An inverse scattering approach based on the differential E-formulation

An inverse scattering technique based on the differential E-formulation in the frequency domain is proposed. The inversion is achieved by minimizing a cost functional, taking into account the discrepancy between measured and estimated field values, while the Helmholtz wave equation is set as constraint. The Fre/spl acute/chet derivatives of the cost functional with respect to the scatterer properties are derived analytically by means of the calculus of variations. Edge elements are used for the numerical treatment of the problem.     

A Nonresonant Perturbation Theory

This paper presents a theory for a nonresonant perturbation technique for the measurement of electric and magnetic field strengths within a device. Most presently employed perturbation field strength measurements require the use of a resonance technique. In the technique discussed here, reflection coefficient measurements are made at the same frequency with, and without, a perturbing object placed at the point at which the field strength is to be measured. By these data, and by the equations derived and presented in this paper, the desired field strength can be calculated. The technique can be used for cavities that are too lossy to support resonance, and is suitable for cavities for which the resonant field configuration differs from the field configuration to be measured. In addition, this technique has the advantage that it permits the measurement of the phase, as well as the amplitude of the field.     

A study of HF field strength variation in India and its prediction

Measured field strength data pertaining to several HF broadcast transmissions monitored in India during 1987-90 has been analysed to study solar activity, local time and seasonal variations in the field-strength. Values tend to saturate at high solar activity levels when sunspot numbers are beyond 120. Variations in field strength have also been studied with varying ratios of operational frequency to maximum usable frequency (f_op/MUF) and equivalent vertical frequency of the links. Based on this study a prediction method for HF field strength has been suggested for India. Comparison of values estimated using this technique has been made with actual measured field strength values for different HF transmissions for different months. This method is to be particularly suitable for field operators     

Computation of harmonic comb filter weights

Harmonic comb filters are characterized in the frequency domain by notches arranged at equal intervals, and centered about DC. For a finite impulse response harmonic comb filter, the characteristic polynomial has its roots on the unit circle at equal angular intervals, symmetrically about 1+j0. A simple formula for the coefficients of such a polynomial and for their derivatives with respect to the frequency interval is presented. This result is applied as well to the computation of Butterworth filter parameters     

Novel nulling/balancing technique for fiber optic magneticgradiometer

A technique for balancing a fiber-optic magnetic gradiometer configured as a Mach-Zehnder interferometer is presented. The method uses different dither frequencies for nulling the DC field experienced by each transducer and a third frequency common to both transducers for detecting the gradient. The method overcomes the hysteresis when each transducer is separately nulled, and the balance is performable in real time. Noise spectra are shown in the near DC and a gradient sensitivity of 0.14 γ/ft-√Hz RMS at 0.22 Hz is demonstrated. This is believed to be the most sensitive direct measurement of a magnetic field gradient by a fiber-optic sensor that has been reported     

A new extraction method to determine bias-dependent source seriesresistance in GaAs FET's

A new method is proposed to determine bias-dependent source resistances for GaAs field-effect transistors (FET's). This method, which is a cold-FET measurement technique, utilizes the relations between the real part of the two-port impedances transformed from the measured S-parameters and their algebraic derivatives. It is based on the fact that the algebraic derivatives of the two-port resistances result in the simple form at the normal cold-FET condition. A bias-independent gate resistance is extracted at the pinched-off cold-FET condition to fulfill necessary and sufficient conditions in extraction. The proposed method is a direct measurement because only algebraic calculation is required, and it is general enough to need only one assumption of the laterally symmetric channel-doping profile. The deleterious results of dispersion (frequency dependence) and negative value in source resistances at the pinched-off cold-FET condition are explained by the effects of the leakage current and the on-wafer pad parasitics, respectively. The problem of deviation of α₂₁ and α₁₂ from 0.5 at the normal cold-FET condition is also resolved by deembedding the on-wafer pad parasitics. This method allows one to extract bias-dependent source resistances for GaAs FET's     

A failure of a numerical electromagnetics code to match a simpleboundary condition

It is shown that it is impossible to satisfy the boundary condition for a perfectly conducting hemisphere-capped dipole without creating a discontinuity in the first derivative of the scattered field on the boundary. Since solutions of Maxwell's equations are analytic in a source-free region, all field derivatives must be continuous in the region where the source-free equations are satisfied. The basis functions used by the generalized multipole technique (GMT) are solutions of Maxwell's source-free equations in a region which includes the scattering surface. Therefore, a GMT solution to the dipole problem can exist only in the sense that the boundary value error approaches zero as the number of basis functions approaches infinity. This in itself is not surprising, but the difficulty of matching the boundary condition at the discontinuity affects the convergence of the technique. For the method-of-moments (MOM) technique, where a source current exists on the scattering surface, it is not clear if a perfect boundary value solution to the dipole problem can be theoretically realized or not     

纳米晶FeCuNbSiB带材巨磁阻抗效应研究

采用微机电系统(MEMS)技术,在玻璃基片上制备了单层结构500℃退火的FeCuNbSiB带材样品。在l～40MHz下,研究了带材的巨磁阻抗(GMI)效应随外加磁场强度以及交流电流频率的变化关系。结果表明:纵向、横向巨磁阻抗效应变化率(GMI值)在5MHz、1.2kA/m和5MHz、8kA/m时,分别达到最大值15.6%和10.6%。     

Direct measurement of base drift field in bipolar transistors

The authors have developed a method to measure an effective base drift field and the base transit-time reduction factor of bipolar transistors, by measuring the excess phase of the base transport factor. This technique relies on measuring small-signal characteristics of the transistor at a low frequency and following the phase of the transconductance at the frequency approaching and exceeding the unit current gain frequency (f_T). With this technique, the authors verify that the effective drift inside the base of Si bipolar transistors decreases with increased base implantation energy and thermal treatment. Such directly measured drift-dependent base transport provides additional insight for optimizing processing used in bipolar technology development     

Use of Asymptotic Waveform Evaluation Technique in the Analysis of Multilayer Structures With Doubly Periodic Dielectric Gratings

The reflection and dispersion characteristics of multilayer structures that involve periodically implanted material blocks are obtained by using the MoM solution of the volume integral equation. The asymptotic waveform evaluation (AWE) technique is utilized to obtain a Pade approximation of the solution in terms of a parameter such as frequency or incident angle. The use of AWE technique enables a fast sweep with respect to the approximation parameter. Moreover, a robust method for extracting the dispersion characteristics of periodic structures via Pade approximation is proposed. The AWE procedure requires the calculation of high order derivatives of the complicated kernel function that consists of Green's functions for stratified medium. These derivatives are calculated by employing the automatic differentiation theory. The reflection coefficient, propagation constant and band diagram of the structure are obtained both via point-by-point simulations and through the use of AWE technique. It is observed that AWE technique increases the computational efficiency without losing accuracy.     

Frequency-domain upper bounds for signals on a multiconductor transmission line behind an aperture

This paper develops a technique for bounding the maximum voltages and currents at terminations of a muiticonductor transmission line (MTL) located behind an aperture-perforated conducting screen excited by an electromagnetic field in the frequency domain. The electromagnetic field is coupled through a small aperture as the excitation of a multiconductor transmission line behind the aperture. A model is presented in terms of external and internal sources which in turn create traveling waves on the multiconductor transmission line. These traveling waves transfer energy to the terminations. The energy at a termination is translated into voltages and currents from which the upper bounds are determined. These upper bounds are obtained using vector norms and associated matrix norms. The formulation is presented in the frequency domain to obtain useful upper bounds for analysis of multiconductor transmission line geometries with aperture excitation.     

Evaluation of MRI RF probes utilizing infrared sensors

The magnetic resonance imaging (MRI) coil's radio frequency (RF) field distribution has a strong effect on image quality as well as specific absorption rate. In this paper, a method of probing a coil's RF field distribution over any unoccupied region of the coil is presented. This technique is based on the use of infrared sensing. The proposed method was implemented and tested on a high field RF volume coil operating at 340 MHz. Very good agreement was achieved between the infrared measurements and numerical data obtained utilizing an in-house three-dimensional finite-difference time-domain package. The results demonstrate that the proposed technique is practical, robust, and efficient in making accurate measurements of the electric field distributions in loaded and unloaded MRI coils.     

基于频率和时间多重相关的实用化红外检测技术

提出了一种基于频率和时间多重相关的红外检测技术，即在原来通用的单一频率调制的基础上加上时间相关，使得各对探测点之间有确定的频率和时间关系。采用该技术组成的二维红外阵列检测系统可以抵抗直流红外辐射干扰以及相邻红外辐射源的干扰，从而使得采用高密度红外列阵的高精度检测成为可能。采用该技术的红外检测系统已经在公路车辆自动识别系统以及模拟雷场系统中得到成功运用，系统工作稳定可靠。     

Singularity in Green's function and its numerical evaluation

The free-space scalar Green's functionghas anR^{-1}singularity, whereRis the distance between the source and observation points. The second derivatives ofghaveR^{-3}singularities, which are not generally integrable over a volume. The derivatives ofgare treated as generalized functions in the manner described by Gel'fand and Shilov, and a new formula is derived that regularizes a divergent convolution integral involving the second derivatives ofg. When the formula is used in the dyadic Green's function formulation for calculating theEfield, all previous results are recovered as special cases. Furthermore, it is demonstrated that the formula is particularly suitable for the numerical evaluation of the field at a source point, because it allows the exclusion of an arbitrary finite region around the singular point from the integration volume. This feature is not shared by any of the previous results on the dyadic Green's function.     

渐近波形估计技术加速的频域有限差分法

本文提出了一种基于三维频域有限差分法(3D-FDFD)和渐近波形估计技术(AWE)计算三维微波结构S参数的快速算法。在分析中,将输入和输出端口的电场分别展开成某抽样频率点处的泰勒级数,通过Pad逼近及奇异值分解技术求出电场与频率的有理函数解析表达式,从而获得频带内的S参数。数值计算结果与已有结果吻台良好,并且与传统FDFD法相比,计算效率提高很多。     

Analysis of frequency selective surfaces using two-parametergeneralized rational Krylov model-order reduction

Generalized rational Krylov model-order reduction techniques are applied to the spectral Galerkin system describing frequency selective surfaces, resulting in surface reflection coefficient models that depend on both the frequency and the incident angle of the exciting wave. The procedure is composed of three steps: construction of the spectral Galerkin system, linearization of that system, and reduction of the linearized system. The linearization of the spectral Galerkin matrix is carried out using two-dimensional (2-D) polynomial interpolation and the generation of a “two-parameter” companion form of the polynomial system. The subsequent model-order reduction is based on the concept of generalized Krylov subspaces, which are defined in the text. It results in a small system with a frequency and incident angle dependent output that matches the two-parameter polynomial interpolant system transfer function and its derivatives at many points in the frequency/incident angle plane. The technique is applied to the characterization of several frequency selective surfaces, and numerical results that demonstrate the accuracy of the techniques over a broad band of frequencies and range of incident angles are presented     

Simultaneous time and frequency domain solutions of EM problemsusing finite element and CFH techniques

This paper describes an efficient method for both time- and frequency-domain solutions of electromagnetic (EM) field problems. In this method EM field problems are formulated using Laplace-domain finite element approach and are solved using complex frequency hopping (CFH) technique. CFH is a moment-matching technique which has been used successfully in the circuit simulation area for solution of large set of ordinary differential equations. Problems consisting of Dirichlet, Neumann and combined boundary conditions can be solved using the proposed algorithm to obtain both time and frequency responses. Several electromagnetic field problems have been studied using the new technique and the speed-up advantage (one to three orders of magnitude) compared to conventional finite element technique is demonstrated. A good agreement between numerical results obtained using the proposed method and the previously published results has been found     

High Frequency Resonance Features of Giant Magnetoresistance Multilayers

High frequency resonance technique was applied to study of magnetic multilayer Co/Cu films possess the giant magnetoresistance. The results of the ferromagnetic resonance study in 60GHz-80GHz and 1OGHz bands revealed low temperature changes of g-factor after annealing and a very small value of high frequency saturation magnetization. An abrupt change of the specimen impedance was registered in low field low field range for 10GHz. The static magnetic characteristics as hysteresis and magnetoresistance loops were obtained at room temperature for the planar orientation of external magnetic field.     

Transmission through axisymmetric, cascaded cylindrical cavities coupled by apertures - part I: structures with coaxial and circular-cylindrical cross-sections

A technique is presented for determining the field in a series of cascaded, axisymmetric cylindrical cavities excited by a /spl phi/-independent source. The constituent cavities are either coaxial or circular-cylindrical. The field in each cavity is expressed in terms of the electric field in the apertures at the interfaces where adjacent cavities join. Coupled integral equations are formulated in the frequency domain from which these aperture fields are computed. From knowledge of the aperture fields, the complete field in the cavity structure can be determined. The frequency-domain data are used to compute the time-domain field in the system of cavities. Cavities were constructed and experiments performed to corroborate the computed data.