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991.
本文提出了一种周期信号等效非均匀采样策略。利用均匀采样方法得到的等效非均匀采样,易于硬件实现。文中推导了非均匀采样的插值公式,并证明了在信号恢复和谐波分析中,非均匀采样策略具有均匀采样方法无可比拟的优越性。 相似文献
992.
993.
994.
Cynthia A. Powell-Doan Arthur H. Heuer 《Journal of the American Ceramic Society》1990,73(12):3670-3676
Characterization of the microstructure and microchemistry of a group of commercial 96% Al2 O3 ceramics, using analytical and conventional electron microscopy techniques, was conducted. A continuous glassy grain-boundary phase was found in all samples, in addition to a number of intragranular and intergranular crystalline second phases; the phases present depended on the original boundary-glass composition. The faceted nature of many of the Al2 O3 -glass interfaces was studied and is thought to be an equilibrium structure. 相似文献
995.
996.
Barry A. Bell 《Journal of research of the National Institute of Standards and Technology》1990,95(4):377-405
Over the last decade the use of digital synthesis and sampling techniques for generating and measuring electrical waveforms has increased dramatically with the availability of improved digital-to-analog (D/A) and analog-to-digital (A/D) converters and related devices. With this evolution has come the need for physical laboratory standards and test methods to support the performance specifications of digital devices and the instruments in which they are used. This article describes the research and development at NIST of several laboratory standards and test systems that utilize “digital technology” for characterizing data converters and for implementing various waveform synthesis and sampling instruments. 相似文献
997.
998.
电站汽轮机轴承座二次灌浆质量影响汽轮机振动,特别是轴承座底板与二次灌浆层接触面的密实度对振动的影响最大。实际施工中,由于各种因素造成二次灌浆层质量问题时有发生,返工重做是最直接、最可靠的处理措施,但如何安全地把设备从二次灌浆层中分离取出一直是工程难题。介绍了采用二次灌浆层取芯隔离安全修复技术成功解决了工程难题。 相似文献
999.
Human CD4 is the receptor for human immunodeficiency virus (HTV).It is well established that the first domain of CD4 binds withhigh affinity to gp120, an envelope protein of HIV, but it hasalso been demonstrated that amino acids located in its seconddomain, within or close to residues 120127 or 163166(lying 15 Å away from the binding site), play a role invirus infectivity. We show here that these two stretches ofamino acids happen to be important for the largest amplitudemotion obtained with the normal-mode theory for the two N-terminaldomains of human CD4: an overall rigid-body displacement ofone domain with respect to the other. Such a hinge-bendingmotion is unexpected since these two domains were found by crystallographersto be tightly abutting. On the other hand, since for severalproteins the hinge-bending motion experimentally observed uponligand binding was found to be similar to the largest amplitudemotion obtained with the normal-mode theory for these proteins,our results suggest that CD4 may undergo such a kind of conformationalchange upon HTV binding. 相似文献
1000.
Acoustic determination of ideal-gas heat capacity and second virial coefficients of some refrigerants between 250 and 420 K 总被引:1,自引:0,他引:1
An apparatus for speed-of-sound measurements with a spherical resonator was adapted for temperatures up to 42(I K. This included new microphones with a special wiring, a pressure indicator which can be thermostatted to 420 K, and some installations to avoid temperature gradients. Calibration of the radius of the resonator with argon was extended to higher temperatures. Speed-of-sound measurements up to 420 K and 0.5 MPa were done onl,l-dilluoroethane (R152a). 1.l,l-trilluoroethane (R 143a ),l,l,l-chlorodifluoroethane (R 142b ), l,1,1,2-tetralluoroethane (R134a), and 2.2.2-trifluoroethanol. The ideal-gas heat capacities coincide with the statistical mechanical values, except for R134a, where our values as well as recent literature data are below the values calculated from spectroscopy. The reduced second virial coefficients can be interpreted in terms of the dipole moment and the angle between dipole moment and molecular axis. For the associated substance trifluoroethanol values of the third virial coefficient are given, which are appreciably negative at low temperatures.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994 Boulder, Colorado, U.S.A. 相似文献