Alireza Baghai-Wadji

Modular two-dimensional analysis of SAW filters. I. Theory

An accurate simulation tool to meet the stringent requirements for broadband surface-acoustic-wave (SAW) filters is described. The primary goal was to develop a modular software tool in which first- and second-order effects can be easily modeled as separate modules providing an overall high-precision model for SAW devices. The model takes into account surface wave diffraction and refraction, quasi-two-dimensional static charge distribution, metal resistance and external load impedances. The angular spectrum of waves formalism is interpreted in such a way as to render possible the simultaneous description of all effects mentioned. A sample of the results is given for the potential distribution.<<ETX>>

S-theorem (on regularization): Green's function-induced distributed elementary sources — Second kind

Standard singular dyadic Greens functions (DGFs) in computational electromagnetics are responses to idealized dipoles - Diracs delta functions. The latter are generalized symbolic functions defined as the limit of a sequence of (η-)parametrized functions. Any member of the sequence, with non-vanishing η, is a function in ordinary sense having finite (non-zero) or infinite support. Utilization of such distributed source functions, rather than symbolic distributions, renormalizes singularities automatically and results in regularized DGFs. In this work a novel physics-inspired distributed elementary source function has been constructed for the first time. Maxwells equations in general media can be split into two complementary systems of partial differential equations: diagonalized-and supplementary forms, D- form and S- form, respectively. Given a boundary value problem, the D-form with respect to a distinguished direction in space allows directly determining field components transversal to the distinguished direction. The remaining two field components (parallel to the distinguished direction) can be determined a posteriori from the transversal components by employing the S-form. Using the S-form, a novel distributed elementary source has been constructed leading to self-consistently regularizing DGFs. The results have been firmly established by providing the complete proof of a theorem.

Three-dimensional electric charge distribution on finitely-thick bus-bars in micro-acoustic devices

The authors original work for three dimensional charge distributions in micro-acoustic devices - dating back to 1986 - has been manifestly extended to include finitely-thick bus-bars. The work has been initiated to investigate the electric charge localization and field enhancement at the electrode/bus-bar junctions depending on the thickness of the metallization in submicron geometries. An easy-to-follow recipe for the construction of relevant Greens functions has been provided. Universal Functions for setting up system matrices in the Method-of-Moments implementations have been constructed. Universal functions (moments of Greens functions) turn out to be extraordinarily smooth and thus easy to compute. Four additional features stand out in the paper: (i) The Greens function for finite-size substrates has also been constructed. (ii) Furthermore, Greens functions for including the metallic package have been derived. (iii) In addition Greens functions for electrodes imbedded in dielectrics have been constructed. (iv) Finally, the Multi-level Multi-pole Method has been introduced, for the first time, in the micro-acoustic device modelling literature, to significantly accelerate computations and ensure O(N) or O(NlogN) computational complexity. The developed package promises to set a new standard for three dimensional accelerated computing of electric field distributions in micro-acoustics. The paper concludes with a brief discussion of universal functions and multipole expansions.

Quasi-static field analysis of SAW devices with arbitrary geometries of electrodes and of metallic enclosures

An easy-to-implement, accurate, and fast method to calculate 2-D quasi-static capacitance and inductance of electrodes in SAW devices is presented. The method is outlined by considering electrodes with any cross-section geometries in dielectric environment, consisting of two anisotropic materials with a plane interface. The analysis allows the inclusion of arbitrarily shaped metallic packages. A significant shift of charges on enclosures, due to the substrate anisotropy, is observed.<<ETX>>

Modelling Network Traffic Using Time Series Analysis: A Review

With the advent of Internet of Things (IoT) technology, the need for tools which facillitate the development and management of network based services has become increasingly important. Issues such as network security and quality of service are no longer just the concern of ISPs and big corporations, but have now become the intimate concern of private users with implications that directly affect the personal lives and businesses of citizens. At the heart of addressing these concerns lies the problem of modelling the networks on which interconnected devices now operate. While the activity of provisioning the networks and responding to threats may still lie in the hands of networking specialists, the ability to at least know when something is amiss remains intrumental to establishing the confidence and peace of mind of IoT users. In this paper we broadly review the historical development of network traffic modelling and trace a path that leads to the use of time series analysis for the said task. A basic introduction to time series analysis is provided in order to facillitate the theoretical discussion regarding the feasibility and suitability of time series analysis techniques for modelling network traffic. The intention is to provide an orientation, for the interested novice, to the domain of time series analysis for network traffic modelling; and to advocate the necessity and utility of further study in these domains for application to IoT concerns.

Enhanced near-field computation in micro-acoustic devices by taming infinities and enabling acceleration what comes next?

Since its introduction into the field of micro-acoustic devices in early 90s the hybridized FEM/BEM (Finite Element Method/Boundary Element Method) has served well in simulating device characteristics. FEM/BEM has become the method of choice, in particular when periodic structures are considered. At the same time steady progress in the miniaturization of devices along with simultaneous involvement of dielectric, elastic, piezoelectric, and metallic materials, as well as the push toward higher frequencies, have increased the expectations from modeling and design tools. Multi-physics problems need to be solved fast, and without compromising the accuracy of the results. Attempts to extend FEM/BEM to three dimensions proved to be nontrivial and are still in their infancy. Consequently, we are left with deep open questions: is it feasible to continue with finer mesh sizes and at the same time larger systems to solve? How to deal with singularities in computational environments where closed-form expressions are not accessible? How to systematically increase the accuracy of the numerical results? Are we at the verge of a need for computational paradigm change? Are the FEM (minimizing energy functional) and the BEM (minimizing residuum functional) still compatible at ultra-small scales? This paper focuses on the thermo-piezoelectric coupling and advances a proven technique, the diagonalization, one significant step further.

Hadamard-Finite-Part simultaneous infrared and ultraviolet self-regularization of Universal functions in electrostatic limit

Divergences in theoretical and computational electromagnetics, or more generally in computational and mathematical physics, can be fascinating and intimidating at the same time. Therefore, it is imperative to understand the genesis of various divergences and develop a unified theory for their treatment. An in-depth analysis of the semantic employed in the contemporary laws of electrostatic, electrodynamics, gravitation theory, Quantum Field Theory and particle physics reveals astonishing similarity amongst various notions of divergences and their regularization and renormalization techniques. In particular in QFT the two most prominent ones are ultraviolet divergences (those associated with large values of momenta (wavenumber); i.e., short wavelengths, hence ultraviolet, and infrared divergences, those associated with small momenta (wavenumber); i.e., large wavelengths, hence infrared. In this contribution, two major results are proven, by considering the solution of Poisson equation in terms of two-dimensional oscillatory integrals: (i) Application of the Method of Moments automatically leads to Hadamard-Finite-Part UV self-regularization, (ii) without deteriorating the field behavior in IR region. (iii) Geometry-independent Universal functions are constructed for the MoMs applications.

Physics-Based Performance Enhancement in Computational Electromagnetics: A Review

Maxwells electrodynamic differential equations in general bi-anisotropic media have been split into an independent 4×4 diagonalized and a dependent 2×4 supplementary system of equations, referred to as the D-and S-forms, respectively. The forms have been utilized to construct standard singular Dyadic Greens functions (DGFs). Problem-tailored expressions for the Diracs delta-function have been obtained using Fourier integral representations for the DGFs. The resulting expressions for the delta-function have been used to regularize the originating DGFs exponentially. On the other hand, employing standard finite-support basis- and testing functions, the DGFs have been regularized algebraically. Since the geneses of the exponential and algebraic regularization techniques are conceptually different they can be employed independently or in unison. Finally, frequency-, material- and geometry independent universal functions have been constructed for accelerated and highly performance-enhanced computation of the self- and mutual interactions in the method of moments applications.

A novel ab-initio finite difference-based method for convenient implementation of the mass-loading effect in microacoustic devices

Accurate, robust and accelerated implementation of the mass-loading effect in non-periodic micro-acoustic device structures continues to be a challenging undertaking. Existing works, nearly exclusively, apply the (Finite Element Method) FEM / (Boundary Element Method) BEM hybrid technique to periodic structures. Application of the FEM/BEM to non-periodic structures is excessively time consuming and leads to comparatively inaccurate results. On the other hand the BEM/BEM monolithic technique, while being impressively accurate, is extraordinarily cumbersome to formulate, and computationally very expensive to handle realistic device models, as the present authors have discussed elsewhere. This work presents a novel technique based on the (Finite Difference Frequency Domain) FDFD / BEM hybrid formulation. The breakthrough result stems from an easy-to-implement formulation of the edge-effects to an arbitrary accuracy and the complete elimination of the corner points from the analysis. These distinguished properties render the implementation of the mass-loading effect amenable to realistic models and parallel computing at the same time. Based on the tables provided for the partial derivatives, the effort for developing the code is negligible: existing software can easily be augmented to account for the mass-loading effect. Numerical results are thoroughly tested by an independently-developed FEM-based package. Excellent numerical results with predictable figures of accuracy have been achieved. The contribution concludes with a brief discussion of the relevance of the conservative FDFD implementation of the mass-loading effect to account for arbitrarily-shaped electrode bounding surfaces.

Introduction to the Special Issue on Modeling, Optimization, and Design of Acoustic Devices

The 18 papers in this special issue focus on modeling, optimization, and design of acoustic devices.