频率

This dissertation studies several kinds of instantaneous frequency estimation methods, pay much attention to the performance of instantaneous frequency estimation algorithm based on the peak of time-frequency distribution. It gives the theory and algorithm of polynomial Wigner-Ville distribution, analyzes its performance to the noise of the moving target in the air.

对常用的几种瞬时频率估计算法进行了研究，重点研究了基于时频分布峰值的瞬时频率估计算法的性能，给出了多项式Wigner-Ville分布的原理与算法，分析了在直升机定位这种特定情况下的有效性。

In order to improve the frequency resolution, realize high precision frequency estimation, this thesis proposes the high resolution polynomial Wigner-Ville distribution using Chirp-Z transform, the validity of this algorithm is verified using synthesized data.

为了提高基于FFT的谱分析算法的分辨率，实现高精度的频率估计，提出了利用Chirp-Z变换的频率高分辨多项式Wigner-ville分布，并用合成的仿真数据，验证了上述算法的有效性和正确性。

It gives performance comparison of instantaneous frequency estimation algorithms based on the peak of Wigner-Ville distribution and fourth-order polynomial Wiger-Ville distribution, analyzes the influence of window width to the estimation performance of instantaneous frequency estimation algorithms based on time-frequency distribution.

在不同飞行状态下，给出了基于Wigner-Ville分布、四阶多项式Wigner-Ville分布峰值的瞬时频率估计算法的性能对比，并分析了窗长对瞬时频率估计性能的影响。

In frequency calculation, the paper takes the positive sequence component as the foundation data, uses the Fourier frequency measurement principle to measure the frequency, thus enhances the frequency measurement stability and precision.

在频率计算上，论文采用以正序分量为基础数据，利用富氏测频原理测算频率，从而提高了测频的稳定度和精度。

External regulator of positive feedback is variation of precipitation amount and frequency of extreme precipitation. It is shown that desertification is predetermined by the decreased precipitation as well as by the decrease in frequency of precipitation. In the second half of 20th century importance of man-induced degradation of dry lands as additional regulator of positive feedback has been increased.

正反馈的外部调节因子是降水量的变化和极端降水事件的出现频率，降水量减少以及降水频率的降低直接影响沙漠化的程度。20世纪下半叶，人类因素导致的干旱土地退化作为另外的正反馈调节因子，其重要性正在增加。

In this topic, the dynamic analysis methods for piezoelectric vibrator are studied systematically based on the theoretical model, FEM numerical experimentation and FEM governing equation for given compound-mode vibrator, and some valuable conclusions are obtained. The main work accomplished is summarized as follows: 1.Elaborate the main modeling methods for piezoelectric vibrator and the significance and necessity to study the dynamic characteristics of piezoelectric vibrator which emphasize the urgency of this paper. 2.Take the bending deformation induced by piezoelectric ceramic as example, the energy transfer mechanism of electric energy to mechanical energy are analyzed; the motion and force transfer mechanism are analyzed for the longitudinal-bending vibrator. 3.Based on mode assumption and Hamilton principle, the coupling model of piezoelectric vibrator of linear USM is built; moreover, the equivalent circuit model is obtained and a coupling equation represents the relation between electric parameters and mechanical parameters is derived which provides foundation to match the vibrator and driving circuit. 4.Combine the constitutive equation of piezoelectric ceramic with elastic-dynamical equation, geometric equation in force field and the Maxwell equation in electric field and the corresponding boundary condition equation, the FEM control equation for piezoelectric vibrator of USM to solve dynamic electro-mechanical coupling field is established by employing the principle of virtual displacement. The equation lays the foundation to study the non-linear constitutive equation of piezoelectric ceramic driven by high-power. 5.Define the dynamic indexes of characteristic of vibrator and carry out variable parameters simulation by calculating the model parameters and the electric characteristics of vibrator are simulated according to the equivalent circuit model. By numerical experimentation, the working mode of vibration of vibrator and the shock excitation results of the working frequency band which provides the mode frequency to realize bimodal are analyzed. Detailed calculation of the electro-mechanical coupling field parameters is made by programming the FEM control equation.

本课题从理论模型、有限元数值试验、有限元控制模型等方面以复合振动模式振子为例对超声电机压电振子的动力学特性及其分析方法进行了全面系统地研究，得出了许多有价值的结论，主要概括如下： 1、阐述了目前针对超声电机压电振子的主要建模方法，对压电振子动态特性的研究意义和必要性进行了论述，突出了本文研究内容的迫切性； 2、以压电陶瓷诱发弹性体发生弯曲变形为例，分析了压电陶瓷通过诱发应变来实现机电能量转换的机理；对基于纵弯模式的压电振子的运动及动力传递机理进行了分析； 3、基于模态假定，利用分析动力学的Hamilton原理，建立了面向直线超声电机压电振子的机电耦合动力学模型，并据此建立了压电振子的等效电路模型，导出了电参量与动力学特性参量的耦合方程，为压电振子与驱动电路的匹配提供了依据； 4、从压电陶瓷的本构方程出发，综合力场的弹性动力学方程、几何方程、电场的麦克斯韦方程以及相应的边界条件方程，采用虚位移原理，建立了压电振子动态问题机电耦合场求解的有限元控制方程，为研究其大功率驱动下的非线性本构模型奠定了基础； 5、界定压电振子的动力学特性指标，对压电振子的机电耦合动力学模型参数进行计算及变参数仿真；依据等效电路模型，对压电振子的电学特性进行了仿真分析；通过有限元数值实验，对压电振子工作模态附近的模态振型及工作频率附近的频段进行了激振效果分析，找出了实现模态简并的激振频率；利用有限元控制方程，通过编程计算，对压电振子的力电耦合场参数进行了详细计算，得出了一些有价值的结论。

Locking laser source to a proper frequency standard can obviously improve the laser's frequency stability.

将激光器锁定到合适的参考频率标准上，可以有效地改善激光器的频率稳定性。

It is favorable on the increase of leaf chlorophyll content,accumulation of day matter and WUE of cucumber under the proper irrigation frequency under drip irrigation manner and irrigation frequency six day.

适宜的灌溉频率也即是在滴灌条件下灌溉频率为6天对于增加叶片的叶绿素含量、干物质积累和水分利用率是有利的。

Experiments show that the micromechanical tunable resonator has a resonant frequency of 4.8 kHz and a Q-factor of 500, and the tunable-range of resonant frequency is about 6.8% at the tuning voltage of 0-25 V.

梳状式可调变频率微共振器的Q值可达500，共振器共振频率约为4.8kHz，调变电压在0-25V时，可调变率达6.8%。

During these processes, the system needs to meet the Lamb-Dicke limit, and the Rabi oscillation frequency should be far smaller than the phonon frequency.

在此过程中，系统需满足Lamb-Dicke极限，并要求光场的Rabi振荡频率远远小于离子的振动频率。

I am accused of being overreligious," she said in her quiet, frank manner,"but that does not prevent me thinking the children very cruel who obstinately commit such suicide.""

客人们在卡罗利娜·埃凯家里，举止就文雅一些，因为卡罗利娜的母亲治家很严厉。

Designed by French fashion house Herm è s, this elegant uniform was manufactured in our home, Hong Kong, and was the first without a hat.

由著名品牌 Herm è s 设计，这件高贵的制服是香港本土制造，是我们第一套不配帽子的制服。