全文获取类型
收费全文 | 36374篇 |
免费 | 1665篇 |
国内免费 | 177篇 |
专业分类
化学 | 18210篇 |
晶体学 | 255篇 |
力学 | 926篇 |
综合类 | 1篇 |
数学 | 2346篇 |
物理学 | 6460篇 |
无线电 | 10018篇 |
出版年
2023年 | 329篇 |
2022年 | 422篇 |
2021年 | 688篇 |
2020年 | 602篇 |
2019年 | 630篇 |
2018年 | 488篇 |
2017年 | 504篇 |
2016年 | 1005篇 |
2015年 | 827篇 |
2014年 | 1091篇 |
2013年 | 1902篇 |
2012年 | 2268篇 |
2011年 | 2468篇 |
2010年 | 1609篇 |
2009年 | 1630篇 |
2008年 | 2216篇 |
2007年 | 2153篇 |
2006年 | 2019篇 |
2005年 | 1924篇 |
2004年 | 1665篇 |
2003年 | 1405篇 |
2002年 | 1353篇 |
2001年 | 1021篇 |
2000年 | 877篇 |
1999年 | 641篇 |
1998年 | 502篇 |
1997年 | 534篇 |
1996年 | 551篇 |
1995年 | 441篇 |
1994年 | 425篇 |
1993年 | 406篇 |
1992年 | 392篇 |
1991年 | 335篇 |
1990年 | 265篇 |
1989年 | 231篇 |
1988年 | 216篇 |
1987年 | 160篇 |
1986年 | 122篇 |
1985年 | 198篇 |
1984年 | 151篇 |
1983年 | 131篇 |
1982年 | 151篇 |
1981年 | 116篇 |
1980年 | 103篇 |
1978年 | 107篇 |
1977年 | 104篇 |
1976年 | 106篇 |
1975年 | 118篇 |
1974年 | 91篇 |
1973年 | 120篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
We shall answer several questions concerning additive or multiplicative commutators in a division ring which are algebraic of bounded degree over its center. 相似文献
62.
Hayoung Lee 《Mathematical Methods in the Applied Sciences》2004,27(3):249-287
The motion of a collisionless plasma is modelled by the Vlasov–Maxwell system. In this paper, solutions of the relativistic Vlasov–Maxwell system are considered in two space dimensions. The speed of light, c, appears as a parameter in the system. With representations of the electric and magnetic fields, conditions are established under which solutions of the relativistic Vlasov–Maxwell system converge pointwise to solutions of the non‐relativistic Vlasov–Poisson system as c tends to infinity, at the asymptotic rate of 1/c. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
63.
64.
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
65.
Fast DCT algorithm with fewer multiplication stages 总被引:1,自引:0,他引:1
Yeonsik Jeong Imgeun Lee Hak Soo Kim Kyu Tae Park 《Electronics letters》1998,34(8):723-724
A novel fast DCT scheme with reduced multiplication stages and fewer additions and multiplications is proposed. The proposed algorithm is structured so that most multiplications tend to be performed at the final stage, which reduces the propagation error that could occur in the fixed-point computation. Minimisation of the multiplication stages can further decrease the error 相似文献
66.
Dongwook Lee Kiseon Kim 《Electronics letters》1998,34(15):1474-1476
The authors suggest a novel virtual circuit connection method based on the reverse traversing technique to minimise the waste of network bandwidth resources, when the Internet protocol multicast is interoperated using the resource reservation protocol over an asynchronous transfer mode network. Simulation results show that, as the number of receivers increases, the bandwidth requirements on all links of the network of the proposed scheme become more advantageous than those of other conventional methods 相似文献
67.
Shih-Kai Lee Mao-Chao Lin 《Communications, IEEE Transactions on》1995,43(5):1917-1925
We propose a hybrid ARQ scheme which uses QPSK modulation for the first transmission and BPSK modulation for retransmissions. The throughput performance of the proposed ARQ scheme is better that those of ARQ schemes purely using QPSK modulation or purely using BPSK modulation for transmissions. Furthermore, the proposed scheme does not require any operation of error correction as usually required in hybrid ARQ schemes. Therefore, the ARQ scheme proposed in this paper can be easily implemented 相似文献
68.
Kyeongho Lee Yeshik Shin Sungjoon Kim Deog-Kyoon Jeong Kim G. Kim B. Da Costa V. 《Solid-State Circuits, IEEE Journal of》1998,33(5):816-823
In a high-resolution flat panel system, a conventional interface that directly connects a liquid crystal display (LCD) controller to a flat panel cannot overcome the problems of excess EMI (electromagnetic interference) and power caused by full-swing transmission signals in parallel lines. This paper presents a high-speed digital video interface system implemented with a low-cost standard CMOS (complimentary metal-oxide-semiconductor) technology that can mitigate EMI and power problems in high-resolution flat panel display systems. The combined architecture of the high-speed, small number of parallel lines and low-voltage swing serial interface can support resolutions from VGA (640×480 pixels) up to XGA (1024×768 pixels) with significant power improvement and drastic EMI reduction. To support high-speed, low-voltage swing signaling and overcome channel-to-channel skew problems, a robust data recovery system is required. The proposed digital phase-locked loop enables robust skew-insensitive data recovery of up to 1.04 GBd 相似文献
69.
This paper studies a particular single-stage power-factor-correction (PFC) switching regulator employing a discontinuous-conduction-mode (DCM) boost-input cell and a continuous-current-mode (CCM) forward output cell. Although this single-stage PFC regulator can provide a reasonably high power factor when its PFC stage is operating in discontinuous mode, substantial reduction in line-current harmonics is possible by applying a suitable frequency-modulation scheme. This paper derives a frequency-modulation scheme and proposes a practical implementation using a simple translinear analog circuit. A quantitative analysis on the total harmonic distortion (THD) of the line current when the circuit is subject to a limited range of frequency variations is presented along with some considerations for practical design. Experimental data obtained from a prototype confirms the effectiveness of the proposed frequency-modulation scheme. The proposed analog translinear circuit allows custom integrated circuit implementation, making it a viable low-cost solution to the elimination of line-current harmonics in switching regulators 相似文献
70.
Young-Wan Choi O-Kyun Kwon El-Hang Lee 《Photonics Technology Letters, IEEE》1993,5(12):1406-1409
The authors report significantly improved performances of a symmetric self-electrooptic-effect device (S-SEED), with high on-off contrast ratio (>30:l) and large optical bistability loop widths (ΔP=44%) at an applied bias of Va=0 V, i.e., with no power supply. The S-SEED is made of extremely shallow quantum wells (ESQWs) in an asymmetric Fabry-Perot (AFP) cavity structure. At Va =5 V ΔP increased by up to 95%, preserving the high contrast ratio. The reflectivity changes at Va=0 and 5 V were about 15% and 30%, respectively. These are believed to be the largest values ever reported for such structures 相似文献