5.4 GHz Lamb Wave Resonator on LiNbO₃ Thin Crystal Plate and Its Application

Abstract

A cognitive radio system operating at frequencies from 400 MHz to 6 GHz requires tunable filters with a wide variable frequency range. Wide band resonators are required to realize such tunable filters consisting of resonators. However, it is difficult to fabricate a very high frequency and wide band resonator, such as 6 GHz and wider than 10%, respectively. The first antisymmetric (A 1 ) mode of a Lamb wave on a LiNbO 3 thin plate or film having a high velocity and a large electromechanical coupling factor is a suitable wave for fabricating an ultrawide band and high frequency resonator. This time, we attempted to fabricate a higher frequency Lamb wave resonator on a thin LiNbO 3 single-crystal plate instead of the LiNbO 3 film, though it has been considered difficult to form a very thin LiNbO 3 crystal plate. A new one-port Lamb wave resonator showed a high frequency in the 5 GHz range, a wide bandwidth of 12%, and a high impedance ratio of 62 dB at resonant and antiresonant frequencies. Moreover, we applied the resonator to a ladder-type tunable filter, and obtained the tunable range of 9% from 5.58 to 6.06 GHz theoretically.