全文获取类型
收费全文 | 662625篇 |
免费 | 8768篇 |
国内免费 | 1995篇 |
专业分类
电工技术 | 12748篇 |
综合类 | 724篇 |
化学工业 | 97260篇 |
金属工艺 | 26955篇 |
机械仪表 | 22892篇 |
建筑科学 | 17097篇 |
矿业工程 | 2492篇 |
能源动力 | 16272篇 |
轻工业 | 58820篇 |
水利工程 | 6165篇 |
石油天然气 | 6761篇 |
武器工业 | 81篇 |
无线电 | 83667篇 |
一般工业技术 | 128123篇 |
冶金工业 | 116899篇 |
原子能技术 | 10599篇 |
自动化技术 | 65833篇 |
出版年
2021年 | 4306篇 |
2019年 | 4085篇 |
2018年 | 13639篇 |
2017年 | 14315篇 |
2016年 | 11092篇 |
2015年 | 5669篇 |
2014年 | 8840篇 |
2013年 | 27670篇 |
2012年 | 17101篇 |
2011年 | 28052篇 |
2010年 | 23108篇 |
2009年 | 24473篇 |
2008年 | 25413篇 |
2007年 | 27390篇 |
2006年 | 18304篇 |
2005年 | 19830篇 |
2004年 | 17815篇 |
2003年 | 17067篇 |
2002年 | 15848篇 |
2001年 | 15364篇 |
2000年 | 14505篇 |
1999年 | 15092篇 |
1998年 | 34603篇 |
1997年 | 25106篇 |
1996年 | 19834篇 |
1995年 | 15494篇 |
1994年 | 13795篇 |
1993年 | 13336篇 |
1992年 | 10247篇 |
1991年 | 9711篇 |
1990年 | 9149篇 |
1989年 | 8762篇 |
1988年 | 8386篇 |
1987年 | 7253篇 |
1986年 | 7107篇 |
1985年 | 8577篇 |
1984年 | 8004篇 |
1983年 | 7100篇 |
1982年 | 6602篇 |
1981年 | 6668篇 |
1980年 | 6252篇 |
1979年 | 6050篇 |
1978年 | 5675篇 |
1977年 | 6832篇 |
1976年 | 8913篇 |
1975年 | 4928篇 |
1974年 | 4804篇 |
1973年 | 4731篇 |
1972年 | 3773篇 |
1971年 | 3405篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
The radiation patterns of simple rectangular waveguide feeds have been measured at V band. A low-cost test arrangement, operating in the far field but without any absorbers, is shown to give better than 0.5 dB repeatability and more than 30 dB of dynamic range. The general rules of waveguide-feed design for reflector antennas are applicable, but the tendency of narrow feed patterns is somewhat emphasized compared to observations in earlier work at C or X band. For horizontal polarization, the difference is 5 dB at 90/spl deg/ between WR-28 and WR-90 hardware. This is partly caused by an increase in the relative guide material thickness at shorter wavelengths. No clear connection between pattern widths of the two linear polarizations was observed if the guide height was increased, the largest momentary deviation being 3 dB for a rectangular 7.1 by 7.1 millimeter feed. Changes in the guide width had a monotone effect up to 3 dB at 90/spl deg/ offset. Very small feed aperture sizes, around /spl lambda//4 or less, did not show respective widening of 1 dB patterns, although such expected effects were visible at lower amplitude levels. 相似文献
962.
On the complexity of sphere decoding in digital communications 总被引:14,自引:0,他引:14
Sphere decoding has been suggested by a number of authors as an efficient algorithm to solve various detection problems in digital communications. In some cases, the algorithm is referred to as an algorithm of polynomial complexity without clearly specifying what assumptions are made about the problem structure. Another claim is that although worst-case complexity is exponential, the expected complexity of the algorithm is polynomial. Herein, we study the expected complexity where the problem size is defined to be the number of symbols jointly detected, and our main result is that the expected complexity is exponential for fixed signal-to-noise ratio (SNR), contrary to previous claims. The sphere radius, which is a parameter of the algorithm, must be chosen to ensure a nonvanishing probability of solving the detection problem. This causes the exponential complexity since the squared radius must grow linearly with problem size. The rate of linear increase is, however, dependent on the noise variance, and thus, the rate of the exponential function is strongly dependent on the SNR. Therefore sphere decoding can be efficient for some SNR and problems of moderate size, even though the number of operations required by the algorithm strictly speaking always grows as an exponential function of the problem size. 相似文献
963.
Zogas D.A. Karagiannidis G.K. Kotsopoulos S.A. 《Wireless Communications, IEEE Transactions on》2005,4(2):374-379
Motivated by the importance of Nakagami-n (Rice) and Nakagami-q (Hoyt) statistical models to describe channel fading in land, mobile, terrestrial, and satellite telecommunications, we present an alternative moments-based approach to the performance analysis of equal-gain combining (EGC) receivers over independent, not necessarily identically distributed Rice- and Hoyt-fading channels. Exact closed-form expressions for the moments of the signal-to-noise ratio (SNR) at the output of the combiner are derived and significant performance criteria such as, the average output SNR, the amount of fading and the spectral efficiency at the low power regime, are studied. Moreover, using Pade rational approximation to the moment-generating function of the output SNR, the average symbol error probability and the outage probability are evaluated. We also study the suitability of modeling a Hoyt-fading environment by a properly chosen Nakagami-m model, as far as the error performance of the EGC is concerned. 相似文献
964.
Towards real-time business intelligence 总被引:1,自引:0,他引:1
Analysing data to predict market trends of products and services and to improve performances of enterprise business systems has always been part of running a competitive business. But it is becoming essential nowadays that not only is the analysis done on real-time data, but also actions in response to analysis results can be performed in real time and instantaneously change parameters of business processes. This paper discusses issues and problems of current business intelligence systems, and then outlines our vision of future real-time business intelligence. We present a list of emerging technologies which could contribute to the realisation of real-time business intelligence and some examples of applying them to improve BT’s systems and services. 相似文献
965.
Turner S.E. Elder R.B. Jr. Jansen D.S. Kotecki D.E. 《Microwave and Wireless Components Letters, IEEE》2005,15(3):144-146
A 41-GHz 4-b adder-accumulator test circuit implemented in InP double heterojunction bipolar transistor (DHBT) technology using 624 transistors is reported. High clock rates are obtained by combining the logic functions into pipelined latches. The adder-accumulator contains a single-level parallel-gated carry circuit that is used as a step toward reduced power consumption. The carry circuit has a maximum clock frequency of 55 GHz. The accumulator architecture employs modular, pipelined 2-b adders and is cascadable to 2 N-bits. The test circuit includes a 4-b digital to analog converter (DAC) that facilitates demonstration of high-speed operation. 相似文献
966.
The application of micro- and nanofabrication processes to the development of hybrid synthetic and biological systems may enable the production of new devices such as controllable transporters, gears, levers, micropumps, or microgenerators powered by biological molecular motors. However, engineering these hybrid devices requires fabrication processes that are compatible with biological materials such as kinesin motor proteins and microtubules. In this paper, the effects of micro- and nanofabrication processing chemicals and resists on the functionality of casein, kinesin, and microtubule proteins are systematically examined to address the important missing link of the biocompatibility of micro- and nanofabrication processes needed to realize hybrid system fabrication. It is found that both casein, which is used to prevent motor denaturation on surfaces, and kinesin motors are surprisingly tolerant of most of the processing chemicals examined. Microtubules, however, are much more sensitive. Exposure to the processing chemicals leads to depolymerization, which is partially attributed to the pH of the solutions examined. When the chemicals were diluted in aqueous buffers, a subset of them no longer depolymerized microtubules and in their diluted forms still worked as resist removers. This approach broadens the application of micro- and nanofabrication processes to hybrid synthetic and biological system fabrication. 相似文献
967.
This paper discusses principles for allocating multiple bearer services onto different subsystems in multiaccess wireless systems. Based on the included subsystem's multiservice capacities, under certain constraints near-optimum subsystem service allocations that maximize combined multiservice capacity are derived through simple optimization procedures. These favorable service allocations are either extreme points where services, as far as possible, are allocated to the subsystems best at supporting them, or they are characterized by the relative efficiency of supporting services being equal in all subsystems. The consequences of this include that services should typically be mixed in subsystems with convex capacity regions and isolated in subsystems with concave capacity regions. Simple user assignment algorithms based on this are also discussed. Additionally, illustrating the main findings of the analysis, some system examples are given, including a case study with combined global system for mobile communications (GSM) and wideband code-division multiple-access (WCDMA) systems. The gain of using the proposed service allocation principles compared to a reference case of maintaining equal service mixes in all subsystems depends on the shape of the subsystem capacity regions; the more different the capacity regions, the larger the gain. In the GSM and WCDMA case study, capacity gains of up to 100% in terms of supported data users for a fixed voice traffic load are achieved. 相似文献
968.
Cochannel narrowband interference can limit the performance of direct sequence spread spectrum (DSSS) and high frequency (HF) systems. Narrowband interference (NBI) can be single tone, chirped or frequency shift keyed (FSK) in nature and numerous techniques for its removal have been proposed. Linear adaptive prediction filters based on autoregressive modelling have been suggested owing to their ability to perform in a non-stationary environment. In the FSK narrowband interference case, adaptive filters are susceptible to excess residual errors owing to instantaneous frequency step changes and the finite convergence time required for the filter to adapt to a new interference frequency. The signal degradation owing to this type of interference becomes greater in high SNR regimes and has been found to be a function of the frequency parameters of the FSK interference signal. The paper discusses the convergence and frequency tracking properties of the recursive least squares (RLS) adaptive lattice filter using a posteriori estimation errors in the presence of FSK narrowband interference. An optimal exponential weighting factor that balances convergence time and steady state error is derived for this case of NBI. Results are compared to those of a previously proposed fast converging minimum frequency error (FCMFE) RLS lattice filter. 相似文献
969.
The shape demagnetizing factors of a planar magnetic insulator are of some interest in the design of a number of microwave ferrite components. One means of experimentally evaluating these quantities, in the case of a disk geometry, is to measure the Kittel line and to make use of a standard identity. Another method, introduced here, is to extract the axial demagnetizing factor by having recourse to the intersections of the split branches of a gyromagnetic resonator. The letter includes measurements on the axial demagnetizing factor of an irregular hexagonal geometry for three different values of saturation magnetization. 相似文献
970.
In "Stationary distributions for the random waypoint mobility model" (TMC, Vol. 3, No, 1), Navidi and Camp find the stationary distribution of the random waypoint model, with or without pause on a rectangular area. In this short note, we show that, under the stationary regime, speed and location are independent. 相似文献