Long range RFID transmitter power tracking loop

• US 8,010,067 B2
• Filed: 10/16/2006
• Issued: 08/30/2011
• 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.
• Primary Examiner(s): Chen, Junpeng
• Application Number:
  US11/581,310
• Publication Number:
  US 20080088457A1
• Time in Patent Office: 1,779 Days
• Field of Search: 455/404.2, 455/41.1, 455/553.1, 455/126, 4551271-1273, 455/456, 455/41.3, 455/456.1, 455/91, 455/101, 455/103, 455/107, 455/114.1, 4551151-1154, 455/123, 455/125, 455/127.2, 455/129, 340/10.1, 343/893
• US Class Current: 455/127.3
• 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 the RF signal for transmission to the 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;

wherein the first transmission medium and the second transmission medium are cables of different lengths;

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 a same signal strength, the first and second RF signal compensating circuits each comprising;

an automatic gain control circuit for automatically adjusting a gain applied to the RF signal to maintain the same signal strength at the transmission antennas, the automatic gain control circuit comprising;

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

a second static gain control circuit, in parallel with the first gain control circuit, for applying a second predetermined static gain to the RF signal; and

a switch configured to manually select either the first static gain control circuit or the second static gain control circuit to apply the first or second predetermined static gain to the RF signal.

2. The RFID system of claim 1, wherein the first static gain control circuit outputs a gain at a predetermined level and the second static gain control circuit outputs a gain at a second predetermined level lower than the predetermined level of the first static gain control circuit.

3. An RFID transmitter power tracking loop for use in RFID systems that are capable of determining locations 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 spaced apart transmitter antennae;

the coaxial cable for each of the at least two spaced apart 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 of the at least two spaced apart transmitter antenna for maintaining a predetermined static RF signal strength provided to each of the at least two spaced apart transmitter antenna regardless of RF signal attenuation over the coaxial cable;

each RF signal circuit comprising;

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 a gain control signal for controlling a gain of the high power amplifier;

a gain control circuit that provides the gain control signal for enabling a desired RF output signal strength to be maintained and coupled to the associated transmitting antennae;

the gain control circuit comprising;

a comparator having first and second inputs;

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

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

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 predetermined RF output signal strength from the high power amplifier than the predetermined RF output signal strength caused by the first adjustable gain control circuit.

4. The RFID transmitter power tracking loop of claim 3 further comprising a variable potentiometer for providing the manually set control signal that is applied to the second comparator input.

5. The RFID transmitter power tracking loop of claim 4 further comprising:

a switch coupled between the first and second adjustable gain control circuits to enable manual selection of the manually set control signal or the different gain control output signal to be applied to a respective transmitter antenna.

6. A method of maintaining RF signal strength substantially constant at an output of one end of a plurality of coaxial cables of differing lengths connected to at least one transmitter antenna over a wide range of RF signal input levels caused by attenuations of RF signals depending upon the lengths of the plurality of coaxial cables carrying the RF signals to the at least one transmitter antenna comprising the steps of:

for each of the plurality of coaxial cables;

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

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

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

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 at least one transmitter antenna over a wide range of RF signal input levels to the RF signal circuit that vary with length of the plurality of coaxial cables, signal frequency, coaxial cable specifications, electronics aging, and temperature variations, wherein the gain control signal is manually adjustable for each of the plurality of coaxial cables so as to maintain substantially a same constant RF signal strength at the output of each of the plurality of coaxial cables,

wherein the comparing step further comprises the steps of;

coupling the proportional dc signal based on the detected RF output signal level from a high power amplifier in the RF signal circuit as one input to a comparator having first and second inputs;

providing the manually generated dc signal representing the desired RF dBm level to the other input of the comparator with a use of a first gain control circuit having a variable potentiometer that is manually adjusted;

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

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

coupling an output of a selected one of the first and second gain control circuits to the comparator.