COM-Analysis of SAW Devices

Abstract

This lecture focuses on coupling-of-modes (COM) analysis of surface acoustic wave (SAW) transducers and devices. COM approximation is a closed-form technique for modeling systems with spatially varying properties. In particular, COM analysis provides a convenient tool for modeling low-loss RF SAW filters while taking into account interelectrode reflections caused by mass-electrical load effects.

COM Analysis of SAW Reflective Arrays

First, COM equations are deduced and applied to the analysis of SAW reflective arrays. The lecture considers an analytic solution of the homogeneous differential equations describing a passive reflective grating. Furthermore, reflection and transduction properties of the reflecting grating are discussed.

COM Analysis of SAW Transducers

Next, a general solution of the inhomogeneous differential equations describing an interdigital transducer (IDT) is considered together with an additional equation accounting for the terminal current flowing into the IDT. Both radiation and reception modes are analyzed. Consequently, the closed-form mixed scattering matrix (P-matrix) of the transducer is constructed.

COM Parameters and Their Determination

In COM analysis, COM parameters play the key role. COM equations are characterized by four independent parameters determined a priori: self- and cross-coupling coefficients, SAW excitation function, and static capacitance. In general, these parameters depend on frequency, substrate and electrode materials, and transducer geometry such as metallization ratio, pitch, and metal thickness.

Furthermore, physical meaning of the COM parameters is explained. In addition, the lecture discusses theoretical and experimental techniques for determining COM parameter values and illustrates them with practical examples.

Simulation and Design Examples of COM Analysis

Finally, practical COM applications are demonstrated through the analysis of SAW reflectors, self-matched IDTs, long resonant IDTs with internal reflections, one- and two-port resonators, and Double Mode SAW (DMS) filters.

Good agreement between theoretical predictions and experimental results is observed.

Contents

1. Introduction

2. Basic approximations and equations in COM analysis of SAW devices

3. Closed-form COM analysis of SAW wave propagation

3.1 Modeling of periodic SAW reflective arrays

3.2 COM modeling of SAW transducers

3.2.1 Transducer terminal current

3.2.2 SAW excitation (radiation)

3.2.3 SAW detection (reception)

4. Mixed scattering matrix in COM analysis of SAW transducers

5. Determination of COM parameters for SAW devices

5.1 Self-coupling coefficient

5.2 Cross-coupling coefficient

5.3 Excitation function and element factor

5.4 Static capacitance

6. COM analysis applications for SAW devices

6.1 SAW reflectors

6.2 Self-matched SAW transducers

6.3 Long resonant SAW transducers

6.4 One- and two-port SAW resonators

7. Simulation and design examples of COM analysis

8. Conclusions