Long range RFID transmitter power tracking loop

• US 20080088457A1
• Filed: 10/16/2006
• Published: 04/17/2008
• Est. Priority Date: 10/16/2006
• Status: Active Grant

• Current Assignee: ABL IP Holding LLC (Acuity Brands, Inc.)
• Original Assignee: GOLIATH Solutions LLC (Osa Acquisition, LLC)
• Inventors: Pyne, John W.
• Granted Patent: US 8,010,067 B2
• Time in Patent Office: Days
• US Class Current: 340/572.7
• CPC Class Codes: G08B 13/2462 Asset location systems combined with EAS inventory, tracking, logistic G06Q10/00; entrance control systems G07C9/00
• CPC Class Codes: H03G 3/3042 in modulators, frequency-changers, transmitters or power amplifiers transmission power control in bidirectional transmission systems H04W52/04
• CPC Class Codes: H04B 5/77 for interrogation
• CPC Class Codes: H04B 7/0602 using antenna switching H04B7/0686 takes precedence; antenna beam directivity switching H01Q3/24
• CPC Class Codes: H04B 7/0615 of weighted versions of same signal
• CPC Class Codes: H04W 52/52 using AGC [Automatic Gain Control] circuits or amplifiers

Abstract

A high power amplifier having an automatic gain control circuit, the high power amplifier being coupled between the output end of an RF signal carrying coaxial cable and a transmitting antenna to automatically compensate for RF signal attenuation over the length of the coaxial cable.

Claims

1. An RFID system configured to determine a location of an RFID tag in a retail environment, comprising

a central transmitter for generating an RF signal for transmission to an RFID tag;

a first transmission antenna coupled to the central transmitter via a first transmission medium;

a second transmission antenna coupled to the central transmitter via a second transmission medium;

a first RF signal compensating circuit coupled between the central transmitter and the first transmission antenna that applies a first variable gain to the RF signal to compensate for attenuation of the RF signal caused by the first transmission medium; and

a second RF signal compensating circuit coupled between the central transmitter and the second transmission antenna that applies a second variable gain to the RF signal to compensate for attenuation of the RF signal caused by the second transmission medium;

the first and second RF signal compensating circuits causing the first and second transmission antennas to broadcast the RF signal with substantially the same signal strength;

the RFID system relying in part on the substantial equality of the signal strengths in determining the location of the RFID tag in the retail environment.

2. The RFID system of claim 1, wherein the first and second RF signal compensating circuits comprise:

a first adjustable gain control circuit for applying a first adjustable gain to the RF signal;

a second adjustable gain control circuit for applying a second adjustable gain to the RF signal; and

a switching circuit configured to select either the first adjustable gain circuit or the second adjustable gain circuit to apply the variable gain to the RF signal.

3. The RFID system of claim 2, wherein the first adjustable gain control circuit is configured to apply a first range of gains to the RF signal and the second adjustable gain control circuit is configured to apply a second range of gains to the RF signal, the first range of gains being different that the second range of gains.

4. The RFID system of claim 1, wherein the first and second transmission medium are coaxial cables of different lengths.

5. The RFID system of claim 1, wherein the location of the RFID tag is determines based on received signal strength indicator (RSSI) values.

6. An RFID transmitter power tracking loop for use in RFID systems that are capable of determining the location of RFID tags within a given area comprising:

a central transmitter within the given area for transmitting signals to at least two spaced apart transmitter antennae located within the given area;

a coaxial cable connected between the central transmitter and each of the at least two transmitter antennae;

the coaxial cable for each of the transmitter antennae being of a different length and different attenuation; and

an RF signal circuit coupled between each RF signal output from a central transmitter coaxial cable and each RF signal transmitter antenna for maintaining a desired RF signal strength provided to each of the at least two spaced transmitter antenna regardless of RF signal attenuation over the coaxial cable.

7. The tracking loop of claim 6 wherein each RF signal circuit further comprises:

a high power amplifier for receiving the RF output signal from a given one of the central transmitter coaxial cables and providing an amplified RF signal to its associated transmitter antenna;

a gain control port associated with the high power amplifier for receiving an input signal for controlling the gain of the high power amplifier; and

a gain control circuit that provides a gain control signal to the high power amplifier gain control port for enabling a desired RF output signal strength to be maintained and coupled to the transmitting antennae.

8. The tracking loop of claim 7 wherein the gain control circuit comprises:

a comparator having first and second inputs;

a reference signal representing the actual RF signal output level of the high power amplifier coupled to one input of the comparator; and

a first adjustable gain control circuit including a manually set control signal representing the desired gain of the high power amplifier applied to the second input of the comparator such that the comparator output provides the desired gain to the high power amplifier.

9. The tracking loop of claim 8 further comprising a variable potentiometer for providing the manually set control signal that is applied to the second comparator input.

10. The tracking loop of claim 9 further comprising:

a second adjustable gain control circuit coupled in parallel with the first adjustable gain control circuit for providing a different gain control output signal to cause a different RF output signal strength from the high power amplifier than the desired minimum RF output signal strength caused by the first adjustable gain control circuit; and

a switch coupled between the first and second adjustable gain control circuits to enable manual selection of the amount of RF signal gain to be applied to a respective transmitter antenna.

11. A system for automatically compensating for RF signal transmission attenuation over a coaxial cable to a signal transmitting antenna in a given area comprising:

the signal transmitting antenna being coupled to one end of the coaxial cable that provides the attenuated RF signal transmission;

a high power amplifier coupled between the one end of the coaxial cable and the transmitting antenna;

a gain control circuit for providing a dc signal representing the actual RF signal level output from the high power amplifier; and

a comparator for comparing the provided dc signal with a manually set dc signal representing a desired gain of the high power amplifier and generating an output signal that is used to control the gain of the high power amplifier to compensate for RF signal transmission attenuation.

12. A method of maintaining RF signal strength substantially constant at the output of one end of a coaxial cable connected to at least one transmitter antenna over a wide range of RF signal levels caused by the attenuation of RF signals depending upon the length of the coaxial cable carrying the RF signals to the transmitter antenna comprising the steps of:

coupling one of the wide range of RF signal input levels to the input of an RF signal circuit whose output is coupled to a transmitter antenna;

detecting the RF level generated at the output of the RF signal circuit for the given RF input signal dBm level and converting the detected RF signal output to a proportional dc signal;

comparing the proportional dc signal representing the detected RF dBm level with a manually generated dc signal representing a desired RF dBm level to generate a gain control signal; and

coupling the gain control signal to the RF signal circuit to cause a desired RF output signal to be generated at the output of the RF signal circuit for powering the transmitter antenna over a wide range of RF signal strengths input to the RF signal circuit that vary with length of coaxial cable, signal frequency, coaxial cable specifications, electronics aging, and temperature variations.

13. The method of claim 12 further comprising the steps of:

receiving the RF output signals from the output of the one end of each coaxial cable with a high power amplifier as the RF signal circuit;

controlling the gain of the high power amplifier with a gain control signal applied to a signal port of the high power amplifier; and

providing a gain control signal to the signal port of the high power amplifier with a first manually adjustable gain control circuit for enabling the desired RF output signal strength to be maintained and coupled to the transmitting antenna.

14. The method of claim 13 wherein the step of providing a gain control signal further comprises the steps of:

coupling a reference dc signal based on the actual RF output signal from the high power amplifier as one input to a comparator having first and second inputs. and

generating a manually set control signal that is coupled to the other input of the comparator to cause the comparator to generate a gain control signal sufficient to cause the high power amplifier to generate and maintain the desired RF output signal strength to be delivered to the transmitter associated antenna.

15. The method of claim 14 further comprising the step of providing the manually set control signal to the other input of the comparator with the use of a first gain control circuit having a variable potentiometer that is manually adjusted.

16. The method of claim 15 further comprising the steps of:

coupling a second gain control circuit in parallel with the first gain control circuit;

adjusting the gain of the second gain control circuit to be different from the set gain of the first gain control circuit; and

coupling the output of a selected one of the first and second gain control circuits to the comparator for causing a desired signal power level from the High Power Amplifier to the transmitting antenna.

17. A method for automatically compensating for RF signal attenuation loss over a coaxial cable coupled at one end to a signal transmitting antenna in a given area comprising the steps of:

coupling a high power amplifier between the one end of the coaxial cable and the signal transmitting antenna;

converting the actual RF signal level output from the high power amplifier to a variable dc level signal; and

comparing the variable dc level signal with a manually set desired RF signal level output from the high power amplifier to generate a control signal that is used to automatically vary the gain of the high power amplifier to compensate for RF signal transmission attenuation over the coaxial cable and provide a substantially constant output level RF signal to the signal transmitting antenna.