Ultrawide band ladder filter using SH0 plate wave in thin LiNbO3 plate and its application

Currently, a cognitive radio using vacant frequency band of digital TV (DTV) channels (TV white space: 470 to 710 MHz in Japan) is receiving a lot of attention. The cognitive radio strongly requires a tunable filter covering a wide frequency range of DTV band and bandwidth tunability. Authors previously fabricated ultra-wide band 0-th mode shear horizontal (SH₀) plate wave resonators for the DTV band. A T type ladder filter, which has a low minimum insertion loss and an ultra-wide bandwidth covering the DTV channels, was numerically synthesized from the measured characteristics of the resonators. Combining the wideband filter and a band rejection filter composed of several resonators with different frequency, the band pass filters with center frequencies of 530 and 660 MHz, which are corresponding with lowest and highest frequencies in DTV band, were obtained. This paper shows that the combination of the wideband filter and both type band rejection filters connected with variable capacitors enables a tunable filter with large tunable range of frequency and bandwidth, and a large rejection of the outside of DTV band.