A spurious-free, steep band rejection filter using a LiTaO₃/quartz heteroacoustic layer surface acoustic wave resonator

We have developed prototype parallel and series types of band rejection filters at 0.9 GHz using a heteroacoustic layer (HAL) surface acoustic wave (SAW) resonator. The HAL SAW resonator is made on 0.63 μm thick 25°YX LiTaO₃ supported with a 42°45′Y 90°X quartz substrate. Steep attenuation was obtained due to the large impedance ratio of the HAL SAW resonator. There is no obvious spurious response up to 6 GHz thanks to the spurious-free characteristic of the HAL SAW resonator, and temperature frequency stability is good. In addition, the attenuation at frequencies other than the rejection frequency is small. This HAL SAW band rejection filter may be useful for carrier aggregation and Internet of Things systems.