交联聚乙烯热老化与绝缘性能的关联关系

为研究交联聚乙烯（cross-linked polyethylene,XLPE）绝缘材料的热分解活化能、电气特性和力学特性随热老化程度变化的规律,对交流电力电缆绝缘用XLPE材料在110 ℃下开展加速热老化实验。采用热失重（thermogravimtric analyzer, TGA）测试手段,对XLPE在20~600 ℃的热分解行为进行研究;采用交流击穿测试、宽频介电谱测试及体积电阻率测试,研究老化后XLPE试样的电气特性;采用拉伸实验测试,研究老化后XLPE试样的力学特性。结果表明：热老化使得XLPE的交联结构和结晶状态被破坏,XLPE活化能呈减小趋势。由于氧化反应快速进行,使得XLPE分子链发生断裂,交联结构变弱,导致XLPE绝缘材料严重劣化,其活化能、击穿强度、体积电阻率、弹性模量和断裂伸长率随老化时间增长呈下降趋势,而介电常数、介电损耗和电导率呈增加趋势。     

A 90–96 GHz CMOS down-conversion mixer with high conversion gain and excellent LO–RF isolation

A 90–96 GHz down-conversion mixer for 94 GHz image radar sensors using standard 90 nm CMOS technology is reported. RF negative resistance compensation technique, i.e. NMOS LC-oscillator-based RF transconductance (GM) stage load, is used to increase the output impedance and suppress the feedback capacitance C_gd of RF GM stage. Hence, conversion gain (CG), noise figure (NF) and LO–RF isolation of the mixer can be enhanced. The mixer consumes 15 mW and achieves excellent RF-port input reflection coefficient of ?10 to ?36.4 dB for frequencies of 85–105 GHz. The corresponding -10 dB input matching bandwidth is 20 GHz. In addition, for frequencies of 90–96 GHz, the mixer achieves CG of 6.3–9 dB (the corresponding 3-dB CG bandwidth is greater than 6 GHz) and LO–RF isolation of 40–45.1 dB, one of the best CG and LO–RF isolation results ever reported for a down-conversion mixer with operation frequency around 94 GHz. Furthermore, the mixer achieves an excellent input third-order intercept point of 1 dBm at 94 GHz. These results demonstrate the proposed down-conversion mixer architecture is very promising for 94 GHz image radar sensors.     

Photoconductivity measurements in optical fibre preforms

The AC photoconductivity and the photodielectric shift in UV-illuminated germano-silicate preforms and optical fibres has been measured using a noncontact microwave resonator technique at 2.9 GHz. The highest photoconductivity measured was 1.5*10/sup -6/ ( Omega cm)/sup -1/ and the largest measured photodielectric shift corresponded to an increase of the refractive index of 1.6*10/sup -2/. A permanently induced increase of the refractive index, 6.1*10/sup -3/, was also observed. The dark DC conductivity was measured to be 10/sup -15/ ( Omega cm)/sup -1/.<>     

Maximally Informative Feature and Sensor Selection in Pattern Recognition Using Local and Global Independent Component Analysis

In pattern recognition, a suitable criterion for feature selection is the mutual information (MI) between feature vectors and class labels. Estimating MI in high dimensional feature spaces is problematic in terms of computation load and accuracy. We propose an independent component analysis based MI estimation (ICA-MI) methodology for feature selection. This simplifies the high dimensional MI estimation problem into multiple one-dimensional MI estimation problems. Nonlinear ICA transformation is achieved using piecewise local linear approximation on partitions in the feature space, which allows the exploitation of the additivity property of entropy and the simplicity of linear ICA algorithms. Number of partitions controls the tradeoff between more accurate approximation of the nonlinear data topology and small-sample statistical variations in estimation. We test the ICA-MI feature selection framework on synthetic, UCI repository, and EEG activity classification problems. Experiments demonstrate, as expected, that the selection of the number of partitions for local linear ICA is highly problem dependent and must be carried out properly through cross validation. When this is done properly, the proposed ICA-MI feature selection framework yields feature ranking results that are comparable to the optimal probability of error based feature ranking and selection strategy at a much lower computational load.     

可调谐三段式DBR激光器波长锁定系统

提出了一种可精确锁定可调谐三段式DBR激光器输出波长的锁定系统。使用PC采集波长漂移信号并自动调节DBR激光器相区注入电流,使其输出波长在24～26℃的温度区间,稳定于四个C波段ITU 100GHz标准信道波长。波长误差在0.01～0.03nm范围内,边模抑制比大于40dB。该方案对可调谐三段式DBR激光器的实用化具有重要意义。     

基于端口的VLAN划分与信息共享的实现

为解决以太网的广播问题和安全性问题,可采用基于端口的虚拟局域网VLAN划分技术。笔者阐述了VLAN划分的意义,介绍了利用交换机进行基于端口的VLAN划分时交换机配置及PC配置的详细操作。VLAN划分后利用ping命令验证其正确性,测试表明VLAN中可以实现信息安全共享,满足实际应用要求。     

提高信号到达角估计精度的新方法

现有的基于矢量天线估计信号到达角的方法都没有利用阵列的孔径信息,这样的估计虽然是无模糊的,但精度不高.该文在现有估计方法的基础上进一步利用阵列的孔径信息,通过迭代搜索的方法改善到达角估计的精度,数值模拟结果证明该方法是有效的.     

Construction of low-density parity-check codes by superposition

This paper presents a superposition method for constructing low-density parity-check (LDPC) codes. Several classes of structured LDPC codes are constructed. Codes in these classes perform well with iterative decoding, and their Tanner graphs have girth at least six.