6E-1 A Small-Sized SAW Duplexer on a SiO2/IDT/LiNbO3 Structure for Wideband CDMA Application

A small-sized SAW duplexer for Wideband CDMA (W-CDMA) application at 2 GHz band has been developed. The W-CDMA application has a wide duplex gap between transmitting (Tx) band and receiving (Rx) band. To realize a small-sized SAW duplexer, a substrate with high electromechanical coupling coefficient (K2) and small temperature coefficient of frequency (TCF) is needed. Our W-CDMA SAW duplexer was fabricated on a SiO₂/IDT/5degYX-LiNbO₃ structure. Using this structure, an appropriate high K2 and small TCF could be achieved, but the spurious caused by Rayleigh-mode would occur. This spurious appears at lower side than the resonance frequency of SAW resonator, and degrades performances of the duplexer. We have successfully developed the technique to suppress the spurious by controlling a shape of SiO₂ film on IDT electrodes. And, the spurious dependences of IDT designs, pitch and metallization ratio, were cleared. The spurious suppression technique has been applied to the W- CDMA SAW duplexer. The duplexer has excellent performance of low insertion loss, high attenuation and small TCF. The Tx and Rx insertion loss are 1.2 dB and 1.9 dB, respectively. The size of the duplexer could be miniaturized to 2.5times2.0 mm². Additionally, the duplexer has sufficient power durability by using the multi-layer electrodes of AlMgCu/Ti. These techniques could be applied to the other wide-band SAW applications for realizing good performances.