RFID tag having parallel resonant circuit for magnetically decoupling tag from its environment

• US 6,072,383 A
• Filed: 11/04/1998
• Issued: 06/06/2000
• Est. Priority Date: 11/04/1998
• Status: Expired due to Term

• Current Assignee: CHECKPOINT SYSTEMS, INC. (CCL Industries Incorporated), Mitsubishi Materials Corporation
• Original Assignee: CHECKPOINT SYSTEMS, INC. (CCL Industries Incorporated)
• Inventors: Gallagher, William F. III, Inui, Shinichiro
• Primary Examiner(s): MULLEN, THOMAS J
• Application Number:
  US09/185,775
• Time in Patent Office: 580 Days
• Field of Search: 340/10.2, 340/10.1, 340/505, 340/572.5, 340/825.3, 340/825.34, 342/51, 29/829, 29/832
• US Class Current: 340/10.2
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs
• CPC Class Codes: G06K 7/10336 the antenna being of the near field type, inductive coil

Abstract

An RFID tag circuit provides magnetic decoupling and amplitude modulation. The circuit is made up of a first inductor and a second inductor connected in series, a first capacitor, a second capacitor, and a switch. The circuit includes a first resonant circuit formed from a parallel connection of the series connected first and second inductors, and the first capacitor. The first resonant circuit has a primary resonant frequency. The circuit also includes a second circuit formed from a series connection of the second capacitor and the switch. The series connection of the second capacitor and the switch are connected in parallel to the second inductor. One end of the series connected second capacitor and switch is connected to the common connection between the series connected first and second inductors. When the switch is open, the second circuit has a minimal or no effect on the tag and the first resonant circuit resonates at the primary resonant frequency when the tag is exposed to an external field at or near the primary resonant frequency. When the switch is closed, the second circuit defines a high impedance parallel resonant circuit which functions to block or minimize current flow at the primary resonant frequency, thereby preventing the tag from drawing any significant amount of power from the external field and from resonating at the primary resonant frequency. The tag is thereby decoupled from its environment.

Claims

1. A transponder including a first inductor and a second inductor connected in series, a first capacitor, a second capacitor, and a switch, the transponder comprising:

(a) a first resonant circuit formed from a parallel connection of (i) the series connected first and second inductors, and (ii) the first capacitor, the first resonant circuit having a primary resonant frequency; and

(b) a second circuit formed from a series connection of the second capacitor and the switch, the series connection of the second capacitor and the switch being connected in parallel to the second inductor, one end of the series connected second capacitor and switch being connected to the common connection between the series connected first and second inductors,

wherein when the switch is open, the second circuit has a minimal or no effect on the transponder and the first resonant circuit resonates at the primary resonant frequency when the transponder is exposed to an external field at or near the primary resonant frequency, and when the switch is closed, the second circuit defines a high impedance parallel resonant circuit which functions to block or minimize current flow at the primary resonant frequency, thereby preventing the transponder from drawing any significant amount of power from the external field and from resonating at the primary resonant frequency, the transponder thereby being decoupled from its environment.

2. A transponder according to claim 1 wherein the primary resonant frequency is:

##EQU7## where L_T =L_1T +L_2T +M₁₂, and L_1T is the inductance of the first inductor,

L_2T is the inductance of the second inductor,

M₁₂ is the mutual inductance between the first and second inductors, and

C_T is the capacitance of the first capacitor.

3. A transponder according to claim 2 wherein the parallel resonant frequency of the second circuit when the switch is closed is:

##EQU8## where L_2T is the inductance of the second inductor,

M₁₂ is the mutual inductance between the first and second inductors,

M₀₂ is the mutual inductance between the second inductor and an inductor of an interrogation antenna; and

C_SW is the capacitance of the second capacitor,

wherein the values of the inductors and capacitors are selected so that f_PAR is equal to f₀.

4. A transponder according to claim 1 wherein the transponder is a radio frequency intelligent tag circuit for an RFID tag.

5. A transponder according to claim 4 further comprising:

(c) an integrated circuit connected to the first resonant circuit for outputting data, the changing state of the switch causing the data to be modulated and transmitted from the transponder to an interrogation reader.