Bulk wave transponder

• US 5,359,250 A
• Filed: 03/04/1992
• Issued: 10/25/1994
• Est. Priority Date: 03/04/1992
• Status: Expired due to Fees

• Current Assignee: Measurement Specialties Limited (TE Connectivity Limited)
• Original Assignee: The Whitaker Corporation (Meggitt PLC)
• Inventors: Toda, Minoru
• Primary Examiner(s): Dougherty, Thomas M.
• Application Number:
  US07/845,685
• Time in Patent Office: 965 Days
• Field of Search: 310/313 R, 310/313 A, 310/313 D, 342/378, 342/44, 342/51, 333/141, 333/142, 333/145, 333/149
• US Class Current: 310/313.00R
• CPC Class Codes: G01S 13/755 using delay lines, e.g. acoustic delay lines
• CPC Class Codes: H03H 9/6406 Filters characterised by a particular frequency characteristic

Abstract

An electronic article surveillance system including a bulk acoustic wave transponder 10 coupled to an antenna 12, an interrogator 14, directional antenna 16 and an output connector 18. The transponder comprises a silicon or glass substrate 20, bus electrodes 22, 24, a launching IDT T0 and a series of "connected" and "unconnected" IDTs T1-T4. The connected/unconnected state of the respective IDTs T1-T4 may be detected and employed to generate an identification code comprising a series of binary digits. A thin copolymer piezoelectric film 26 covers the substrate 20 and electrodes T0-T4.

Claims

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 (T₁, T₂, T₃ . . . 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;

space="preserve" listing-type="equation">D.sub.i =2d/tan(arcos(ν

a/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.