Small and low-loss intermediate frequency surface acoustic wave filters using zinc oxide film on quartz substrate

A ZnO/quartz substrate is expected to be applied to a intermediate frequency (IF) surface acoustic wave (SAW) filter for which a small size, a low insertion loss, and a good temperature coefficient of frequency (TCF) are necessary because it has a large k_s and an excellent TCF. However it was clarified that the SAW filter consisting of a single solid electrode or single-phase unidirectional transducer (SPUDT) on ZnO/quartz substrate often has a large spurious response due to the Love wave. The new cutting angle and the propagating direction (26-27°Y-X) for quartz were developed in order to suppress the spurious response. The developed W-CDMA IF 380 MHz filter was mounted on a small package of 3.2 × 5.0 mm². It had a low loss of 5.8 dB, and high adjacent channel selectivity (ACS) of 36.1 dB/35.0 dB. Then, the developed N-CDMA IF 183 MHz filter was mounted on a small package of 5 × 7 mm². It had a low loss of 4.3 dB. In both cases, the frequency shift per 1°C was less than 0.37 ppm/°C.