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排序方式: 共有251条查询结果,搜索用时 15 毫秒
81.
WAVEFORM RELAXATION METHODS AND ACCURACY INCREASEWAVEFORMRELAXATIONMETHODSANDACCURACYINCREASE¥SongYongzhong(NanjingNormalUniv... 相似文献
82.
The asymmetry and fringe inclination of the sawtooth-like waveform observed in the self-mixing signal obtained from a semiconductor laser with optical feedback enable the direction discrimination of line-of-sight motion of external target surfaces. We investigated the effects of linewidth enhancement factor and feedback strength on the asymmetry and the direction of fringe inclination of the signal by numerical simulations. For the first time, we show that direction of the fringe inclination is determined not only by the direction of the target motion but also by the value of the linewidth enhancement factor. 相似文献
83.
利用倍频电路产生十路不同频率的相干信号,通过滤波、移相、幅度调整、加法器、乘法器等电路可实现用多路正弦信号做波形合成、两路信号的同向振动合成,李萨茹图形、调幅波等实验及演示。 相似文献
84.
Erkki Vilkman Eija-Riitta Lauri Paavo Alku Eeva Sala Marketta Sihvo 《Journal of voice》1999,13(2):303-312
The effects of prolonged (5x45 minute) reading (vocal loading) on fundamental frequency (F0), sound pressure level (SPL), subglottal (intraroral) pressure (p), and two glottal flow waveform parameters (AC amplitude of glottal flow, f, and negative peak amplitude of differentiated flow (d) of normal female and male subjects (N = 80) were studied. Two rest (morning and noon) and three loading (two in the morning and one in the afternoon) samples were recorded and analyzed. The glottal waveforms were obtained by inverse filtering of the acoustic pressure waveforms of speaking voice samples. The analyses were based on measurement and inverse filtering of the first stressed syllable of "paappa" words repeated 3x5 times for normal, as soft as possible, and as loud as possible phonation. In normal phonation the parameter values changed statistically significantly due to loading. In many cases the values obtained in the morning samples changed after the first loading session. This is interpreted as a vocal "warming-up effect." Especially in soft phonation p, d, and f were sensitive indicators of vocal loading. In both normal and soft phonation, the SPL, p, d, and f values tended to rise due to prolonged reading in the morning and afternoon samples, indicating increased effort (normal phonation) and a rise in the phonatory threshold (soft phonation). The lunch break vocal rest ("rest effect") considerably affected the parameter values in many cases. 相似文献
85.
基于开关电容矩阵(Switched Capacitor Array, SCA)的波形数字化技术是未来物理实验装置前端读出电子学的重要发展方向之一。本工作设计了一种基于SCA芯片DRS4(Domino Ring Sampler 4)的超高速波形数字化数据采集电路。DRS4单通道可以最高以5 GHz的采样率对探测器输出信号进行全波形采样。该电路主要由模拟调理电路、DRS4电路、ADC(Analog-to-Digital Converter)电路、FPGA(Field-Programmable Gate Array)、DAC(Digital-to-Analog Converter)电路、触发电路和接口电路等组成。性能测试表明,该电路通道的噪声小于0.5 mV,积分非线性 (Integral Non-Linearity, INL) 优于1%。本电路模拟输入带宽高,采样率700 MHz~5 GHz可调,具有良好的非线性。该数据采集电路具有一定的通用性,适用于多种类型的物理实验装置中的探测器读出。 相似文献
86.
并行交替采样(Time-Interleaved Analog-to-Digital Conversion, TIADC)可成倍提升采样系统的采样率,是高速波形数字化系统设计中的一项重要技术。然而不同采样通道间存在失配误差,这会使得系统性能相对于单个ADC明显下降,因此TIADC系统设计中还需要对失配误差进行修正。而对TIADC系统进行科学的性能测试与评估是评价系统设计质量以及推进下一步优化设计的必备环节。针对TIADC系统的性能测试与评估,详细介绍了各项性能指标以及测试方法,完成了一个由2片国产高速ADC组成TIADC系统的一系列性能测试,并利用完美重构算法对该系统的失配误差进行了修正。经测试,该系统可实现5 Gsps的等效采样率,并可基于该算法实现宽带内失配误差修正,修正后系统各项动态性能均得到显著提高,其中有效位(Effective Number of Bits, ENOB)性能达9.2 bits@247 MHz,8.9 bits@857 MHz,与单个ADC性能指标相当。 相似文献
87.
Yu-Bing Li 《中国物理 B》2023,32(1):14303-014303
High-resolution images of human brain are critical for monitoring the neurological conditions in a portable and safe manner. Sound speed mapping of brain tissues provides unique information for such a purpose. In addition, it is particularly important for building digital human acoustic models, which form a reference for future ultrasound research. Conventional ultrasound modalities can hardly image the human brain at high spatial resolution inside the skull due to the strong impedance contrast between hard tissue and soft tissue. We carry out numerical experiments to demonstrate that the time-domain waveform inversion technique, originating from the geophysics community, is promising to deliver quantitative images of human brains within the skull at a sub-millimeter level by using ultra-sound signals. The successful implementation of such an approach to brain imaging requires the following items: signals of sub-megahertz frequencies transmitting across the inside of skull, an accurate numerical wave equation solver simulating the wave propagation, and well-designed inversion schemes to reconstruct the physical parameters of targeted model based on the optimization theory. Here we propose an innovative modality of multiscale deconvolutional waveform inversion that improves ultrasound imaging resolution, by evaluating the similarity between synthetic data and observed data through using limited length Wiener filter. We implement the proposed approach to iteratively update the parametric models of the human brain. The quantitative imaging method paves the way for building the accurate acoustic brain model to diagnose associated diseases, in a potentially more portable, more dynamic and safer way than magnetic resonance imaging and x-ray computed tomography. 相似文献
88.
89.
Yudong YANG;Zhiyi WEI 《光子学报》2022,51(1):155-165
Ultrashort laser pulses are powerful and important tools for scientific researches in many areas in that they allow studying ultrafast dynamics in materials with extreme time resolution. Different experiments across different research fields ask for laser pulses with very different characteristics. Ultrafast laser pulse shaping, where the amplitude, phase or polarization of laser pulses are modulated to fulfill various requirements of different experiments, is widely used. On the other hand, the pure quest of the technology development and the desires for studying even faster dynamics in materials jointly motivate the development of ultrafast laser technology. The record of the shortest pulse duration was continuously renewed. Eventually, ultrafast lasers step into the few cycle regime thanks to the introduction of Ti:Sapphire lasers. When the pulse duration approaches the oscillation period of the laser carrier wave, the differences between few cycle pulses and longer pulses emerge. One of the most notable differences is that even for two few cycle pulses with identical envelopes, the electric fields underneath can be utterly different. Hence, full control over few cycle pulses requires direct control over the electric field, which implies the technological leap from laser pulse shaping to sub-cycle laser field shaping. Sub-cycle laser field shaping technology not only enables full control over laser pulses, but also makes possible direct manipulation of strong-field physics process via tailored optical waveforms, which fundamentally enhances the toolbox for controlling light and matter interaction.Preliminary laser field shaping can be achieved via the Carrier Envelope Phase (CEP) of laser pulses, which is sufficient to significantly affect the electric field and alter the outcomes of light and matter interactions. Therefore, CEP stabilization is crucial for laser field shaping. Currently, CEP locking methods can be categorized into active stabilization and passive stabilization. Active CEP stabilization requires feedback loops which lock the CEP mostly by tuning the inter-cavity group velocity dispersion. In contrast, the passive CEP stabilization exploits the phase relation between different beams in nonlinear optics process, where the idler beam of OPA/DFG is naturally CEP stabilized if the signal beam and the pump beam shares identical CEP fluctuation. Additionally, controlling the spectral phase precisely further enhances the shaping capability that the electric field can be shaped to deviate notably from sinusoidal oscillation. Complete characterization of such few-cycle/single-cycle pulses is indispensable for utilizing them in experiments. Typical ultrashort pulse characterization methods measure the pulse envelopes but the exact shape of the electric fields. New methods which measure the electric field have to be developed. The field-sensitive methods are usually based on high harmonic generation, either by exploiting the process itself or by employing the XUV radiation from HHG.Laser field shaping targets extending the capability of direct electric field control in radio frequency to optical frequency. Customizing optical waveforms builds on the generation of extremely broadband spectrum and precise control of the spectral phase. Since laser pulses with broad bandwidth correspond to pulses which are temporally compressible to very short duration, sub-cycle laser field shaping and sub-cycle laser pulse generation share common technological ground. However, generating spectrum experimentally with bandwidth supporting sub-cycle laser pulses with a single light source is, if not impossible, extremely difficult. On the other hand, coherent combination, or synthesis, of several few-cycle pulses of different colors is the enabling technology for extremely broadband spectrum and intense sub-cycle laser pulses. Different approaches have been proposed along the development of optical waveform synthesis. The optical waveform synthesizer based on noble gas filled hollow-core fibers is one of the most successful attempts, which leads to fruitful results. However, the HCF approach has its own limits which are, e.g. the pulse energy and the bandwidth. To overcome such limits, OP(CP)As are introduced for the waveform synthesis. After conceptual demonstration with small OPAs, the signal beam, the idler beam and even the pump beam of more powerful OP(CP)As are employed for coherent synthesis, which takes advantage of the fact that the beams are inherently synchronized. The full potential of a parametric waveform synthesizer is however yet to explore. Hence, a waveform synthesizer consists of several different OP(CP)As was built, which outputs millijoule level sub-cycle pulses and the waveform can be varied by tuning the synthesis parameters. With the intense sub-cycle pulses, isolated attosecond pulses are directly generated without the assistance of additional gating methods. Moreover, tunable isolated attosecond pulses are conveniently delivered via varying the synthesis parameters. In the meantime, simulations are performed to illustrate the shaping of the generated attosecond pulses by tailored waveforms. 相似文献
90.
采用模压烧结工艺制备不同W含量的Al/W活性材料,基于分离式霍普金森压杆(SHPB)技术,采用紫铜片和橡胶片进行波形整形,研究不同配比Al/W材料的动态压缩和破坏特性。实验结果表明:随着W含量增多,Al/W材料内部的孔洞、微裂纹逐渐增多;W含量不同,Al/W材料的动态压缩变形和破坏特性呈现明显的差异;W的质量分数为44%和64%时,不同应变率下Al/W的应力-应变曲线呈现出弹性-塑性强化的变形特点,破坏应变随应变率增加而增大; W的质量分数为83%的Al/W材料则表现为塑性阶段的应变软化特点;当W的质量分数达到91%时,Al/W材料达到破坏强度后便迅速失效,破坏应变保持在0.03左右。W含量增加时,Al/W变形模式的转化是增强相W和材料内缺陷相互竞争的结果。
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