System and method for sensitivity optimization of RF receiver using adaptive nulling

• US 20060033607A1
• Filed: 04/27/2005
• Published: 02/16/2006
• Est. Priority Date: 04/30/2004
• Status: Active Grant

• Current Assignee: WJ Communications, Inc. (Qorvo, Inc.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Bellantoni, John Vincent
• Granted Patent: US 7,369,811 B2
• Time in Patent Office: Days
• US Class Current: 340/10.100
• 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

Abstract

The present invention provides sensitivity enhancement for a single antenna RFID interrogating device by separately coupling a nulling signal formed using a portion of a transmit signal into a receiver. The phase and amplitude of nulling signal can be adjusted so that the nulling signal cancels that a reflected transmit signal from the antenna, resulting in the small backscattered signal from the distant RFID tag to be more easily detected, and improving the sensitivity of the RFID receiver.

Claims

1. An RFID interrogating device connected to an antenna, comprising:

an RF transmitter configured to form a transmit signal;

a first directional coupler configured to extract a first portion of the transmit signal;

a vector modulator configured to adjust the first portion of the transmit signal in phase and amplitude to form a nulling signal; and

a second directional coupler configured to combine at least a portion of the nulling signal with a second portion of the transmit signal reflected from the antenna and at least a portion of a responding signal from an RFID tag received by the antenna.

2. The RFID interrogating device of claim 1, wherein the phase and amplitude of the first portion of the transmit signal is adjusted such that the nulling signal can be used to substantially cancel the second portion of the transmit signal reflected from the antenna.

3. The RFID interrogating device of claim 2, further comprising:

a detector configured to detect a feedback portion of a combined signal from an output of the second directional coupler; and

a controller configured to receive an input signal formed from the feedback portion of the combined signal and to form at least one output signal based on the input signal; and

wherein the at least one output signal is used by the vector modulator to adaptively attenuate the first portion of the transmit signal and to adaptively change the phase of the first portion of the transmit signal.

4. The RFID interrogating device of claim 3, further comprising at least one digital to analog converter configured to convert the at least one output signal from the controller into a first electrical measure and a second electrical measure that are provided to the vector modulator.

5. The RFID interrogating device of claim 4, wherein the vector modulator comprises:

a splitter configured to receive the first portion of the transmit signal from the first directional coupler and to split the first portion of the transmit signal into a first branch signal and a second branch signal;

an upper branch coupled to the first voltage and configured to receive the first branch signal from the splitter and to form a first output signal from the first branch signal, a magnitude and polarity of the first output signal being dependent on the first voltage;

a lower branch coupled to the second voltage and configured to receive the second branch signal from the splitter and to form a second output signal from the second branch signal, a magnitude and polarity of the second output signal being dependent on the second voltage; and

a 90°

hybrid coupler coupled to the upper and lower branches and configured to combine the first and second output signals in quadrature.

6. The RFID interrogating device of claim 5, wherein the upper or lower branch comprises:

a hybrid coupler having an input port, a transmitted port, a coupled port, and an output port, the input port being coupled to the splitter; and

a pair of p-intrinsic-n diodes coupled to the transmitted port and the coupled port, respectively.

7. The RFID interrogating device of claim 6, wherein the hybrid coupler is a 90°

hybrid coupler.

8. The RFID interrogating device of claim 6, wherein the upper or lower branch further comprises:

a pair of inductors serially coupled with each other and between the transmitted port and the coupled port of the hybrid coupler, a circuit node between the pair of inductors being coupled to the first or second voltage through a series resistor.

9. The RFID interrogating device of claim 8, wherein the upper or lower branch further comprises:

a capacitor coupled between the series resistor and ground.

10. The RFID interrogating device of claim 1, further comprising a circulator or a third directional coupler configured to couple at least a portion of the transmit signal reflected from the antenna and at least a portion of the responding signal from the RFID tag received by the antenna into an input port of the second directional coupler.

11. An RFID reader connected to an antenna, comprising:

an RF transmitter configured to form a transmit signal for interrogating an RFID tag; and

an RF receiver configured receive a responding signal from the RFID tag; and

wherein a nulling signal is formed from a first portion of the transmit signal and is coupled into the receiver, the nulling signal is adapted in phase and amplitude to substantially cancel a second portion of the transmit signal reflected from the antenna.

12. The RFID reader of claim 11 wherein the first portion of the transmit signal is extracted from the transmit signal.

13. The RFID reader of claim 11 wherein the receiver receives the responding signal from the RFID tag together with at least a portion of the nulling signal and the second portion of the transmit signal.

14. The RFID reader of claim 13, wherein adaptation of the nulling signal is controlled using at least one electrical measure, which is determined based on a feedback portion of a signal entering the receiver.

15. The RFID reader of claim 14, wherein the at least one electrical measure comprises a first voltage and a second voltage, and adaptation of the nulling signal is performed in a vector modulator receiving the first and second voltage.

16. A method of interrogating a passive RFID tag performed by an RFID reader connected to an antenna, comprising:

forming a transmit signal;

extracting a first portion of the transmit signal;

adjusting the first portion of the transmit signal in phase and amplitude to form a nulling signal; and

combining at least a portion of the nulling signal with a second portion of the transmit signal reflected from the antenna.

17. The method of claim 16, wherein the adjusting step comprises:

splitting the first portion of the transmit signal into a first branch signal and a second branch signal;

forming a first output signal from the first branch signal, a magnitude and polarity of the first output signal being dependent on a first voltage;

forming a second output signal from the second branch signal, a magnitude and polarity of the second output signal being dependent on a second voltage; and

combining the first and second output signals in quadrature.

18. The method of claim 17, wherein the first voltage and the second voltage are determined based on a feedback of a combined signal including the nulling signal and the second portion of the transmit signal.

19. The method of claim 16, wherein the adjusting step comprises adjusting the first portion of the transmit signal in phase and amplitude to form a nulling signal that substantially cancels the second portion of the transmit signal.

20. The method of claim 19, wherein the combining step comprises combining at least a portion of the nulling signal with the second portion of the transmit signal and at least a portion of a responding signal from the RFID tag received by the antenna, whereby the sensitivity of the RFID reader towards the responding signal is substantially enhanced.