全文获取类型
收费全文 | 36731篇 |
免费 | 5366篇 |
国内免费 | 4321篇 |
专业分类
化学 | 21152篇 |
晶体学 | 446篇 |
力学 | 1267篇 |
综合类 | 218篇 |
数学 | 2856篇 |
物理学 | 9093篇 |
无线电 | 11386篇 |
出版年
2024年 | 114篇 |
2023年 | 792篇 |
2022年 | 981篇 |
2021年 | 1293篇 |
2020年 | 1347篇 |
2019年 | 1348篇 |
2018年 | 1147篇 |
2017年 | 1187篇 |
2016年 | 1565篇 |
2015年 | 1731篇 |
2014年 | 2189篇 |
2013年 | 2656篇 |
2012年 | 3059篇 |
2011年 | 3263篇 |
2010年 | 2469篇 |
2009年 | 2404篇 |
2008年 | 2737篇 |
2007年 | 2440篇 |
2006年 | 2210篇 |
2005年 | 1864篇 |
2004年 | 1439篇 |
2003年 | 1200篇 |
2002年 | 1184篇 |
2001年 | 945篇 |
2000年 | 772篇 |
1999年 | 637篇 |
1998年 | 525篇 |
1997年 | 416篇 |
1996年 | 370篇 |
1995年 | 326篇 |
1994年 | 324篇 |
1993年 | 259篇 |
1992年 | 221篇 |
1991年 | 201篇 |
1990年 | 153篇 |
1989年 | 137篇 |
1988年 | 81篇 |
1987年 | 74篇 |
1986年 | 61篇 |
1985年 | 61篇 |
1984年 | 44篇 |
1983年 | 27篇 |
1982年 | 27篇 |
1981年 | 15篇 |
1980年 | 23篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1976年 | 15篇 |
1975年 | 9篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
121.
The distributions of limit cycles of cubic vector fields (P2, Q3) are considered in this paper, where P2 and Q3 are polynomials of x and y of order two and three, respectively. It is possibly seven different distributions of limit cycles given in [1]. We now prove that in which three kinds of distributions are impossible and other four kinds all can be realized by concrete vector fields of (P2,Q3). Some related results are also given. 相似文献
122.
Eunseok Song Yido Koo Yeon-Jae Jung Deok-Hee Lee Sangyoung Chu Soo-Ik Chae 《Solid-State Circuits, IEEE Journal of》2005,40(5):1094-1106
This paper describes a single-chip CMOS quad-band (850/900/1800/1900 MHz) RF transceiver for GSM/GPRS applications. It is the most important design issue to maximize resource sharing and reuse in designing the multiband transceivers. In particular, reducing the number of voltage-controlled oscillators (VCOs) required for local oscillator (LO) frequency generation is very important because the VCO and phase-locked loop (PLL) circuits occupy a relatively large area. We propose a quad-band GSM transceiver architecture that employs a direct conversion receiver and an offset PLL transmitter, which requires only one VCO/PLL to generate LO signals by using an efficient LO frequency plan. In the receive path, four separate LNAs are used for each band, and two down-conversion mixers are used, one for the low bands (850/900 MHz) and the other for the high bands (1800/1900 MHz). A receiver baseband circuit is shared for all four bands because all of their channel spaces are the same. In the transmit path, most of the building blocks of the offset PLL, including a TX VCO and IF filters, are integrated. The quad-band GSM transceiver that was implemented in 0.25-/spl mu/m CMOS technology has a size of 3.3/spl times/3.2 mm/sup 2/, including its pad area. From the experimental results, we found that the receiver provides a maximum noise figure of 2.9 dB and a minimum IIP3 of -13.2dBm for the EGSM 900 band. The transmitter shows an rms phase error of 1.4/spl deg/ and meets the GSM spectral mask specification. The prototype chip consumes 56 and 58 mA at 2.8 V in the RX and TX modes, respectively. 相似文献
123.
Ju Han Lee You Min Chang Young-Geun Han Sang-Hyuck Kim Haeyang Chung Sang Bae Lee 《Photonics Technology Letters, IEEE》2005,17(1):34-36
We experimentally demonstrate a simple and novel scheme for tunable real-repetition-rate multiplication, based on the combined use of fractional Talbot effect in a linearly tunable chirped fiber Bragg grating (FBG) and cross-phase modulation (XPM) effect in a nonlinear optical loop mirror (NOLM). By tuning the group-velocity dispersion of the chirped FBG fabricated with the S-bending method using a uniform FBG, we obtain high quality pulses at pseudorepetition rates of 20/spl sim/50 GHz from an original 8.5-ps 10-GHz soliton pulse train. We subsequently convert this pseudorate multiplication into a real-rate multiplication using XPM effect in an NOLM. A wavelength tuning is also achieved over a /spl sim/15-nm range. 相似文献
124.
125.
Song Jiang 《Mathematische Nachrichten》1998,190(1):169-183
We consider initial boundary value problems for the equations of the one-dimensional motion of a viscous, heat-conducting gas with density-dependent viscosity that decreases (to zero) with decreasing density. We prove that if the viscosity does not decrease to zero too rapidly, then smooth solutions exist globally in time. 相似文献
126.
本文提出了一种直接把红外前视传感器的输出作为测量值来跟踪红外目标的扩展型卡尔曼滤波跟踪算法。在滤波处理时,设置可变的测量矩阵,其目的在于提高运算速度和抑制噪声干扰。通过具体的实例,对所提跟踪算法的特点和有效性进行了细致的分析。 相似文献
127.
Modern digital communication systems rely heavily on baseband signal processing for in-phase and quadrature (I-Q) channels, and complex number processing in low-voltage CMOS has become a necessity for channel equalization, timing recovery, modulation, and demodulation. In this work, redundant binary (RB) arithmetic is applied to complex number multiplication for the first time so that an N-bit parallel complex number multiplier can be reduced to two RE multiplications (i.e., an addition of N RB partial products) corresponding to real and imaginary parts, respectively. This efficient RE encoding scheme proposed can generate RB partial products with no additional hardware and delay overheads. A prototype 8-bit complex number multiplier containing 11.5 K transistors is integrated on 1.05×1.33 mm2 using 0.8 μm CMOS. The chip consumes 90 mW with 2.5 V supply when clocked at 200 MHz 相似文献
128.
气流式加速度传感器的敏感机理 总被引:2,自引:0,他引:2
介绍了在研制气流式加速度传感器过程中,为对气流式加速度传感器进行建模,作者对气流式加速度传感器的工作机理进行了理论性探讨。在分析过程中以流体力学、传热学和惯性理论为基础并结合实验结果,建立了适合于气流式加速度传感器的气体运动方程和能量方程。同时还讨论了热线测量加速度的方法及根据实验分析了输入加速度对腔体内温度及传感器输出电压的影响,从而为进一步研究一系列气流式传感器的工作机理提供了方法。 相似文献
129.
130.