全文获取类型
收费全文 | 658900篇 |
免费 | 6847篇 |
国内免费 | 1979篇 |
专业分类
化学 | 304041篇 |
晶体学 | 8714篇 |
力学 | 25998篇 |
综合类 | 17篇 |
数学 | 68885篇 |
物理学 | 176548篇 |
无线电 | 83523篇 |
出版年
2021年 | 5361篇 |
2020年 | 5961篇 |
2019年 | 6469篇 |
2018年 | 8363篇 |
2017年 | 8096篇 |
2016年 | 12255篇 |
2015年 | 7531篇 |
2014年 | 11970篇 |
2013年 | 28492篇 |
2012年 | 21631篇 |
2011年 | 26380篇 |
2010年 | 18940篇 |
2009年 | 19732篇 |
2008年 | 25845篇 |
2007年 | 26538篇 |
2006年 | 24999篇 |
2005年 | 22786篇 |
2004年 | 21139篇 |
2003年 | 19012篇 |
2002年 | 18573篇 |
2001年 | 20550篇 |
2000年 | 16423篇 |
1999年 | 13279篇 |
1998年 | 11364篇 |
1997年 | 11088篇 |
1996年 | 10519篇 |
1995年 | 9710篇 |
1994年 | 9484篇 |
1993年 | 9096篇 |
1992年 | 9942篇 |
1991年 | 9957篇 |
1990年 | 9478篇 |
1989年 | 8944篇 |
1988年 | 8738篇 |
1987年 | 8034篇 |
1986年 | 7521篇 |
1985年 | 9611篇 |
1984年 | 9824篇 |
1983年 | 7933篇 |
1982年 | 8082篇 |
1981年 | 8064篇 |
1980年 | 7501篇 |
1979年 | 7863篇 |
1978年 | 8138篇 |
1977年 | 7985篇 |
1976年 | 7888篇 |
1975年 | 7338篇 |
1974年 | 7287篇 |
1973年 | 7451篇 |
1972年 | 5116篇 |
排序方式: 共有10000条查询结果,搜索用时 32 毫秒
991.
A new approach is suggested to reduce the optical beat interference (OBI) in subcarrier multiplexed (SCM) wavelength-division multiple access (WDMA) networks. The idea is to deliberately introduce independent random polarization fluctuations in the electric fields transmitted on each optical channel. Random polarization results in an expanded OBI spectrum, and hence, less OBI power at the reference user receiving filter output. Electro-optic phase modulation is used to introduce polarization randomness in the fields before they are coupled into the fibers, A two-user system was simulated. Simulation results show the drastic reduction in OBI power spectral density using appropriate PN signals 相似文献
992.
The impacts of CVD tungsten polycide (WSix) on MOSFET performance and reliability are studied in this letter. The WSix process is shown to enhance the S/D lateral extent for both N- and P-channel devices via CGD and Leff measurements, confirming previous suspicion. This enhanced S/D extent is found to be easily modulated by drain-to-gate bias, which is favorable for achieving both higher drive currents and higher S/D punch-through voltages than those of non-WSix devices. Both electron and hole mobility for the WSix device are also slightly higher and closer to the published data compared to the non-WSix case. These effects together yield about >5% improvement for nMOSFET and >10% improvement for pMOSFET in drive current at a given punch-through voltage. The channel hot-electron lifetime for the n-channel WSix device is about 10 times higher than that of the non-WSix one. These enhancements in both performance and reliability make the WSix device very attractive fog VLSI CMOS technologies 相似文献
993.
Sousa E.S. Jovanovic V.M. Daigneault C. 《Vehicular Technology, IEEE Transactions on》1994,43(4):837-847
The paper describes a set of measurements performed in the Toronto area, in order to assess the impact of multipath propagation on the performance of the TIA IS-54 digital standard. Five existing cells were systematically surveyed: two in downtown Toronto, two in the suburbs, and one in a suburban/rural area. A sweeping correlator apparatus with a 0.1 μs resolution and 910 MHz carrier frequency, and an omnidirectional antenna was used. In one of the cells, the measurements were repeated with a 60 degree beamwidth directional antenna. A fairly sophisticated thresholding technique was applied in order to reduce the impact of noise. Generally, the measurements with the omnidirectional antenna exhibit multipath propagation with considerably smaller excess delays than some reported, but are consistent with earlier results obtained in the U.S. and Europe. Sectorization was found to considerably reduce the multipath effects. Very large delay components appear to be due mostly to the combination of large transmitter-receiver distances and a large degree of shadowing, and could probably be avoided to a large extent by standard cellular engineering techniques. In view of these results, it seems that the long delay (high selectivity) problem has been somewhat overemphasized in the past, at least for areas like Toronto. Problems associated with short delays (flat fading), on the contrary, appear to have been underestimated in the case of dense urban environments 相似文献
994.
Investigation of multicarrier modulation has demonstrated the feasibility of using this modulation format to deliver relatively high channel bit rates (24 kbps) to a low cost, pager-like personal communications service (PCS) terminal that enjoys long battery life (e.g., one month on a few pen-light cells). This is to be compared to the 1200 bps carried by typical current paging systems or the 6250 bps attained by the new ERMES paging system. While more complex modulation formats, such as PSK and QAM, might achieve higher bit rates in less (power, size, cost) constrained environments, amplitude keying of the component subcarriers allows simple noncoherent energy detection that is found to be robust in this short message delivery application 相似文献
995.
Golestaneh S. Hafez H.M. Mahmoud S.A. 《Vehicular Technology, IEEE Transactions on》1994,43(4):946-954
In FDMA cellular radio systems, the adjacent channel interference (ACI) is suppressed by the IF filter and also by a distance factor. Channels immediately adjacent to each other in frequency are assigned to different cells and, as such, the distance plays a role in reducing their mutual interference. The authors study the feasibility of optimizing the frequency spacing between carriers such that the combined effect of distance and IF filter will be enough to control the interference while allowing the maximum number of channels in a given bandwidth. They also study the decision feedback equalizer (DFE) as a means to reduce any excess ACI that results from reducing the channel spacing. The paper starts with a model that estimates the cumulative probability density of the signal to adjacent channel interference ratio (SIR) as a function of the channel spacing. They then present the results of a computer simulation study to estimate the performance of DFE in mitigating the effect of ACI. The results indicate that, under the conditions of slow flat fading and selection diversity, the DFE is very effective in reducing the effect of excess ACI interference which allows for a considerable reduction in the frequency spacing 相似文献
996.
Statistical theory and its applications have played a significant role in the advancement of methodology and techniques for solving various problems in reliability engineering. Different criteria of aging have been introduced for modelling and characterizations of wear properties. Some real life engineering reliability operations arising in maintenance and replacement policies formulations are investigated as convolution, mixture and formation of coherent systems. Shock models are studied for shocks arriving according to Poisson process and causing damages with specific form of aging. Cumulative damage models have been also studied by many researchers. Statistical hypotheses testing are used to test whether a certain, censored or uncensored data has exponential or a specific aging propety.In this paper we present a unified state of the art type of survey for the research and developments in these problems. In particular we expose the main recent contributions, trace the trend of research and highlight the possible direction of further research. 相似文献
997.
G.W.A.D. 《Microelectronics Reliability》1994,34(11)
998.
A generalized formulation of several circuit design problems, such as manufacturing yield optimization, circuit performance variability minimization, deterministic and statistical minimax design, Income Index maximization, Taguchi approach, etc., is developed. Several other “intermediate” problems can be defined in a sense similar to the one used in Zadeh's fuzzy set theory. A specific problem is identified by the selection of a generalized membership function ω(·) of the acceptability region, and a sequence of the values of the “smoothing” parameter β. Generalized gradient formulas are developed, and various possible algorithmic implementations discussed. As a result, trade-offs between different design strategies can be investigated by circuit designers, within one coherent methodology. 相似文献
999.
Measures of direct importance of unrepairable system components, based on the multilinear expansions, are introduced. Various characteristics of system quality of performance are discussed. Cases of binar and multistate components are studied. The measure of specific importance, considering the cost of reliability component's improvement, is defined. The multilinear expansion for a system of multistate components, giving the possibility to estimate measures of the direct importance of components and levels of components performance as well, is discussed. The limit transition to continuum structures is performed. Some measures of importance for repairable systems are introduced. 相似文献
1000.