Surface acoustic wave filter for improving flatness of a pass band and a method of manufacturing thereof

摘要

A surface acoustic wave filter has a plurality of interdigital transducers arranged in a ladder circuit structure and includes a series arm and at least two parallel arms. Each of the interdigital transducers has a first comb-shaped electrode and a second comb-shaped electrode, each of which has a plurality of electrode fingers and a bus bar connected to first ends of the plurality of electrode fingers. The first and second comb-shaped electrodes interdigitate with each other so that second ends of the plurality of electrode fingers of the first comb-shaped electrode extend toward the bus bar of the second comb-shaped electrode. A gap is created in the interdigital transducers defining the surface acoustic wave resonators that are connected to the parallel arms to cause ripples to be distributed in the pass band of the surface acoustic wave filter such that the ripples improve the flatness of the pass band. The gap is defined between the bus bar of the first comb-shaped electrode and the second ends of the plurality of electrode fingers of the second comb-shaped electrode. The gap width should be greater than about 0.50 lambd, and preferably should be in the range of about 1.0 lambd to about 5.0 lambd, wherein lambd is a wavelength of a surface acoustic wave excited on the piezoelectric substrate. Further, the gap in the interdigital transducer of the one-port SAW resonator that is connected at one parallel arm is different from the corresponding gap in the interdigital transducer of the one-port SAW resonator connected at the other parallel arm.