A wideband multi-mode SAW filter employing pitch-modulated IDTs on cu-grating/15°YX-LiNbO3-substrate structure

This paper describes an attempt to develop extremely wideband multi-mode surface acoustic wave (MMS) filters in the gigahertz range. The Cu-grating/15°YX-LiNbO₃ (15-LN)-substrate structure is employed. First, a design tool developed for the present purpose is detailed. Precise simulation is performed using a modified coupling-of-modes model, in which the coupling between propagating surface and bulk acoustic waves is taken into account. Parameters necessary for the simulation are determined experimentally. Next, this simulation tool is used to design a wideband MMS filter employing pitch-modulated IDTs proposed by the authors. It is shown that a fractional bandwidth of more than 12% is achievable by successfully using six SAW resonances supported in the MMS structure. The designed MMS filter was fabricated on a Cu-grating/15-LN-substrate structure. The passband width of 12.6% and the minimum insertion loss of 1.2 dB were experimentally obtained around 850 MHz. The measured result was in good agreement with the simulation.