Small and low-loss IF SAW filters with excellent temperature coefficient consisting of zinc oxide film on quartz substrate

One of the authors reported that a surface acoustic wave (SAW) substrate with an excellent temperature coefficient of frequency (TCF) and a large electromechanical coupling factor could be realized by a combination of a specific quartz (cut-angle and propagation-direction) substrate having a positive TCF and a ZnO film having a negative TCF from theoretical and experimental investigations. The authors have tried to apply this ZnO/quartz substrate to various SAW filters at an intermediate frequency stage (IF). But, it was clarified that the SAW filter consisting of a single electrode (strip) or single-phase unidirectional transducer (SPUDT) on these substrates often had a large spurious response due to the Love wave which was a higher-order-mode of the Rayleigh wave. The new cut angle of quartz was developed in order to suppress its response. The IF filters for wide-band code division multiple access (W-CDMA: 380 MHz) and narrow-band CDMA (N-CDMA: 183 MHz) were developed by combining the new cut angle quartz and the ZnO film. As a result, small-sized IF filters without a spurious response due to the Love wave and with excellent TCF, low insertion loss, and cost advantages in the applications were obtained. Their frequency shift per 1°C from -20°C to 80°C was less than 0.37 ppm/°C. Their insertion losses were 3.6 to 4.4 dB lower than those of conventional SAW filters consisting of the ST-X quartz substrate. Adjacent channel sensitivities (ACSs) of the W-CDMA filter were always larger than 36.1 dB/35 dB in the temperature range of -20 to 80°C.