Wojciech Gwarek

Analysis of arbitrarily shaped two-dimensional microwave circuits by finite-difference time-domain method

A version of the finite-difference time-domain method adapted to the needs of S-matrix calculations of microwave two-dimensional circuits is presented. The analysis is conducted by simulating the wave propagation in the circuit terminated by matched loads and excited by a matched pulse source. Various aspects of the methods accuracy are investigated. Practical computer implementation of the method is discussed, and an example of its application to an arbitrarily shaped microstrip circuit is presented. It is shown that the method in the proposed form is an effective tool of circuit analysis in engineering applications. The method is compared to two other methods used for a similar purpose, namely the contour integral method and the transmission-line matrix method. >

Wide-band S-parameter extraction from FD-TD simulations for propagating and evanescent modes in inhomogeneous guides

This paper proposes a new method of S-parameter extraction from finite-difference time-domain simulations. Unlike the previously published methods, the present method extracts the frequency-dependent mode impedance and propagation constant directly from the three-dimensional simulations. This makes it accurate and computationally effective in wide-band analysis. This paper provides examples of calculations including difficult cases of inhomogeneous or lossy structures, with the frequency band spanning below and above the cutoff frequency. Special attention is given to the S-parameter extraction for evanescent modes. It is shown that the available literature provides insufficient and sometimes confusing background in this regard. Thus, a new consistent theoretical background is presented.

Polarization sensitive detection of 100 GHz radiation by high mobility field-effect transistors

Detection of 100 GHz electromagnetic radiation by a GaAs/AlGaAs high electron mobility field-effect transistor was investigated at 300 K as a function of the angle α between the direction of linear polarization of the radiation and the symmetry axis of the transistor. The angular dependence of the detected signal was found to be A0 cos2(α−α0)+C with A0, α0, and C dependent on the electrical polarization of the transistor gate. This dependence is interpreted as due to excitation of two crossed phase-shifted oscillators. A response of the transistor chip (including bonding wires and the substrate) to 100 GHz radiation was numerically simulated. Results of calculations confirmed experimentally observed dependencies and showed that the two oscillators result from an interplay of 100 GHz currents defined by the transistor impedance together with bonding wires and substrate related modes.

Application of the FD-TD method to the analysis of circuits described by the two-dimensional vector wave equation

A class of microwave circuits described by a two-dimensional vector wave equation is defined. It is proposed to refer to them as vector two-dimensional or 2-DV circuits to distinguish them from circuits described by a two-dimensional scalar wave equation (typically referred to as 2-D circuits). It is shown that the 2-DV class contains some planar circuits filled with anisotropic media, two-dimensional waveguide discontinuities, and circular waveguide discontinuities. Calculation of dispersion characteristics of inhomogeneously filled hollow waveguides is an eigenvalue problem belonging to the 2-DV class. Application to the finite-difference-time-domain (FDTD) method to the analysis of 2-DV circuits is described. Examples show the efficiency of the method for several types of circuit. >

Generalized TLM algorithms with controlled stability margin and their equivalence with finite-difference formulations for modified grids

Generalized TLM formulation based on modified grids of 2-D shunt nodes or 3-D expanded modes are proposed. Generalization consists of permitting flexible control of the numerical stability margin (and thus a time-step for a particular discretization), and of introducing enhanced models for curved boundaries. Formal equivalence between generalized TLM and FDTD algorithms based on the same grid is proved. Simple rules for transforming circuit models (from TLM to FDTD and vice versa) and for their equivalent excitation are given. It is demonstrated that the application of the generalized algorithm reduces computer resources required for the TLM analysis of a circular waveguide by an order of magnitude. >

Higher-Order Modelling of Media Interfaces for Enhanced FDTD Analysis of Microwave Circuits

The 1st and 2nd order accurate models of media interfaces are originally developed and incorporated into the FDTD method. Both models are shown to provide unambiguous results and increased fidelity of the FDTD method for inhomogeneous microwave circuits. They combine computational effectiveness of standard FDTD with accuracy equal or superior to that of nonorthogonal FDTD and FETD formulations. For practical problems the 1st order model proves satisfactory, the 2nd order model becomes feasible for coarse discretization.

Computer-aided analysis of arbitrarily shaped, coaxial discontinuities

The author proposes a method of analyzing a coaxial discontinuity arbitrarily shaped in two dimensions (radial and longitudinal) but maintaining its axial symmetry. It is shown that under such assumptions the equations to be solved correspond to the equations describing an equivalent planar circuit filled with a nonuniform medium. These equations are solved by a version of the finite-difference time-domain method. The method produces a universal computer algorithm capable of solving a wide range of practical problems with no analytical preprocessing. The examples presented show that the method can be effectively used in engineering applications. >

A differential method of reflection coefficient extraction from FDTD simulations

A novel concept for accurate extraction of the reflection coefficient from electromagnetic simulations is proposed and implemented into the finite-difference time-domain (FDTD) method. It uses only the values of an arbitrary tangential E-field component and its derivative with respect to the longitudinal direction at one selected point, and of an arbitrary tangential H-field component and its derivative with respect to the longitudinal direction at another point. No a priori knowledge of reference impedances is needed, and the reflection coefficient is extracted from a single run of FDTD. The proposed method is directly applicable to arbitrarily shaped and inhomogeneous transmission lines.

On the nature of solutions produced by finite difference schemes in time domain

This paper is a review discussing properties of field solutions produced by various finite difference schemes in the time domain. Considered algorithms include standard FDTD as well as its modifications based on different sets of electromagnetic equations and/or different discretization in space. By developing their complete dispersion relations, conclusions are drawn regarding divergence, curl, and energy conserva tion by each of the emulated eigenmodes separately. This is in extension to previous works which were concerned with the total solution, and permits to formulate conditions for restricting the total solutions to physical uncoupled solenoidal and potential modes. Original classification of spurious modes for the time domain finite difference modelling is also proposed. It is shown that spurious compensating modes can propagate over the mesh in the case of penalty schemes with p≠0, and that spurious degenerate modes at high- and low-frequencies appear in the case of condensed node discretization. Copyright

A Broadband Absorber With a Resistive Pattern Made of Ink With Graphene Nano-Platelets

A novel type of a broadband flexible electromagnetic absorbing panel made of a resistive pattern printed on a dielectric spacer backed by a conducting surface is investigated in this paper. It is shown that an appropriate choice of the shape and sheet resistance of the pattern allows extending an absorption spectrum up to an octave and beyond, which is very competitive over alternative solutions. The pattern is made of ink with graphene nano-platelets, which allows achieving expected sheet resistance with decent repeatability, what is confirmed with measurements undertaken with the DC point probe and microwave dielectric resonator techniques. An exemplary sample of the proposed absorber is manufactured and characterized, indicating a very good agreement between theoretical study and experiments.