Karl-Christian Wagner

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 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.

High selectivity SAW duplexer for W-CDMA Band VIII

Special requirements for Band VIII Wideband CDMA (W-CDMA) duplexers are the relatively large bandwidths of 3.9% for transmitting (TX) and 3.7% for receiving (RX) filters as well as the low duplex distance of 1.1%. Moreover, compared to systems at higher frequencies, for 1 GHz systems increased focus is put on miniaturization. In order to achieve good RX suppression within the TX band, the RX filter is comprised of one port resonators as well as a DMS track whereas the TX filter is a pure ladder type filter. The center frequencies of these building blocks show different dependencies with respect to metallization ratio and layer thickness. The consideration of this relationship within the design process was necessary to achieve a very low shift of the right skirt of the TX filter relative to the left skirt of the RX filter. Our duplexer is realized in a 3.0x2.5 mm2 package on LTCC ceramic and offers a suppression of the W-CDMA RX-signal of 50 dB within the TX band. Typical values for attenuation of the W-CDMA signal in the transmission band are 1.7 dB for the TX path and 2.0 dB for the RX path. The comparison between simulation and measurement shows the applicability of the design concept which is the most appropriate one for duplexers with small duplex distance and good RX suppression within the TX band.

Liquid crystal thermography on coated SAW devices

Reliability of micro-electronic devices is one of the most important issues in modern technologies and is significantly influenced by the thermal behavior of the components. In this context, we present Liquid Crystal Thermography (LCT) not only as an easy-to-use but even comparably low-cost approach for temperature measurements. This technique is based on thermochromic liquid crystals which exhibit temperature-dependent colors by selectively reflecting incident white light. We describe and demonstrate this method in exemplary investigations of selfheating effects in a half-section ladder-type Surface Acoustic Wave (SAW) filter with silicon dioxide coating. Conventionally, mean temperature values are obtained by evaluating measured frequency shifts under load by means of the Temperature Coefficient of Frequency (TCF). Moreover, LCT provides spatially resolved measurements of the temperature distribution on the component and serves as an independent scheme for thermal characterization in contrast to TCF based evaluations. The results of LCT measurements and temperature simulations are compared and show good agreement.

Two dimensional periodic array of reflection centers on electrodes in SAW resonators

Periodic arrays of metallic patches on the electrodes of a “Rayleigh-type SAW resonator” are investigated to suppress undesired modes above resonance frequency. Both, the metallization ratio(ηt) and the period (PL) of the patches in transversal direction, were varied. Simulations were performed employing the well established 2D P-matrix model. A well-designed periodic array is capable to suppress all transversal modes within the frequency band for ky = [0, π/pT]. By evaluation of the overlap integral of the excitation and mode profile, and consequently, the analysis of the single mode contribution to the excitation strength, it is demonstrated that higher modes are responsible for remaining spurious peaks in the resonators frequency response. The analysis of the contribution of the transversal modes to the admittance provides insight how the Δν/ν waveguide, formed by the areas with and without patches, can be engineered to suppress bound and leaky continuum modes.

6D-5 A 2D P-Matrix Model for the Simulation of Waveguiding and Diffraction in SAW Components (Invited)

The 2D P-matrix model has proven to predict 2D propagation effects in a wide spectrum of SAW components for different applications reliably. With reasonable computational effort, a very accurate simulation of the electrical characteristics of SAW devices is achievable. After an overview of different possible approaches proposed in the literature for the inclusion of 2D effects into device simulation, the method implemented and used by Epcos is described in some detail. Comparison to other methods, especially the 2D COM model is made. The modeling of the coupling of forward and backward propagating modes in reflective gratings, which is crucial for obtaining high quality results, is highlighted. A wide variety of different application examples (inline filters using SPUDTs, resonators, filters with slanted finger transducers) will demonstrate that the 2D P-matrix model not only gives reliable predictions of electrical characteristics, but lends detailed insight into the physics of a device under consideration. Thus, it sets up a basis allowing systematic approaches for the suppression of transversal modes