A SAW duplexer with superior temperature characteristics for US-PCS

The system requirements for US-PCS are very hard to fulfill with conventional SAW filters, since the duplex gap between Tx and Rx bands is only about 1% wide. Furthermore, current PCS CDMA systems impose strong demands on isolation and rejection levels. Typical SAW devices on rotated Y-X LiTaO₃ cannot meet these requirements over the whole operating temperature range, because of the large temperature coefficient of frequency (TC_f) of the substrate. SiO₂ films can be applied to lower the effective TC_f. However, since the necessary film thickness is rather high, this causes fundamental changes in the electro-acoustic properties of this new material system. Using one-port resonators and ladder-type base sections as test structures, we have conducted a series of experiments to identify interrelations between parameters defining the layer system and its electro-acoustic properties like TC_f, piezoelectric coupling factor, reflectivity, and propagation loss. We have determined requirements for realisation of a SAW duplexer in the US-PCS hand and have designed a suitable layer system. We also demonstrate that the effects of a SiO₂ layer can be simulated accurately by a standard P-matrix model with suitably adjusted parameters. Finally, we present a complete SAW duplexer device for the PCS band featuring a TC_f of -17 ppm/K.