AU2007216685A1 - System and method for tuning RFID resonant frequency - Google Patents

Description

Australian Patents Act 1990 Regulation 3.2 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title System and method for tuning RFID resonant frequency The following statement is a full description of this invention, including the best method of performing it known to me/us:- P/00/011 5102 P VVPC>OCS KMMSpeoficaburs\Dl f Ay One\20197578 doc-9/612007 -1e, IN SYSTEM AND METHOD FOR TUNING RFID RESONANT FREQUENCY

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Background of the Invention 00 The invention proposed herein is related to a resonant-frequency-tuning system and IND method for RFID manufacturing through frequency measurement method. More i 5 particularly the invention relates to the RFID technology in which an LC resonant circuit is Smade up of an inductor and a capacitor. However, the system also supports other tunable C0 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 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.

In a passive RID 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 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 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\KMFASpeallabOns\DrmerAy OneX0197578 doc.9/1R2007 -2- Smodulated RF signal from the transponder is usually maintained in the form of frequency IDshift keying (FSK) RF magnetic field conforming to the protocol recognized by the reader.

0 The reader recovers the small modulated data from the signal picked up at the reader n antenna and performs decoding until the telegram conducted by the microchip has been 00 ID 5 completed. The communication through the magnetic field between the transponder and the reader/interrogator is the key characteristic in the low frequency (100kHz -1SOkl-z) and high frequency RFID applications (13.56 MHz). H-owever, the read distance of the Smagnetically 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.

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 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 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 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 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 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'XMKSpeaficatons\Olwfs Ay One%20197578 do-9/612O7 -3- If the transponder's Q is greater than 30, and its resonant frequency is exactly the same as IOthe reader's reference frequency, the read distance is noticeably increased. In contrast, if 0 the resonant frequency is slightly deviated from the reference frequency due to component 'n variations, the efficiency in accumulated energy is reduced and the read distances even in 00 O 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 C 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 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 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 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 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.

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 \WPDOCSXMK-fSpeoficabons\'overs Ay One\01 91578 doc-9110f7 -4indicator (SSI) to measure the radio-frequency signal strength transmitted by a tag and give IDan 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.

Vn The maximum voltage reflects the tag's best resonant frequency in communicating with 00 ID 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 Smicroprocessor uses these data to tune each tag.

In contrast, the invention proposed herein involves the system and method in tuning 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 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: 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 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 M PDOCSAKMKSpeafcsbons0@rs Ay n2019757a doc-91,2O7 Stuning network to transmit the data back to the reader. The tuning value (or tuning word) IOstored in the memory is loaded by the digital controller to configure the tuning network.

0 The digital controller uses the latches to hold the tuning value read from the memory Sduring the normal operation. Hence, the tuning network consists of an array of capacitors 00 O 5 whose capacitances are configured to add in parallel to the external LC tank circuit ranging

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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. T'he calculation calls for a minimum of 2 points of measurement data at 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- 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 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, 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.

P XWPDOCSXMFASpeofiaons\Drwerl Ay On20197578 doc-916I2D7 -6- SPreferably, the RFID microchip with resonant-frequency tuning components, is O composed of: 0 a capacitor array, orderly scaled in the binary weighted manner designed to have n balanced parasitic capacitance by equally dividing each of the tuning capacitors and 00 O 5 switches into two parts reversely connecting each other over both the terminals of the tank

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circuit; a capacitor volatile switch array able to connect or disconnect each capacitor in the Ssaid capacitor array in parallel to the tank circuit; (,i an oscillator able to oscillate at the frequency set by the resonant frequency of the 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 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 the two-point frequency measurement and using the extracted relationship to interpolate or extrapolate the appropriate configuration value.

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 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~AMMSPeafichons\lrS Ay One\20197578 doc-91/2007 -7- A FIG. 4A shows frequency responses of the tanks having different resonant NOfrequencies; FIG. 4B shows corresponding relationships of the tanks between the square of the n resonant frequency and tuning capacitance; 00 O 5 FIG. 5A shows the resonant-frequency tuning concept by using data from two

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frequency measurement; FIG. 5B shows the resonant-frequency tuning example by using highest and lowest Sconfigurable frequencies; and (Ni FIG. 6 shows a flow diagram for the operation of the reader and the tag under the 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 frequencytuning network and relevant circuits integrated. In normal operations, the RFID reader 1 1 5 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 1 by means of amplitude or frequency modulation through the magnetic field 3 also.

The RFID reader 1 consists of an oscillator 4, a driver 5, a resonant capacitor 6, an 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 bidirectional 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 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 the modulation index. A higher modulation index indicates a larger difference and also P XWPDOCSXKMHASpgficabons\DrMaS Ay OM20197578 do-9/6f2D07 -8- ;3 implies that the intensity during the modulation is much more reduced. A dominating INO modulation index nowadays is 100% (on-off keying or OOK), whereby the driver 0 completely stops driving the antenna during the modulation, thus eliminating the magnetic Vt) field. Another wide-spread index is 10%, of which the magnetic field intensity is reduced 00 O 5 by increasing the output resistance of the driver 5. The tag distinguishes such differences

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and further translates them into system commands and data.

0 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 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 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 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: to measure a period of the amplified 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 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 XWPDOCS\KWASpafi~hawDlmem Ay On.2 0197578 docM2I07 -9- 3 17 is required. The resonant frequency of the RFID tag 2 in use must be set to be closely IOmatched with the carrier frequency transmitted from the reader in order to obtain the maximum energy transfer, thus maximizing the operating distance. However, component n variations in the tank 13 cause the resonant frequency to shift from the target frequency f0 00 NO 5 as shown in FIG. 4A.

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The tunable RFID microchip 16 consists of a frequency-tuning network 18, latches O 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 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.

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.

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 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\KMM-,Speofcahons~rmers Ay O e O197578 doc-9/612007 3 changed by a tuning command from the reader I which is suited for the trial-and-error D operation used in the tuning algorithm. As soon as the RFID tag 2 receives the tuning 0 command, the digital controller 22 decodes the tuning value and passes it to the latches 23 t directly. The RFID reader 1 can then determine the suitable tuning value instantly by 00 ID 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 PI'MOS switches exhibit parasitic capacitance added to the tank 13.

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.

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 f0 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.

The frequency responses 42 and 43 shows that the more the resonant frequency shift, the less the RF amplitude at the frequency fO 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 proportional to the sum of a discrete capacitance (Cr) and the tuning capacitance 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 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 product of the inductance Lr and the capacitance Cr. Therefore, each tank requires P \WPDOCS\KMMSpeaicaon\Drmem Ay One\2017578 doc-962007 -11different tuning capacitance ctl, ct2, ct3) to adjust its resonant frequency to the carrier Nfrequency 1O.

To determine a suitable tuning capacitance, the tuning reader utilizes a predictable 00 characteristic of each tank obtained by measuring the resonant frequencies twice with two

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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 Sused for interpolation if the targeted carrier frequency resides between two measured C1 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 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 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 RFID tag to configure the tuning capacitance with the tuning value p1 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 fO used by the reader is precise, the reader can calculate the suitable tuning value from the measured relationship and the reference frequency f0. 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- all tuning P XWPDOCS\KMFASpeaficabons\D s AV One\20197578dc-9/S/2007 -12r. capacitor connected to the tank) are measured to get the characteristic curve. The reader IDcan calculate the suitable tuning value (nt) at the coordinate 31 from the measured relationship and the resonant frequency f0 as shown in equation 0 The operation of the reader and tag under the proposed tuning process can be ID 5 summarized in a flow diagram 41 illustrated in FIG. 6. First, the reader transmits a tuningvalue 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 C 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.

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 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 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.

Claims (4)

1. 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- 00 tunable microchip resides, communicating with the reader contactlessly.

2. A system as claimed in claim 1, wherein, the RFID reader communicates with the RFID tag by the means of the amplitude modulation or frequency modulation, being able Sto 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.

3. A system as claimed in claim 1 or 2, wherein the RFID microchip with resonant- frequency tuning components, 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 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 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 command and has the nonvolatile memory for storing the configuration permanently the configuration.

4. 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 the two-point frequency measurement and using the extracted relationship to interpolate or extrapolate the appropriate configuration value.