On the concepts used in return stroke models applied in engineering practice

In this paper, we investigate the similarities between two basic concepts used in engineering return stroke models. In one type, the current propagation (CP) models, the return stroke channel merely acts as a medium for the current propagation with the driving source being at the ground. In the other type, the current generation (CG) models, the current sources (corona current) are assumed to be distributed along the return stroke channel. Our analysis shows that popular CP models, such as transmission line (TL), the modified transmission-line exponential (MTLE) model , and the modified transmission-line linear (MTLL) can be converted to CG models leaving the temporal and spatial variation of the return stroke current the same. In this alternative representation, the equivalent corona currents of the TL, MTLE, and MTLL models are bipolar, indicating initial deposition and subsequent removal of positive charge from the channel. This knowledge is applied to construct a simple CG model which generates electromagnetic fields identical to those obtained with the TL model at short times, but free from the difficulties associated with the latter at longer times.     

On the Relationship Between the Signature of Close Electric Field and the Equivalent Corona Current in Lightning Return Stroke Models

Engineering return stroke models can be categorized either as current generation (traveling current source type) models or current propagation (transmission line type) models. The current generation models are described among other parameters by a corona current distributed along the channel. Recent studies show that there is equivalence between the models of current generation and current propagation types. Due to this equivalence, any engineering return stroke model of current propagation type can be described in terms of an equivalent corona current per unit channel length. The measurements conducted within 10–500 m from triggered lightning flashes show that the electric field of subsequent return strokes at these distances flattens within 15 $mu$ s or so. In this paper, the constraints imposed by this feature on the temporal and spatial variation of the equivalent corona current are investigated. The results show that in order for the close fields to flatten within 15 $mu$s or so, the equivalent corona current, should be bipolar and the corona current wave shape at late times should be identical to that of the longitudinal current time derivative. This is in contrast to most of the engineering models of current generation type, in which the corona current is assumed to be unipolar.      

Comments on 'On the concepts used in return stroke models applied in engineering practice'

For original paper see V. Cooray, ibid., vol.45, no.1, p.101-8, (2003). Cooray's paper is an important contribution to the lightning return stroke modeling literature. It shows that any engineering model implying a lumped current source at the lightning channel base - any transmission-line (TL) type model - can be formulated in terms of sources distributed along the channel and progressively activated by the upward-moving return stroke front. Comments are made on Cooray's division of the total charge density into its "deposited" and "transferred" components. The charge-density formulation in engineering return-stroke models by other ways is discussed.     

A device model for the amorphous-silicon staggered-electrodethin-film transistor

A model for the amorphous-silicon (a-Si) staggered-electrode thin-film transistor (TFT) that incorporates gate-voltage dependent mobility for channel current and space-charge-limited current effects for the source and drain contacts is discussed. This model is in excellent agreement with TFT data over a wide range of applied voltages and for various channel lengths. For the devices measured, the TFT current depends more sensitively on effective channel mobility than on space-charge-limited current through the a-Si layer, but the latter is responsible for current crowding at low drain voltage. Because of the two-dimensional current flow under the contacts, their equivalent lumped element model exhibits a different power law behaviour than that for one-dimensional current flow in an n⁺-i-n⁺ structure. It also shows that a peak in the differential conductance curve at low drain voltage is a sensitive indicator of current crowding and implies a superlinear equivalent lumped element in series with the intrinsic TFT     

Organic Field‐Effect Transistors and Unipolar Logic Gates on Charged Electrets from Spin‐On Organosilsesquioxane Resins

Controllable shifting of threshold voltage and modulation of current in organic field‐effect transistors (OFETs) is demonstrated, resulting in the formation of unipolar inverters by making use of space‐charge electrets. Prior to the deposition of the organic semiconductor (OSC), negative corona charges are injected and trapped in the bulk of the organosilsesquioxane glass resin gate dielectrics. The effective surface potential is controlled by the corona‐charging and subsequent annealing process. It is found that the shift of the transfer characteristics is governed by the electrostatic induction effects of the charged gate electrets, and this observed shift can be related to the surface potential of the layer next to the transistor channel. The process control, efficiency, and long‐term stability of charge storage in spin‐on organosilsesquioxane glass resins are sufficient to enable the construction of simple unipolar inverters and to allow for circuit tuning. New OFET unipolar inverters with an enhancement‐mode driver and a depletion‐mode load are presented, composed of only two simple OFETs with the same channel dimensions and the same p‐type OSC on charged electrets. This design allows the implementation of full‐swing organic logic circuits and illustrates a potential process simplification for organic electronics.     

全双极WTA神经网络模型的光学实现

本文给出一种用光学方法实现全双极WTA三层神经网络模型的新方案及其实验结果.在全双极情况下.中间层第i个神经元的输入值由两部分组成,一部分为单极神经元状态时中间层该神经元的输入值,另一部分为对应存储模式的反模式和的二分之一.其中常数1/2可通过将每一输入神经元分成相等的两部分,使其中一部分为全透,另一部分为完全不透来实现.实验表明全双极系统比原先的单极系统具有更高的存储容量和寻址能力.     

Circuit topologies for single-phase voltage-doubler boostrectifiers

A new family of single-phase voltage-doubler PWM (pulse width modulated) boost rectifiers is presented. By examining the switching states of several standard single-phase boost rectifier circuits, three characteristic PWM voltage switching patterns are identified: unipolar PWM; bipolar PWM; and phase-adjusted unipolar PWM. From this analysis, an equivalent family of voltage-doubler rectifiers is derived. When high output voltages are required, voltage-doubler rectifiers are shown to be able to generate AC line currents with the lowest current distortion. The circuits presented are examined using circuit simulators and experimental results     

Optimal current for minimum thermal noise operation ofsubmicrometer MOS transistors

The wide band noise voltage (equivalent thermal noise voltage at the gate) of a submicron MOSFET, working in saturation, exhibits a minimum value at a certain drain current. This is supported by measurements and theoretical analysis based on a suitable thermal noise model. This macroscopic noise model attributes the thermal noise of the drain current to the superposition of two noise sources originating from two separate regions of the transistor's channel (a gradual channel approximation region and a saturation region). The existence of a minimum of the noise spectral density at an optimum drain current (I_opt), is well proved by measurements and is contradictory to the predictions of the current simulation program with integrated circuit emphasis (SPICE) models. An empirical way for evaluating analytically I_opt is given. The fact of the existence of a noise minimum for a submicron MOSFET, brings a phenomenological equivalence to the bipolar transistor and GaAs MESFET when they are employed at the first stage of an amplifier     

基于变换域的OFDM信道估计算法分析

首先对目前OFDM系统中常用的信道估计算法进行了介绍,其次对离散采样信道做出说明,并以此为基础,着重分析传统的基于离散傅里叶变换的信道估计滤波算法,即对接收导频LS(最小二乘)算法估计,通过IDFT转换到时域滤波后,由DFT转换到频域获取信道系数.最后给出了一种基于变换域滤波估计算法.由理论分析和工程实践平台中的仿真验证得出,基于提出的滤波矩阵,此DFT变换域算法性能较传统的DFT滤波方法有较大提升.     

Doppler Shift and Capacity Analysis of MIMO Antenna Arrays in a Novel 3D Geometric Channel Model

One of the fundamental research areas in wireless communications is the development of realistic models that can efficiently and accurately describe the wireless propagation channel. In multi-path environments, the received signal frequency constantly varies as a result of the relative motion between the receiver and transmitter. In this paper, we bridged a novel 3D geometric channel model and the multiple-input and multiple-output (MIMO) antenna array, analyzed deeply the probability density functions (PDFs) of the Doppler shift (DS), its variance, and characteristic function and etc. For the particular channel model, analytical expressions for PDFs of the channel model and the DS of mobile station (MS) due to its motion, have been derived. Based on the analysis, we investigated MIMO receiving performance. Also developed a geometric channel model, and was firstly in the asymmetric geometry literature due to a directional antenna array employed at base station. For this channel model, it was assumed that each multi-path component of the propagating signal undergoes one bounce. It was also assumed that the scattering objects around MS, could be expressed as Gaussian and exponential density models, which are more suitable to simulate outdoor and indoor environments.

     

Dynamic-stress-induced dielectric breakdown in ultrathinnitride/oxide stacked films deposited on rugged polysilicon

The dielectric breakdown characteristics of thin reoxidized Si₃N₄ films on both smooth and rugged poly-Si have been studied under dynamic stressing (unipolar and bipolar) with frequencies of up to 500 kHz. For capacitors with smooth poly-Si the time to breakdown (t_BD) increases with frequency under unipolar stressing with positive gain bias, whereas it slightly decreases with frequency under unipolar and bipolar stressing with negative gate bias. For capacitors with rugged poly-Si, t_BD increases with frequency in all cases     

Influence of pulsed DC current stress on electromigration results in AlCu interconnections; analysis of thermal and healing effects

Electromigration experiments have been performed on AlCu interconnect structures subjected to unipolar or bipolar current stresses. Activation energy values were similar between AC and DC stress suggesting that the same failure mechanisms occurred in both cases. From 100 Hz to 1 MHz, the results followed an average current model for which the sample is stressed by an efficient current density averaged over the period and Joule heating is taken into account with an original thermal model. The average current model was also verified for bipolar currents and provided very large increase of median time to failure when symmetric signals were considered. Asymmetric signals lead to unipolar electromigration behavior. At 1 Hz, the results tend toward an on time model for which matter diffusion occurred only during the application of the current.     

Vector Channel Estimation and Multiuser Detection for Multicarrier DS CDMA in Time and Frequency Selective Fading Channels

In a multicarrier direct-sequence code-division multiple access (MC DS CDMA) system, different fading channels for different users and/or different carriers are correlated in general; thus a vector channel model is more appropriate than disjoint scalar channel models. For multiuser MC DS CDMA systems, we propose (1) a generalized vector autoregressive model which accounts for correlation between different user/carrier fading channels, (2) the use of a two-phased algorithm to obtain the proposed model’s parameters, and (3) a receiver structure that consists of a generalized decorrelator followed by maximal-ratio combining (MRC) of uncorrelated carrier channel outputs of each user. The estimated fading coefficients provide the necessary quantities to MRC. The computer simulation results show that the proposed scheme has performance close to the case in which the channel is perfectly known, and outperforms separate scalar channel estimation case.     

Issues of Modeling Fading Dispersive Channels in DS／CDMA Communications

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Laser Radar Receiver Channel With Timing Detector Based on Front End Unipolar-to-Bipolar Pulse Shaping

An integrated receiver channel for a pulsed time-of-flight laser range finder is presented based on a timing discrimination principle in which the incoming unipolar detector current pulse is converted to a bipolar pulse at the front end of the receiver channel. Thus no optical or electrical gain control is needed within the dynamic range of the receiver, which according to measurements is 1:3000 with a timing walk error of $pm$55 ps ( $pm$8 mm in distance). The minimum detectable input signal current is about 1.3 $muhbox{A}$ at an SNR of 10 with a bandwidth of 200 MHz. The circuit is realized in a 0.35$ muhbox{m}$ SiGe BiCMOS process and consumes 220 $~$mW of power.      

青藏高原东北部双极性窄脉冲放电物理参数模型反演

利用青藏高原东北部青海地区三维定位数据和双极性窄脉冲电场波形同步数据,基于三种模型和粒子群优化算法,拟合并反演了双极性窄脉冲放电通道中电流峰值、通道尺度等特征参数,统计分析了不同传输线模型拟合得到的双极性窄脉冲的物理参数.结果表明：在基于流光-先导的准静电场传统空气热击穿机制的TL模型和MTLK模型中,相比TL模型,MTLK模型考虑了电流传输过程中能量的损耗,更符合实际;而基于准静电场相对逃逸雪崩击穿机制的MTLEI模型,相比较另两种传输线模型,表现为更窄放电通道内存在长时间随着高度呈指数递增的电流强度.三种传输线模型反演得到的电偶极矩范围为100～800 C·m,而更高处发生的双极性窄脉冲事件(narrow bipolar event, NBE)通常具有更大电偶极矩.     

光码分多址系统中双极性码的应用研究

基带双极性光码分多址(OCDMA)系统，采用码的极性转换思想，实现在考虑信道非理想下的单极性信道中传输双极性码。研究了Gold序列，m序列在系统中的应用和传输信道对光码分多址系统性能的影响。仿真得到了双极性码在光码分多址系统的自相关和互相关特性。基于不同码字的光码分多址系统用户数与误码率的关系和基于非理想传输信道的系统性能影响曲线。结果表明，Gold序列和m序列适合双极性光码分多址系统，但Gold序列较m序列更适合应用到双极性光码分多址系统，当系统激活用户数为20，系统误码率分别达到了10^-9，10^-7，非理想传输信道对系统性能有较大的影响(恶化近20dB)，在系统研究中不可忽略。     

Analysis and design of the dual-gate inversion layer emitter transistor

The dual-gate inversion layer emitter transistor (DGILET) is a device in which the injection of minority carriers takes place from an inversion layer formed under a MOS gate. Therefore, the device can be switched between MOS and bipolar modes using the gate giving the means to achieve a superior combination of low conduction losses and low switching losses. The structure of the device and operation in both the unipolar and bipolar modes are described in detail. Devices have been fabricated on bulk silicon wafers using junction isolation and experimental results confirm the expected superior performance. In particular, the results confirm predictions that if the inversion layer injector is properly designed, the voltage snapback that occurs during the transition between unipolar and bipolar modes can be completely suppressed. This can be achieved with a compact structure in contrast to the extended structures required in anode-shorted lateral insulated gate bipolar transistor (LIGBTs). An equivalent circuit for the DGILET is presented and the control of the minority carrier injection is also analyzed. Experimental results show that the DGILET can switch at speeds approaching those characteristic of MOSFETs with operating current densities comparable to LIGBTs. The results show that the DGILET offers lower overall losses than an LIGBT at switching frequencies above about 10 kHz.     

Pacemaker interference by 60-Hz contact currents

Contact currents occur when a person touches conductive surfaces at different potentials, thereby completing a path for current flow through the body. Such currents provide an additional coupling mechanism between the human body and external low-frequency fields. The resulting fields induced in the body can cause interference with implanted cardiac pacemakers. Modern computing resources used in conjunction with millimeter-scale human body conductivity models make numerical modeling a viable technique for examining any such interference. An existing well-verified scalar-potential finite-difference frequency-domain code has recently been modified to allow for combined current and voltage electrode sources, as well as to allow for implanted wires. Here, this code is used to evaluate the potential for cardiac pacemaker interference by contact currents in a variety of configurations. These include current injection into either hand, and extraction via: 1) the opposite hand; 2) the soles of both feet; or 3) the opposite hand and both feet. Pacemaker generator placement in both the left and right pectoral areas is considered in conjunction with atrial and ventricular electrodes. In addition, the effects of realistically implanted unipolar pacemaker leads with typical lumped resistance values of either 20 kohms and 100 kohms are investigated. It is found that the 60-Hz contact current interference thresholds for typical sensitivity settings of unipolar cardiac pacemaker range from 24 to 45 microA. Voltage and electric field dosimetry are also used to provide crude threshold estimates for bipolar pacemaker interference. The estimated contact current thresholds range from 63 to 340 microA for bipolar pacemakers.     

Interface trap generation by FN injection under dynamic oxide fieldstress

Interface trap generation under dynamic (bipolar and unipolar) and dc oxide field stress has been investigated with the charge pumping technique. It is observed that regardless of stress type, whether dc or dynamic (bipolar or unipolar), and the polarity of stress voltage, interface trap generation starts to occur at the voltage at which Fowler-Nordheim (FN) tunneling through the oxide starts to build up. For positive voltage, interface trap generation is attributed to the recombination of trapped holes with electrons and to the bond breaking by the hydrogen (H and H⁺) released during stressing. For negative voltage, in addition to these two mechanisms, the bond breaking by energetic electrons may also contribute to interface trap generation. The frequency dependence of interface trap generation is also investigated. Interface trap generation is independent of stressing frequency for unipolar stress but it shows a frequency dependence for bipolar stress