Clinton S. Hartmann

An Analysis of SAW Interdigital Transducers with Internal Reflections and the Application to the Design of Single-Phase Unidirectional Transducers

A new coupling-of-modes (COM) analysis of SAW interdigital transducers (IDTs) is presented. The analysis includes the effects of multiple reflections within a transducer. The fundamental equations are derived and some simple closed-form results a re given. In addition, some general properties having regard to the relative phase of transduction and reflection c oefficients within a transducer are derived. A new class of unidirectional transducers is proposed which require only one matching network and do not have the crossovers required in current three-phase designs. These single-phase unidirectional transducers (SWDTs) utilize internal r eflections within the transducer to achieve unidirectional behavior. Practical SWDT configurations are proposed and some experimental results presented. The application of the new COM analysis to synthesis is discussed.

Overview of design challenges for single phase unidirectional SAW filters

A comprehensive discussion of the design synthesis challenges for single-phase unidirectional transducers (SPUDTs) is presented. The complexity of SPUDT filter design results from the fact that a SPUDT deliberately includes reflections internal to the transducer to cancel the effects of regeneration reflection. This destroys the validity of the impulse-response model which is the basis for all conventional SAW (surface acoustic wave) transducer designs. Although a recent generalization of the impulse response model includes effects of uniform internal reflections, it is of limited usefulness, because a high-performance SPUDT filter requires both weighted reflection and weighted transduction. Presented is a six-step SPUDT synthesis procedure which includes a new method for determining transduction and reflection weighting functions that satisfy a desired amplitude and phase response, achieve low insertion loss, achieve low triple transit, allow use of simple matching networks, and properly account for the interactions between transduction and reflection effects under matched conditions. Fundamental insertion-loss limitations for SPUDT filters are discussed and areas for future SPUDT research are outlined. Good experimental performance is demonstrated for a bandpass filter that uses a new SPUDT structure, the EWC/SPUDT, the geometry of which is also shown.<<ETX>>

A coupling-of-modes analysis of chirped transducers containing reflective electrode geometries

An accurate and numerically efficient model for analyzing chirped transducers incorporating internal reflections is presented. The model results in a complete three-port characterization of the transducer. The three-port description of the transducer (two acoustic ports and one electric port) is obtained by representing the response of the chirped transducer as that of a set of cascaded uniform sections. The three-port solutions for a uniform section of transducer are presented as the results of a coupling-of-modes (COM) analysis. The solution for the transducers acoustic admittance, a quantity not previously documented in the literature, is included. For verification of the analysis technique, experimental results for a pair of linearly chirped transducers fabricated on 34 degrees quartz are presented. These transducers include a conventional two-electrode per-wavelength transducer and a single-phase unidirectional transducer (SPUDT). Excellent correlation between the theoretical solutions and the experimental measurements is shown.<<ETX>>

Fundamental Limitations on Reading Range of Passive IC-Based RFID and SAW-Based RFID

For the first time, fundamental read-range limitations are calculated for two major types of passive (i.e. battery-less) RFID systems. Many applications require large read range. But, in shorter range applications excess read range provides read-path signal margin which enhances reading reliability, signal penetration, and/or flexibility in RFID system design, layout, and installation. Analysis was restricted to passive technologies because passive tags are required for the majority of future RFID applications. The analysis shows 2.44 GHz surface acoustic wave (SAW) based RFID potentially has a 30X read range superiority compared to passive semiconductor IC-based RFID operating near 900 MHz.

Matching of single-phase unidirectional SAW transducers and a demonstration using a low-loss EWC/SPUDT filter

The authors present the theoretical criterion for calculating the electrical port load required for matching a SPUDT (single-phase unidirectional transducer) and an efficient procedure for tuning a SPUDT filter. The theoretical development identifies a potentially useful class of SPUDTs that requires no matching network to achieve a high degree of triple transit cancellation. The sensitivity of the SPUDT filters insertion loss to parasitic resistance is explained. It is found that the insertion loss of a SPUDT can increase as the thickness of the metallic overlay is increased (contrary to conventional surface acoustic wave technologies). The process of matching a SPUDT filter is demonstrated using a 196 MHz filter packaged in an 18-pin DIP package and matched with miniature internal coils. A triple transit suppression of 55 dB was obtained for an insertion loss of 8.6 dB.<<ETX>>