The most frequently used models for surface acoustic wave (SAW) devices are the impulse model, the equivalent circuit models, the Coupling-of-Modes model, and the matrix models. While the impulse-model is only a first order model the other models include second order effects, e.g. reflections, dispersion, and charge distribution effects. The influence of diffraction and refraction on the transfer function of a SAW filter can be described by the angular spectrum of straight-crested waves model. A survey of these different models will be given. The simulation of low-loss filters requires flexible analysis tools, which can cope with different geometries and substrates. Operating with a parameter set, which depends only on the substrate crystal and not on the specific geometry of the SAW filter, is advantageous. Due to the high insertion attenuation of conventional transversal filters the requirements on the accuracy of the analysis are focused on S21, whereas for low-loss filters all elements of the S-matrix are important. The comparison of simulations with a P-matrix model, which fulfills the above mentioned prerequisites, and measurements of different types of low-loss filters, e.g. SPUDT, DMS, and transverse-mode coupled resonator filters are presented

Review of models for low-loss filter design and applications

The advantages of the coupled-mode (COM) formalism and the transmission-matrix approach are combined in order to create exact and computationally efficient analysis and synthesis tools for the design of coupled surface acoustic wave resonator filters. The models for the filter components, in particular gratings, interdigital transducers (IDTs) and multistrip couplers (MSCs), are based on the COM approach that delivers closed-form expressions. To determine the pertinent COM parameters, the COM differential equations are solved and the solution is compared with analytically derived expressions from the transmission-matrix approach and the Green's function method. The most important second-order effects, such as energy storage, propagation loss, and mechanical and electrical loading, are fully taken into account. As an example, a two-pole, acoustically coupled resonator filter at 914.5 MHz on AT quartz is investigated. Excellent agreement between theory and measurement is found. >

Efficient analysis tool for coupled-SAW-resonator filters

An analytic formulation for transduction parameter required by the coupling-of-modes analysis of surface acoustic wave (SAW) transducers is derived by comparing solutions obtained using the coupling-of-modes analysis and an equivalent quasi-static analysis using the Green function method. This analysis results in the derivation of a convenient representation for the transduction strength of periodic SAW transducers. Using the formulation, analytic solutions are obtained for the transduction magnitude and phase of regular SAW transducers. Nonregular transducers are also modeled with the help of an electrostatic field analysis of the transducer's structure. >

Transduction magnitude and phase for COM modeling of SAW devices

A monolithically packaged surface acoustic wave (SAW) radio transponder and pressure sensor are developed for the application to a tire pressure monitoring system (TPMS). The device contains the wireless transponder, which converts analog signal into digital one without any auxiliary electronic circuits and transmits the converted data wirelessly. No power sources are needed for wireless transponder and pressure sensor. The touch-mode pressure sensor converts externally applied pressure into capacitance, and the SAW radio transponder radiates sensor values with pulse train to the interrogation (measurement) unit. The realization of the mechanical A/D conversion is possible since the SAW radio transponder is connected to the touch-mode capacitive pressure sensor. The SAW radio transponder and touch-mode sensor are fabricated using a surface micromachining and a bulk micromachining technologies, respectively. The performance of the integrated, passive and wireless pressure sensor meets the design specifications such as linearity, sensitivity and noise figure. Finally, experimental results for the radio transponder and the sensor without power source are presented.

SAW based passive sensor with passive signal conditioning using MEMS A/D converter

Single-phase unidirectional transducer (SPUDT) filters have been shown to have the advantage of producing low insertion loss, but yet to be simple to manufacture at a relatively low cost. However, the synthesis of SPUDT devices has always been relatively difficult, especially with regard to deriving an optimized transduction and reflection apodization function. A technique for using both overlap and withdrawal weighting for SPUDT filters to realize a specific filter response is presented. Experimental SPUDT filters with insertion losses as low as 4.8 dB and with a sidelobe rejection of greater than 50 dB for various fractional bandwidths on various piezoelectric substrates are presented. The experimental responses of these apodized devices are compared with the theoretical simulated response. The application of these low-loss devices for use with surface-mount packaging is also demonstrated. >

Apodized single-phase unidirectional transducer SAW devices

Procedures are presented for integrating the COM (coupling-of-modes) results of C.S. Hartmann and B.P. Abbott (1988) into a form useful for modeling an entire SAW (surface acoustic wave) device. Conversion of the three-port P-matrix COM solutions into a standard two-port ABCD matrix form is shown. With the resultant ABCD matrix, graphical results are shown for the full SAW device model including second-order effects and matching circuitry. These results are presented for different types of SAW devices such as those with uniform, apodized, and withdrawal-weighted transducers and are compared with conventional approaches to SAW modeling. Experimental results for various device configurations and metal thicknesses are shown for comparison with predicted model results. In general there is close agreement between experiment and the implementation of COM theory reviewed. >

Practical implementation of coupling-of-modes theory for SAW device modeling

Experimental results are presented for a reflective-array SAW (surface acoustic wave) device that is optimized for narrowband filter applications. The filter consists of broadband input and output transducers with two weighted reflector arrays. Previous devices of this type have used reflective dot density and depth profile weighting for the reflective arrays. A unique implementation of this design technique for a 70-MHz narrowband filter is demonstrated, in which position-weighting of the reflector arrays is used. This filter has 0.2% fractional bandwidth and excellent amplitude and phase characteristics. A detailed analysis of the filter by use of a computer simulation and comparisons between the predicted and experimental responses are discussed. The proposed design approach results in a shorter die length as compared to conventional filter design and is more tolerant to fabrication defects such as shorts and opens. >

R.B. Brown

Papers

Practical implementation of coupling-of-modes theory for SAW device modeling

Reflective array SAW narrowband filters