Ultrawide-band resonators using shear horizontal-type plate wave and their application

Currently, mobile or smart phones with multibands and a cognitive radio system require a tunable filter with a wide tunable range to simplify their circuits. An ultrawide-band resonator is an important device to realize the tunable filter. The relative bandwidths (BWs) of 12 to 17% of previously reported surface acoustic wave (SAW) resonators are not sufficient to realize the tunable filter. This time, the authors studied an SH0 mode plate wave in a LiNbO₃ plate. As a result, plate wave resonators with a large electro-mechanical coupling factor (k2) of 0.55 (55%), an ultrawide bandwidth of 29%, a high Q of 700, and a large impedance ratio of 98 dB between resonance (fr) and anti-resonance (fa) frequencies were realized for the first time using 27.5-30YX-LiNbO₃ plates. Applying these resonators to a ladder-type tunable filter, a wide tunable range of 19% was obtained.