Ultra-wideband and high frequency resonators using shear horizontal type plate wave in LiNbO₃thin 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 ( f r ) and anti-resonance frequencies ( f a ) using 0-th mode shear horizontal (SH 0 mode) plate wave in a LiNbO 3 plate. Also, a tunable filter with the tunable range of 13% was demonstrated. However, the frequency ( f r =1.8 MHz and f a = 2.3 MHz) is too low for modern wireless communication. Also, the resonators suffered from ripples due to transverse mode between f r and f a . In this study, high frequency SH 0 mode plate wave resonators ( f r = 430–560 MHz and f a = 530–670 MHz) with a wide bandwidth and a large impedance ratio have been fabricated using an ultrathin LiNbO 3 plate. The ripples were suppressed by an apodized interdigital transducer.