5.4 GHz Lamb wave resonator on LiNbO₃ thin crystal plate and its application

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 (A₁) mode of a Lamb wave on a LiNbO₃ 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 LiNbO₃ single-crystal plate instead of the LiNbO₃ film, though it has been considered difficult to form a very thin LiNbO₃ 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.