全文获取类型
收费全文 | 3081篇 |
免费 | 393篇 |
国内免费 | 158篇 |
专业分类
电工技术 | 355篇 |
综合类 | 327篇 |
化学工业 | 87篇 |
金属工艺 | 106篇 |
机械仪表 | 309篇 |
建筑科学 | 156篇 |
矿业工程 | 74篇 |
能源动力 | 62篇 |
轻工业 | 37篇 |
水利工程 | 75篇 |
石油天然气 | 434篇 |
武器工业 | 39篇 |
无线电 | 743篇 |
一般工业技术 | 458篇 |
冶金工业 | 52篇 |
原子能技术 | 56篇 |
自动化技术 | 262篇 |
出版年
2024年 | 8篇 |
2023年 | 42篇 |
2022年 | 59篇 |
2021年 | 91篇 |
2020年 | 118篇 |
2019年 | 96篇 |
2018年 | 80篇 |
2017年 | 120篇 |
2016年 | 152篇 |
2015年 | 155篇 |
2014年 | 178篇 |
2013年 | 213篇 |
2012年 | 209篇 |
2011年 | 272篇 |
2010年 | 164篇 |
2009年 | 180篇 |
2008年 | 148篇 |
2007年 | 233篇 |
2006年 | 171篇 |
2005年 | 160篇 |
2004年 | 116篇 |
2003年 | 110篇 |
2002年 | 100篇 |
2001年 | 69篇 |
2000年 | 51篇 |
1999年 | 47篇 |
1998年 | 48篇 |
1997年 | 50篇 |
1996年 | 45篇 |
1995年 | 24篇 |
1994年 | 28篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1959年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有3632条查询结果,搜索用时 281 毫秒
51.
针对无线通信和移动通信中的信道干扰和码间干扰等问题,对正交幅度调制(QAM)和OFDM的调制性能进行了研究,首先,在System View仿真环境下,搭建一个通信系统,然后分别将两种调制方式应用其中,最后得到OFDM与QAM的比特误码率(BER)性能曲线。仿真结果表明,OFDM具有良好的误比特率性能,而且在相同的信噪比(SNR)下OFDM的BER性能比QAM更优越。 相似文献
52.
衰减直流(DDC)分量、高次谐波等干扰信号的存在,使得对电网畸变信号中基频分量的幅值、相位检测存在一定误差,其中DDC分量的时间常数通常超过45 ms,持续时间较长.为此,文中首先针对畸变信号中DDC分量提出一种半周期四点采样检测算法,缩短了DDC分量的检测响应时间.其次,针对同时含有DDC分量与高次谐波的畸变信号,提出将畸变信号进行半周期积分后,在dq坐标系下将上述DDC分量检测算法与高次谐波检测算法进行组合,在工频半周期中可同时滤除干扰信号的影响,准确检测到畸变信号中的基频分量.最后,搭建了MATLAB/Simulink半实物实时仿真模型,从检测精度、响应时间等方面验证了所提算法的有效性. 相似文献
53.
54.
锤砧座振动模式及其振幅 总被引:1,自引:0,他引:1
在碰撞理论的基础上,利用单自由度小阻尼衰减振动模式导出锻锤砧座下压和上跳幅值理论计算公式.并与实测结果进行了比较。所得公式有助于某些争论问题的解释。 相似文献
55.
王龙潘明海宋聂 《数据采集与处理》2016,31(4):823-831
为了实现宽带雷达系统中雷达信
号的低失真采集与处理,研究并设计了具有幅相误差校正功能的宽带低杂散采样系统。该系统采用宽带模数转换(Analog to digital converter, ADC)器件和高性能可编程逻辑阵列(Field programmable gate array, FPGA)的实现方案,并从低抖动采样时钟、低噪声电源和防串扰等方面进行了低杂散最优方案研究。为了改善系统的带内传输特性,利用优化算
法设计了有限长冲激响(Finite impulse response,FIR)数字校准滤波器。最后对设计
的系统进行实验测试,结果表明系统瞬时带宽达到800 MHz以上,采样率1.8GS/s,量化位数8位、杂散电平-50 dBc,性能指标满足系统在宽带雷达信号获取、宽带雷达目标成像和宽带雷达目标回波重构等领域的应用。 相似文献
56.
刘彤 《单片机与嵌入式系统应用》2016,(6)
通过对空间矢量脉宽调制算法的分析,研究了数字信号处理器生成SVPWM波形的实现方法及软件算法,设计了基于DSP数字控制的三相逆变电源。相关测试参数和结果表明,该设计提高了直流电压的利用率,使开关器件的损耗更小。此外,还提出了逆变电源闭环控制的PI控制算法,利用DSP强大的数字信号处理能力,提高了系统的响应速度。 相似文献
57.
温度效应造成的半球谐振陀螺振幅和测角误差因素分析 总被引:1,自引:0,他引:1
在半球谐振陀螺工作的过程中,环境温度的变化是不可避免的。温度的变化影响谐振子尺寸变化,导致陀螺产生热漂移。根据振幅检测原理,推导温度效应误差的数学表达式,分析它对测量结果的影响,这对选择合适的补偿算法以消除温度效应误差有一定的指导意义。 相似文献
58.
Ortwin Feustel 《Nexus Network Journal》2009,11(1):7-21
It is commonly believed that the longitudinal axes of churches extend exactly in an east-west direction. However, thorough
investigations have shown that this is not always correct; rather, both southern and northern deviations of up to about 25°
can occur. The angular deviation between the church axis and true east is called the Holy Alignment. This present study presents the possibility that the nave is oriented towards the direction of the sun-rising point on the
name day of the patron saint of the church. If several saints share the patronage, the Holy Alignment equals the algebraic sum of the angular distances for each saint. The orientation of a nave can be analyzed by means of common
mathematical relations used in geodesy, astronomy and gnomonics. In order to perform such an analysis, it is necessary to
know the history of the patronage of the church; the Gauss-Krüger coordinates of the ground plan; and characteristic astronomical
quantities at the time the church was built. A calculated example with the saints Andrew, James and Philip for the year 980
illustrates the analysis.
An appendix deals with the influence of atmospheric refraction on the apparent altitude of the sun near the horizon. 相似文献
59.
Robust reversible watermarking can provide robustness against various attacks besides the ability to recover the cover image. However, robustness and reversibility are somewhat separate in many schemes. The original cover image cannot be recovered even if the watermarked image suffers from a tiny distortion. This paper presents a new robust reversible watermarking scheme by exploring the reversibility of spread-spectrum codes. Watermark bits are embedded by a suggested adaptive spread-spectrum code. The embedding amplitude used in the algorithms is determined by quantizing the source interference of the cover. The proposed scheme is robust to various attacks. Furthermore, since the embedding amplitude is available at the receiver, the original image can be recovered losslessly when there is no attack. Even in the presence of attacks, the original cover images can still be partially recovered. Experimental results demonstrate that the proposed scheme performs well on robustness and watermarked image quality, and provide extra reversibility that resists image distortions. 相似文献
60.
This investigation attempts to understand the corrosion fatigue crack propagation behavior of pipeline steels exposed to near-neutral pH environments. The fatigue loading was designed to simulate the underload-type variable amplitude pressure fluctuations found during pipeline operation. The effects of amplitudes (R ratios) of underload and minor cycles were investigated. It has been found from this investigation that the crack growth rate is enhanced significantly through load interaction of the variable amplitude fatigue. The acceleration factor is found to be up to 2.7 and 5.3 for tests in air and in the near-neutral pH solution, respectively. The crack growth rate decreases with R ratios of underload and minor cycles for tests both in air and in near-neutral pH environments. The latter could enhance crack propagation by a factor of up to 11, as compared with the crack growth rate in air. The critical R ratio of minor cycles at which the minor cycles do not contribute to crack propagation through load interaction was determined to be as high as 0.982, which is much lower than the threshold determined by constant amplitude fatigue. This critical R ratio could be utilized to demarcate stress corrosion cracking and corrosion fatigue, and should be incorporated as one of the design principles for components/structures subjected to variable amplitude cyclic loading. 相似文献