共查询到20条相似文献,搜索用时 0 毫秒
1.
《Electromagnetic Compatibility, IEEE Transactions on》1988,30(4):538-552
The problem of threshold or weak-signal detection in highly nonGaussian EMI is extended to vector fields, and narrow-band signals and interference. The emphasis is on a canonical formulation, illustrated by a number of specific examples. Spatial sampling with adaptive beam forming, as well as temporal sampling and all relevant vector field components, must be included for maximum processing gain. New results for a canonical theory of these vector detection cases are presented. Jointly and asymptotically locally optimum algorithms and performance measures are obtained. These results provide statistical-physical models of the EMI environment, and they include first-order probability distributions of vector EMI noise fields and received processes, with specific examples of EMI fields generated by randomly distributed electric and magnetic dipole sources, as well as more general sources. The effects of beamforming, selfdirecting beams, multiple field components, fading, and Doppler `smear' on signal detectability are included 相似文献
2.
Krassowska W 《IEEE transactions on bio-medical engineering》2003,50(1):33-40
Polarization of individual cells ("sawtooth") has been proposed as a mechanism for field stimulation and defibrillation. To date, the modeling work has concentrated on the myocardium with periodic spatial structure; this paper investigates potentials arising in cardiac fibers with random spatial structure. Ten different random fibers consisting of cells with varying length (l(c) = 100 +/- 50 microm), diameter (d(c) = 20 +/- 10 microm), thickness of extracellular space (t(e) = 1.18 +/- 0.59 microm), and junctional resistance (R(j) = 2 +/- 1 M(omega)) are studied. Simulations demonstrate that randomizing spatial structure introduces to the field-induced potential (V(m)) a randomly varying baseline, which arises due to polarization of groups of cells. This polarization appears primarily in response to randomizing t(e); R(j), l(c), and d(c) have less influence. The maximum V(m) increases from 3.5 mV in a periodic fiber to 20.5+/-4.7 mV in random fibers (1 V/cm field applied for 5 ms). Field stimulation threshold E(th) decreases from 6.9 to 1.59 +/- 0.43 V/cm, which is within the range of experimental measurements. Thresholds for normal and reversed field polarities are statistically equivalent: 1.59 +/- 0.43 versus 1.44 +/- 0.41 V/cm (p value = 0.453). Thus, V(m) arising due to random structure of the myocardium may play an important role in field stimulation and defibrillation. 相似文献
3.
Even in the absence of external perturbation to the human cardiovascular system, measures of cardiac function, such as heart rate, vary with time in normal physiology. The primary source of the variation is constant regulation by a complex control system which modulates cardiac function through the autonomic nervous system. Here, we present methods of characterizing the statistical properties of the underlying processes that result in variations in ECG R-wave event times within the framework of an integrate-and-fire model. We first present techniques for characterizing the noise processes that result in heart rate variability even in the absence of autonomic input. A relationship is derived that relates the spectrum of R-R intervals to the spectrum of the underlying noise process. We then develop a technique for the characterization of the dynamic nature of autonomically related variability resulting from exogenous inputs, such as respiratory-related modulation. A method is presented for the estimation of the transfer function that relates the respiratory-related input to the variations in R-wave event times. The result is a very direct analysis of autonomic control of heart rate variability through noninvasive measures, which provides a method for assessing autonomic function in normal and pathological states. 相似文献
4.
J J Struijk J Holsheimer B K van Veen H B Boom 《IEEE transactions on bio-medical engineering》1991,38(1):104-110
The effect of electrical stimulation with several electrode combinations on nerve fibers with different orientations in the spinal cord was investigated by computing the steady-state field potentials and activating functions. At first an infinite homogeneous model was used while secondly the spinal cord and its surrounding tissues were modeled as an inhomogeneous anisotropic volume conductor. The effect of mediodorsal epidural stimulation was calculated. It was concluded that with cathodal stimulation, mediodorsally in the epidural space, longitudinal fibers are depolarized, but dorsoventral ones are hyperpolarized. With anodal stimulation the opposite will occur. It was found that parameters substantially affecting the potential distribution in the dorsal columns are the conductivity of the white matter and the width and the conductivity of the csf layer. 相似文献
5.
A new effect associated with Metal-Oxide-Silicon Field-Effect-Transistors (MOS-FET's) is presented in this paper. MOS-FET's show an increase of threshold voltage with decreasing ratio of channel width to gate depletion width. This narrow channel effect is explained by means of geometrical edge effects. With decreasing channel width the transition from the field oxide depletion region to the gate oxide depletion region becomes comparable to the gate width and cannot be neglected in the derivation of the threshold voltage equation.A theoretical model is given to explain the influence of decreasing channel width on the threshold voltage as well as on other electrical parameters. This theoretical model is in good agreement with experimental results given in this paper. 相似文献
6.
Anderson W. Shah V. Curtis L. Johnson A. Kilmer J. 《Lightwave Technology, Journal of》1987,5(2):211-217
The mode-field diameter (MFD) of a single-mode fiber can be measured using several different techniques. The extent to which these techniques are consistent depends upon the definition used to compute the MFD and upon the fiber design. In this paper, three measurement techniques (far-field scan, offset joint, and knife-edge scan) are applied to the measurement of both conventional step-index fibers and to non-step-index dispersion-shifted fibers. Using the far-field second moment definition, which is often called the "Petermann 2" definition, measurements made using these three techniques agree to within 1.6 percent. 相似文献
7.
Mode field diameter measurements in single-mode optical fibers 总被引:3,自引:0,他引:3
Artiglia M. Coppa G. Di Vita P. Potenza M. Sharma A. 《Lightwave Technology, Journal of》1989,7(8):1139-1152
The role of the mode field diameter in the characterization of single-mode fibers is examined. The most relevant definitions of this parameter are reviewed, and a comparative analysis of methods for its measurement is performed. All the discussed measurement methods have reached a repeatability and reproducibility which are quite satisfying. Emphasis is given to the requirements posed by the new fiber designs, such as the polarization-maintaining structures. Most of the discussed techniques have been industrialized, and a number of instruments based on them are commercially available; however, it is predicted that the evolution of fiber design will impose new requirements on some of these instruments 相似文献
8.
在近场光学显微镜、高功率光纤耦合器等应用场合,需要对熔锥光纤中的光场分布有更精确的了解。基于锥形函数,应用时域有限差分法模拟了熔锥光纤中的光场分布,分析了熔锥参数对单锥形光纤和双锥形光纤传输效率的影响,并与使用基模高斯近似得到的结果进行了比较。结果表明:使用基模高斯近似分析熔锥光纤的光场会产生较大误差;在形态分布对称的熔锥光纤中光场分布是非对称性的,基模场入射熔锥光纤后会在熔锥光纤中激发出泄漏模,对于单锥形和双锥形结构的熔锥光纤,初始熔融长度和拉伸长度对传输效率均有影响。应根据具体的应用情况,确定熔锥参数,以获得更高的传输效率。 相似文献
9.
动态阈值nMOSFET阈值电压随温度退化特性 总被引:1,自引:0,他引:1
对动态阈值nMOSFET阈值电压随温度退化特性进行了一阶近似推导和分析。动态阈值nMOSFET较之普通nMOSFET,降低了阈值电压温度特性对温度、沟道掺杂浓度及栅氧厚度等因素的敏感程度。讨论了动态阈值nMOSFET优秀阈值电压温度特性的内在机理。动态阈值nMOSFET优秀的阈值电压随温度退化特性使之非常适合工作于高温恶劣环境。 相似文献
10.
The authors have studied activation of nerve fibers by pairs of Pt-Ir wire electrodes implanted within single fascicles of the nerve innervating the gastrocnemius muscle in cats. The purpose of this study was to determine if these intrafascicular electrodes can activate nerve fibers in different fascicles independently of each other and if they can also be used to activate separate subsets of axonal populations within a single fascicle. The average overlap of activated nerve fiber populations was 5.5% between fascicles and 27% within a fascicle, indicating that such selective activation is possible with these electrodes 相似文献
11.
Electric field stimulation of excitable tissue 总被引:6,自引:0,他引:6
Examines the transmembrane voltage response of an unmyelinated fiber to a stimulating electric field from a point current source. For subthreshold conditions, analytic expressions for the transmembrane potential, vm, are developed that include the specific effects of fiber-source distance, h, and time from the onset of the stimulus, T. Suprathreshold effects are determined for two examples by extending the analytical results with a numerical model. The vm , response is a complex evolution in time, especially for small h, that differs markedly from the “activating function”. In general, the subthreshold response is a good predictor of the wave shape of the suprathreshold vm but a poor predictor of its magnitude. The subthreshold response also is a good (but not a precise) predictor of the region where excitation begins 相似文献
12.
We present direct methods for determination of equivalent-step-index (ESI) parameters and file modal field by measurement of the wavelength dependence of the far-field intensity in single-mode fibers. A comparison is made with the spot-size method commonly used for characterization of such fibers. A new technique for measuring the LP11 mode cutoff wavelength is also presented. 相似文献
13.
Struijk J.J. Holsheimer J. Boom H.B.K. 《IEEE transactions on bio-medical engineering》1993,40(7):632-639
In epidural spinal cord stimulation it is likely not only that dorsal column fibers are activated, but also that dorsal root fibers will be involved as well. In this investigation a volume conductor model of the spinal cord was used and dorsal root fibers were modeled by an electrical network including fiber excitation. The effects of varying some geometric fiber characteristics, as well as the influence of the dorsal cerebrospinal fluid layer and the electrode configuration on the threshold stimulus for their excitation, were assessed. The threshold values were compared with those of dorsal column fibers. The results of this modeling study predict that, besides the well known influence of fiber diameter, the curvature of the dorsal root fibers and the angle between these fibers and the spinal cord axis are a major influence on their threshold values. Because of these effects, threshold stimuli of dorsal root fibers were relatively low as compared to dorsal column fibers. Excitation of the dorsal root fibers occurred near the entry point of the fibers 相似文献
14.
The transmitted near- and far-field pattern methods, measuring the mode field diameter of the fundamental LP01 mode in a single-mode fiber are studied. On the basis of measurement error evaluation, the optimum measurement conditions for typical single-mode fibers are determined. Actual measured values for nearly ideal step-index fibers are in good agreement with the theoretical predictions (within 0.2 μm with a standard deviation of 0.05 μm). The mode field diameter measurement by the transmitted field pattern methods is useful for the study of the structural optimization of the single-mode fibers. 相似文献
15.
Based on experimental and theoretical studies of n- and p-channel polysilicon thin film transistors with gate W/L ratios from 0.3 to 3.3, we have demonstrated that the threshold voltage extracted from gate to channel capacitance data results in field effect mobility parameters which are independent of device geometry. The parameters extracted using this Vt allow us to reproduce the I-V characteristics of the n- and p channel TFTs over wide ranges of bias voltages and gate sizes. The Cgc-VGS characteristics of polysilicon TFTs are strongly affected by the trapping and de-trapping of carriers. As a result, the measured Cgc characteristic is a function of measurement frequency and gate length. However, we demonstrate that to the first order, the frequency dispersion of the Cgc curve can be related to the effective carrier transit time determined using the VGS dependent field effect mobility 相似文献
16.
Modeling of magnetic field stimulation of bent neurons 总被引:3,自引:0,他引:3
The authors consider a simple model of magnetic stimulation of a long bent neuron located in a semi-infinite volume conductor with a planar interface. It is shown that the stimulating coil characteristics (size, shape and location) and the neuron shape affect the location of the stimulation. The activating function, defined as the electric field derivative along the neuron, has two components. One component depends on the derivative of the electric field along the straight section of the neuron, and the other on the field magnitude. The maximal stimulation point is at the bent part of the nerve and its position depends on the nerve shape and coil parameters. The analysis also has shown a better performance (a stronger stimulus) for a double-circular (figure eight) coil than for a double-square coil 相似文献
17.
Struijk J.J. Holsheimer J. van der Heide G.G. Boom H.B.K. 《IEEE transactions on bio-medical engineering》1992,39(9):903-912
An electrical network model of myelinated dorsal column nerve fibers is presented. The effect of electrical stimulation was investigated using both a homogeneous volume conductor and a more realistic model of the spinal cord. An important feature of dorsal column nerve fibers is the presence of myelinated collaterals perpendicular to the rostro-caudal fibers. It was found that transmembrane potentials, due to external monopolar stimulation, at the node at which a collateral is attached, is significantly influenced by the presence of the collateral. It is concluded that both excitation threshold and blocking threshold of dorsal column fibers are decreased up to 50% compared to unbranched fibers. 相似文献
18.
19.
《Electron Devices, IEEE Transactions on》1987,34(5):953-956
The transient and steady-state velocity-field characteristics of InP at 300 K are calculated for short samples by using a Monte Carlo model. It is found that the threshold field for negative differential resistance increases with a decrease in the sample length, but the peak-valley velocity ratio is almost independent of the sample length. 相似文献
20.
Evaluation of the cable model for electrical stimulation of unmyelinated nerve fibers 总被引:2,自引:0,他引:2
The cable model, used to calculate the membrane potential of an unmyelinated nerve fiber due to electrical stimulation, is reexamined under passive steady-state conditions. The validity of two of the assumptions of the cable model are evaluated, namely that the membrane potential be a function of the axial coordinate only and that the extracellular potential due to the presence of the nerve fiber be negligible. The membrane potential calculated from the passive steady-state cable model is compared with the membrane potential obtained from an analytical three-dimensional (3-D) volume conductor model of a nerve fiber. It is shown that for very small electrode-fiber distances (of only a few fiber radii), both assumptions are violated and the two models give quite different results. Over a wide range of the electrode-fiber distance (about 0.1 mm to 1 cm), both assumptions are fulfilled and the two models give approximately the same results. For very large distances (more than 10 cm, independent of fiber diameter) only the second assumption is satisfied, but a modification of the activating function of the cable model allows to calculate the membrane potential in agreement with the 3-D model. 相似文献