High performance microwave acoustic components for mobile radios

Abstract

Due to their outstanding characteristics acoustic components, such as SAW and - later on - also BAW filters, have been key components in wireless communication systems from their very beginning. Regarding mobile radio handsets the primary field of application of acoustic components has moved to the RF front-end sections. Here, with the increasing demand for WCDMA-FDD capable mobile phones, duplexers, which allow to separate the TX and RX paths while being simultaneously connected to the antenna, have become indispensable. Regarding the specifications of the different operating bands the requirements on form factors, pass band characteristics, attenuations of the other signals (TX, RX, GPS, WLAN, ...), and isolations make them high performance components requiring a sophisticated acoustic as well as electromagnetic design. Due to the tough requirements imposed on the duplexers in terms of, e.g., center frequencies, spacings of RX and TX bands, relative band widths, and shape factors, they also require application-specific acoustic structures. As a consequence, in contrast to standard filters, duplexers are highly optimized components using leading-edge technologies. In order to achieve the best balance of the characteristic properties of the acoustic structures regarding, for instance, their quality factors, pole-zero distances, and temperature coefficients, the manufacturing processes of the acoustic chips are designed for the specific applications. Furthermore, optimizing the electrical characteristics a co-design of package and acoustic chip is applied. In this paper we provide an overview of the different types of duplexers that have been the result of the focused specialization and optimization efforts. We classify them according to the type of acoustic structures, matching circuitry, and package being used. So far, purely SAW-based and BAW-based duplexers as well as hybrid duplexers using both SAW and BAW chips have been reported. Most of the matching circuitry is integrated in, for instance, an LTCC package using a distributed approach based on a ?/4 line or a lumped approach comprising coils and/or capacitors. Sometimes the matching circuitry is realized externally on the PCB. Furthermore, we present recent integration successes, i.e., duplexer inserts, allowing the seamless integration of duplexers into modules without drawbacks on performance or module thickness.