全文获取类型
收费全文 | 24969篇 |
免费 | 3865篇 |
国内免费 | 3337篇 |
专业分类
化学 | 12807篇 |
晶体学 | 331篇 |
力学 | 1068篇 |
综合类 | 252篇 |
数学 | 1991篇 |
物理学 | 5955篇 |
无线电 | 9767篇 |
出版年
2024年 | 54篇 |
2023年 | 417篇 |
2022年 | 646篇 |
2021年 | 812篇 |
2020年 | 773篇 |
2019年 | 854篇 |
2018年 | 762篇 |
2017年 | 833篇 |
2016年 | 984篇 |
2015年 | 1187篇 |
2014年 | 1435篇 |
2013年 | 1709篇 |
2012年 | 2050篇 |
2011年 | 2138篇 |
2010年 | 1679篇 |
2009年 | 1722篇 |
2008年 | 1847篇 |
2007年 | 1706篇 |
2006年 | 1560篇 |
2005年 | 1357篇 |
2004年 | 1232篇 |
2003年 | 1003篇 |
2002年 | 1133篇 |
2001年 | 1035篇 |
2000年 | 708篇 |
1999年 | 554篇 |
1998年 | 428篇 |
1997年 | 252篇 |
1996年 | 254篇 |
1995年 | 195篇 |
1994年 | 185篇 |
1993年 | 144篇 |
1992年 | 116篇 |
1991年 | 81篇 |
1990年 | 77篇 |
1989年 | 58篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 33篇 |
1985年 | 26篇 |
1984年 | 20篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1957年 | 2篇 |
1936年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
31.
Jianhua Liu Jian Li 《Mobile Computing, IEEE Transactions on》2006,5(9):1133-1142
We consider improving the performance of orthogonal frequency-division multiplexing (OFDM)-based mobile broadband wireless communication (MBWC) systems. We show that a recently approved packet-based MBWC standard can cause the resulting systems to suffer from severe performance degradations for time-varying channels due to the lack of a mechanism for tracking the time-varying channels needed for coherent detection. We consider both packet design and signal processing to deal with time-varying channels. For the packet design, we segment an entire packet into multiple subpackets—with each subpacket having zero tail bits to reset the convolutional channel encoder—so that the detection/decoding result of each subpacket can be used to update the channel response for this subpacket. For signal processing, we use weighted polynomial fitting and prediction at the receiver to improve the channel tracking accuracy. Simulation results show that the coherent detection based on our new scheme can significantly outperform the commonly suggested differential modulation/detection methods. 相似文献
32.
Stewart C. Kai Shen Dwarkadas S. Scott M.L. Jian Yin 《Distributed Systems Online, IEEE》2004,5(10):1-1
The growth of the Internet and of various intranets has spawned a wealth of online services, most of which are implemented on local-area clusters using remote invocation (for example, remote procedure call/remote method invocation) among manually placed application components. Component placement can be a significant challenge for large-scale services, particularly when application resource needs are workload dependent. Automatic component placement has the potential to maximize overall system throughput. The key idea is to construct (offline) a mapping between input workload and individual-component resource consumption. Such mappings, called component profiles, then support high-performance placement. Preliminary results on an online auction benchmark based on J2EE (Java 2 Platform, Enterprise Edition) suggest that profile-driven tools can identify placements that achieve near-optimal overall throughput. 相似文献
33.
Hogari K. Tetsutani S. Jian Zhou Yamamoto F. Sato K. 《Lightwave Technology, Journal of》2003,21(2):540-545
Many cables containing 1.3-/spl mu/m zero-dispersion single-mode (SM) optical fibers are installed in trunk and access networks. Recently, there have been a number of studies on wavelength-division-multiplexing (WDM) systems designed to increase transmission capacity and flexibility. If we can construct WDM systems using SM optical-fiber cable networks designed to transmit using wavelengths in the 1.3-/spl mu/m window (O-band), this will prove very effective in reducing construction costs. It is therefore important to examine the wavelength dependence of the transmission characteristics of SM optical-fiber cables and networks that have already been installed and in which several optical fibers are joined. In this paper, we describe the measured optical characteristics of SM optical-fiber cables and installed optical-fiber cable networks at various wavelengths. The optical characteristics were stable in the 1.46 to 1.625-/spl mu/m wavelength range and we confirmed that the installed SM optical-fiber cable networks could be used for WDM system applications. 相似文献
34.
35.
一种基于PWM的电压输出DAC电路设计 总被引:16,自引:1,他引:15
对实际应用中的脉宽调制(PWM)波形的频谱进行了理论分析.指出通过一个低通滤波器可以把PWM调制的数模转换信号解调出来.实现从PWM到DAC的转换。论文还对转换误差产生的因素进行了分析,指出了减少误差的方法,论文给出了两种从PWM到0~5V电压输出的电路实现方法,第2种电路具有很高的转换精度。 相似文献
36.
37.
38.
用矩阵光学理论计算了由主振荡器和功率放大器组成的种子注入光腔。主振荡器光腔是由曲率半径R1=-250mm,R2=5000mm,相距L=2375mm的球面镜组成的正支非稳腔;功率非稳腔是由曲率半径R3=400mm,R4=5600mm,相距S=3000mm的球面镜构成的负支非稳腔。理论计算表明,该种子注入光腔可输出光束发散角约几十微弧的激光,可满足铜蒸气激光器主振荡器功率放大器(MOPA)的技术要求。 相似文献
39.
40.
深亚微米MOSFET衬底电流的模拟与分析 总被引:1,自引:0,他引:1
利用器件模拟手段对深亚微米MOSFET的衬底电流进行了研究和分析,给出了有效的道长度,栅氧厚度,源漏结深,衬底掺杂浓度以及电源电压对深亚微米MOSFET衬底电流的影响,发现电源电压对深亚微米MOSFET的衬底电流有着强烈的影响,热载流子效应随电源电压的降低而迅速减小,当电源电压降低到一定程度时,热载流子效应不再成为影响深亚微米MOS电路可靠性的主要问题。 相似文献