Melinda Piket-May

FD-TD modeling of digital signal propagation in 3-D circuits with passive and active loads

Most existing computer-aided circuit design tools are limited when digital clock speeds exceed several hundred MHz. These tools may not deal effectively with the physics of UHF and microwave electromagnetic wave energy transport along metal surfaces such as ground planes or in the air away from metal paths that are common at or above this frequency range. In this paper, we discuss full-wave modeling of electronic circuits in three dimensions using the finite-difference time-domain (FD-TD) solution of Maxwells equations. Parameters such as stripline complex line impedance, propagation constant, capacitance per unit length and inductance per unit length can be easily computed as a function of frequency. We also discuss FD-TD Maxwells equations computational modeling of lumped-circuit loads and sources in 3-D, including resistors and resistive voltage sources, capacitors, inductors, diodes, and transistors. We believe that this approach will be useful in simulating the large-signal behavior of very high-speed nonlinear analog and digital devices in the context of the full-wave time-dependent electromagnetic field. >

9 – Computational Electromagnetics: The Finite-Difference Time-Domain Method

Prior to abour 1990, the modeling of electromagnetic engineering systems was primarily implemented using solution techniques for the sinusoidal steady-state Maxwells equations. Before about 1960, the principal approaches in this area involved closed-form and infinite-series analytical solutions, with numerical results from these analyses obtained using mechanical calculators. After 1960, the increasing availability of programmable electronic digital computers permitted such frequency-domain approaches to rise markedly in sophistication. Researchers were able to take advantage of the capabilities afforded by powerful new high-level programming languages such as Fortran, rapid random-access storage of large arrags of numbers, and computational speeds that were orders of magnitude faster than possible with mechanical calculators. In this period, the principal computational approaches for Maxwells equations included the high-frequency asymptotic methods of Keller (1962) as well as Kouyoumjian and Pathak (1974) and the integral equation techniques of Harrington (1968) .

Photonic bandgap structures used as filters in microstrip circuits

The application of photonic bandgap structures (PBGs) as substrates in microstrip circuits has been investigated. The effects of substrate thickness, microstrip transmission line location, and length of the PBG structure were studied using a finite-difference time-domain (FDTD) simulation and experimental measurement. A low-pass filter with a very wide high-frequency rejection bandwidth was constructed from a serial connection of many different PBG structures.

A modified FDTD (2, 4) scheme for modeling electrically large structures with high-phase accuracy

A new fourth-order finite-difference time-domain (FDTD) scheme has been developed that exhibits extremely low-phase errors at low-grid resolutions compared to the conventional FDTD scheme. Moreover, this new scheme is capable of combining with the standard Yee (1966) scheme to produce a stable hybrid algorithm. The problem of wave propagation through a building is simulated using this new hybrid algorithm to demonstrate the large savings in computing resources it could afford. With this new development, the FDTD method can now be used to successfully model structures that are thousands of wavelengths large, using the present day computer technology.

The use of SPICE lumped circuits as sub-grid models for FDTD analysis

A general approach for including lumped circuit elements in a finite difference time domain (FDTD) solution of Maxwells equations is presented. The methodology allows the direct access to SPICE to model the lumped circuits, while the full 3-dimensional solution to Maxwells equations provides the crosstalk and dispersive properties of the microstrips and striplines in the circuit.<<ETX>>

Wide-band slot antennas with CPW feed lines: hybrid and log-periodic designs

In microwave and millimeter wave applications, slot antennas fed by coplanar waveguide (CPW) lines are receiving increasing attention. These antennas have several useful properties, such as a wider impedance bandwidth compared to microstrip patch antennas, and easier integration of solid-state active devices. In this paper novel CPW-fed wideband slot antennas are presented. The design procedure of CPW-fed hybrid slot antennas (HSA) having impedance bandwidths (VSWR<2) up to 57% is described. Theoretical and measured results are shown. We also describe the design procedure of a CPW-fed log-periodic slot antenna (LPSA). The impedance matching and the radiation characteristics of these structures were studied using a method of moment technique. Simulated and measured results for different dielectrics are presented.

Alternating-direction implicit (ADI) formulation of the finite-difference time-domain (FDTD) method: algorithm and material dispersion implementation

The alternating-direction implicit finite-difference time-domain (ADI-FDTD) technique is an unconditionally stable time-domain numerical scheme, allowing the /spl Delta/t time step to be increased beyond the Courant-Friedrichs-Lewy limit. Execution time of a simulation is inversely proportional to /spl Delta/t, and as such, increasing /spl Delta/t results in a decrease of execution time. The ADI-FDTD technique greatly increases the utility of the FDTD technique for electromagnetic compatibility problems. Once the basics of the ADI-FDTD technique are presented and the differences of the relative accuracy of ADI-FDTD and standard FDTD are discussed, the problems that benefit greatly from ADI-FDTD are described. A discussion is given on the true time savings of applying the ADI-FDTD technique. The feasibility of using higher order spatial and temporal techniques with ADI-FDTD is presented. The incorporation of frequency dependent material properties (material dispersion) into ADI-FDTD is also presented. The material dispersion scheme is implemented into a one-dimensional and three-dimensional problem space. The scheme is shown to be both accurate and unconditionally stable.

The impact of a nonideal return path on differential signal integrity

Printed circuit board transmission lines propagating high-speed digital signals are susceptible to nonideal return paths which may affect signal integrity. In an attempt to clarify this complex issue, this paper presents a study of differential signal integrity issues for the familiar situation in which two coupled microstrip traces are in close proximity to a common reference plane. Differential signaling on coupled microstrip lines and stripline configurations are investigated. Finite-difference time domain simulations are used to demonstrate the impact a nonideal return path can have on differential signal integrity.

FDTD analysis of electromagnetic wave radiation from systems containing horn antennas

The application of the finite-difference time-domain (FDTD) method to various radiating structures is considered. These structures include two- and three-dimensional waveguides, flared horns, a two-dimensional parabolic reflector, and a two-dimensional hyperthermia application. Numerical results for the horns, waveguides, and parabolic reflectors are compared with results from using the method of moments (MM). The results for the hyperthermia application are shown as extensions of the previously validated models. This new application of the FDTD method is shown to be useful when other numerical or analytic methods cannot be applied. >

Electrodynamics of visible-light interactions with the vertebrate retinal rod.

We report the initial investigation of the electrodynamics of visible-light interaction with the outer segment of the vertebrate retinal rod based on detailed, first-principles computational electromagnetics modeling. The computational method employs a direct time integration of Maxwell’s equations in a two-dimensional space grid for both transverse-magnetic and transverse-electric vector-field modes. Detailed maps of the optical standing wave within the retinal rod are given for three illumination wavelengths: 714, 505, and 475 nm. The standing-wave data are Fourier analyzed to obtain spatial frequency spectra. Except for isolated peaks, the spatial frequency spectra are essentially independent of the illumination wavelength.