Surface acoustic wave filter capable of widening a bandwidth

Claim 1

A surface acoustic wave filter utilizing at least two longitudinal modes which are different from each other, the wave filter comprising: a piezoelectric substrate; an input interdigital transducer being supplied with an input electric signal and mounted on said piezoelectric substrate for converting said input electric signal into an input surface acoustic wave as a surface acoustic wave propagated thereon, said input interdigital transducer comprising a plurality of input electrode digits which intersect one another and which are equal to N in number where N represents a positive integer greater than two; an output interdigital transducer mounted on said piezoelectric substrate and opposed to said input interdigital transducer and comprising a plurality of output electrode digits which are equal to N in number and which intersect one another, said output interdigital transducer receiving said propagated surface acoustic wave as a received surface acoustic wave and converting said received surface acoustic wave into an output electric signal; a first grating reflector mounted on said piezoelectric substrate and opposed to said input interdigital transducer at an opposite side of said output interdigital transducer; and a second grating reflector mounted on said piezoelectric substrate and opposed to said output interdigital transducer at an opposite side of said input interdigital transducer; each of said input and said output electrode digits being made of a metal material having a discontinuous impedance coefficient K related to acoustic impedance, said filter being structured so that a product of said number N and said discontinuous impedance coefficient K is not smaller than 0.55, each of said input and said output interdigital transducers having a predetermined electrode period λ, said output interdigital transducer being spaced apart from said input interdigital transducer by a first distance L1, said first and said second grating reflectors being spaced apart from said input and said output interdigital transducers, respectively, by a second distance L2, said first distance L1 being defined by an inequality given by: (n/2-4/24)λ≦L1≦(n/2-2/24)λ, where n represents a positive integer, said second distance L2 being defined by an inequality given by: (n/2-2/24)λ≦L2≦(n/2+7/24)λ.