Bulk wave transponder

Claim 1

A transponder, comprising: (a) an antenna; and (b) a bulk wave transducer, coupled to said antenna, comprising a substrate, first and second bus electrodes, a plurality of interdigital transducers (IDTs) so disposed upon said substrate as to generate, in response to an interrogation signal received by said antenna, a response signal conveying an identifying code, and a layer of piezoelectric film in contact with said IDTs; wherein each of said IDTs is coupled to at least one of said first and second bus electrodes; wherein said plurality of IDTs comprises a series of connected and unconnected IDTs (T 1 , T 2 , T 3 . . . T N ) so disposed as to generate a response signal in which is encoded a prescribed sequence of digits, wherein the connected IDTs are connected to both said first and second bus electrodes and the unconnected IDTs are unconnected to said second bus electrode; wherein all of said IDTs are disposed on a single surface of said substrate; wherein each digit (digit i ) corresponds to a unique IDT (T i ) and is capable of representing at least two conditions, including the connected/unconnected state of its corresponding IDT; wherein each digit is carried via a unique frequency (f i ); and wherein the inter-IDT spacings D i are related to the frequencies f i as follows: D.sub.i =2d/tan(arcos(νa/Lf.sub.i)), where ν a represents the velocity of the bulk waves in the substrate, d represents the thickness of the substrate, and L represents two times the spacing between adjacent electrodes of the IDTs plus the width of an electrode.