Method for tuning an oscillating receiver circuit of a transponder built into a RFID system

• US 6,272,321 B1
• Filed: 07/12/1999
• Issued: 08/07/2001
• Est. Priority Date: 09/13/1996
• Status: Expired due to Term

• Current Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Original Assignee: Temic Semiconductor GmbH
• Inventors: Melchior, Marc, Bruhnke, Michael, Schülgen, Andreas, Hirsch, Konrad
• Primary Examiner(s): LE, THANH C
• Application Number:
  US09/254,866
• Time in Patent Office: 757 Days
• Field of Search: 455/41, 455/73, 455/106, 455/107, 455/121, 455/123, 455/129, 455/196.1, 375/192.1, 375/192.2, 375/192.3, 375/304, 375/334, 375/344, 340/80.1, 340/10.2, 340/10.3, 340/10.4
• US Class Current: 455/41.1
• 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 19/0726 the arrangement including a circuit for tuning the resonance frequency of an antenna on the record carrier
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06Q 20/341 Active cards, i.e. cards including their own processing means, e.g. including an IC or chip
• CPC Class Codes: G06Q 20/40975 using encryption therefor
• CPC Class Codes: G07F 7/1008 Active credit-cards provided with means to personalise their use, e.g. with PIN-introduction/comparison system
• CPC Class Codes: G11C 11/419 Read-write [R-W] circuits
• CPC Class Codes: G11C 7/1048 Data bus control circuits, e.g. precharging, presetting, equalising
• CPC Class Codes: G11C 7/1051 Data output circuits, e.g. read-out amplifiers, data output buffers, data output registers, data output level conversion circuits

Abstract

The invention relates to a method for calibrating an oscillating receiver circuit of a transponder in an RFID system to resonance to the frequency of an inquiry signal transmitted by a reader, the oscillating receiver circuit comprising at least one inductive resistor and at least one capacitor. Immediately after the start of reception of the inquiry signal, capacitors are connected in increments to the capacitance of the oscillating receiver circuit until such time as the rectified, smoothed inquiry signal assumes a maximum voltage value. This active self-calibration is performed upon each initialization of the transponder but can also be addressed by having the reader send a special sequence to the transponder.

Claims

1. A circuit for calibrating an oscillating receiver circuit of a transponder in a radial frequency identification system to resonance to the frequency of an inquiry signal generated by a reader, comprising:

an oscillating receiver circuit having at least one inductive resistor and at least one capacitor;

a plurality of capacitors, which can be switched by triggerable switches parallel to the capacitor of the oscillating receiver circuit;

a rectifier stage, connected to the oscillating receiver circuit, rectifying the inquiry signal;

a filter stage, connected to the rectifier stage, smoothing the rectified signal;

a maximum detector, detecting a maximum in the smoothed, rectified signal;

a control logic triggering the switches to engage capacitors as a function of the status of the maximum detector.

2. The circuit of claim 1, wherein the switchable capacitors comprise a series circuit of one capacitor and one switching transistor, and that the switching transistor is triggered by the control logic.

3. The circuit of claim 2, wherein one pair each of identically dimensioned capacitors are triggered by the control logic;

wherein the first series circuit of a capacitor and a switching transistor connects a first terminal of the oscillating receiver circuit to the ground potential; and

wherein the second series circuit of a capacitor and a switching transistor connects a second terminal of the oscillating receiver circuit to the ground potential.

4. The circuit arrangement of claim 3, wherein the control logic includes an n-stage dual counter, whose outputs trigger n pairs of capacitors and switches.

5. A method for calibrating an oscillating receiver circuit of a transponder in a radial frequency identification system to resonance to the frequency of an inquiry signal transmitted by a reader, the oscillating receiver circuit comprising at least one capacitor, the method comprising the steps of:

transmitting the inquiry signal from the reader to the oscillating receiver circuit;

changing capacitance of the oscillating receiver circuit in increments by adding or subtracting capacitances, wherein the capacitances of the capacitor are graduated in binary-weighted form and the capacitors are triggered using a binary counter; and

retaining counter state of the binary counter at the maximum voltage value in order to perform the calibration.

6. The method of claim 5, further comprising the steps of:

performing calibration of the oscillating receiver circuit immediately after start of reception of the inquiry signal.

7. The method of claim 5, further comprising the step of:

transmitting a command from the reader to the oscillating receiver circuit; and

performing the calibration after the reception of the corresponding command sent by the reader.

8. The method of claim 5, further comprising the step of performing the calibration to resonate to the frequency by changing the capacitance so that the calibration begins at a higher frequency and ends at a lower frequency.

9. The method of claim 5, further comprising the step of performing the calibration to resonate to the frequency by changing the capacitance so that the calibration begins at a lower frequency and ends at a higher frequency.

10. A method for calibrating a transponder to the resident frequency of a base station during an initialization phase, comprising the steps of:

(a) providing an oscillating circuit;

(b) rectifying and smoothing a first induced voltage induced in a coil;

(c) storing said first induced voltage in memory;

(d) connecting a capacitor in parallel to the oscillating circuits to increase the capacitance of the transponder;

(e) storing a second induced voltage in memory based on the increased capacitance;

(f) comparing the second induced voltage to the first induced voltage;

(g) replacing the first induced voltage with the second induced voltage in memory;

(h) adding a capacitor if the second induced voltage is greater than the first voltage; and

(i) repeating steps (d)-(i) until the second voltage is less than the first voltage; and

(j) recording the capacitance associated with the first voltage so that the transponder calibrates itself to the resident frequency of the base station.