US 4,060,833 AGranted

Transducer arrangement for a surface acoustic wave device to inhibit the generation of multiple reflection signals

AssigneeRCA Corporation

InventorLubomyr Stephen Onyshkevych

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Abstract

The intermediate frequency (IF) portion of a television receiver includes a surface acoustic wave (SAW) filter comprising first, second and third transducers mounted on a piezoelectric substrate. Each transducer includes at least two interdigitized comb-shaped conductors. The teeth of the comb-shaped conductors of the first transducer overlap in accordance with a predetermined envelope to define a desired IF response. The first and second transducers are connected to IF circuit portions of the receiver. The third transducer is positioned from the second transducer so that regenerated acoustic signals reflected from the first and third transducers to the second transducer are substantially 180.degree. out of phase at a predetermined frequency. The aperture W.sub.3, i.e., the overlap of the teeth of the comb-shaped elements, of the third transducer is selected with respect to the aperture W.sub.1 of the first transducer and the magnitude of the impedance of the combination of the first transducer and the electrical circuit connected to it so that regenerated acoustic signals reflected from the first and third transducers are substantially equal in magnitude at the predetermined frequency. The regenerated acoustic signals reflected from the first and third transducers are received by the second transducer to develop an electrical output signal substantially free of the influence of ghost-producing multiple reflection signals. The spacing between pairs of interdigitized teeth of the third transducer may be selected so that the center frequency associated with the third transducer is the frequency of the picture carrier, the frequency at which reflected signals are most readily manifested as ghost images.

Apparatus comprising:

a piezoelectric substrate capable of supporting acoustic signals;
first, second and third transducers associated with one surface of said substrate, each of said transducers including at least one pair of interdigitized comb-shaped conductors, each of said transducers having a complex characteristic impedance associated therewith;
first and second electrical circuits respectively connected between said comb-shaped conductors of said first and second transducers, each of said circuits having a complex impedance associated therewith;
said first and second transducers being space apart a predetermined distance and aligned with respect to one another so that acoustic signals may be propagated between them;
said third transducer having no electrical connections thereto;
said third transducer being space apart from said second transducer by said predetermined distance modified by an incremental distance and aligned with respect to said second transducer so that acoustic signals may be propagated between them, said incremental distance being selected with respect to the phase shift provided by said complex characteristic impedance of said first transducer and said complex impedance of said first circuit so that the acoustic signals reflected from said third transducer are substantially out of phase with acoustic signals reflected from said first transducer at a predetermined frequency;
said third transducer having an aperture selected with respect to the aperture of said first transducer and the magnitudes of said complex characteristic impedance of said first transducer and said complex impedance of said first circuit so that the magnitude of acoustic signals reflected from said third transducer is substantially the same as the magnitude of acoustic signals reflected from said first transducer at said predetermined frequency;
and
said second transducer receiving and substantially cancelling reflected acoustic waves from said first transducer and said third transducer.

6 more elements

Abstract

Claims11