Acoustic surface wave device

Claim 1

An acoustic surface wave device comprising a body of piezoelectric material on one surface of which is formed an acoustic surface wave transducer including at least one apodized interdigital array of electrodes in which a first and second set of electrodes are respectively connected to a corresponding one of two opposite bus-bars such that, in operation, at least one bus-bar is ungrounded and is therefore capable of varying in voltage at signal frequency relative to a quasi-steady ground potential established on or about the electrode-bearing surface region of said body at least in the vicinity of said acoustic surface wave transducer, so that an acoustic surface wave directed along an acoustic surface wave propagation direction can be propagated or received via a communicating end of the acoustic surface wave transducer, that portion of an ungrounded bus-bar adjacent a communicating end of the acoustic surface wave transducer being directed inwardly into the acoustic surface wave beam aperture substantially to the central axis of the apodized array, said transducer further including a velocity compensation region comprising dummy electrodes parallel to the electrodes of the interdigital array and connected to a grounded bus-bar, the outer communicating end of the velocity compensation region being at right angles to the acoustic surface wave propagation direction, characterized in that a grounded bus-bar portion is located adjacent the said inwardly directed portion of the ungrounded bus-bar situated within the acoustic aperture and is uniformly spaced therefrom in the acoustic surface wave propagation direction, facing boundary edges of the adjacent portions of ungrounded and grounded bus-bars situated within the communicating aperture of the transducer being inclined to the direction at right angles to the acoustic surface wave propagation direction, so that the innermost and outermost ends of each respective said edge are correspondingly displaced relative to one another in the acoustic surface wave propagation direction by at least 0.8λ, and the inclination of either said edge is at no point less than that of a notional straight line joining corresponding notional innermost and outermost points which are displaced in the propagation direction by 0.8λ, the arrangement being such that the main response lobe, for undesired acoustic surface waves generated or received by an acoustic surface wave transducing element formed by said facing boundary edges of the bus-bars situated within the communicating aperture, is directed obliquely to the acoustic surface wave propagation direction.