Acoustic wave bandpass electrical filters

Claim 1

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 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 tranducing means has a fundamental passband which is substantially the desired passband of the filter with a maximum amplitude response substantially at a frequency f o and has a corresponding third harmonic passband, and in which the coupling means consists of 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 consisting of one strip from each array, at least the majority of the strips of both arrays of the coupling means having a mark-to-space ratio of substantially one-to-one, characterised in that f o /f s is in the range 0.80 to 1.14 but excluding substantially 1.0, f s being the frequency at which the centre-to-centre spacing of at least the majority of said strips is half a wavelength of said acoustic waves, whereby a stopband in the amplitude-frequency response of the coupling means between 2f s and 4f s suppresses the amplitude-frequency response of the input and output transducing means over a range of frequencies including 3f o ±0.5f o .