of the four contact type connectors. The worst value at any temperature from -40 to 80°C can be estimated, by measuring the return loss of these connectors at room temperature     

Adaptive Sampling Rate Correction for Acoustic Echo Control in Voice-Over-IP

Hands-free terminals for speech communication employ adaptive filters to reduce echoes resulting from the acoustic coupling between loudspeaker and microphone. When using a personal computer with commercial audio hardware for teleconferencing, a sampling frequency offset between the loudspeaker output D/A converter and the microphone input A/D converter often occurs. In this case, state-of-the-art echo cancellation algorithms fail to track the correct room impulse response. In this paper, we present a novel least mean square (LMS-type) adaptive algorithm to estimate the frequency offset and resynchronize the signals using arbitrary sampling rate conversion. In conjunction with a normalized LMS-type adaptive filter for room impulse response tracking, the proposed system widely removes the deteriorating effects of a frequency offset up to several Hz and restores the functionality of echo cancellation.     

LiNbO_3:Fe晶体的光致声发射现象

本文观察到了LiNbO_3:Fe晶体在光致准击穿时的一种声发射现象,产生的低频声波调制了输入晶体的超高频声信号。其频率随入射光强辐照时间增加而增大,其周期与同时测量的光生伏特电流、光致折射率变化以及某一点衍射光强的跃变周期相同。文中对比现象作了,探讨认为它起因于准击穿时跃变空间电荷场的逆压电效应。     

Bounds on estimating the acoustic attenuation of small tissue regions from reflected ultrasound

The acoustic attenuation coefficient of most soft biological tissues has an approximately linear-with-frequency attenuation characteristic. The slope of the attenuation coefficient with frequency, denoted by β, has been observed to vary with the severity of liver disease. Two approaches for estimating the β value from reflected ultrasound signals are examined: the spectral-difference approach, which estimates β from the slope of the difference between log-spectra from two locations within the liver, and the spectral-shift approach, which estimates β from the downward shift experienced by the propagating pulse spectrum with penetration into the liver. This paper considers signals reflected from a small tissue region, defined by a cell measuring D by D centimeters in the plane of the sonogram, thus determining the feasibility of producing attenuation images. Lower bounds on the variance of the two β estimators are calculated by deriving maximum-likelihood estimators and by locating the tissue cell in the focal plane of the transducer. If W denotes the usable bandwidth in the reflected signals, the bounds are shown to be proportional to (WD)^-3for β estimates calculated from individual reflected signals, and (WD)^-4for estimates from all the signals reflected from the tissue region. With currently available technology, clinically useful results can be obtained for cell sizes measuring approximately 2.0 cm on a side.