The approach to realize the characteristics of SAW resonator with the temperature compensation and steepness for PCS duplexer

The SAW resonators with a low temperature coefficient of frequency (TCF) and steep characteristics have been studied in order to realize the SAW duplexers for CDMA based PCS in the United States (US-PCS). The width of cross-band that is a band between transmission and reception band, is very narrow of 20MHz in US-PCS at 1.9GHz. On the other hand, the SAW filter fabricated on a rotated Y-cut LiTaO₃ substrate (LT) shows a large shift of frequency characteristics on the temperature change. Therefore a SAW duplexer for US-PCS requires SAW filters which have steeper cut-off characteristics and better temperature characteristics compared with conventional ones. To solve these problems, we have studied the resonators of SiO₂/IDT/LT structure. It has been known that low TCF can be realized with SiO₂IDT/LT structure. Additionally, steepness becomes sharp due to the capacitance ratio γ increasing because of SiO₂ coating. In this paper, we will show that it is possible to achieve the resonator characteristics that are suitable to design the filter for the US-PCS SAW duplexer by optimizing the temperature and frequency characteristics of the SAW resonator covered with SiO₂.