Günter Kovacs

Accurate FEM/BEM-simulation of surface acoustic wave filters

The accurate analysis of surface acoustic wave (SAW) filter is very challenging task for SAW designers. Especially the prediction of losses in RF-filters employing leaky surface acoustic waves (LSAWs) with aperiodically arranged metallic electrodes such as DMS-filters is not possible with conventional CMO-, P-matrix or equivalent-circuit models, since these approaches ignore relevant second-order effects: neither the weak guiding of the LSAWs near the upper stop-band edge nor radiation into bulk-waves are accounted for correctly. However, bulk-wave radiation causes increased insertion loss (in particular near the upper pass-band edge) and deteriorates the steepness of the upper skirt. Therefore, it is a matter of special importance. We have developed and implemented an accurate model for acoustic tracks with the 2D approximation only: to describe the electro-acoustical properties of the piezoelectric substrate a boundary element formulation is used, which is based on a semi-infinite dyadic Greens function, avoiding any compromises in the characterization of the physics of bulk-wave interactions. The mechanical behaviour of the electrodes is described with finite elements since this approach provides high flexibility with respect to the geometry and material composition. Due to the fact that the FEM/BEM model does not make any a priori assumptions concerning crystal cuts and/or wave types employed in the simulated device, it is widely applicable. Since well-adapted shape functions are used for the BEM-part of the model, the size of the discretized system is kept to a minimum, thereby enabling the simulation of acoustic tracks with up to 400 electrodes in reasonable CPU times on a PC. To guarantee an accurate description of real SAW-devices, a model for finite finger resistance as well as a tool for the description of the electromagnetic effects on the chip and in the package is attached to the FEM/BEM-model. Comparison between simulations and measurements on real SAW-devices show (i) very good agreement and (ii) proof the effectiveness of the developed simulation tool.

A generalised P-matrix model for SAW filters

The P-matrix model, which can be seen as a discrete version of the continuous COM model, is a well established tool to analyse the electro-acoustic properties of IDTs and reflector gratings. It relates both outgoing acoustic surface waves and the electric current to both incoming acoustic surface waves and the electric voltage. If an acoustic track consists of several IDTs and reflectors, it can be described by cascading the P-matrices of its building blocks. However, this approach has its clear limitations. Let us consider, e.g., a DMS track: if a voltage is applied to one IDT it will generate electric charges not only on the fingers of this IDT but also on its neighbouring fingers. These charges cause capacitive coupling between neighbouring elements and change the electro-acoustic excitation and detection of forward and backward propagating surface waves. Both effects are ignored in the building block model. The resulting inaccuracies are most pronounced, if short elements, such as in DMS tracks, are used. In this paper, I will present a generalisation of the classical P-matrix model, which relates the outgoing acoustic surface waves and all electric currents to the incoming acoustic surface waves and all electric voltages in a general acoustic track. This track might consist of an arbitrary number of IDTs, reflectors, and delay lines. To demonstrate the advantage of this model it will be compared to the simpler building block model and to measurements for two typical filters.

Finite element simulation of bulk- and surface acoustic wave (SAW) interaction in SAW devices

Though loss parameters in SAW propagation are used as input for fast analysis models for the simulation of SAW devices, the quantitatively correct description of loss mechanisms is still a challenging task. Therefore, it is important to develop exact measurement and simulation methods which are able to determine these loss parameters accurately. By the use of special boundary conditions, the finite element method (FEM) is able to fulfill these requirements. Regarding the periodic substructure of common SAW devices, we have incorporated periodic boundary conditions (PBCs) in our simulation code. The piezoelectric substrate of SAW devices operating at high frequencies can be modeled as a semi-infinite half space. Therefore, we introduce newly evolved absorbing boundary conditions. They allow us to examine effects emerging from bulk acoustic wave (BAW) radiation (e.g. bulk wave onset frequencies) and assess propagation loss due to leakage and bulk wave conversion accurately.

Low loss recursive filters for basestation applications without spurious modes

In SAW filters spurious waveguide modes may seriously affect the performance: Undesired lobes in the upper pass- or stopband and ripples in the group delay are typical for devices suffering from waveguiding effects. In order to suppress spurious modes and to excite only the fundamental symmetric mode various approaches as overlap weighting or sawtooth shaping of the busbars in order to destroy the waveguide have been applied. Alternatively recently it was suggested to employ the first antisymmetric mode by splitting the acoustic track into two subtracks, which are longitudinally shifted by half a wavelength with respect to each other. The very reason of the excitation of higher modes is, however, the fact that the excitation profile is of different shape than the fundamental symmetric or antisymmetric mode. In most cases the excitation profile is of rectangular shape while the waveguide modes usually are of sinusoidal shape. While the approaches of prior arts aimed at changing the excitation profile to maximize excitation of the fundamental symmetric or antisymmetric mode we suggest an approach where the waveguide is changed in order to shape the fundamental symmetric mode rectangularly. The approach is applied to low loss recursive inline filters which are employed as intermediate frequency filters in GSM infrastructure systems. The method proves to be particulary successful in reducing group delay ripple and sidelobes. I. INTRODUCTION Spurious modes are a serious problem in SAW filters. They may lead to ripples in the passband, distortions of the group delay time and reduced out of band suppression.

A SAW duplexer with superior temperature characteristics for US-PCS

The system requirements for US-PCS are very hard to fulfill with conventional SAW filters, since the duplex gap between Tx and Rx bands is only about 1% wide. Furthermore, current PCS CDMA systems impose strong demands on isolation and rejection levels. Typical SAW devices on rotated Y-X LiTaO/sub 3/ cannot meet these requirements over the whole operating temperature range, because of the large temperature coefficient of frequency (TC/sub f/) of the substrate. SiO/sub 2/ films can be applied to lower the effective TC/sub f/. However, since the necessary film thickness is rather high, this causes fundamental changes in the electro-acoustic properties of this new material system. Using one-port resonators and ladder-type base sections as test structures, we have conducted a series of experiments to identify interrelations between parameters defining the layer system and its electro-acoustic properties like TC/sub f/, piezoelectric coupling factor, reflectivity, and propagation loss. We have determined requirements for realisation of a SAW duplexer in the US-PCS hand and have designed a suitable layer system. We also demonstrate that the effects of a SiO/sub 2/ layer can be simulated accurately by a standard P-matrix model with suitably adjusted parameters. Finally, we present a complete SAW duplexer device for the PCS band featuring a TC/sub f/ of -17 ppm/K.

Simulation of waveguiding in SAW devices on substrates with anisotropic slowness and excitation

The 2D P-matrix simulation method is extended to deal with SAW substrates exhibiting a strong anisotropy in slowness and excitation such as YZ-cut lithium niobate.In these substrates the parabolic approximation of the slowness curve is in general not valid. Instead, the exact form of the slowness as obtained from solutions of the basic wave equations of the piezoelectric half space needs to be employed. This has two major implications: Firstly, the imaginary branches cannot be simply determined as the complex roots of a parabolic equation. Secondly, the resulting transversal modes in general are not orthogonal as in the parabolic case. We determined the imaginary branches by analytical continuation of the exact slowness curve. The problem of non-orthonormal basis functions was dealt with by a projection technique including bounded as well as continuum modes. Besides the general anisotropy of the slowness also the anisotropy of excitation is taken into account. Thereto a convolution of the angle dependent excitation characteristic with the transversal charge distribution is determined. The method is verified at the example of very narrow resonators on YZ-cut lithium niobate, which exhibit strong diffraction effects. With respect to the parabolic approximation a significant improvement of simulation quality is observed.

DMS filter with reduced resistive losses

Resistive losses in the metal fingers of an acoustic track are an essential loss mechanism, limiting the electrical performance of RF SAW filters. In this paper, we present two alternative models to account for finite finger resistance in IDT tracks. The more comprehensive one assumes an individual, continuous function of current and voltage for each finger. By dividing the track into infinitesimal transversal strips, we obtain a system of coupled differential equations which can be integrated analytically. Applying the proper boundary conditions at the upper and the lower finger ends, we obtain the nodal admittance matrix of the acoustic track. We also developed a second model, which uses an equivalent circuit for each finger and is a very good approximation for small and moderate values of finger resistance. The accuracy of both models are demonstrated and compared to experimental results for an exemplary DMS filter. Furthermore, we show how resistive losses can be minimised by using DMS tracks with five or more IDTs, which are essentially a parallel connection of conventional 3-IDT DMS tracks.

A powerful novel method for the simulation of waveguiding in SAW devices

A new method to simulate waveguiding in SAW filters is presented and applied to mobile communication and consumer electronics filters. It is based on the P-matrix method and discretizes the filter into longitudinal and transversal sections. Diffraction effects are introduced by considering free waveguide propagation within the longitudinal sections. This method is an extension of the pseudoinverse diffraction matrix method by Rooth et al. In contrast to the transmission matrix form of that scheme the presented method uses an intrinsically reciprocal 2D scattering matrix description. It is shown that the dispersion relations of the method are equivalent to those obtained by a 2D COM approach and hence both methods are equivalent. The scheme has proved to predict 2D effects in a wide range of filter types reliably. This is exemplified for recursive filters, as well as filters with apodized and fan-type IDTs built on quartz and lithium niobate. In order to better understand the filter behaviour visualizations of the wavefields are presented.

Concepts for RF Front-Ends for Multi-Mode, Multi-Band Cellular Phones

This paper describes front-end architectures of modern multi-mode, multi-band cellular phones and will discuss the requirements on RF filtering in such applications. Special focus will be on dual mode (GSM and WCDMA) cellular phones with four GSM and three WCDMA bands. On the one hand driven by forward integration - e.g., from PA function via transmit front-end to a fully integrated radio - on the other hand influenced by new requirements of 3G systems and the integration of complementary access, filtering components such as SAW and BAW filters have to solve several increased requirements simultaneously. New technologies are required to follow the demand of increased RF performance, reduced PCB area consumption and continuously decreasing component costs.

Effects of bulk wave radiation on IDT admittance on 42/spl deg/YX-LiTaO/sub 3/

This paper investigates the effects of bulk acoustic wave (BAW) radiation on the admittance of interdigital transducers (IDTs) with significant internal reflections of shear horizontal (SH) type leaky surface acoustic waves (SAWs) on 42/spl deg/YX-LiTaO/sub 3/(42-LT). Theoretical analysis is made by using the discrete Green function theory, and synchronous one-port resonators are analyzed. It is shown that the BAW radiation significantly affects the IDT characteristics even for resonators; under certain circumstances, BAWs launched from an IDT are converted into SH-type SAWs by the strong internal reflections, and they interact with the BAWs radiated by the IDT. Then, the net amount of the radiated BAW power is highly dependent upon the number of IDT finger pairs. For the precise simulation of devices based on the SH-type SAWs with strong internal reflections, the BAW radiation should carefully be taken into consideration. If the BAW radiation is ignored, the radiated power of the SH-type SAWs may seem to be negative above the BAW cut-off frequency.