Abstract
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 anti-resonance 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 =1.8 MHz and fa = 2.3 MHz) 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 = 430–560 MHz and fa = 530–670 MHz) 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.
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