Application of the TLM method to two-dimensional scattering problems

In this paper the application of the TLM method to electromagnetic scattering problems is investigated. To achieve the full computational benefits of using a time-domain numerical method, pulse excitation can be applied together with Fourier transformation of an output impulse response to yield frequency-domain results over a wide bandwidth. The configuration of the mesh is the same as that used in FD–TD scattering simulations (i.e., separation of the simulation space into total and scattered field regions). To terminate the exterior boundaries of the mesh, absorbing boundary conditions are applied to absorb the outward travelling waves. The effect of the absorbing boundaries on both time- and frequency-domain results is investigated. Frequency-domain scattered far-field patterns are presented for perfectly conducting square and circular cylinders illuminated by an incident plane wave (TM case). Forward and back scattering cross-section results obtained from a single TLM simulation indicate both the accuracy and efficiency of the method for calculating frequency-domain results over a wide bandwidth.     

A quasi‐planar incident wave excitation for time‐domain scattering analysis of periodic structures

We present a quasi‐planar incident wave excitation for time‐domain scattering analysis of periodic structures. It uses a particular superposition of plane waves that yields an incident wave with the same periodicity as the periodic structure itself. The duration of the incident wave is controlled by means of its frequency spectrum or, equivalently, the angular spread in its constituting plane waves. Accuracy and convergence properties of the method are demonstrated by scattering computations for a planar dielectric half‐space. Equipped with the proposed source, a time‐domain solver based on linear elements yields an error of roughly 1% for a resolution of 20 points per wavelength and second‐order convergence is achieved for smooth scatterers. Computations of the scattering characteristics for a sinusoidal surface and a random rough surface show similar performance. Copyright © 2006 John Wiley & Sons, Ltd.     

A study of parallel tuner with voltage‐dependent capacitance

A study of varactor tuned LC circuits is presented. Nonlinear time domain circuit differential equation is rewritten in terms of phase plane variables, which can then be solved in closed form. General expressions are derived, which are applicable to any capacitance–voltage relationship. Two types of circuit structures, namely single‐ended and balanced, with MOS diodes as the variable capacitance elements, are specifically considered. The nature of the voltage waveforms across the two circuits is determined by phase plane plots. Variation of voltage with time is calculated numerically. It is shown that the voltage waveform for the single‐ended circuit is asymmetric, with higher harmonics present. Furthermore, the fundamental resonant frequency is dependent on amplitude of oscillation and could decrease to 94% of its small signal value for large voltage swings. Near 34% control over frequency is calculated, for a bias voltage range of 8 to 1. On the other hand, the balanced structure results in symmetric voltage waveform, with negligible harmonic content. Dependence of frequency on amplitude is weak, only decreasing to 98% of its small signal value, for the largest swings. The tuning range is marginally improved by the balanced structure. The results are compared with those obtained from Fourier‐based calculations and experimental data in literature, and good agreement is obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

方孔金属线网屏两侧电磁场的精确表达式

利用层阻抗边界条件模拟方孔金属线网屏对电磁场的影响.推导出平面线网屏两侧谐变垂直电、磁偶极子的反射和透入电磁场精确解析表达式.应用圆柱波函数的球面波展开式和超几何函数理论,场分量中的Sommerfeld型积分表示为绝对、快速收敛的球面波函数集展开式,其展开系数是以线网几何参量为宗量的勒让德函数.该文的结果便于场的计算和分析并能够为线网屏平面的电磁屏蔽和场的反射等相关工程电磁场问题的研究提供的电磁场解析表达式.     

基于FDTD的粗糙地面宽带前向电磁散射研究

采用指数型分布粗糙面模拟实际的粗糙地面,运用FDTD方法研究了超宽带高斯脉冲电磁波入射粗糙地面时的宽带前向电磁散射问题;通过数值计算得到前向散射系数随频率的变化曲线,并与时谐场源激励下的结果进行了对比;分析了不同的粗糙地面高度起伏相关长度、均方根高度,土壤湿度和入射角下前向散射系数随频率的变化关系,得到了指数型分布粗糙地面的宽带前向电磁散射特性。数值计算结果表明各参数的变化对前向电磁散射系数的影响比较明显,入射波不同极化方式下的前向散射系数也有显著变化。     

Digital filter model of power transmission line

An analytical formulation for power system electromagnetic transient analysis has been developed recently in which successive transformations are made between the frequency domain, the z plane, and the time domain. In this sequence, transmission line forward impulse responses and surge impedence functions formed in the z plane are similar to response functions typical of digital filters. From this starting point, the equations of a multiconductor power transmission line are given in scattering form. It is then shown how these lead to a digital filter model, which draws on the now standard canonical realizations of digital filter design. Procedures are given by which the z-plane functions of this filter representation are related to power transmission line parameter sets over the range of frequencies relevant to power system electromagnetic transients. The paper then considers two application areas of the filter models derived.     

超宽带脉冲信号的一种频域采样和接收方法

对纳秒级的超宽带脉冲信号进行采样需要设计采样速率高达数十GHz,且必须具有较低功耗的超高速模数变换器,提出了一种频域模数变换的方法,使超宽带脉冲信号通过一个频域滤波器组,并采用一组低速模数变换器在频域对脉冲信号并行采样,大大地降低了接收机对前端模数变换器采样速率的要求。并给出了频域同步、频域信道估计和频域解调的设计算法,计算机仿真结果证明了在能够大大降低采样速率的情况下,频域采样超宽带脉冲通信接收机具有和传统时域高速采样接收机相近的误码率性能。     

Modeling of ultra‐wideband indoor channels with the modified leapfrog ADI‐FDTD method

Full‐wave time‐domain electromagnetic methods are usually effective in rigorously modeling and evaluating ultra‐wideband (UWB) wireless channels. However, their computational expenditures are expensive, when they are used to deal with electrically large‐size problems consisting of fine structures. In order to reduce computational time, the unconditionally stable leapfrog alternating‐direction implicit finite‐difference time‐domain (leapfrog ADI‐FDTD) method has been proposed recently. In this paper, the leapfrog ADI‐FDTD algorithm is developed for simulating lossy objects, such as office walls, floors, and ceilings, for UWB communication channel characterization. It leads to effective UWB channel characterization with power‐decay time constant, path loss exponent, and probability distribution of power gain. In comparison with the conventional FDTD, the proposed method can achieve 60% saving in computational time while retaining good accuracy. Copyright © 2014 John Wiley & Sons, Ltd.     

Alpha-stable分布粗糙土壤表面电磁散射研究

采用空间域合成法生成高度起伏遵守Alpha-stable分布的粗糙土壤表面,运用矩量法研究了锥形入射波入射时粗糙土壤表面的电磁散射特性.数值计算了散射系数随散射角的变化关系,分析了稳定性系数、归一化尺度参数、互相关长度、入射波频率、土壤湿度对散射系数的影响.结果表明,稳定性系数、归一化尺度参数、互相关长度、入射波频率、土壤湿度对散射系数均有较大的影响,遵守不同的规律.     

粗糙面与上方目标复合的宽带电磁散射研究

采用指数型分布粗糙面和Monte Carlo方法模拟实际的粗糙地面,运用时域有限差分法研究了超宽带高斯脉冲电磁波入射时粗糙地面与上方等边三角形截面柱的宽带后向电磁散射问题;通过数值计算得到了后向复合散射系数随频率的变化曲线,分析了不同的粗糙地面高度起伏均方根、相关长度,土壤湿度和三角形边长、倾角、高度及介电常数条件下后向散射系数随频率的变化关系,得到了指数型分布粗糙地面与上方目标的宽带后向电磁散射特性。     

超宽带信号频谱实现研究

为了给不同的应用环境下所要采用的超宽带信号波形的选择提供依据,本文采用数字方法产生了超宽带(ultra wideband＞UWB)无线通信系统中常用的基带高斯脉冲,在此基础上,分别利用带通滤波和上变频方式实现了中心频率和带宽可控的UWB信号脉冲,给出了仿真结果,结果表明,带通滤波方式功率谱能量较低,适用于短距离低功率系统,但系统实现简单.上变频方式功率谱较高,适用于高速远距离应用,且频谱控制更加灵活,但系统复杂度略高.与传统波形设计方法相比,这些方法从频域出发,频谱实现简单灵活并且系统易于实现.     

Monte Carlo simulation of terahertz quantum cascade lasers: The influence of the modelling of carrier-carrier scattering

A theoretical investigation of electron-electron scattering in quantum cascade lasers is presented. The devices are studied by means of an ensemble Monte Carlo simulation that includes all relevant scattering mechanisms. The energy levels and wave functions are determined by a self-consistent resolution of the Schrödinger and Poisson equations. The influence of the modelling of carrier-carrier scattering is discussed on the example of a resonant-phonon structure operating at 3.4 THz. To demonstrate the usefulness of such a model for optimization purpose, an alternative design operating at a lower frequency is proposed. Our model predicts that a significant population inversion can be achieved at about 1 THz.     

Shadow-corrected bistatic scattering from randomly very rough surfaces

Contents  The like- and cross-polarized scattering coefficients of electromagnetic waves from random rough surfaces corrected by shadowing functions are studied in this paper. Due to the use of approximate tangential surface fields in the derivative of all the wave scattering from randomly rough surfaces, the shadowing functions should be added to correct the scattering especially from surfaces with large rms surface slope. For correct prediction to the scattering characteristics we apply the suitable angular shadowing function to the single and multiple scattering coefficients respectively. The comparisons of scattering coefficients with or without shadowing functions are shown in this paper to evaluate the conditions of angular shadowing functions and the effect of shadowing function for the scattering from random very rough surfaces. We also compare the shadow-corrected scattering coefficients with the measured data and find the good agreements between them. Received: 12 December 2000     

用超宽频带局部放电检测技术评定电机线棒老化状态的实验研究

在大型电机运行过程中 ,由于电、热、机械和环境应力的联合作用 ,其绝缘性能会逐渐下降。在实验室中通过多因子加速老化实验模拟了电机线棒在实际运行中的老化。对不同老化程度的线棒 ,使用超宽频带局部放电检测技术测试了试样的局部放电 ,并分析了放电信号在时域和频域随老化时间的变化情况。实验结果表明 ,随着试样线棒绝缘的不断劣化 ,放电脉冲时域波形的高频振荡不断加剧 ,脉冲宽度不断减小 ;同时 ,频域中的高频分量越来越丰富 ,谱峰逐渐增多 ,并且在高频段出现特征峰分布     

超宽带频率非平稳信道硬件实时模拟研究

利用信道模拟器在实验室复现电波传播环境是超宽带通信设备测试的重要手段。 本文针对超宽带信道的频率非平稳 特性,提出了一种基于频域平稳区间叠加的超宽带信道模型,并据此设计了基于软件无线电平台的硬件实时模拟系统。 该系统 使用多路数据并行处理架构实现对高速率数据的实时处理,并引入迭代算法和时分复用结构,既保证信道衰落生成的实时性也 降低了硬件资源消耗。 实测结果表明,本文模拟器可支持模拟 600 MHz 的超宽带信道衰落,信道衰落谱型与理论仿真结果吻 合,并且输出的信道冲激响应与实测结果基本一致。 该系统可为超宽带通信系统的优化、验证和评估提供一种有效的手段。     

Calculation of HVDC-converter harmonics in frequency domain withregard to asymmetries and comparison with time domain simulations

In order to study the effects of large HVDC power converters on the feeding AC networks, it is of importance to explain and to calculate harmonic phenomena which are a result of power converter operation. During commissioning of real HVDC power converters, it could be seen that harmonics resulting from asymmetry in the system voltages or from asymmetry in power converter operation led to significant difficulties concerning the system design. For this reason, not only the effects of characteristics but also the effects of noncharacteristic converter harmonics must be taken into account. The aim is to describe the steady state harmonic behavior of the converter. The harmonic spectra are not determined by time domain analysis but instead the solution is found by frequency domain calculations. This can result in reduced calculation time in comparison to conventional Fourier analysis of the time functions. The power converter is interpreted as an amplitude modulator with voltage and current converter functions which describe the coupling of the DC circuit and the AC network through the power converter. To verify the theory, comparisons of frequency domain with time domain calculations were carried out     

线列阵管中校准方法研究

本文提出了一种在密闭管中测量线阵的自由场复数灵敏度和指向性图的方法。在管中沿管轴方向布置一系列发射换能器和监测水听器 ,根据实测得到的传递函数矩阵和所要模拟的平面波频率、幅度和入射角计算得到一组发射换能器驱动信号的幅度和相位 ,然后以此信号激励每个发射换能器 ,就能在管中形成一个等同于任意角度入射的自由场平面波声场的声压分布。已经开发了一个初型实验系统 ,系统频率范围 2 0～ 6 0 0Hz ,管中所得的测量结果与理论曲线符合很好     

A numerical analysis and expository interpretation of thediffraction of light by ultrasonic waves in the Bragg and Raman-Nathregimes using multiple scattering theory

In this paper, we examine some of the fundamental properties of Bragg and Raman-Nath diffraction of light by ultrasonic waves by revisiting the well-known multiple plane wave scattering theory developed by Korpel and Poon in 1980. The purpose is to provide a clear and unambiguous insight into the variety of physical and geometrical configurations associated with the process of optical diffraction from Bragg and Raman-Nath ultrasonic cells, treating each domain separately. Despite well-established theoretical models, there is a tendency to sometimes erroneously associate general Bragg domain diffraction (as opposed to exact Bragg diffraction where the incident angle is Bragg-matched and the interaction width is infinite) with only two diffracted orders that vary sinusoidally with peak phase shift of the light and distance of propagation. In numerical analyses of the coupled equations, there is also a tendency to sometimes limit the number of orders to a few lower ones. With the enthusiasm to arrive at a solution, this truncation is sometimes applied in the Raman-Nath regime as well. In doing so, higher Raman-Nath-scattered orders are implicitly assumed to be progressively weaker and, therefore, negligible. In complex acoustooptic systems, such approximations can lead to serious errors. With an aim toward rectifying these and other common misconceptions, a thorough numerical analysis of uniform plane wave acoustooptic diffraction in the two well-known regimes is presented and the limits of such analysis are examined     

Spectral domain analysis of large (M)MIC‐structures using novel quadrature methods

The spectral domain approach for the solution of integral equations is especially attractive for planar circuits and antennas in multilayered environments characterized by analytically derived Green's functions. However, for circuits with large lateral dimensions the spectral integrands are strongly oscillatory and accurate evaluation of the integrals is extremely difficult and often time‐consuming. In this paper, we propose higher‐order Legendre quadrature rules optimized for the integration of the oscillating integrands using the Filon principle. The Legendre–Filon quadrature weights are computed for the specific oscillations by solving a small linear system of equations. The method is applied within a spectral domain solution approach evaluating the spectral integrals in cartesian wavenumbers and using asymptotic extraction techniques as well as complex integration path deformation for improved convergence. Since the asymptotic extraction causes a singularity in the origin of the wavenumber domain that is not annihilated by cartesian wavenumber integration, a special procedure for the treatment of this singularity is also introduced. The overall method needs very few sampling points for the numerical evaluation of the spectral integrals but is still applicable to very large structures as compared to the wavelength. Copyright © 2004 John Wiley & Sons, Ltd.     

Power spectral density of a UWB signal with discrete quantized pulse positions

The FCC specification for ultra-wideband (UWB) emissions states that the effective isotropic radiated power (EIRP) cannot exceed 41.3 dBm per 1 MHz bandwidth in the frequency region of 3.1 to 10.6 GHz. Hence, to achieve adequate transmit power in a UWB data link it is necessary to use a wide-bandwidth modulation scheme. This requirement presents a pair of technical challenges which are addressed in this paper. The first challenge is to realize a wide-bandwidth radiated transmitter pulse shape which adequately covers the UWB band from 3.1 to 10.6 GHz. The second challenge is to structure the modulation such that the discrete line component of the power spectral density (PSD) of the transmitted signal is minimized. This is necessary as the discrete spectral components are essentially wasted power and limit the output transmitted power due to the FCC EIRP mask. In this paper the radiated EIRP of a UWB signal based on pulse position modulation (PPM) is explored. Previous researchers have derived the PSD under the supposition that the PPM pulse delay is continuous. However, simpler and more insightful expressions are possible for the PPM PSD if the pulse delays are restricted to quantized steps, as is assumed in this paper. A network-analysis approach for determining the EIRP of the unmodulated radiated pulses is given based on spectral measurements of an experimental UWB transceiver pair. These EIRP characteristics are applied to the derived equations for the continuous and discrete portions of the PSD of the UWB signal. From this, insights emerge into optimum PPM transmitter implementation that maximizes transmit power and minimizes power losses due to the discrete spectral lines. As demonstrated in this paper, such optimization necessitates the joint design of the UWB transmitter radiated pulse shape and PPM structure.