Surface acoustic wave filter and multistage surface acoustic wave filter

A surface acoustic wave filter on a piezoelectric substrate comprising a first surface acoustic wave resonator having reflector electrodes on both sides of a first electrode for exciting a surface acoustic wave and a third surface acoustic wave resonator having reflector electrodes on both sides of the third electrode being disposed with respect to each other in the positions in which a propagation direction of the respective surface acoustic waves becomes parallel,

• a first bus bar electrode included in said first electrode and a third bus bar electrode included in said third electrode being mutually separated and disposed in opposed manner,
• a second surface acoustic wave resonator having a plurality of strip line electrodes, an electrode for connecting the one end parts of both ends of those plural strip line electrodes, and electrodes for connecting the other end parts, being formed between the opposed first bus bar electrode and the third bus bar electrode,
• said first surface acoustic wave resonator and said third surface acoustic wave resonator being disposed nearby to each other and to the second surface acoustic wave resonator and acoustically coupled,
• one input terminal of balanced type input terminals, said one input terminal being constructed by using an electrical connection between leading out electrodes led out from at least two places on said first bus bar electrode, and
• one output terminal of balanced type output terminals, said one output terminal being constructed by using an electrical connection between leading out electrodes led out from at least two places on said third bus bar electrode, thereby performing balanced operation.

A surface acoustic wave filter on a piezoelectric substrate comprising first and third surface acoustic wave resonators each having a reflector electrode on both sides of an IDT electrode as an inter-digital transducer electrode, said resonators being disposed on a piezoelectric substrate in positions in which directions of propagation of the respective surface acoustic waves are parallel with each other, and

• a plurality of strip line electrodes being disposed in parallel between said first and third surface acoustic wave resonators in the same electrode period as those of the first and third surface acoustic wave resonators, said plural strip line electrodes being connected to one another by bus bar electrodes to form a second surface acoustic wave resonator having periodic structured electrode rows, said first and third surface acoustic wave resonators being disposed nearby to said second surface acoustic wave resonator to make an acoustic coupling, and the adjacent bus bar electrodes between said surface acoustic wave resonators being electrically separated, and all periodic structured electrodes of said second surface acoustic wave resonators being grounded, wherein
• an electrode finger crossing width of IDT electrodes constituting the first and third surface acoustic wave resonators is W1, and a strip line length of said periodic structured electrode rows constituting the second surface acoustic wave resonator is W2, and the relative size of W1 to W2 is set to 1<W2/W1.