窗口Fourier变换反演公式的级数表示——献给陆善镇教授75华诞

本文研究连续窗口Fourier变换的反演公式.与经典的积分重构公式不同,本文证明当窗函数满足合适的条件时,窗口Fourier变换的反演公式可以表示为一个离散级数.此外,本文还研究这一重构级数的逐点收敛及其在Lebesgue空间的收敛性.对于L^2空间,本文给出重构级数收敛的充分必要条件.     

Necessary conditions for Schatten class localization operators

We study time-frequency localization operators of the form , where is the symbol of the operator and are the analysis and synthesis windows, respectively. It is shown in an earlier paper by the authors that a sufficient condition for , the Schatten class of order , is that belongs to the modulation space and the window functions to the modulation space . Here we prove a partial converse: if for every pair of window functions with a uniform norm estimate, then the corresponding symbol must belong to the modulation space . In this sense, modulation spaces are optimal for the study of localization operators. The main ingredients in our proofs are frame theory and Gabor frames. For and , we recapture earlier results, which were obtained by different methods.

     

随机激励下受热翼面的模态频率特性试验研究

现代高速飞行器结构热模态频率特性试验研究,对这类飞行器设计校核和飞行安全具有重要意义。根据飞行过程中遭受的气动加热特性设计了瞬态热环境模拟系统,同时,根据高温环境的特点对测试中的激励和测量方式进行了重新设计,成功地将普通激振器应用于高温结构模态试验,最终将热环境模拟系统与振动测试系统组合,形成一套考虑瞬态热影响的热模态试验系统,实现了瞬态热环境下结构模态的地面测试。对一个切尖三角翼测量了各个加热区的温度随加热时间的变化,验证了加热温度控制的精确性;在纯随机激励下对测得的激励和振动响应信号采用短时傅里叶变换(Short Time Fourier Transformation,STFT)进行时变模态参数辨识,获得了前四阶模态频率随加热时间的变化,并与结构有限元数值计算结果进行了比较,试验与计算结果吻合得很好,验证了该试验方法对热模态测试问题的有效性和准确性。通过分别对瞬态和稳态热环境下结构模态频率试验和计算结果的分析,探讨了结构瞬态温度场对模态频率影响的机理,揭示了结构内部存在的热应力和材料属性的变化,是决定模态频率随加热时间变化趋势的内在原因。     

Short-Time Decoherence of Solid-State Qubit at Optimal Operation Points

We investigate the short-time decoherence of a solid-state qubit under Ohmic noise at optimal operation points. The decoherence is analyzed by maximum norm of the deviation density operator. It is shown that at the temperature T = 3 mK, the loss of the fidelity due to decoherence is much smaller than the DiVincenzo low decoherence criterion, which means that the mode/may be an optimal candidate of qubit for quantum computation.     

The studies of high-temperature and short-time annealing,phase transition process,and magnetic property for LaFe11.7Si1.3 compound

The high-temperature phase transition is analyzed according to the DSC of as-cast LaFe_11.7 Si_1.3 compound and the X-ray patterns of LaFe_11.7Si_1.3 compounds prepared by high-temperature and short-time annealing. Large amount of 1:13 phase begins to appear in LaFe_11.7Si_1.3 compound annealed near the melting point of LaFeSi phase (about 1422?K). When the annealing temperature is close to the temperature of peritectic reaction (about 1497?K), the speed of 1:13 phase formation is the fastest. The phase relation and microstructure of the LaFe_11.7Si_1.3 compounds annealed at 1523?K (5?h), 1373?K (2?h)?+?1523?K (5?h), and 1523?K (7?h) +1373?K (2?h) show that longer time annealing near peritectic reaction is helpful to decrease the impurity phases. For studying the influence of different high-temperature and short-time annealing on magnetic property, the Curie temperature, thermal, and magnetic hystereses, and the magnetocaloric effect of LaFe_11.7Si_1.3 compound annealed at three different temperatures are also investigated. Three compounds all keep the first order of magnetic transition behavior. The maximal magnetic entropy change ΔS_M (T, H) of the samples is 12.9, 16.04, and 23.8?J?kg^?1?K^?1 under a magnetic field of 0–2?T, respectively.     

无源雷达目标信号时延与多普勒频率联合估计

提出一种基于STFRFT的无源雷达目标时延与多普勒联合估计新方法。首先,给出了一种STFRFT时频分辨率量化方法及其数学表达式;其次,利用STFRFT的投影包络对回波信号时延与Doppler频移进行了联合估计;最后,讨论分析了脉内信噪比、高斯窗参数等对所提方法估计性能的影响。仿真结果表明,与CAF方法相比,所提方法更容易获得高精度的时延与Doppler频移的联合估计,具有更高的可靠性。     

二分量胶体悬浮系统的短时间动力学

研究了二分量带电粒子悬浮系统的短时间平动和转动自扩散系数.由于存在静电相互作用和流体力学作用,扩散系数与两种粒子的尺寸比,它们的体积分数,以及所带的有效电荷都有关.计入了流体力学相互作用对扩散张量的二体贡献和首项三体贡献.计算结果表明,流体力学作用对于带电粒子系统的影响要小于它对硬球粒子系统的影响.扩散系数随两种粒子的尺寸比和它们的体积分数变化的关系可以用有效硬球模型来解释,而其定性结果与实验相符合.     

传输高稳定原子钟信号的光纤模拟通信系统

采用光纤模拟通信技术,对某微波通信站高稳定的原子钟信号进行光纤传输,设计专用光纤模拟通信设备.并分析影响原子钟信号短期频率稳定度的主要因素.同时给出减少影响的有效方法.通过设备研制和实验对比.结果证明.经光纤模拟传输后.原子钟信号的短期频率稳定度达到某微波通信站的应用要求.     

一种改进的跳频信号参数估计方法

针对在跳频信号跳变时刻和跳变频率估计方面实时性和估计精度无法同时兼顾的问题,提出了一种基于短时傅立叶变换(STFT)和多重信号分类(MUSIC)算法的跳频信号参数估计方法。在建立跳频信号数学模型的基础上,利用STFT选取较大时间窗对整个信号在时域进行粗搜索,生成时频谱图,提取时频脊线从而获得跳变时刻,然后选取较小时间窗在已知跳变时间段利用STFT进行跳变时刻的细估计,并利用MUSIC算法进行频率的精确估计。该方法利用STFT的二次估计,减少了MUSIC搜索范围,从而降低了时间开销。仿真表明该算法的跳变时刻频率估计精度高,实时性能满足参数测量需求。