一种定征复合板材粘接层性质的非线性超声兰姆波方法

借助于兰姆波频散曲线及导波激发的模式展开分析方法,对基频兰姆波时域信号及二次谐波时域信号的发生过程进行了直观的论述。结合Ritec-SNAP系统的测量功能,详细分析了二次谐波时域脉冲包络积分表达式的物理意义;该积分表达式可表征基频兰姆波时域脉冲传播过程中的二次谐波发生效率,以及基频与二倍频兰姆波模式之间的频散程度。在基频与二倍频兰姆波相速度相等(或近似相等)的频率附近,实验观察到显著的且无模式混叠的二次谐波信号,显示出在兰姆波的传播过程中的确可存在强烈的非线性效应。对于三种不同粘接情形的复合板材,实验结果表明,采用本文引入的非线性兰姆波应力波因子,结合二次谐波幅频曲线峰值所对应的频率值,可有效地对板材粘接层性质进行表征。     

脉冲发电机电源宽频变化情况下锁相技术仿真

针对在基波频率变化情况下传统锁相环无法进行正确锁相的问题,提出了基于同步参考坐标系的追踪型三相软件锁相环。三相电压信号(V_a、V_b、V_c)经同步坐标变换后,得到两相静止坐标(V_α、V_β)信号,再经过旋转坐标变化后获得(V_d、V_q)信号;V_q信号经过低通滤波后,进行动态信号延迟,其延迟的时间由输出的频率信号获得;把延迟前后的信号做差,经过PID调节后输出为角频率,对角频率积分后获得稳定基波正序相位。为了验证该算法,基于matlab建立了仿真模型。仿真结果表明,该锁相环在电源基波频率线性变化情况下其相移时间小于0.2ms,在脉冲发电机电压基波频率非线性变化情况下需要经过一个周波也能获得精准地锁相,其运用于有源电力滤波器有效地抑制了脉冲发电机的输出谐波。     

基于电光晶体马赫-曾德干涉仪的载波包络偏移频率调节方法

基于电光晶体马赫-曾德(M-Z)干涉仪的载波包络相位偏移频率(carrier-envelop offset frequency, f₀)调节方法是一种新颖的f₀调节方法.该方法通过改变脉冲包络而不改变载波频率实现对f₀信号的调节.本文对该方法所涉及的偏振控制装置进行了仿真,分析了其中波片光轴偏差对输出激光偏振方向和偏振度的影响.在实验上提出了一种光轴校准方法以减小波片光轴偏差带来的影响,并对比了抽运电流调节方法和基于电光晶体M-Z干涉仪的f₀调节方法对f₀信号和光梳与激光拍频信号(beat note, f_b)的影响.实验结果表明改变抽运电流,在f₀调节量为9 MHz的情况下,对f_b影响为7 MHz.而在相同f₀调节量下,电光晶体M-Z干涉仪f₀调节方法对f_b的影响为0.2 MHz,仅为抽运电流对f_b影响的1/35,从而验证了基...     

用于俄歇电子测量的廉价的锁相放大器

一、锁相放大器结构及其主要电路 关干锁相放大器的原理可见文献[1—3].本文中所描述的锁相放大器,简化了结构.它采用换程开关,使振荡器产生二个频率f0和f0/2,分别工作于基频模式和倍频模式,以适用于一次和二次微商的测量要求.整机有前置放大器、窄带放大器、相敏检波器(由混频     

兰姆波非线性效应的实验观察

对兰姆波非线性效应进行了实验观察。首先,对实验所采用的Ritec系统进行了介绍,尤其对实验系统的声学部分进行了详细考虑,特别是对二次谐波的时域响应进行了简洁直观的分析。根据兰姆波具有强烈非线性效应的条件,采用斜劈换能器发射和接收兰姆波。改变发射与接收斜劈换能器的间距,对一定频率范围内的兰姆波及二次谐波在固体板表面的振幅进行了测量。在基频与二倍频兰姆波相速度相等所对应的频率附近,分析了二次谐波振幅随传播距离的定量关系,从实验上证实了二次谐波在传播过程中具有积累增长效应的结论。     

数字正交锁相放大器的研制及其在甲烷检测中的应用

利用可调谐二极管激光吸收光谱技术检测气体浓度时,为了从红外传感信号中提取一次及二次谐波信号来表征气体浓度,研发了一种基于数字信号处理器的数字正交锁相放大器.介绍正交锁相放大原理,设计谐波提取算法,给出数字正交锁相放大器的软硬件实现方案.利用配备的浓度为1%~5%的甲烷样品以及研制的锁相放大器,开展气体实验.实验结果显示,当甲烷浓度为5%时,在二次谐波对应的频率点处,测得的系统信噪比为34dB,表明设计的锁相放大器具有较好的性能;测得的二次与一次谐波信号峰峰值的比值与气体浓度成线性关系;考虑动态配气以及气体沿管道传输的时间,检测系统的响应时间约为96~98s;气体浓度为20 000ppm时,测试浓度波动范围为-92ppm~+118ppm;根据Allan方差预测的系统检测下限为29.52ppm.与模拟锁相放大器以及商用锁相放大器相比,本文研制的数字正交锁相放大器硬件结构简单、体积小、成本低、易于集成,在红外气体检测领域具有很好的应用前景.     

极紫外阿秒脉冲在高次谐波产生过程中引起的非偶极效应

当红外强激光和极紫外(XUV)阿秒脉冲共同作用于原子分子时，电离出去的电子通常会吸收和辐射激光光子而发生能量扩展.讨论了由于XUV阿秒脉冲的短波长与扩展后的电子波包尺度可相比拟时在高次谐波产生过程中引起的非偶极效应.采用H⁺₂作为模型分子，并把分子轴置于激光场的传播方向，通过解二维含时薛定谔方程并比较考虑非偶极效应和采用偶极近似两种方法计算得到的结果，两者相比，前者的谐波强度降低，谐波频率向低级次稍有移动，电子能谱的能带内出现了更多的光电子峰.在相同的光电子能量处，两种方法计算得到的信号强度相差2—5倍.并且这种非偶极效应随着红外基频光光强的增大而增强，随阿秒脉冲波长的增大而减弱. 关键词： 非偶极效应 光场空间不均匀性 阿秒脉冲 高次谐波产生     

普通话元音和辅音的频谱分析及共振峯的测算

本文介绍利用声学实验室常备仪器来作语音频谱的分析方法.根据包络峯内所包含的三个谐波的频率和振幅的关系来计算共振峯的频率、振幅和带宽值,给出便于计算的简单公式.文中讨论了难于分辨的共振峯,归纳为单、双两种共振峯结构,各分为不同类型,分别处理.最后提供普通话元音、浊辅音的共振峯数据和清辅音的能量集中区数据.     

利用低频场控制轨道直接产生低于50阿秒的单个脉冲

提出一种直接得到脉宽小于50as的单个脉冲的新方法.利用波长为800nm脉宽为5fs的超短脉冲叠加上波长为10μm,强度为超短脉冲强度的10％且脉宽足够长的低频场脉冲驱动氦原子,谐波谱可以拓展到I_p+8.2U_p,同时,高于4.2U_p的谐波几乎是连续的.如果继续调节超短脉冲的载波包络相位,超连续谱可以继续拓宽,从而可以得到低于50as的单个脉冲. 关键词： 阿秒脉冲 超连续谱     

有机电致发光器件的动态电学特性

利用交流阻抗谱技术，研究了有机发光二极管ITO/Alq₃(90 nm)/Al的载流子传导机理.根据器件对不同频率的响应曲线及其等效电路模型，该器件可看作是由并联的电阻R_p和电容C_p再与电阻R_s串联而成，并根据实验数据求出了R_p，C_p和R_s的数值.实验结果表明器件的载流子传输机理属于指数分布式的陷阱电荷限制电流，其介电弛豫时间随偏压的增加而逐渐减小. 关键词： 3')" href="#">Alq₃ 陷阱电荷限制电流 交流阻抗谱 有机发光二极管     

Responses to amplitude-modulated tones in the auditory nerve of the cat.

Sinusoidally amplitude-modulated (AM) tones are frequently used in psychophysical and physiological studies, yet a comprehensive study on the coding of AM tones in the auditory nerve is lacking. AM responses of single auditory-nerve fibers of the cat are studied, systematically varying modulation depth, frequency, and sound level. Synchrony-level functions were nonmonotonic with maximum values that were inversely correlated with spontaneous rate (SR). In most fibers, envelope phase-locking showed a positive gain. Modulation transfer functions were uniformly low pass. Their corner frequency increased with characteristic frequency (CF), but changed little for CFs above 10 kHz. The highest modulation frequencies to which phase locking occurred were more than 0.8 oct lower than the highest frequencies to which phase locking to pure tones occurs. Cumulative, or unwrapped, phase increased linearly with modulation frequency: The slope was inversely related to CF, and slightly higher than group delays reported for pure tones. High SR, low CF fibers showed the poorest envelope phase locking. In some low CF fibers, phase locking increased at high levels, associated with "peak-splitting" phenomena. Changes in average rate due to modulation were small, and could be enhancement or suppression.     

Design optimization for microwave coupling structures of a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions

A new coupling scheme of the external microwave signal for a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions has been proposed. It is suggested to couple the meander-like array inductively to a parallel microwave transmission line. Within the resistively shunted junction model different types of microwave coupling structures were analyzed with the goal to reduce the mutual phase locking and to achieve high output power and phase-locking stability. A harmonic balance technique was used to derive an analytic expression connecting the amplitudes of microwave voltages and currents in the array and the frequency of the external signal. Numerical simulations were performed to determine the boundaries of stable locking as a function of circuit parameters.     

Phase-locking of a second-harmonic gyrotron oscillator using aquasi-optical circulator to separate injection and output signals

Phase-locking in a 34.5-GHz special complex cavity gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency was studied. Injection of the locking power was made via a quasi-optical circulator connected to the gyrotron output. Locking bandwidth was measured by comparing the phase of the injection signal and output signal using a balanced mixer. Locking was observed with input power level as low as 40 dB below the gyrotron output power. The locking bandwidth is, however, narrower than in gyrotrons operating at the fundamental cyclotron frequency which may be attributed to the longer resonant cavity in the second harmonic gyrotron and the corresponding larger value of external quality factor. The measurements are roughly in agreement with predictions of Adler's phase-locking equation which is given for our system in terms of powers propagating in the output waveguide toward and away from the gyrotron cavity     

Spectral characteristics and synchrony in primary auditory-nerve fibers in response to pure-tone acoustic stimuli

Under pure-tone stimulation, the spectrum of the period histogram recorded from primary auditory-nerve fibers at low and medium frequencies contains components at DC, at the applied tone frequency (the fundamental), and at a small number of harmonics of the tone frequency. The magnitudes and phases of these spectral components are examined. The spectral magnitudes of the fundamental and various harmonic components generally bear a fixed proportionality to each other over a broad range of signal conditions and nerve-fiber characteristics. This implies that the shape of the underlying rectified wave remains essentially unchanged over a broad range of stimulus intensities. For high-frequency stimuli, the fundamental and harmonic components are substantially attenuated. We provide a theoretical basis for the decrease of the spectralcomponent magnitudes with increasing harmonic number. For low-frequency pure-tone signals, the decrease is caused principally by the uncertainty in the position of neural-event occurrences within the half-wave-rectified period histogram. The lower the stimulus frequency, the greater this time uncertainty and therefore the lower the frequency at which the spectral components begin to diminish. For high-frequency pure-tone signals, on the other hand, the decrease is caused principally by the frequency rolloff associated with nervespike time jitter (it is then called loss of phase locking or loss of synchrony). Since some of this jitter arises from noise in the auditory nerve, it can be minimized by using peak detection rather than level detection. Using a specially designed microcomputer that measures the times at which the peaks of the action potentials occur, we have demonstrated the presence of phase locking to tone frequencies as high as 18 kHz. The traditional view that phase locking is always lost above 6 kHz is clearly not valid. This indicates that the placeversus-periodicity dichotomy in auditory theory requires reexaraination.     

光纤激光器自组织相干阵列的理论分析

 分析了光纤激光器自相干阵列最简单的情况——两个光纤激光器之间的相位锁定过程。建立了该过程的数学模型，并对它的时间特性和输出光束的模式进行了理论和数值分析，讨论了运行频率差和耦合系数对锁相过程的影响。对于运行频率相等的两个光纤激光器，当两个光纤激光器初始相位差不为0时，相位锁定后的激光器阵列输出模式为异相模；对于运行频率不等的情形，给出了锁相运行对两个光纤激光器运行频率差的要求，并分析了降低这种要求的几种方法。     

Rydberg原子的电磁诱导透明光谱的噪声转移特性

基于马赫-曾德尔干涉仪和平衡零拍探测技术研究了Cs原子6S_1/2↔6P_3/2↔62D_5/2Rydberg态阶梯型三能级系统电磁诱导透明效应中耦合光场的噪声向探测光场相位噪声的转移特性.实验中探测光频率锁定在Cs原子6S_1/2↔6P_3/2态共振跃迁线上,通过扫描6P_3/2到62D_5/2态跃迁的耦合光频率,测量了Rydberg态电磁诱导透明光谱.利用探测光经过声光调制器后的一级衍射光实现了马赫-曾德尔干涉仪的相位锁定,测量了不同锁定相位情况下的电磁诱导透明光谱,实验结果与阶梯型三能级系统的理论计算结果符合得很好.在此基础上详细研究了耦合光频率共振在6P_3/2到62D_5/2态跃迁线上时,耦合光频率噪声向探测光相位噪声的转移特性,发现耦合光频率噪声转移效率在高频处显示出较明显的抑制.同时观察到耦合光在不同失谐情况时,随着耦合光功率的改变,探测光相位噪声的变化特征表现出明显差异.     

Complex tone processing in primary auditory cortex of the awake monkey. I. Neural ensemble correlates of roughness

Previous physiological studies [e.g., Bieser and Muller-Preuss, Exp. Brain Res. 108, 273-284 (1996); Schulze and Langner, J. Comp. Physiol. A 181, 651-663 (1997); Steinschneider et al., J. Acoust. Soc. Am. 104, 2935-2955 (1998)] have suggested that neural activity in primary auditory cortex (A1) phase-locked to the waveform envelope of complex sounds with low (<300 Hz) periodicities may represent a neural correlate of roughness perception. However, a correspondence between these temporal response patterns and human psychophysical boundaries of roughness has not yet been demonstrated. The present study examined whether the degree of synchronized phase-locked activity of neuronal ensembles in A1 of the awake monkey evoked by complex tones parallels human psychoacoustic data defining the existence region and frequency dependence of roughness. Stimuli consisted of three consecutive harmonics of fundamental frequencies (f(0)s) ranging from 25 to 4000 Hz. The center frequency of the complex tones was fixed at the best frequency (BF) of the cortical sites, which ranged from 0.3 to 10 kHz. Neural ensemble activity in the thalamorecipient zone (lower lamina III) and supragranular cortical laminae (upper lamina III and lamina II) was measured using multiunit activity and current source density techniques and the degree of phase-locking to the f0 was quantified by spectral analysis. In the thalamorecipient zone, the stimulus f0 at which phase-locking was maximal increased with BF and reached an upper limit between 75 and 150 Hz for BFs greater than about 3 kHz. Estimates of limiting phase-locking rates also increased with BF and approximated psychoacoustic values for the disappearance of roughness. These physiological relationships parallel human perceptual data and therefore support the relevance of phase-locked activity of neuronal ensembles in A1 for the physiological representation of roughness.     

Analytical treatment of synchronization of spin-torque oscillators by microwave magnetic fields

An analytical approach is presented for the study of synchronization effects in spin-transfer-driven nanomagnets subjected to radio-frequency magnetic fields. The conditions are derived and discussed under which the current-induced magnetization precession is synchronized by the radio-frequency field. Exact analytical results are obtained for the case when the problem exhibits uniaxial symmetry around the axis perpendicular to the device plane. It is demonstrated that the magnetization dynamics under nonzero current and nonzero rf field is identical in structure to that under zero current. On this basis, analytical predictions are obtained for: the existence of phase-locking between current-induced magnetization precession and rf field oscillations; the frequency pulling effect in proximity of phase locking; the occurrence of hysteresis effects in phase-locking as a function of the spin-polarized current. The proposed approach is valid for arbitrary rf field amplitude and current intensity.     

Autoresonant excitation of multiphase waves in the sine-Gordon model

Autoresonant excitation and control of multiphase waves of the sine-Gordon equation is discussed. The autoresonance (a persistent nonlinear phase-locking) is achieved by applying chirped frequency wave-like perturbations and passage through resonances with inactive (dormant) degrees of freedom in the system. Successful phase locking requires the amplitude of the driving perturbation to exceed a threshold that scales as 3/4 power of the driving frequency chirp rate. The spectral theory of the inverse scattering transform method is used for diagnostics and analysis of the excited solutions.