全文获取类型
收费全文 | 5944篇 |
免费 | 927篇 |
国内免费 | 333篇 |
专业分类
化学 | 193篇 |
晶体学 | 12篇 |
力学 | 283篇 |
综合类 | 52篇 |
数学 | 598篇 |
物理学 | 2944篇 |
综合类 | 3122篇 |
出版年
2024年 | 32篇 |
2023年 | 51篇 |
2022年 | 124篇 |
2021年 | 107篇 |
2020年 | 146篇 |
2019年 | 134篇 |
2018年 | 139篇 |
2017年 | 196篇 |
2016年 | 245篇 |
2015年 | 196篇 |
2014年 | 367篇 |
2013年 | 338篇 |
2012年 | 296篇 |
2011年 | 447篇 |
2010年 | 293篇 |
2009年 | 364篇 |
2008年 | 354篇 |
2007年 | 401篇 |
2006年 | 342篇 |
2005年 | 303篇 |
2004年 | 233篇 |
2003年 | 274篇 |
2002年 | 258篇 |
2001年 | 233篇 |
2000年 | 227篇 |
1999年 | 159篇 |
1998年 | 138篇 |
1997年 | 131篇 |
1996年 | 103篇 |
1995年 | 88篇 |
1994年 | 61篇 |
1993年 | 70篇 |
1992年 | 58篇 |
1991年 | 45篇 |
1990年 | 53篇 |
1989年 | 52篇 |
1988年 | 39篇 |
1987年 | 35篇 |
1986年 | 17篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1973年 | 2篇 |
1955年 | 3篇 |
排序方式: 共有7204条查询结果,搜索用时 328 毫秒
41.
The theory of nonequilibrium potentials or quasipotentials is a physically motivated approach to small random perturbations of dynamical systems, leading to exponential estimates of invariant probabilities and mean first exit times. In the present article we develop the mathematical foundation of this theory for discrete-time systems, following and extending the work of Freidlin and Wentzell, and Kifer. We discuss strategies for calculating and estimating quasipotentials and show their application to one-dimensionalS-unimodal maps. The method proves to be especially suited for describing the noise scaling behavior of invariant probabilities, e.g., for the map occurring as the limit of the Feigenbaum period-doubling sequence. We show that the method allows statements about the scaling behavior in the case of localized noise, too, which does not originally lie within the scope of the quasipotential formalism. 相似文献
42.
陈欣汉 《汕头大学学报(自然科学版)》1994,(1)
本文就信号、噪声通过线性工作系统设备过程中信噪比变化的问题,提出一个简单和实用的模型,建立相应的计算公式,并在视频领域和音频领域对公式展开讨论,进一步阐述公式的实用性,可作为工作系统的设计和调整的参考,以祈获得更好的社会效益和经济效益. 相似文献
43.
R. S. Kaler 《Fiber and Integrated Optics》2006,25(1):41-57
In this article, the comparison of large signal theory and small signal theory has been done with dispersive propagation of optical signal with IMDD (Intensity Modulation Direct Detection) systems for semiconductor lasers with higher-order dispersion terms. The expressions for an exact large signal theory and small signal theory including higher-order dispersion terms for propagation of an optical wave with sinusoidal amplitude and frequency modulation in a dispersive fiber have been derived. It is observed that small signal theory is more sensitive compared to large signal theory in terms of intensity modulation/direct detection systems. Also, it is reported that for large signal analysis the higher-order effects of dispersion can be ignored, whereas for small signal theory, the higher-order effects can be ignored for lower modulation frequencies only. The variation in the transfer function for various values of modulation indices are greater for small signal analysis than for large signal analysis. Also, as the intensity modulation index is increased, there is a decrease in the value of transfer function. The large signal model approximates the small signal model for lower values of the intensity modulation index. 相似文献
44.
通过求解BogoliubovedeGennes(BdG)方程,利用推广的BlonderTinkhamKlapwijk(BTK)理论,计算铁磁绝缘层铁磁d波超导结中的微分电导、平均电流和散粒噪声功率.研究表明,系统的微分电导和散粒噪声与平均电流的比值都随中间铁磁层厚度作周期性振荡,振荡的幅度随绝缘层势垒增高而变大,随铁磁层中磁交换劈裂的增强而变小.
关键词:
微分电导 散粒噪声 磁交换劈裂 相似文献
45.
46.
采用矩阵奇异值分解(singular value decomposition, SVD)的方法,对高温射频超导量子干涉仪(HTc rf-SQUID)采集到的单通道心磁信号进行处理.证明了对于近似周期性的心磁信号,在无参考噪声的情况下矩阵奇异值分解的方法与自适应窄带陷波相结合有较好的消除广谱噪声的效果.
关键词:
高温射频超导量子干涉仪
心磁信号
奇异值分解
噪声消除 相似文献
47.
48.
Non-Markovian Brownian motion in a periodic potential is studied by means of an electronic analogue simulator. Velocity spectra, the Fourier transforms of velocity autocorrelation functions, are obtained for three types of random force, that is, a white noise, an Ornstein—Uhlenbeck process, and a quasimonochromatic noise. The analogue results are in good agreement both with theoretical ones calculated with the use of a matrix-continued-fraction method, and with the results of digital simulations. An unexpected extra peak in the velocity spectrum is observed for Ornstein-Uhlenbeck noise with large correlation time. The peak is attributed to a slow oscillatory motion of the Brownian particle as it moves back and forth over several lattice spaces. Its relationship to an approximate Langevin equation is discussed. 相似文献
49.
Dai Lu David Rutledge Milan Kovacevic Jon Hacker 《International Journal of Infrared and Millimeter Waves》2002,23(5):693-704
This paper presents an active patch array designed at 24 GHz. It can be used as a front-end component for a phased array. A series resonant array structure is chosen which is compact and easy excite. With 5 elements, the array proved a 12-dB antenna gain. A power amplifier and a low noise amplifier are designed on a single GaAs chip (PALNA). Bias switch is used in the PALNA, which greatly reduces the switch loss in a transceiver and increases the efficiency. 20-dB small signal gain is achieved in both power amplifier and low noise amplifier. The active patch array is built by the combination of the patch array and PALNA. The measured active gain of this antenna is 35-dB for the PA mode and 31-dB for the LNA mode. This active patch array can obtain an EIRP of 34 dBm with a total radiated power of 22dBm and a maximum PAE of 32%. To check the noise performance, we applied sources at both normal temperature and 77K (liquid nitrogen) and extracted the noise figure (3.5 dB) of the active antenna by the Y factor method. The results proved that the active antenna is working efficiently as both a transmitting and receiving antenna. 相似文献
50.
OrCAD9.2是国际通用电子设计自动化(EDA)软件包,文中介绍了用该软件对电路进行噪声计算的方法,并给出用模拟分析计算的典型电路噪声,证明这种方法在分析复杂电路的噪声时方便、简单、准确。 相似文献