全文获取类型
收费全文 | 31435篇 |
免费 | 2721篇 |
国内免费 | 1506篇 |
专业分类
电工技术 | 2017篇 |
技术理论 | 2篇 |
综合类 | 2082篇 |
化学工业 | 4901篇 |
金属工艺 | 1493篇 |
机械仪表 | 1995篇 |
建筑科学 | 3018篇 |
矿业工程 | 821篇 |
能源动力 | 828篇 |
轻工业 | 2471篇 |
水利工程 | 533篇 |
石油天然气 | 1355篇 |
武器工业 | 232篇 |
无线电 | 3698篇 |
一般工业技术 | 3534篇 |
冶金工业 | 2273篇 |
原子能技术 | 399篇 |
自动化技术 | 4010篇 |
出版年
2024年 | 149篇 |
2023年 | 568篇 |
2022年 | 1049篇 |
2021年 | 1438篇 |
2020年 | 1036篇 |
2019年 | 857篇 |
2018年 | 920篇 |
2017年 | 1031篇 |
2016年 | 962篇 |
2015年 | 1181篇 |
2014年 | 1555篇 |
2013年 | 1785篇 |
2012年 | 2013篇 |
2011年 | 2138篇 |
2010年 | 1996篇 |
2009年 | 1879篇 |
2008年 | 1735篇 |
2007年 | 1637篇 |
2006年 | 1571篇 |
2005年 | 1340篇 |
2004年 | 891篇 |
2003年 | 832篇 |
2002年 | 820篇 |
2001年 | 740篇 |
2000年 | 667篇 |
1999年 | 719篇 |
1998年 | 785篇 |
1997年 | 641篇 |
1996年 | 574篇 |
1995年 | 427篇 |
1994年 | 368篇 |
1993年 | 265篇 |
1992年 | 174篇 |
1991年 | 127篇 |
1990年 | 91篇 |
1989年 | 102篇 |
1988年 | 85篇 |
1987年 | 65篇 |
1986年 | 56篇 |
1985年 | 49篇 |
1984年 | 13篇 |
1983年 | 15篇 |
1982年 | 16篇 |
1981年 | 14篇 |
1980年 | 27篇 |
1977年 | 44篇 |
1976年 | 90篇 |
1962年 | 42篇 |
1961年 | 39篇 |
1960年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations 相似文献
53.
54.
在本试验中,采用智能同步广播激励器和CCTV传输的1MHz标频,实现了中波同步广播同频保护率趋于0dB.并提出了自适应相位跟踪同步的概念。 相似文献
55.
OSEK/VDX——汽车电子系统的开放式平台 总被引:6,自引:0,他引:6
介绍了当前在国际汽车工业界日益占据主导地位的汽车电子系统开放式平台--OSEK/VDX体系.详细地论述了这种体系的由来、运行机制和最新的发展动向,以及它对汽车电子技术发展的深远影响. 相似文献
56.
57.
Zuochang Ye Wenjian Yu Zhiping Yu 《Microwave Theory and Techniques》2006,54(5):2128-2137
An efficient algorithm for extraction of three-dimensional (3-D) capacitance on multilayered and lossy substrates is presented. The new algorithm presents a major improvement over the quasi-3-D approach used in a Green's function-based solver and takes into consideration the sidewalls of 3-D conductors. To improve the efficiency of the computation and the transformation of the Green's function, a nonuniform grid is adopted. The most computationally intensive part in the transformation of the Green's function is computed separately as technology-independent matrices Tk foremost. Once computed, Tk can be stored and used for any technology, thus the storage requirement and computational complexity are reduced from O (S/sup 2/) and O (S/sup 2/ log S/sup 2/), respectively, to just O [(log S/sub max/)/sup 2/]. Extensive tests have been performed to verify the new algorithm, and its accuracy has been established by comparing with other programs. 相似文献
58.
本文利用最优回归设计“310”方案,研究了黑土地甜菜 Zn、K_2O、B的效果及合理施用问题。得出甜菜块根产量、含糖、产糖量、灰分 K 和有害 N 与 ZnK_2O、B 肥回归关系显著的回归模型,分析了 Zn、K_2O、B 对上述五个指标的效应,明确了 Zn 是影响各项指标的主要因子,并采用频率筛选法,提出了兼顾上述五项指标的优质高产施肥措施。 相似文献
59.
本文结果表明经一次大剂量全身照射后的机体血液流变的主要特性为全血表观粘度先高粘后低粘的双向性变化,从照后第一天起即升高。随着放射病病程的发展,在早期有一个高粘峰,其峰均值比照前升高47.80%。但因不同动物、不同剂量、不同均速,不同血源而变化不同。 相似文献
60.
Qiang Wang Yi Shen Ye Zhang Jian Qiu Zhang 《IEEE transactions on instrumentation and measurement》2003,52(4):1041-1047
Hyperspectral image fusion is a key technique of hyperspectral data processing. In recent years, many fusion methods have been proposed, but there is little work concerning evaluation of the performances of different image fusion methods. In this paper, a method called quantitative correlation analysis (QCA) is proposed, which provides a quantitative measure of the information transferred by an image fusion technique into the output image. Using the proposed method, the performances of different image fusion methods can be compared and analyzed directly based on the images of before and after performing the fusion. The correlation information entropy, based on the developed QCA, is also proposed and testified by numerical simulations. Typical hyperspectral data are applied to the proposed method. The results show that the method is effective, and its conclusions agree with the classification results in applications. 相似文献