JP2008077554A - Ic chip for rfid tag and rfid tag - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an RFID (Radio Frequency Identification) tag capable of charging a capacitor in an IC chip with the radio wave of a frequency to be used for communication at the same time with communication. <P>SOLUTION: In the RFID tag provided with an antenna for communication, an antenna for charging, an IC chip and a base material for supporting them, the IC chip is provided with a capacitor, a circuit for communication and a circuit for charging connected to the capacitor, and a back scatter included in the circuit for communication, the back scatter can change the impedance of the IC chip by power supply from the capacitor, the antenna for communication is connected to the circuit for communication of the IC chip, the antenna for communication is connected to the circuit for charging of the IC chip, respectively, and the impedance of the antenna for charging and the circuit for charging is not complex conjugate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an RFID tag used in a radio frequency identification (RFID) system and an IC chip used therefor, and more particularly, an IC chip having a passive backscatter modulation (backscatter) technique and a charging antenna. And an RFID tag.

  An RFID (Radio Frequency Identification) (radio frequency identification) system is a system that adds information (identification information) for identifying an individual such as a device, a baggage, a human being, and a living organism to a solid by an inexpensive device called a tag. In this RFID system, wireless communication is performed between a wireless transceiver called a reader / writer and one or more RFID tags. The RFID tag has an IC chip and an antenna for receiving a signal from a reader / writer.

  In an RFID system, a reader / writer communicates with an RFID tag using a modulated radio signal or a non-modulated signal, and the RFID tag responds with a modulated radio signal. In many cases, communication of an RFID system is performed by a half-duplex method in which transmission / reception is performed by a single antenna. Therefore, a communication sequence (order in accordance with a law) performed in the RFID system is determined by the data in the RFID tag. It is divided into a write sequence for writing and a read sequence for reading data from the RFID tag.

  Here, a part of the RFID tag adopts an ASK (Amplitude Shift Keying) method and performs wireless communication with the reader / writer. The ASK method is a method of performing communication by generating a code of High or Low depending on whether radio waves from a reader / writer are absorbed or reflected by an IC chip built in an RFID tag. FIG. 3 shows that when the signal wave is High, the ASK amplitude takes the maximum value, and when the signal wave is Low, the ASK amplitude takes the minimum value.

  In order for the IC chip to absorb the radio wave coming from the reader / writer, it is said that it is most efficient if the impedance of the antenna and the IC chip is complex conjugate. That is, wireless communication can be performed if the impedance of the antenna and the IC chip is close to the complex conjugate.

An IC chip mounted on an RFID tag includes several transistors, a backscatter (impedance converter), a capacitor, an arithmetic device, and a memory. FIG. 4 shows a circuit diagram of a conventional IC chip. FIG. 5 is a block diagram schematically showing the function of the IC chip. Here, the impedance of the chip is the impedance of the backscatter. The impedance of the chip is set to Z _L = R _L −jX _L, and the impedance of the antenna included in the RFID tag is set to Z _a = R _a −jX _a . Here, Z _L is the impedance of the chip, R _L is the resistance of the chip, X _L is the inductance of the IC chip, j is an imaginary number, Z _a is the impedance of the antenna, R _a is the resistance of the antenna, and X _a is the antenna resistance Inductance. When the antenna receives the radio wave from the reader / writer and induces the electromotive force V _S , the current I flowing through the circuit of the IC chip is

It becomes. When the impedance Z _L and the antenna impedance Z _a of this time the IC chip is to take the complex conjugate,
At this time, the current I supplied to the IC chip is
It becomes. The available power Pi (power supplied to the IC chip) at this time is
And take the maximum value.

  In addition, from the formula (1), when the impedance of the antenna and the IC chip greatly deviates, the denominator of the formula (1) becomes large and the current I hardly flows. At this time, the radio wave propagating from the reader / rider hardly reflects on the antenna of the RFID tag and is reflected. That is, in order to reflect the radio wave propagated from the reader / writer, it is understood that the impedance mismatch between the antenna and the chip is greatly shifted.

  Here, when impedance mismatching is performed, it is difficult to make the impedance of the antenna variable, so the impedance of the IC chip is usually changed. The part having the function of making this impedance variable is the backscatter. The backscatter is incorporated in the circuit of the IC chip, and the impedance of the IC chip is changed by changing its own impedance according to a command from the arithmetic unit.

  Here, a certain amount of electric power is required to operate the backscatter and change the impedance of the IC chip. However, when the RFID tag itself does not have a battery or an external power source, this power is obtained from a capacitor inside the IC chip. Usually, the impedance of the antenna and the IC chip is set to match.

  In the case where a radio wave transmitted from a reader / writer is absorbed by an antenna, a part of the energy of the radio wave can be charged in a capacitor inside the IC chip. When the radio wave from the reader / writer is reflected, the backscatter obtained from the capacitor inside the IC chip changes the impedance, thereby changing the impedance of the IC chip to an impedance different from that of the antenna (impedance Mismatch). And the radio wave is reflected.

  In this method, when a signal that reflects radio waves is continuous, there is a problem that the backscatter and the arithmetic unit will eventually use up the electric charge stored in the capacitor, and the impedance of the IC chip cannot be changed.

  In order to solve this, there was a method of transmitting radio waves for charging before starting communication, and starting communication after charging a capacitor in the IC chip, but radio waves of the same frequency were used for communication and charging. Since it is used and absorbed by the same antenna, there is a problem that it takes time to complete communication.

For this reason, there has been proposed an RFID tag including an antenna that receives a charging radio wave separately from an antenna that receives a communication radio wave (see Patent Document 1). The RFID tag includes a charging antenna that absorbs a charging radio wave transmitted at a frequency different from the signal radio wave and stores it as electric power. Since the antenna for receiving the signal radio wave does not react to the charging radio wave, the antenna for receiving the charging radio wave is also set so that the impedance does not react to the signal radio wave, The capacitor can be charged by simultaneously transmitting charging radio waves during signal communication, and the communication time can be shortened. However, this method has a problem that the apparatus becomes large because a transmitter for charging other than a reader / writer for sending signals must be provided.
JP-A-5-285798

  An object of the present invention is to provide an RFID tag including an antenna and an IC chip, and capable of charging a capacitor inside the IC chip with radio waves having a frequency used for communication simultaneously with communication. is there.

In order to solve these problems, an IC chip for an RFID tag according to the present invention is an IC including a capacitor, a communication circuit and a charging circuit connected to the capacitor, and a backscatter incorporated in the communication circuit. The chip is characterized in that the backscatter can change the impedance of the IC chip by supplying power from a capacitor.
With this configuration, an antenna with an appropriate impedance can be connected to each circuit, and the impedance matching between the communication antenna and the communication circuit can be adjusted. The reflection / absorption timing can be shifted with the antenna.

Furthermore, in order to solve these problems, the RFID tag according to the present invention is an RFID tag including a communication antenna, a charging antenna, an IC chip, and a base material that supports these antennas, A capacitor, a communication circuit and a charging circuit connected to the capacitor, and a backscatter incorporated in the communication circuit. The backscatter can change the impedance of the IC chip by supplying power from the capacitor. The communication antenna is connected to the communication circuit of the IC chip, the charging antenna is connected to the charging circuit of the IC chip, and the impedance between the charging antenna and the charging circuit is not complex conjugate.
With this configuration, the charging antenna does not completely absorb radio waves from the reader / writer, and a reflected signal can be returned.

Furthermore, the RFID tag according to the present invention is characterized in that impedances of the charging antenna and the charging circuit are equal only in an imaginary part and not in a real part.
More preferably, the impedance Z _{ca of the} charging antenna
(R _ca : Resistance of charging antenna, X _ca : Reactance of charging antenna)
And impedance Z _{cc of the} charging circuit
(R _cc : Resistance of charging circuit, X _cc : Reactance of charging circuit)
Toga
And the real number A is the following inequality
It is characterized by satisfying.

  With this configuration, for impedance matching between the charging antenna provided for charging the IC chip and the charging circuit, the impedance of the charging antenna is made equal to the charging circuit only in the imaginary part, and the real part is charged. The method is slightly shifted from the impedance of the circuit. In this way, the communication antenna of the RFID tag can generate a reflected wave, and the charging antenna can store charges in a capacitor in the IC chip.

Furthermore, in the RFID tag of the present invention, the impedance between the communication antenna and the communication circuit is complex conjugate when a backscatter is not operating.
With this configuration, radio waves from the reader / writer can be absorbed with maximum efficiency, electric charges can be stored in the capacitor inside the IC chip, and power can be supplied to the backscatter.

  Since the backscatter provided in the IC chip for RFID tag of the present invention can change the impedance of the IC chip by supplying power from the capacitor, an antenna having an appropriate impedance is connected to each circuit, and the communication antenna and the communication circuit are connected. Therefore, even if the same radio wave is received, the reflection / absorption timing can be shifted between the communication antenna and the charging antenna.

  Further, the backscatter in the IC chip included in the RFID tag of the present invention can change the impedance of the IC chip by supplying power from the capacitor. The communication antenna is the IC chip communication circuit, and the charging antenna is the IC chip. Because the impedance of the charging antenna and the charging circuit is not complex conjugate, the charging antenna does not completely absorb the radio wave from the reader / writer and returns a reflected signal. Can do.

  Furthermore, since the RFID tag of the present invention has only the imaginary part and the real part are not equal in impedance between the charging antenna and the charging circuit, the radio wave is always absorbed from the communication radio wave at a constant rate and charged in the IC chip. At the same time, when necessary for reflection, a reflected wave can be returned. Furthermore, by making the real part of the impedance of the charging antenna and the impedance of the charging circuit constant, the RFID tag communication antenna can generate a sufficient reflected wave, and the charging antenna is a capacitor in the IC chip. In addition, a sufficient charge for driving the backscatter and the arithmetic unit can be stored.

Furthermore, the RFID tag of the present invention absorbs radio waves from the reader / writer with maximum efficiency because the impedance between the communication antenna and the communication circuit is complex conjugate when the backscatterer is not operating, and the IC chip. Electric charge can be stored in the internal capacitor and power can be supplied to the backscatter.
Furthermore, since the capacitor in the IC chip can be charged only by radio waves of the signal frequency, the reader / writer structure can be simplified. In addition, since charging is performed simultaneously with communication, communication time can be shortened.

Hereinafter, an example of the present invention will be described with reference to the drawings.
FIG. 1 is a top view of an RFID tag 10 according to the present invention. The RFID tag 10 of the present invention further includes a charging antenna 16 in addition to the IC chip 10 and the signal antenna 12 for absorbing radio waves from the reader / writer as a signal. As a base material that supports the antenna and the IC chip, an organic substrate, paper, PET film, or the like is used. For example, the IC chip 12 is flip-chip mounted on the communication antenna 14 and the charging antenna 16. As long as the antenna and the IC chip are connected to each other, they may be arranged on either side of the base material. Here, the communication antenna 14 and the charging antenna 16 use a printed circuit board as an RFID tag substrate, and a dipole antenna printed and wired on the antenna is used as the antenna. . The impedance of both the signal antenna and the charging antenna can be adjusted by changing the folded length, material, width, etc. of the antenna.

  FIG. 2 is a diagram schematically showing the function of the IC chip mounted on the RFID tag of the present invention. The IC chip 20 includes several transistors, a capacitor 24, an arithmetic unit 26, a memory 28, and a backscatter 22. The circuit inside the IC chip is divided into two systems: a signal circuit connected to the signal antenna and a charging circuit connected to the charging antenna. Either circuit can deliver charge to the capacitor.

  A backscatter 22 is incorporated in the signal circuit. The back scatter 22 can change the impedance of the signal circuit by receiving a command from the arithmetic unit 26 and changing its own impedance by the electric power from the capacitor 24. On the other hand, since the charging circuit does not include a backscatter, its impedance does not change.

The impedances of the communication antenna and the communication circuit are adjusted so as to be matched (complex conjugate) when the backscatter is not operating. That is, the impedance Z _sc of the communication circuit when the backscatter is not operating is
(R _sc : Resistance of communication circuit, X _sc : Reactance of communication circuit)
Then, the impedance Z _sa of the communication antenna is
It is.

  The charging antenna is designed so that the impedance does not perfectly match the charging circuit. If the impedance of the charging antenna and the charging circuit is designed to be complex conjugate, when the radio wave to be reflected is transmitted, the signal antenna reflects the impedance by mismatching due to the backscatter function. This is because a charging antenna without a back scatter absorbs it.

In the present invention, the impedances of the charging antenna and the charging circuit are adjusted so that only the imaginary part is equal. That is, the impedance Z _cc of the charging circuit is
(R _cc : Resistance of charging circuit, X _cc : Reactance of charging circuit)
Then, the impedance Z _ca of the charging antenna is
(R _ca : Resistance of charging antenna, X _ca : Reactance of charging antenna)
In the case of
The impedance of the charging antenna is adjusted so that
By adjusting the impedance of the charging antenna and the charging circuit so as to have such a relationship, the radio wave for communication can always be absorbed little by little and the capacitor can be charged.

  When the radio wave transmitted from the reader / writer is to be absorbed by the IC chip, the command for operating the back scatter 22 is not issued from the arithmetic unit 26, so the impedances of the communication antenna and the communication circuit match. Therefore, radio waves are absorbed by the IC chip, a non-reflective signal is returned to the reader / writer, and a part of the energy obtained by communication is charged in the capacitor 24 inside the IC chip 20.

At this time, the impedances of the charging antenna and the charging circuit are not perfectly matched but only the imaginary part is equal. Therefore, when the electromotive force Vc is induced in the charging antenna, the current I _c flowing in the charging circuit is expressed by the equation From (1)
It becomes. here,
If (A is an arbitrary real number), Equation (4) is
It becomes. When the charging antenna and the charging circuit are matched, A = 1, so if the power when A = 1 is P _icmax
Therefore, the electric power P _ic supplied to the capacitor at this time is
It becomes.

When P _ic occupies a large ratio with respect to P _icmax , most of the signal radio wave is always absorbed as electric power, so that there is a problem that a signal that must be reflected is weakened. Therefore _{P ics} is 1 or less is preferable half the _{P Icmax.} Depending on the combination of the signals, in the case of absorption signals because it is charged from the signal antenna to the capacitor, can be stored in the capacitor sufficient power as long as it is 1 or more P _ics is a third of the P _Icmax. Ie
A satisfying this range is given by the following inequality.
A satisfying this is in the range of about 1.83 to about 2.46.

  When radio waves transmitted from the reader / writer are to be reflected without being absorbed by the IC chip, the arithmetic device 26 issues a command to operate the back scatter 22, and the back scatter 22 uses its power from the capacitor 24 to The impedance is changed, and the impedance of the signal circuit is mismatched with the signal antenna (not complex conjugate). When the impedance of the signal antenna and the impedance of the IC chip are mismatched, the radio wave is returned to the reader / writer as a reflected wave, and power is not supplied from the signal circuit to the IC chip.

  The history of absorption / reflection of signal radio waves is stored as data in the memory 28 when the memory 28 can be rewritten.

  Even when the radio wave is reflected, the charging antenna always supplies a constant power as shown by the equation (7). As a result, even when the RFID tag is communicating by reflecting radio waves from the reader / writer, energy is charged in the capacitor in the IC chip in the RFID tag, and reflection is continued as a code to the reader / writer. Even in this case, the IC chip can operate.

  In addition, a reader / writer that communicates with such an RFID tag only needs to include a communication transmitter, and a transmitter for sending a charging radio wave to the RFID tag can be eliminated.

  The IC chip and RFID tag of the present invention can be used for entrance / exit management such as exhibitions, cargo distribution, livestock and food history management, inventory management, and the like. In particular, it is compact without a battery, and because communication and charging can be performed simultaneously, the communication time can be shortened. Therefore, it is preferably used for applications such as admission tickets, cards, and tags that have small tags and must be processed in large quantities. be able to.

It is the schematic diagram which looked at the RFID tag of this invention from the upper surface. It is the block diagram which showed typically the function inside IC chip of this invention. It is explanatory drawing which shows the communication method of an ASK system. It is a circuit diagram of a conventional IC chip. It is the block diagram which showed typically the function inside the conventional IC chip.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... RFID tag 12 ... IC chip 14 ... Communication antenna 16 ... Charging antenna 20 ... IC chip 50 of this invention ... Conventional IC chip 22, 52 ... Back scatter 24, 54 ... Capacitor 26, 56 ... Arithmetic device 28, 58 ... Memory

Claims (5)

  An IC chip including a capacitor, a communication circuit and a charging circuit connected to the capacitor, and a back scatter incorporated in the communication circuit. The back scatter is an impedance of the IC chip by supplying power from the capacitor. An IC tag for an RFID tag, wherein the IC tag can be changed. An RFID tag including a communication antenna, a charging antenna, an IC chip, and a base material supporting these,
The IC chip includes a capacitor, a communication circuit and a charging circuit connected to the capacitor, and a back scatter incorporated in the communication circuit. The back scatter changes the impedance of the IC chip by supplying power from the capacitor. Is possible,
The communication antenna is connected to the IC chip communication circuit, and the charging antenna is connected to the IC chip charging circuit.
An RFID tag, wherein an impedance between a charging antenna and a charging circuit is not complex conjugate.   The RFID tag according to claim 2, wherein impedances of the charging antenna and the charging circuit are equal only in an imaginary part and not equal in a real part. Impedance Z _{ca of the} charging antenna
(R _ca : Resistance of charging antenna, X _ca : Reactance of charging antenna)
And impedance Z _{cc of the} charging circuit
(R _cc : Resistance of charging circuit, X _cc : Reactance of charging circuit)
Toga
And the real number A is the following inequality
The RFID tag according to claim 2, wherein:   5. The RFID tag according to claim 2, wherein an impedance between the communication antenna and the communication circuit is a complex conjugate when a backscatter is not operating. 6.