AU2007216685B2 - System and method for tuning RFID resonant frequency - Google Patents

Abstract

P-WPDOC\M H\Speficatons\Droers Ay One\20197575 doc-9/5/2007 This RFID (radio frequency identification) resonant-frequency-tuning system is disclosed for use in RFID tag manufacturing. The RFID resonant-frequency-tuning system consists of a custom designed RFID reader incorporating a frequency counter, a calculation 5 unit, a tuning algorithm, and a RFID tag in which the tuning system facilities are embedded. The reader starts the resonant frequency tuning process by sending a specific command with an arbitrary tuning value to the RFID tag to configure its chip in-built capacitance of the resonant circuit. When the reader ceases the transmission, the tag responds by oscillating magnetic flux coupled back to the reader. The reader antenna picks 10 up the signal whereas the post processing system and circuitry measures the frequency transmitted from the tag being tuned. A tuning process is contactless and requires at least two frequency measurement points derived from two chosen tuning values to determine the best possible frequency tuning target value. At the end of the tuning process, the reader writes the optimum tuning value into the tag's non-volatile memory inside the microchip 15 respective to capacitance chosen. During regular operation, the tune data can be retrieved from the microchip at a start-up time to combine the chosen capacitance with the external antenna inductor to achieve the target resonant frequency. The proposed system and method renders a tight control of the resonant frequencies for every tag during mass production. 812 9- 7 --- 2 11Mod Frequency ControllerDe o RF1 2"-'xC, 2"2xCO C" Voc 1 5 -20- 2-_I _ 191 [ - ..o. T T 202 22 21 RF Interface, Controller 13 & 16\ 1,23 Memory F n-bit Latch RF1 2"-xC_ CO 1 1 19 HV o o0 22'-, RF Interface. 21 Controller 13 26 26& 23 Memory F n-bit Latch

Description

P :WPDOCS\KMKSpeaficatis\Droers Ay One\20197578 doc-9/6/2007 SYSTEM AND METHOD FOR TUNING RFID RESONANT FREQUENCY Background of the Invention The invention proposed herein is related to a resonant-frequency-tuning system and method for RFID manufacturing through frequency measurement method. More 5 particularly the invention relates to the RFID technology in which an LC resonant circuit is made up of an inductor and a capacitor. However, the system also supports other tunable oscillation fashions including the electromagnetic wave resonators whereby the external components to be tuned are part of the tags' tuning system. Description of the Prior Art 10 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. 15 In a passive REID system employing a RFID microchip with a LC resonant circuit or a "Tank" circuit, the RFID microchip only relies on the energy transmitted or induced from a RFID reader or an interrogator in the form of magnetic field coupling. Therefore, no external energy power source is required. A "RFID tag" or "tag" can be made in various forms depending on applications and consists of a microchip attached to the LC resonator 20 circuit. The RFID microchip communicates with the reader through the tank circuit performing a transponder function whereby the receiving and transmitting features are achieved in a single unit. The LC tank loads the reader's magnetic field in a resonating manner while the microchip converts RF ac power to a dc supply. After the dc power is accumulated sufficiently, the microchip then generates a modulating signal in one of many 25 load modulation schemes through magnetic field feedback coupled to the RFID reader. For example, in a sequential RFID system, the microchip gradually collects the magnetic field energy until the stored power attains an adequate level, during the reader's field gap interval, the microchip then begins transmitting its data back to the reader. The P \WPDOCS\KMH\Speoficatons\DroverS Ay One\20197578 doc-9/6/2007 -2 modulated RF signal from the transponder is usually maintained in the form of frequency shift keying (FSK) RF magnetic field conforming to the protocol recognized by the reader. The reader recovers the small modulated data from the signal picked up at the reader antenna and performs decoding until the telegram conducted by the microchip has been 5 completed. The communication through the magnetic field between the transponder and the reader/interrogator is the key characteristic in the low frequency (100kHz -150kl-z) and high frequency RFID applications (13.56 MHz). However, the read distance of the magnetically coupling system especially when designed for small portable readers is relatively short. Typically, the read distance achieved is in the range of 5 to 20 centimeters. 10 Such distance deems inadequate in many applications. The physical limit in antenna size does not permit sufficient energy to power the transponder hence becomes the main limiting factor for most of small reader systems. Many passive RFID designs attempt to improve the transponders' efficiency to extend the read distance as much as possible. Nevertheless, due to the fact that the tag's 15 read distance is principally dependent on how efficiently can the tag aggregate the energy from the receiving magnetic field. Hence the key technique has been by far emphasizing on maximizing or increasing the inductor's quality factor (Q) of the resonant circuit. The higher the Q being utilized, the less the energy loss which directly renders the longer read distance. However, if the high-Q inductors are used, the risk of the tags falling short of the 20 read distance within the production batch can be imminent. Although the discrete inductors and capacitors possess large variations, most manufacturers unavoidably use these normal inductors and capacitors to minimize their costs. The problem lies in the typical inductors and capacitors tolerances mostly used in tag component manufacturing. The tolerance between 5% to 10% of such components can 25 yield the tags' resonant frequencies which significantly deviate from the required system reference frequency. This mainly is caused by the differences in the antenna tuning frequencies between the reader and the tags. The higher the Q of the transponder's antenna, the more dramatic loss in energy transfer can occur at the transponders. In practice, the tag manufacturers tend to avoid this problem by selecting the inductors' Q to 30 no more than 30 to lessen the wide read distance variation spread of the transponders within the production batch. Consequently, the read distance achieved drops considerably.

P:\WPDOCS\KMH\Speafications\Drovers Ay One\20197578 doc-9/6/2OO7 -3 If the transponder's Q is greater than 30, and its resonant frequency is exactly the same as the reader's reference frequency, the read distance is noticeably increased. In contrast, if the resonant frequency is slightly deviated from the reference frequency due to component variations, the efficiency in accumulated energy is reduced and the read distances even in 5 the same batch spread out and drop. This effect is a fundamental drawback of the high-Q LC tank antenna system during the mass production. High-Q LC tank inevitably needs precise frequency tuning in order to operate efficiently. The tuning process involves adjusting the inductance or the capacitance of the tank until its resonant frequency matches the frequency of the RFID reader's generated RF 10 field. The procedure for inductance and the capacitance's fine tuning may be achieved by physically adjusting the inductor's and the capacitor's geometry. Another method commonly used nowadays is to gradually add a proper small capacitor to incrementally adjust the capacitance until the near exact resonant frequency is obtained. However, a production using these tuning methods is costly, complicated, and time consuming. A more 15 pragmatic method is to embed a small tuning capacitor array into the microchip and store a tuning configuration value within a non-volatile memory. This memory in turn sets the capacitors array combination which enables the tank circuit to achieve the required system frequency. The tuning capacitors can be connected or disconnected using electronic switches controlled by the signals derived from a tuning control circuitry. During the 20 production, manufacturers can use a specially arranged reader which is designed for tag tuning purpose to tune every single tag. Such calibrating reader determines the tags' proper tuning capacitance by configuring the capacitor array in the microchip to globally cancel out both variations of the capacitance and the inductance composed in the tank circuit and the microchip. The reader then registers the stored tuning configuration for every tag into 25 the tags' own memory. The LC tolerances thus result in individually different tuning configurations. With this embedded tuning process, the transponders need no physical discrete components changes, simplifying the manufacturing process steps. After the tuning process, all of the tags resonant frequencies and their read distances are well within the target, become closely matched in the same batch. 30 An example of a frequency-tuning system for RFID tags has been proposed in the US5.396.251 patent. The system consists of a RFID reader equipped with a signal strength P \WPDOCS\KMF-fSpeoficabons\Drovers Ay One\20197578 doc-9/6/2007 -4 indicator (SSI) to measure the radio-frequency signal strength transmitted by a tag and give an output voltage that is proportional to the signal strength. The system commands the tag to adjust its own resonant frequency until the maximum voltage is obtained from the SSI. The maximum voltage reflects the tag's best resonant frequency in communicating with 5 the reader. However, this method complicates the tuning system because such system requires the SSI and an analog-to-digital converter (ADC) to convert the SSI output voltage into digital data. The reader's control unit implemented by a microcontroller or a microprocessor uses these data to tune each tag. In contrast, the invention proposed herein involves the system and method in tuning 10 the resonant frequency of the RFID tag by means of frequency measurement. The system accuracy relies only on a precision clock source whose frequency derived precisely from a moderately accurate XTAL reference source matches the target resonant frequency. The reader picks up the signal generated by the tag at the reader antenna and shapes it into a digital clock signal to be counted by the controller for further processing. The system does 15 not require any calibration and is well suited to critical tunable RFID tags mass manufacturing. Summary of the Invention The invention proposed herein involves a system and method in tuning the resonant frequency of RFID tags. The tuning system contains: 20 a RFID reader or an interrogator which can communicate with a RFID tag through the radio frequency (RF) signal or the magnetic field using either the amplitude modulation (AM) or the frequency modulation (FM) scheme. The entire system measures the exact frequency of the signal sending back from the RFID tag under tuning by using high frequency clock sampling method. In addition, the reader can also send tuning-related 25 commands to the tag enabling the control unit to perform a tuning process according to the method proposed herein. The system also contains a RFID microchip in which a resonant frequency-tuning network, a sinusoidal LC oscillator, a nonvolatile memory, latches, and a digital controller are embedded. The oscillator uses the LC tank whose resonant frequency is tuned by the P \PDOCS\KMHSpeaficabons\rvrs Ay On\20197578 doc-96/2O07 -5 tuning network to transmit the data back to the reader. The tuning value (or tuning word) stored in the memory is loaded by the digital controller to configure the tuning network. The digital controller uses the latches to hold the tuning value read from the memory during the normal operation. Hence, the tuning network consists of an array of capacitors 5 whose capacitances are configured to add in parallel to the external LC tank circuit ranging from a minimum to a maximum value in binary weighted manner. The reader's control unit is designed in conjunction with the tuning algorithm which calculates for the tuning word that matches the tag's almost exact resonant frequency. The calculation calls for a minimum of 2 points of measurement data at 10 different tuning values to construct a characteristic curve between the resonant frequency and the tuning word. In this invention, the tuning word is a binary string controlling the capacitance combination of the tuning network directly. The purpose of the invention is as follows 1. Developing the production system that allows a precision tuning of a high 15 Q resonant circuit and is able to serve the RFID tag production in large scale, which results in the uniformity of the reading distance. Such a system provides RFID-tag manufacturers with shorter production and test time comparing to the conventional method of physical modification of the resonant circuit to obtain a desired resonant frequency. 2. Developing a specific RFID reader used in the tuning process that is 20 optimum yet simple enough and able to complete the tuning operation by itself without human intervention. In one broad form, the present invention provides a resonant-frequency tuning system, being composed of a RFID reader capable of configuring the resonant frequency of a RFID tag, and the RFID tag in which a frequency-tunable microchip resides, 25 communicating with the reader contactlessly. Preferably, the RFID reader communicates with the RFID tag by the means of the amplitude modulation or frequency modulation, being able to measure the frequency of the magnetic field appearing on the antenna and configure the resonant frequency of the RFID tag to match the reference frequency.

-6 Preferably, the RFID microchip with resonant-frequency tuning components, is composed of: a capacitor array, orderly scaled in the binary weighted manner designed to have balanced parasitic capacitance by equally dividing each of the tuning capacitors and 5 switches into two parts reversely connecting each other over both the terminals of the tank circuit; a capacitor volatile switch array able to connect or disconnect each capacitor in the said capacitor array in parallel to the tank circuit; an oscillator able to oscillate at the frequency set by the resonant frequency of the 10 RFID tag which is to be measured by the RFID reader; a latch, in a presence of the power supply, temporarily holding a frequency configuration retrieving from a nonvolatile memory or from a command sent by the RFID reader during the tuning procedure; and a digital subsystem that is able to respond to a resonant-frequency configuration 15 command and has the nonvolatile memory for storing the configuration permanently the configuration. In a further broad form, the present invention provides a procedure used in configuring the resonant frequency to the closest match of the reference frequency by finding a relationship between the resonant frequency and the configuration value through 20 the two-point frequency measurement and using the extracted relationship to interpolate or extrapolate the appropriate configuration value. In a further broad form, the present invention provides a resonant-frequency tuning system including an RFID reader and an RFID tag, the RFID tag including a resonant frequency tuning module, wherein: 25 the RFID reader is configured to: transfer a first interrogation signal indicative of a first frequency configuration command to the RFID tag; receive a first response from the RFID tag having a first resonant frequency; transfer a second interrogation signal indicative of a second frequency 30 configuration command to the RFID tag; receive a second response from the RFID tag having a second resonant frequency; - 6A determine an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; transfer a third interrogation signal indicative of the optimal resonant 5 frequency command; and the RFID tag is configured to: configure the resonant frequency tuning module in accordance with the first frequency configuration command; transfer the first response; 10 configure the resonant frequency tuning module in accordance with the second frequency configuration command; transfer the second response; configure the resonant frequency tuning module in accordance with the third frequency configuration command. 15 In yet a further broad form, the present invention provides a method of tuning a resonant frequency of an RFID tag using an RFID reader, the RFID tag including resonant frequency tuning module, wherein the method includes: the RFID reader transferring a first interrogation signal indicative of a first resonant frequency configuration command to the RFID tag; 20 the RFID tag configuring the resonant frequency tuning module in accordance with the first resonant frequency configuration command; the RFID reader receiving a first response from the RFID tag having a first resonant frequency; transferring a second interrogation signal indicative of a second resonant frequency 25 configuration command to the RFID tag; the RFID tag configuring the resonant frequency tuning module in accordance with the second resonant frequency configuration command; the RFID reader receiving a second response from the RFID tag having a second resonant frequency; 30 the RFID reader determining, using extrapolation or interpolation, an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; the RFID reader transferring a third interrogation signal indicative of the optimal resonant frequency configuration command, wherein the RFID tag configures the resonant frequency tuning module in accordance with the third resonant frequency command.

- 6B In another broad form, the present invention provides a reader for tuning an RFID tag having a resonant frequency tuning module, wherein the RFID reader is configured to: transfer a first interrogation signal indicative of a first frequency command to the 5 RFID tag; receive a first response from the RFID tag having a first resonant frequency in accordance with the first frequency command; transfer a second interrogation signal indicative of a second frequency command to the RFID tag; 10 receive a second response from the RFID tag having a second resonant frequency in accordance with the second frequency command; determine an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; transfer a third interrogation signal indicative of the optimal resonant frequency 15 command such that the resonant frequency tuning module is configured in accordance with the third resonant frequency command. In a further broad form, the present invention provides a method performed by a RFID reader for tuning an RFID tag having a resonant frequency tuning module, wherein 20 the method includes: transferring a first interrogation signal indicative of a first frequency command to the RFID tag; receiving a first response from the RFID tag having a first resonant frequency in accordance with the first frequency command; 25 transferring a second interrogation signal indicative of a second frequency command to the RFID tag; receiving a second response from the RFID tag having a second resonant frequency in accordance with the second frequency command; determining an optimal resonant frequency for the RFID tag based on the first and 30 second resonant frequencies; transferring a third interrogation signal indicative of the optimal resonant frequency command such that the resonant frequency tuning module is configured in accordance with the third resonant frequency command.

- 6C Brief Description of the Drawings The present invention will become more fully understood from the following description of preferred but non-limiting embodiments thereof, described in connection 5 with the accompanying wherein: FIG. 1 shows a RFID-frequency tuning system; FIG. 2 shows a structure of a resonant-frequency-tunable RFID microchip; FIG. 3 shows the resonant frequency tuning circuit which has switching devices made up of array of PMOS transistors; P:\WPDOCS\M Specficatons\Drvers Ay One\20197578 doc-9/6/2007 -7 FIG. 4A shows frequency responses of the tanks having different resonant frequencies; FIG. 4B shows corresponding relationships of the tanks between the square of the resonant frequency and tuning capacitance; 5 FIG. 5A shows the resonant-frequency tuning concept by using data from two frequency measurement; FIG. 5B shows the resonant-frequency tuning example by using highest and lowest configurable frequencies; and FIG. 6 shows a flow diagram for the operation of the reader and the tag under the 10 resonant-frequency tuning process. Detailed Description of the Preferred Embodiments A resonant frequency tuning system for RFID tags proposed herein, shown in FIG. 1, consists of a RFID reader 1 for the tuning purpose and a RFID tag 2 being a frequency tuning network and relevant circuits integrated. In normal operations, the RFID reader 1 15 uses a magnetic field 3 to deliver energy to the RFID tag 2 and by modulating on the same magnetic field 3, data is transmitted to the tag. The RFID tag 2, in return, transmits an identification number or ID kept in its nonvolatile memory back to the reader I by means of amplitude or frequency modulation through the magnetic field 3 also. The RFID reader I consists of an oscillator 4, a driver 5, a resonant capacitor 6, an 20 inductor 7, a modulator 8, a receiver 9, a demodulator 10, a controller 11 and a frequency counter 12. The oscillator 4 is used to generate a carrier frequency used in the bi directional communication from the reader to the tag. The carrier frequency when optimized is hence a target frequency of the tag to achieve the optimum system performance after the tuning process. The driver 5 forces a resonant circuit with the carrier 25 signal generating the magnetic field 3 on the inductor 7 used as an antenna. To transmit data from the reader to the tag, the modulator 8 is employed to reduce the intensity of the magnetic field generated by the driver 5, producing two different intensities of the magnetic field. The intensity differences represent digital logics low and high or so called the amplitude modulation whose associated merit figure is also know as 30 the modulation index. A higher modulation index indicates a larger difference and also P \WPDOCS\KMFSpoficatons\Droers Ay One\20197578 doc-9/6/2007 -8 implies that the intensity during the modulation is much more reduced. A dominating modulation index nowadays is 100% (on-off keying or OOK), whereby the driver 5 completely stops driving the antenna during the modulation, thus eliminating the magnetic field. Another wide-spread index is 10%, of which the magnetic field intensity is reduced 5 by increasing the output resistance of the driver 5. The tag distinguishes such differences and further translates them into system commands and data. In the amplitude modulation scheme, the data transmission from the tag to the reader is achieved by an equivalent resistive loading of magnetic coupling system. The load modulates a ripple over the resonating voltage for more than 20 Volts at the receiving 10 antenna. The receiver circuit 9 detects and amplifies such a small ripple into a large-swing signal able to be decoded by the demodulator 10. The output data from the demodulator 10 are further processed by the controller 11 if needed for tuning or displaying purposes. In the frequency modulation scheme, the data transmission from the tag is done by a capacitive loading which creates a frequency shift in the transmitting signal. During the 15 transmission, the driver 5 stops energizing also known as a periodic field gap, while the receiver 9 listens to the signal appeared on the reader antenna. Upon the signal arrival, the small signal is amplified into a digitally recognized signal and further processed downstream. The frequency counter 12 employs a higher frequency clock generated from a crystal oscillator, is designed for two purposes: (i) to measure a period of the amplified 20 signal then sends the result to the controller 11 for further demodulation, and (ii) to measure the resonant frequency generated from the RFID tag in the frequency tuning process. The entire tuning mechanism is designed to result in a system's accuracy better than 0.1% of the measured frequency so that the controller 11 can adjust the resonant frequency to be well within the required specification. Hence, to further improve the 25 measured frequency accuracy, the counter 12 then performs oversampling by counting over multiple periods and the result being averaged. The RFID tag 2 in FIG. 2 consists of the external resonant circuit 13 or tank, made up of an inductor 14 and a capacitor 15, and a RFID microchip 16 with an adjustable capacitor array for tuning. In some applications, an external supply-decoupling capacitor P.\WPDOCS\KWASpooficaosDroer Ay On.\20197578 doc-9/6/2007 -9 17 is required. The resonant frequency of the RFID tag 2 in use must be set to be closely matched with the carrier frequency transmitted from the reader in order to obtain the maximum energy transfer, thus maximizing the operating distance. However, component variations in the tank 13 cause the resonant frequency to shift from the target frequency fD 5 as shown in FIG. 4A. The tunable RFID microchip 16 consists of a frequency-tuning network 18, latches 3, a capacitive load for frequency modulation 19 or a resistive load for the amplitude modulation, a full-wave rectifier 20, a sinusoidal oscillator 21, and a subsystem 22. The subsystem 22 comprises a tank interfacing circuit, a digital controller and a generic 10 programmable nonvolatile memory. The RFID microchip 16 transmits data back to the reader by switching the resonant circuit load or so-called load modulation. FIG.2 shows an implementation of the frequency modulation using a capacitor array and its associated controllable switch array. The switch array periodically connects the capacitors to the tank 13 causing the resonant frequency to shift down, thus developing a data pattern. 15 The full-wave rectifier 20 converts an AC signal on the tank 13 into a DC supply for the microchip's internal circuitries 22. Keeping the oscillatory tank to sustain, the sinusoidal oscillator 21 converts the energy stored in the DC power supply to create an alternating sinusoidal voltage on the tank 13 which in turn generates the magnetic field 3 used as a medium to communicate back to the reader 1. 20 To maximize the communication distance, the frequency tuning network 18 is inserted into the microchip. The frequency-tuning network 18 in FIG 3 consists of a group of capacitors 25 whose capacitances are orderly scaled in the binary-weighted manner and a switching circuit 24 selectively connecting capacitors in parallel to the tank according to the n-bit tuning value kept in latches 23. The embedded capacitors allow the tuning process 25 to be performed accurately and readily without requiring any external component. During the microchip's startup, the tuning value stored inside the nonvolatile memory 22 is read by the controller and held by the latches 23 to configure the frequency-tuning network 18. The tuning value is usually pre-programmed by the RFID reader 1 during the production. To facilitate the tuning process, the tuning value held by the latches 23 can be temporarily P \WPDOCS\KMH\Spooficahons\rovers Ay One\20197578 doc-9/6/2007 -10 changed by a tuning command from the reader I which is suited for the trial-and-error operation used in the tuning algorithm. As soon as the RFID tag 2 receives the tuning command, the digital controller 22 decodes the tuning value and passes it to the latches 23 directly. The RFID reader I can then determine the suitable tuning value instantly by 5 measuring the frequency and change the tuning value in several iterations until the optimum tuning value is obtained without nonvolatile memory program step. Therefore, the manufacturing time is reduced. The switching circuit 24 can be realized from a set of PMOS transistors 26. However, the PMOS switches exhibit parasitic capacitance added to the tank 13. 10 Furthermore, the on-chip capacitors 25 also append parasitic capacitances on each plate. In order to balance such parasitic capacitances, each of tuning capacitors 25 and switches 24 are equally divided into two parts reversely connecting each other as shown in FIG. 3. Similarly, the capacitive loads 19 are implemented in pairs similarly arranged with switch PMOS's. 15 FIG. 4A shows frequency responses of three tanks having different resonant frequencies (fres). The tank that receives the highest RF amplitude has the resonant frequency fD which is exact on to the carrier frequency generated from the reader as shown by the frequency response 27. Note that the RF amplitude of the tank is reduced precipitously nonlinearly when the resonant frequency deviates from the carrier frequency. 20 The frequency responses 42 and 43 shows that the more the resonant frequency shift, the less the RF amplitude at the frequency fD will be developed on the tags tank. To tune the resonant frequency of each tank, the relationship between the resonant frequency and the capacitance of the frequency-tuning network 18 must be established. Such relationship relies on the tank characteristic that the square of the resonant frequency is inversely 25 proportional to the sum of a discrete capacitance (Cr) and the tuning capacitance (Ct), as shown in FIG. 4B. However, the relationship varies from tank to tank due to component variations from inductance Lr (of the inductor 14) and capacitance Cr (of the capacitor 15) depicted by a group of three different curves 29 in FIG 4B. The slopes of these curves are equal to the corresponding inductance (Lr) of each tank while the offsets depend on the 30 product of the inductance Lr and the capacitance Cr. Therefore, each tank requires P \WPDOCS\KMFASpeaficabons\Drver Ay One\20197578 doc-9/6/2007 -11 different tuning capacitance (e.g. etl, ct2, ct3) to adjust its resonant frequency to the carrier frequency f1. To determine a suitable tuning capacitance, the tuning reader utilizes a predictable characteristic of each tank obtained by measuring the resonant frequencies twice with two 5 different pre-defined tuning capacitances Ct. Two-point data from the frequency measurement above define coordinates of the frequency-Ct characteristic. The data can be used for interpolation if the targeted carrier frequency resides between two measured frequencies or extrapolation if it is beyond the measured frequencies. The calculation helps eliminate the search time required by the binary-search method during the trial-and-error 10 cycles when the tuning resolution is better than 3 bits. The calculation is simple yet can be implemented in the microcontroller with little effort. In the proposed tuning method, the tuning capacitance Ct has been digitized into an n-bit binary code with each step denoted by the tuning value (nt) as shown in FIG 5. The least significant bit of the tuning value nt represents the smallest capacitance in the tuning 15 network 18. Due to the oscillating nature of the tank 13, which governs the relationship between the resonant frequency and the tuning capacitance, and since the tuning capacitance (Ct) and the tuning value (nt) are directly proportioned, thus the tuning value nt is inversely proportional to the square of the resonant frequency. To construct a relationship curve of a specific RFID tag, the RFID reader sends two commands to the 20 RFID tag to configure the tuning capacitance with the tuning value pl and p2. Then, the RFID reader pauses for an energy gap and measures the resonant frequencies of responses of each command fpl and fp2. The tuning value pl and its result fpl define a coordinate 32 while the tuning value p2 and its result fp2 define a coordinate 33 in FIG 5A. Since the reference frequency fi used by the reader is precise, the reader can calculate the suitable 25 tuning value from the measured relationship and the reference frequency fD. The calculation yields the tuning value p0 at the coordinate 31. An example of the proposed tuning method is shown in FIG. 5B, the highest resonant frequency fmax at a coordinate 34 (nt=O : no tuning capacitor connected to the tank) and the lowest resonant frequency fmin at a coordinate 35 (nt=2n-l : all tuning -12 capacitor connected to the tank) are measured to get the characteristic curve. The reader can calculate the suitable tuning value (nt) at the coordinate 31 from the measured relationship and the resonant frequency fO as shown in equation (1). The operation of the reader and tag under the proposed tuning process can be 5 summarized in a flow diagram 41 illustrated in FIG. 6. First, the reader transmits a tuning value setting command to the RFID tag to release all the tuning capacitors connected to the tank. Then, the reader system during the magnetic field gap interval then measures the frequency of the signal generated by the oscillator in the RFID microchip. The frequency measured at this stage is the highest resonant frequency that the RFID tag can produce. 10 Next, the RFID reader sends another tuning-value setting command to the RFID tag to connect all the tuning capacitors to the tank, and then measures the second frequency. The resonant frequency measured at this stage is the lowest resonant frequency that the RFID tag can produce. With two resonant frequencies measured, the controller 11 can calculate for the suitable quantized tuning value (nt) which set the resonant frequency of the tag as 15 close as possible to the reference frequency fO. Finally, the reader sends a programming command to the tag to store the calculated tuning value into the non-volatile memory from which such value will automatically be loaded to configure the frequency-tuning network at the start-up. Whilst the present invention has been herein described with reference to preferred 20 embodiments thereof, it will be understood by persons skilled in the art that numerous variations and modifications may be made thereto. All such variations and modifications should be considered to fall within the scope of the invention as broadly hereinbefore described, and as hereinafter claimed. Throughout this specification and the claims which follow, unless the context 25 requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference in this specification to any prior publication (or information derived 30 from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims (15)

1. A resonant-frequency tuning system including an RFID reader and an RFID tag, the RFID tag including a resonant frequency tuning module, wherein: 5 the RFID reader is configured to: transfer a first interrogation signal indicative of a first frequency configuration command to the RFID tag; receive a first response from the RFID tag having a first resonant frequency; transfer a second interrogation signal indicative of a second frequency 10 configuration command to the RFID tag; receive a second response from the RFID tag having a second resonant frequency; determine an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; 15 transfer a third interrogation signal indicative of the optimal resonant frequency command; and the RFID tag is configured to: configure the resonant frequency tuning module in accordance with the first frequency configuration command; 20 transfer the first response; configure the resonant frequency tuning module in accordance with the second frequency configuration command; transfer the second response; configure the resonant frequency tuning module in accordance with the 25 third frequency configuration command.

2. The system according to claim 1, wherein the RFID reader is configured to determine the optimal resonant frequency using interpolation or extrapolation. 30

3. The system as claimed in claim 1 or 2, wherein, the RFID reader communicates with the RFID tag by the means of the amplitude modulation or frequency modulation.

4. The system as claimed in any one of claims 1 to 3, wherein the RFID tag includes an tank circuit, wherein the resonant frequency tuning module includes: -14 a capacitor array including a plurality of capacitors; a switch array able to connect or disconnect one or more capacitors of the capacitor array to the tank circuit; and a controller electrically connected to the switch array, wherein the controller 5 electrically actuates the switch array in response to the first, second and third frequency configuration commands to connect one or more of the capacitors of the capacitor array to the tank circuit.

5. The system as claimed in claim 4, wherein one or more of the capacitors of the 10 capacitor array are connected in parallel to the tank circuit during tuning.

6. The system as claimed in claim 4 or 5, wherein the capacitors of the capacitor array are orderly scaled in a binary weighted manner and configured to provide balanced parasitic capacitance where the capacitors are divided two parts reversely connected to the 15 tank circuit.

7. The system as claimed in any one of claims 4 to 6, wherein the system includes a latch, in a presence of the power supply, temporarily holding a frequency configuration retrieved from a nonvolatile memory or from a frequency configuration command received 20 from the RFID reader during tuning.

8. The system according to any one of claims 4 to 7, wherein the controller includes nonvolatile memory for storing a configuration of the resonant frequency tuning module in a non-volatile manner. 25

9. A method of tuning a resonant frequency of an RFID tag using an RFID reader, the RFID tag including resonant frequency tuning module, wherein the method includes: the RFID reader transferring a first interrogation signal indicative of a first resonant frequency configuration command to the RFID tag; 30 the RFID tag configuring the resonant frequency tuning module in accordance with the first resonant frequency configuration command; the RFID reader receiving a first response from the RFID tag having a first resonant frequency; -15 transferring a second interrogation signal indicative of a second resonant frequency configuration command to the RFID tag; the RFID tag configuring the resonant frequency tuning module in accordance with the second resonant frequency configuration command; 5 the RFID reader receiving a second response from the RFID tag having a second resonant frequency; the RFID reader determining, using extrapolation or interpolation, an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; the RFID reader transferring a third interrogation signal indicative of the optimal 10 resonant frequency configuration command, wherein the RFID tag configures the resonant frequency tuning module in accordance with the third resonant frequency command.

10. A reader for tuning an RFID tag having a resonant frequency tuning module, wherein the RFID reader is configured to: 15 transfer a first interrogation signal indicative of a first frequency command to the RFID tag; receive a first response from the RFID tag having a first resonant frequency in accordance with the first frequency command; transfer a second interrogation signal indicative of a second frequency command to 20 the RFID tag; receive a second response from the RFID tag having a second resonant frequency in accordance with the second frequency command; determine an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; 25 transfer a third interrogation signal indicative of the optimal resonant frequency command such that the resonant frequency tuning module is configured in accordance with the third resonant frequency command.

11. A method performed by a RFID reader for tuning an RFID tag having a resonant 30 frequency tuning module, wherein the method includes: transferring a first interrogation signal indicative of a first frequency command to the RFID tag; receiving a first response from the RFID tag having a first resonant frequency in accordance with the first frequency command; -16 transferring a second interrogation signal indicative of a second frequency command to the RFID tag; receiving a second response from the RFID tag having a second resonant frequency in accordance with the second frequency command; 5 determining an optimal resonant frequency for the RFID tag based on the first and second resonant frequencies; transferring a third interrogation signal indicative of the optimal resonant frequency command such that the resonant frequency tuning module is configured in accordance with the third resonant frequency command. 10

12. A resonant-frequency tuning system substantially hereinbefore described with reference to the accompanying drawings.

13. A method of tuning a resonant frequency of an RFID tag using an RFID reader 15 substantially hereinbefore described with reference to the accompanying drawings.

14. A reader for tuning an RFID tag having a resonant frequency tuning module substantially hereinbefore described with reference to the accompanying drawings. 20

15. A method performed by a RFID reader for tuning an RFID tag substantially hereinbefore described with reference to the accompanying drawings.