Surface acoustic wave filter device

A surface acoustic wave filter device comprising:

• a substrate comprising a piezoelectric material and having a surface;
• an interdigital-type input side transducer applied on said surface of the substrate and including an input side positive electrode having a plurality of electrode fingers, an input side negative electrode having a plurality of electrode fingers each arranged between successive electrode fingers of said input side positive electrode, and an input side floating electrode having a plurality of electrode fingers arranged between successive electrode fingers of said input side negative and positive electrodes;
• an interdigital-type output side transducer applied on said surface of the substrate and including an output side positive electrode having a plurality of electrode fingers, an output side negative electrode having a plurality of electrode fingers each arranged between successive electrode fingers of said output side positive electrode, and an output side floating electrode having a plurality of electrode fingers arranged between successive electrode fingers of said output side negative and positive electrode, said electrode fingers of positive and negative electrodes of each of said input side and output side transducers being spaced from each other by a center distance of .lambda./2 in a propagation direction of a surface acoustic wave which propagates on said surface of the substrate, wherein .lambda. is a wavelength of a fundamental surface acoustic wave, and;
• said piezoelectric material of the substrate comprises quartz or a material having an electromechanical coupling factor which is substantially on the same order as that of quartz;
• said floating electrodes of said input side and output side transducers are short-circuited floating electrodes;
• each electrode finger of said positive electrodes, negative electrodes and floating electrodes of said input side and output side transducers has a width "d" as defined in the propagation direction of the surface acoustic wave, said width "d" satisfying a condition 0.8.times..lambda./12.ltoreq.d.ltoreq.1.3.times..lambda./12;
• each electrode finger of the floating electrode of the input side transducer has an edge on its propagation direction-side of the surface acoustic wave, said edge being spaced from an intermediate position between an adjacent electrode finger of the positive electrode and an adjacent electrode finger of the negative electrode, in a direction which is opposite to the propagation direction of the surface acoustic wave;
• and
• each electrode finger of the floating electrode of the output side transducer has an edge on its side opposite to the propagation direction of the surface acoustic wave, said edge being spaced from an intermediate position between an adjacent electrode finger of the positive electrode and an adjacent electrode finger of the negative electrode, in the propagation direction of the surface acoustic wave.