Ultra-wideband and high frequency resonators using shear horizontal type plate wave in LiNbO₃ thin plate

A cognitive radio system requires a tunable filter with a wide tunable range to compose a simple front-end. Because the tunable range is limited by the bandwidth (BW) of resonators, an ultra-wideband resonator is an important device to implement the tunable filter. The authors previously obtained low frequency resonators with a wide BWs of 26 to 29% and a large impedance ratios of 92 to 98 dB between resonance ( fr) and antiresonance frequencies ( fa) using 0-th mode shear horizontal (SH0 mode) plate wave in a LiNbO₃ plate. Also, a tunable filter with the tunable range of 13% was demonstrated. However, the frequency ( fr =1.8MHz and fa = 2.3MHz) is too low for modern wireless communication. Also, the resonators suffered from ripples due to transverse mode between fr and fa. In this study, high frequency SH0 mode plate wave resonators ( fr = 430-560MHz and fa = 530-670MHz) with a wide bandwidth and a large impedance ratio have been fabricated using an ultrathin LiNbO₃ plate. The ripples were suppressed by an apodized interdigital transducer.