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排序方式: 共有1307条查询结果,搜索用时 15 毫秒
1.
This work aims to shed more light on the issue of why smart antennas haven't yet penetrated the market despite all the research efforts that have been put into the technology. The paper looks at this issue from various perspectives and subdivides the discussions into fixed or nomadic and high mobility applications because of the different types of markets, demands, and technological barriers. 相似文献
2.
R. H. Howell P. A. Sterne M. J. Fluss J. H. Kaiser K. Kitazawa H. Kojima 《Journal of Superconductivity and Novel Magnetism》1994,7(1):169-173
High-statistics (>4 × 108 counts), room-temperature measurements of the electron-positron momentum density of La2?x Sr x CuO4 have been performed for samples with Sr concentrations of x=0.0, 0.1, 0.13, and 0.2. These spectra have been analyzed in conjunction with theoretical calculations of the electron-positron momentum density. The metallic samples show features consistent with the presence of a Fermi surface, but its evolution with increasing Sr concentration does not follow the predictions of band theory. These results may indicate the effects of electron-electron correlation on the electron momentum distribution in the Cu-O plane. 相似文献
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U. H. Pi D. H. Kim Z. G. Khim U. Kaiser M. Liebmann A. Schwarz R. Wiesendanger 《Journal of Low Temperature Physics》2003,131(5-6):993-1002
We have studied vortex dynamics in Bi2Sr2CaCu2O8 single crystal with low density columnar defects by using a magnetic force microscope. Single crystal Bi2Sr2CaCu2O8 sample was irradiated by 1.3 GeV uranium ion to form artificial pinning centers along the crystalline c-axis. The irradiation dose corresponded to a matching field of 20 gauss. The radius of an individual vortex is approximately 140 nm, which is close to the penetration depth of this material. Magnetic force microscope (MFM) images show that intrinsic crystalline defects such as stacking fault dislocations are very effective pinning centers for vortices in addition to the pinning centers due to ion bombardment. By counting the number of vortex, we found that the flux trapped at each pinning center is a single flux quantum. At higher magnetic field, the vortex structure showed an Abrikosov lattice disturbed only by immobile vortices located at pinning centers. When increasing or decreasing the external magnetic field, the spatial distribution of vortices showed a Bean model like behavior. 相似文献
5.
METEOR: a constraint-based FIR filter design program 总被引:3,自引:0,他引:3
It is proposed to specify a filter only in terms of upper and lower limits on the response, find the shortest filter length which allows these constraints to be met, and then find a filter of that order which is farthest from the upper and lower constraint boundaries in a minimax sense. The simplex algorithm for linear programming is used to find a best linear-phase FIR filter of minimum length, as well as to find the minimum feasible length itself. The simplex algorithm, while much slower than exchange algorithms, also allows the incorporation of more general kinds of constraints, such as concavity constraints (which can be used to achieve very flat magnitude characteristics). Examples are given to illustrate how the proposed and common approaches differ, and how the proposed approach can be used to design filters with flat passbands, filters which meet point constraints, minimum phase filters, and bandpass filters with controlled transition band behavior 相似文献
6.
Physical wavelets are acoustic or electromagnetic waves, resulting from the emission of a time signal by a localized acoustic or electromagnetic source moving along an arbitrary trajectory in space. Thus, they are localized solutions of the wave equation or Maxwell's equations. Under suitable conditions, such wavelets can be used as “basis” functions, to construct general acoustic or electromagnetic waves. This gives a local alternative to the construction of such waves in terms of (nonlocal) plane waves, via Fourier transforms. We give a brief, self-contained introduction to physical wavelets, and apply them to remote sensing. We define the ambiguity functional, generalization of the radar and sonar ambiguity functions, which applies not only to wideband signals, but also to targets and radar platforms executing arbitrary nonlinear motions 相似文献
7.
Hansen J.H.L. Gavidia-Ceballos L. Kaiser J.F. 《IEEE transactions on bio-medical engineering》1998,45(3):300-313
Traditional speech processing methods for laryngeal pathology assessment assume linear speech production with measures derived from an estimated glottal flow waveform. They normally require the speaker to achieve complete glottal closure, which for many vocal fold pathologies cannot be accomplished. To address this issue, a nonlinear signal processing approach is proposed which does not require direct glottal flow waveform estimation. This technique is motivated by earlier studies of airflow characterization for human speech production. The proposed nonlinear approach employs a differential Teager energy operator and the energy separation algorithm to obtain formant AM and FM modulations from filtered speech recordings. A new speech measure is proposed based on parameterization of the autocorrelation envelope of the AM response. This approach is shown to achieve impressive detection performance for a set of muscular tension dysphonias. Unlike flow characterization using numerical solutions of Navier-Stokes equations, this method is extremely computationally attractive, requiring only a small time window of speech samples. The new noninvasive method shows that a fast, effective digital speech processing technique can be developed for vocal fold pathology assessment without the need for direct glottal flow estimation or complete glottal closure by the speaker. The proposed method also confirms that alternative nonlinear methods can begin to address the limitations of previous linear approaches for speech pathology assessment 相似文献
8.
The growth structure of MgF2 and NdF3 films grown on polished CaF2(111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF2 and NdF3 films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K. 相似文献
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