Acoustic wave bandpass electrical filters

Claim 1

In an acoustic wave bandpass electrical filter including a substrate able to propagate acoustic waves at a surface thereof, input transducing means arranged to launch acoustic wave energy along a first propagation track at said surface, output transducing means arranged to receive acoustic wave energy from a second propagation track at said surface, and electrical coupling means arranged on said surface so as to receive acoustic wave energy from the input transducing means in the first track and to relaunch at least part of that energy as acoustic wave energy towards the output transducing means in the second track, in which the combined amplitude-frequency response of the input and output transducing means has a first frequency passband which is substantially the desired passband of the filter and further frequency passbands having a harmonic relationship to said first passband, said coupling means comprising an array of conducting strips across the first track and an array of conducting strips across the second track, the conducting strips of each array being discrete parallel strips electrically insulated from each other and strips of the two arrays being electrically connected in pairs with each pair including one strip from each array, the improvement wherein the strips of the coupling means are arranged in at least one first group, the first group comprising three sub-groups and each sub-group comprising strips from both arrays, the sub-group spacing in the first track being different from the sub-group spacing in the second track, and the composition of the three sub-groups of the first group and their separation along the first and second tracks being selected to provide a stopband in the amplitude-frequency response of the coupling means which suppresses a selected said further passband.