High frequency resonators with wide bandwidth using SH0 mode plate wave in thin LiNbO3

Currently, an ultrawide band resonator used for a tunable filter with a wide tunable range is strongly required, because a mobile phone with multi-bands and a cognitive radio system require such a tunable filter to simplify their wirelss front-end circuits. The authors developed resonators with ultrawide bandwidths of 26% to 29% and large impedance ratios of 92 to 98 dB between resonance (fr) and anti-resonance frequencies (f_a) using a 0-th shear horizontal (SH₀) mode plate wave in a 28-30°YX-LiNbO₃ plate. However, the resonators were of low frequency as f_r = 1.8 MHz and f_a = 2.3 MHz and suffer from ripples due to transverse mode between f_r and f_a. In this study, high frequency SH₀ mode plate wave resonators (f_r = 440 ~ 560 MHz) with a wide bandwidth and a large impedance ratio and without ripples have been fabricated using an ultrathin LiNbO₃ plate and an apodized interdigital transducer.