W.J.R. Hoefer

Tunable microwave and millimeter-wave band-pass filters

The authors present an overview of tunable microwave and millimeter-wave bandpass filters realized in different technologies. Some general design principles are described. Recent progress in the performance of various tunable filters is reported. The authors survey magnetically tunable filters (ferrimagnetic resonance filters, magnetostatic-wave filters, evanescent waveguide filters, E-plane printed circuit filters), electronically tunable filters, and mechanically tunable filters. The typical performance parameters are summarized. This comparison shown that none of these devices can simultaneously satisfy all requirements for perfect tunable filters. For microwave systems where multioctave tuning is essential, a YIG filter is an obvious choice. In systems where the requirement of high power handling capability combined with low insertion loss, predominates, mechanically tunable filters and magnetically tunable E-plane filters are recommended. If the tuning speed is a crucial requirement, varactor-tuned filters or E-plane filters with ferrite toroids are devices of choice. For millimeter-wave design, the most promising structures are ferrimagnetic resonance filters utilizing hexagonal ferrite resonators or, up to 60 GHz, magnetically tunable E-plane printed circuit filters. >

A local mesh refinement algorithm for the time domain-finite difference method using Maxwell's curl equations

An efficient local mesh refinement algorithm, subdividing a computational domain to resolve fine dimensions in a time-domain-finite-difference (TD-FD) space-time grid structure, is discussed. At a discontinuous coarse-fine mesh interface, the boundary conditions for the tangential and normal field components are enforced for a smooth transition of highly nonuniform held quantities. >

A new finite-difference time-domain formulation and its equivalence with the TLM symmetrical condensed node

A finite-difference time-domain (FD-TD) formulation is described. It is shown that the finite-difference time-domain formulation is equivalent to the symmetrical condensed node model used in the transmission line matrix (TLM) method. The TLM method can be formulated exactly in a finite-difference form in terms of total field quantities. It is shown that, due to a better field resolution and fulfilment of continuity conditions, the FD-TD formulation or its TLM equivalent model give better convergence and accuracy than the traditional FD-TD method. This is illustrated by numerical results pertaining to a finned waveguide. >

Transmission line matrix modeling of disperse wide-band absorbing boundaries with time-domain diakoptics for S-parameter extraction

A numerical modeling procedure based on Johns time-domain diakoptics approach with space interpolation techniques for efficient transmission-line matrix (TLM) analysis of two-dimensional microwave circuits is discussed. Frequency-dispersive boundaries are represented in the time domain by their characteristic impulse response or numerical Greens function (Johns matrix). Almost perfect wide-band absorbing boundary conditions have been obtained with this technique, permitting accurate characterization of waveguide discontinuities and compounds. The application of these techniques saves considerable computer run time and memory when compared with conventional TLM analysis. >

A two-dimensional transmission line matrix microwave field simulator using new concepts and procedures

A two-dimensional field simulator for microwave circuit modeling is described. It incorporates a number of recently developed concepts and advanced transmission line matrix (TLM) procedures. In particular, a discrete Greens function concept based on P.B. Johns and K. Akhlarzads time-domain diakoptics is realized, providing a high level of processing power through modularization of large structures at the field level, simulation of wideband matched loads or absorbing walls, modeling of frequency-dispersive boundaries in the time domain, and large-scale numerical preprocessing of passive structures. Nonlinear field modeling concepts are also implemented in the TLM field simulator. It can analyze two-dimensional circuits of arbitrary geometry containing both linear and nonlinear media. The circuit topology is input graphically. Both time-domain and frequency-domain responses can be computed and displayed. The capabilities and limitations of the simulator are discussed, and several microstrip and waveguide components are modeled to demonstrate its important features. >

Huygens and the computer-a powerful alliance in numerical electromagnetics

A time domain technique based on equivalent transmission line interconnections, referred to as the transmission line matrix model (TLM), is presented. The author retraces the history of TLM, from the original concepts of Huygens to the pioneering work of Peter B. Johns, and on to the latest developments in TLM modeling. The most important two-dimensional and three-dimensional TLM algorithms are explained, and new concepts in time-domain representation of frequency-dispersive boundaries are discussed. Some typical applications are described. >

The compensation of coarseness error in 2D TLM modeling of microwave structures

The origin of the coarseness error in two-dimensional TLM (transmission line matrix) meshes is investigated, and a method for compensating the coarseness effect without increasing the computational expenditure is presented. The coarseness error can be eliminated by modifying the properties of the nodes situated at sharp corners or edges. The compensation is achieved by adding reactive stubs to the corner nodes. As a result, relatively coarse TLM meshes may be used to obtain highly accurate results. The efficiency and accuracy of this method are demonstrated by comparison with analytically exact solutions. The savings in computational expenditure are typically three orders of magnitude in 2D-TLM simulations.<<ETX>>

TLM synthesis of microwave structures using time reversal

Presents a novel numerical synthesis technique based on the reversal of the transmission line matrix (TLM) method process in time. It allows the designer to generate the geometry of a passive circuit from its desired frequency response using alternate forward and backward time domain simulation. The essential steps of the procedure are explained and validated using, as an example, the synthesis of an inductive obstacle in a waveguide.<<ETX>>

A new boundary description in two-dimensional TLM models of microwave circuits

A boundary representation for the two-dimensional transmission line matrix (TLM) method of numerical analysis is described. In conventional TLM simulations, boundary conditions are realized by introducing the appropriate impulse reflection coefficients halfway between two nodes. Since the total field quantities are only defined on the nodes, their values at the boundary are not directly available from TLM solutions. The TLM procedure is modified so that boundaries can be placed across the nodes. The boundary conditions in TLM can then be formulated in terms of the field boundary conditions derived from Maxwells equations rather than in terms of impulse reflection coefficients. The essential differences between the conventional TLM and the proposed procedure are presented. Examples are given for several typical problems, and the results obtained with the two methods are compared. These are found to be in excellent agreement. >

New procedures for 2-D and 3-D microwave circuit analysis with the TLM method

Four contributions to numerical field modeling with the TLM (transmission line matrix) method are presented: (1) the formulation of a 3-D Johns matrix (or numerical Greens function) for wideband non-TEM (transverse electromagnetic)-absorbing boundary conditions using the 3-D condensed TLM node; (2) use of a tapered Johns matrix (or numerical Greens function) for improving the return loss of frequency dispersive absorbing boundaries; (3) a recursive algorithm for wideband non-TEM absorbing boundary modeling; and (4) a pseudoparallel iteration scheme for the simultaneous processing of TLM substructures. These procedures are essential for efficient time-domain modeling of 3-D waveguide discontinuities of arbitrary geometries. Their application saves considerable computer run-time and memory when compared with conventional TLM analysis.<<ETX>>