WO2004015614A1

WO2004015614A1 - Rf identification system with multiple readers

Info

Publication number: WO2004015614A1
Application number: PCT/ZA2003/000106
Authority: WO
Inventors: Raymond Catherall Atkins; Liviu Susnea; Marthinus Jacobus Rudolph Oosthuizen
Original assignee: Supersensor (Pty) Ltd
Priority date: 2002-08-08
Filing date: 2003-08-08
Publication date: 2004-02-19
Also published as: AU2003266171A1

Abstract

A reader arrangement (50) for an RFID system for interrogating at least one transponder (16.1 to 16.n) utilizing radio frequency signals comprises a first reader (52) for transmitting a first RF signal (64) having a first frequency fR1. The arrangement comprises at least one further reader (54) for transmitting a second RF signal (68) having a second frequency fR2. The first frequency fR1 and the second frequency fR2 are generated by a common oscillator, are the same, synchronized with one another and equal to a desired frequency, thereby to avoid beat signals in a baseband resulting from differences in the frequencies caused by component tolerance in the know arrangements.

Description

RF IDENTIFICATION SYSTEM WITH MULTIPLE READERS

TECHNICAL FIELD

This invention relates to wireless, more particularly radio frequency

electronic identification (RFID) systems comprising a plurality of

spaced readers.

BACKGROUND ART

Systems of the aforementioned kind are known in the art. A plurality

of readers are used to interrogate transponders or tags in a

transponder population, thereby to read response data (typically in the

form of an identification code) transmitted by the transponders, to

identify the transponders. In a known system, each transponder

backscatter modulates the interrogation signal in the form of an

energizing signal transmitted by the reader with the response data and

the reader then demodulates the backscattered signal to extract the

base band data signal. The readers each comprises an own

temperature compensated crystal oscillator (TCXO) for generating the

interrogation signal which is intended to have a frequency of say

915MHz. However, due to component tolerances, there usually is a

difference in the frequency of the interrogation signal transmitted by

one reader and that of the interrogation signal transmitted by another reader. If the reader arrangement is such that one reader picks up the

signal transmitted by the other reader, the difference in the

aforementioned frequencies manifests itself in unwanted beat signals

in the aforementioned base band. These signals are often very large

compared to the response signals, which makes extraction of the data

in the response signals difficult, if not impossible.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a reader

arrangement, an RFID system and method of reading transponders

with which the applicant believes the aforementioned disadvantages

may at least be alleviated.

SUMMARY OF THE INVENTION

According to the invention there is provided a reader arrangement for

interrogating at least one transponder utilizing electromagnetic signals,

the arrangement comprising:

a first reader for transmitting a first electromagnetic signal;

at least one further reader for transmitting a second

electromagnetic signal; and a common oscillator for use in generating the first and second

signals, to ensure that respective frequencies of the first and

second signals are substantially equal to a desired frequency.

The oscillator may be a temperature compensated crystal oscillator.

The common oscillator may generate a signal having an oscillator

frequency which is at least one order lower than the desired frequency

and a frequency multiplier may be provided to multiply the oscillator

frequency to be equal to said desired frequency.

In one embodiment the oscillator and multiplier may form part of a

single and common oscillator arrangement and which is connected via

a link to at least said at least one further reader.

In another embodiment the oscillator may be located at a central site

and at least said at least one further reader may be a remote reader

comprising a respective frequency multiplier for multiplying the

oscillator frequency.

In yet another embodiment the common oscillator may generate

reference information and each reader may comprise a local signal generator responsive to the reference information to generate the first

and second signals having the desired frequency.

The common oscillator may be connected to a wireless transmitter for

broadcasting to the readers the reference information which may

comprise a reference frequency.

The electromagnetic signals are preferably radio frequency signals.

Said desired frequency may be between 800MHz and 1 ,2GHz and

preferably is about 915MHz.

The at least one transponder may be a passive transponder which

derives power to energize local circuits of the transponder from at

least one of said first and second signals.

Also included within the scope of the invention is a reader

arrangement for interrogating at least one transponder utilizing

electromagnetic signals, the arrangement comprising:

a first reader for transmitting a first electromagnetic signal

having a first frequency;

at least one further reader for transmitting a second

electromagnetic signal having a second frequency; and the first frequency and the second frequency being the same

and equal to a desired frequency.

According to another aspect of the invention there is provided a

method of reading at least one transponder, the method comprising

the steps of:

providing a first reader to broadcast a first interrogation signal;

providing at least one further reader to broadcast a second

interrogation signal; and

- utilizing a common oscillator in generating the first and second

signals thereby to ensure that respective frequencies of the first

and second signals are substantially equal to a desired

frequency.

Also included within the scope of the present invention is a method of

reading at least one transponder, the method comprising the steps of:

utilizing at least a first reader and a second reader to interrogate

the at least one transponder; and

causing a first frequency of an interrogation signal transmitted

by the first reader to be the same as a second frequency of a

second interrogation signal transmitted by the second reader. BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only,

with reference to the accompanying diagrams, wherein

figure 1 is a block diagram of a prior art electronic radio frequency

identification system;

figure 2 is a block diagram of a first embodiment of a reader

arrangement and system according to the invention;

figure 3 is a block diagram of a second embodiment of a reader

arrangement and system according to the invention; and

figure 4 is a block diagram of a third embodiment of a reader

arrangement and system according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In figure 1 there is shown a known electronic radio frequency

identification (RFID) system 10.

The system 10 comprises a first reader 12 and a second reader 14 for

reading and identifying transponders 16.1 to 16.n in a transponder

population 16. The readers are similar in configuration and therefore

reader 12 only is shown in more detail and will be described in more

detail hereinafter. Reader 12 comprises an antenna 18 connected via a

directional coupler 20 to a temperature compensated crystal oscillator (TCXO) 22 and an amplitude detector 24. The detector is connected to

a filter 26 and the filter is connected to signal processing circuitry 28.

As stated in the introduction of this specification, the TCXO 26

generates an energizing or interrogation signal 30 which is broadcast via

antenna 18. The signal 30 has a frequency f_R1 equal to a nominal

frequency f₀ plus an error₁ - component determined by tolerances in

component values in the TCXO. Similarly, energizing or interrogation

signal 32 broadcast by reader 14 has a frequency f_R2 equal to the

nominal frequency f₀ plus a normally different error₂ - component

determined by component tolerances of the local TCXO of the second

reader 14. In a known system the nominal frequency is 915MHz and

the error-components may be between 0 and 10 KHz. Hence there may

be a difference between the frequencies of the energizing signals 30 and

32 of between 0 and 20 KHz. The transponders 16.1 to 16.n

backscatter modulate the energizing signals with data streams. The rate

of data modulation is about 16 KHz.

Also as stated in the introduction of this specification, with an

arrangement as shown in figure 1 , where the reader 12 also receives the

energizing signal of reader 14, the aforementioned difference in the

frequencies of the energizing signals manifests itself at the output of non-linear amplitude detector 24 in the form of a beat frequency falling

within the base band, where the transponder data signal is to be

retrieved after filtering at 26.

In figure 2 there is shown a first embodiment of a reader arrangement

50 according to the invention and comprising readers according to the

invention. The arrangement 50 comprises a first reader 52 and a

second reader 54 for interrogating transponders 16.1 to 16.n.

However, the readers derive their energizing signals from a remote and

common TCXO 56. TCXO 56 is connected to first reader 52 by line

58 and to second reader 54 by line 60. The TCXO outputs a signal f,

having a frequency of 35MHz, for example. An a = 26 multiplier 62 is

utilized at reader 52 to generate the energizing signal 64 with desired

frequency to be broadcast by reader 52. An identical multiplier 66 is

utilized at reader 54 to generate the energizing signal 68 with desired

frequency to be broadcast by reader 54. Since the error component

caused by component tolerances in TCXO 56 is common to both

readers, the resulting energizing signals have substantially the same

frequency and are also synchronized with one another. The result is

that the unwanted beat signal is avoided. The desired frequency may

fall in the range 800MHz to 1 ,2GHz and preferably is about 915MHz. In figure 3, a second embodiment of the system and reader

arrangement is shown. The arrangement 70 comprises a first reader

72 and a second reader 74. Again neither reader comprises a local

oscillator for generating their respective energizing signals 76 and 78.

Instead a remote master oscillator arrangement 80 is provided. The

oscillator arrangement comprises a TCXO 82 generating a signal

having a frequency f₂ of about 32MHz plus a possible error

component. An α = 28.6 multiplier 84 increases the frequency to the

desired frequency of about 915MHz. In other applications a known

frequency synthesizer using a phase locked loop may be used to

produce the desired frequency. An output of the oscillator arrangement

80 is connected to the two distributed readers 72 and 74 respectively.

At the readers the signals from arrangement 80 are suitably amplified

to be broadcasted as energizing signals 76 and 78 respectively. Since

the resulting energizing signals have substantially the same frequency

and are synchronized with one another, the aforementioned unwanted

beat signals are again avoided.

In figure 4 there is shown a fourth embodiment of the arrangement

according to the invention. The arrangement 90 comprises a master or

reference oscillator 92 connected to an antenna 94. In some forms of

this embodiment the reference oscillator may form part of a reader for interrogating a transponder population 16. In other forms, such as that

shown in figure 4, it merely serves to generate a reference signal or

reference information relating to a refrence signal and which is

broadcast at 96 to a plurality of readers 98.1 to 98. n in a reader

arrangement. Each of the readers comprises a TCXO arrangement

100.1 to 100.n. Each arrangement 100.1 to 100.n is adapted to lock

in known manner onto the reference signal 96 to generate a respective

energizing signal 102.1 to 102.n having the desired frequency based

on the frequency of the reference signal, so that the frequencies of the

respective energizing signals are substantially equal and synchronized,

and so that the aforementioned unwanted beat signals are again

avoided.

Claims

1 . A reader arrangement for interrogating at least one transponder

utilizing electromagnetic signals, the arrangement comprising:

a first reader for transmitting a first electromagnetic signal;

- at least one further reader for transmitting a second

electromagnetic signal; and

a common oscillator for use in generating the first and

second signals, to ensure that respective frequencies of the

first and second signals are substantially equal to a desired

frequency.

2. An arrangement as claimed in claim 1 wherein the oscillator is a

temperature compensated crystal oscillator.

3. An arrangement as claimed in any one of claims 1 and 2

wherein the common oscillator generates a signal having an

oscillator frequency which is at least one order lower than the

desired frequency and wherein a frequency multiplier is provided

to multiply the oscillator frequency to be equal to said desired

frequency.

4. An arrangement as claimed in claim 3 wherein the oscillator and

multiplier form part of a single and common oscillator

arrangement and which is connected via a link to at least said at

least one further reader.

5. An arrangement as claimed in claim 4 wherein the oscillator is

located at a central site and wherein at least said at least one

further reader is a remote reader comprising a respective

frequency multiplier for multiplying the oscillator frequency.

6. An arrangement as claimed in claim 1 or claim 2 wherein the

common oscillator generates reference information and wherein

each reader comprises a local signal generator responsive to the

reference information to generate the first and second signals

having the desired frequency.

7. An arrangement as claimed in claim 6 wherein the common

oscillator is connected to a wireless transmitter for broadcasting

the reference information comprising a reference frequency to

the readers.

8. An arrangement as claimed in any one of claims 1 to 2 wherein

the electromagnetic signals are radio frequency signals.

9. An arrangement as claimed in claim 8 wherein said desired

frequency is between 800MHz and 1 ,2GHz.

10. An arrangement as claimed in any one of the preceding claims

wherein the at least one transponder is a passive transponder

which derives power to energize local circuits of the transponder

from at least one of said first and second signals.

1 1 . A reader arrangement for interrogating at least one transponder

utilizing electromagnetic signals, the arrangement comprising:

a first reader for transmitting a first electromagnetic signal

having a first frequency;

at least one further reader for transmitting a second

electromagnetic signal having a second frequency; and

the first frequency and the second frequency being the same

and equal to a desired frequency.

12. A method of reading at least one transponder, the method

comprising the steps of: providing a first reader to broadcast a first interrogation

signal;

providing at least one further reader to broadcast a second

interrogation signal; and

- utilizing a common oscillator in generating the first and

second signals, thereby to ensure that respective

frequencies of the first and second signals are substantially

equal to a desired frequency.

13. A method of reading at least one transponder, the method

comprising the steps of:

utilizing at least a first reader and a second reader to

interrogate the at least one transponder; and

causing a first frequency of an interrogation signal

transmitted by the first reader to be the same as a second

frequency of a second interrogation signal transmitted by

the second reader.