RF tag

An RF tag that includes a limiter and is capable of satisfactorily adjusting an impedance of a matching circuit is provided. The RF tag includes a variable load unit of which an amount of power consumption is changeable and a control unit that is capable of executing an impedance adjusting process of adjusting the impedance of the matching circuit such that an output voltage of a rectification circuit becomes a maximum and executes the impedance adjusting process after reducing the output voltage of the rectification circuit to a voltage less than a limiter voltage by adjusting the amount of power consumption of the variable load unit when a predetermined condition is satisfied.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Japan application serial no. 2017-177013, filed on Sep. 14, 2017. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND

Technical Field

The disclosure relates to an RF tag.

Description of Related Art

In recent years, the development of RF tags has been remarkable, and RF tags are used for mounting wireless electronic components by applying a wireless power transmission function in addition to their original use of object identification. In such applications, electrical loads such as a CPU and a sensor are included as wireless electronic components, and it is preferable to further increase the electric power received by the RF tags such that electric power is stably supplied for operations of such loads. As an RF tag that realizes the effect described above, an RF tag in which a matching circuit having variable impedance (variable impedance circuit) is disposed in a front stage of a rectification circuit is known.

In such an RF tag, in a case in which a deviation of the impedance of the matching circuit from an optimal value is large, an output voltage of the rectification circuit (hereinafter, also expressed as a power supply voltage) may be easily caused to be equal to or less than an operation lower limit voltage of each circuit included in the RF tag in accordance with environmental changes. More specifically, since the impedance of the matching circuit is adjusted to an optimal value, an RF tag having a power supply margin of 7 dB will be considered. Here, a power supply margin is the reduced amount of input power that causes a power supply voltage (the output voltage of the rectification circuit) to be reduced to the operation lower limit voltage in units of dB.

In a case in which input power to the RF tag drops by 6 dB due to an environmental change (appearance of an obstacle inhibiting the propagation of radio waves toward the antenna, water adhesion to the antenna, or the like), as illustrated inFIG. 1A, the power supply voltage is lowered but not to the operation lower limit voltage. Accordingly, the RF tag continues to operate.

On the other hand, in a case in which the impedance of the matching circuit is not adjusted to an optimal value, and thus, the power supply margin of the RF tag is only 4 dB, as illustrated inFIG. 1B, when the input power drops by 6 dB, the power supply voltage becomes equal to or lower than the operation lower limit voltage. As a result, the RF tag does not operate.

In this way, in a case in which a deviation of the impedance of the matching circuit from the optimal value is large, the power supply voltage may be easily caused to be equal to or lower than the operation lower limit voltage of each circuit included in the RF tag due to environmental change. For this reason, it is preferable to satisfactorily adjust the impedance of the matching circuit included in the RF tag.

Meanwhile, the impedance of the matching circuit included in the RF tag is, generally, adjusted such that the output voltage of the rectification circuit is maximal (for example, see Japanese Patent Application Laid-open No. H7-111470). In a case in which the output voltage of the rectification circuit can be directly detected, the impedance of the matching circuit can be adjusted to the optimal value using the adjustment process described above. However, as illustrated inFIG. 2, in an RF tag including a limiter that is used for preventing application of an excessive voltage to each circuit or the like, the power supply voltage is limited by the limiter. For this reason, in the adjustment process described above, the impedance of the matching circuit of the RF tag cannot be adjusted to the optimal value.

More specifically, as the matching circuit, generally, a circuit (seeFIG. 5A) combining two inductors and a variable-capacitance capacitor is employed. In addition, as the adjustment process, generally, a process in which the capacitance C of the variable-capacitance capacitor included in the matching circuit is reduced to a certain value C0, and the capacitance C is increased until the power supply voltage does not change is performed. Accordingly, in a case in which the impedance of the matching circuit of the RF tag including a limiter is adjusted in the adjustment process, as schematically illustrated inFIG. 3, the adjustment of the impedance (the capacitance C of the variable-capacitance capacitor) of the matching circuit ends in a stage in which the power supply voltage rises up to a saturation voltage (limiter voltage), and thus, the impedance of the matching circuit cannot be adjusted to the optimal value.

The disclosure provides an RF tag including a limiter and is capable of satisfactorily adjusting the impedance of a matching circuit.

SUMMARY

According to the disclosure, there is provided an RF tag to which power is wirelessly fed from a reader/writer including: a rectification circuit that rectifies an AC signal supplied from an antenna and outputs a DC voltage supplied to each unit inside the RF tag as a power supply voltage; a limiter that limits the output voltage of the rectification circuit to a predetermined voltage or less; a matching circuit, of which an impedance is changeable, disposed between the antenna and the rectification circuit; a variable load unit, of which an amount of power consumption is changeable, consuming power output from the rectification circuit; and a control unit that is capable of executing an impedance adjusting process of adjusting the impedance of the matching circuit such that the output voltage of the rectification circuit increases and executes the impedance adjusting process after reducing the output voltage of the rectification circuit to a voltage less than the predetermined voltage by adjusting the amount of power consumption of the variable load unit when a predetermined condition is satisfied.

DESCRIPTION OF THE EMBODIMENTS

“An impedance adjusting process of adjusting the impedance of the matching circuit such that the output voltage of the rectification circuit increases” is a process in which more satisfactory adjustment of the impedance can be achieved when the process is performed after the output voltage of the rectification circuit is decreased to a voltage that is not limited by the limiter. The RF tag according to the disclosure has a function of “executing the impedance adjusting process after reducing the output voltage of the rectification circuit to a voltage less than the predetermined voltage by adjusting the amount of power consumption of the variable load unit”. Therefore, according to the disclosure, an RF tag including a limiter and is capable of satisfactorily adjusting the impedance of the matching circuit can be realized.

In addition, the variable load unit of the RF tag according to the disclosure may include a variable load for impedance adjustment (for adjustment of the amount of power consumption) and a load (an IC, a sensor, or the like) for the original use of the RF tag or may include only a variable load for impedance adjustment (for adjustment of the amount of power consumption). Furthermore, the “predetermined condition” according to the disclosure may be any condition. More specifically, the “predetermined condition”, for example, may be a condition that “a predetermined time has elapsed after previous adjustment of impedance was performed”, a condition that “a predetermined direction is input from another device (a reader/writer or the like), or a condition that “the power supply voltage becomes less than the limiter voltage”.

The RF tag according to the disclosure may employ a configuration in which “the control unit repeats the process of executing the impedance adjusting process after reducing the output voltage of the rectification circuit to the voltage less than the predetermined voltage by adjusting the amount of power consumption of the variable load unit when the predetermined condition is satisfied until the output voltage of the rectification circuit after the process becomes a voltage less than the predetermined voltage”. In addition, the RF tag according to the disclosure may employ a configuration in which “the variable load unit is a unit of which a resistance value is changeable, and the control unit repeats the process of executing the impedance adjusting process after reducing the output voltage of the rectification circuit to the voltage less than the predetermined voltage by adjusting the amount of power consumption of the variable load unit when the predetermined condition is satisfied until the resistance value of the variable load unit after the process becomes a predetermined value or less” or a configuration in which “the control unit repeats the process of executing the impedance adjusting process after reducing the output voltage of the rectification circuit to the voltage less than the predetermined voltage by adjusting the amount of power consumption of the variable load unit when the predetermined condition is satisfied a predetermined number of times or until a predetermined time elapses. By employing such a configuration, an RF tag capable of adjusting the impedance of the matching circuit more satisfactorily is acquired.

According to the disclosure, an RF tag including a limiter and is capable of satisfactorily adjusting the impedance of a matching circuit can be provided.

Hereinafter, an embodiment of the disclosure will be described with reference to the drawings.

FIG. 4illustrates the configuration and the use form of an RF tag10according to one embodiment of the disclosure. As illustrated in the drawing, the RF tag10includes a matching circuit11, a demodulation circuit12, a rectification circuit13, a modulation circuit14, an impedance adjustment control circuit15, a limiter16, an ADC17, a control unit18, a variable load21, and a load25.

The matching circuit11is a variable impedance circuit used for impedance matching between an antenna20and a circuit disposed inside the RF tag10. The matching circuit11included in the RF tag10according to this embodiment, as illustrated inFIG. 5A, is a circuit including two inductors51and52and a variable-capacitance capacitor53of which a capacitance can be changed in accordance with a control signal supplied from the outside. The matching circuit11uses a circuit combining capacitors C1to C5having mutually-different capacitance values and switches SC1to SC5as illustrated inFIG. 5Bas the variable-capacitance capacitor53. Here, the matching circuit11may be a circuit that has a different circuit configuration (for example, a circuit configured such that the capacity of an inductor can be changed) as long as it is a circuit of which an impedance can be changed using a control signal supplied from the outside.

The demodulation circuit12(FIG. 4) is a circuit that extracts a command transmitted by a reader/writer (R/W)85(host device90) from a radio wave received by the antenna20by demodulating an output of the antenna20that is input through the matching circuit11. The modulation circuit14is a circuit that is used for transmitting information to the reader/writer85by performing modulation according to the information to be notified to the reader/writer85for a reflected wave of a carrier wave transmitted from an antenna80of the reader/writer85.

The rectification circuit13is a circuit that generates DC power used for operating each unit (the demodulation circuit12, the modulation circuit14, the load25, and the like) disposed inside the RF tag10by rectifying AC power output by the antenna20that has received the radio wave. As this rectification circuit13, for example, a circuit having a configuration illustrated inFIG. 6, in other words, a diode charge pump in which unit rectification circuits551to55Neach configured by two diodes (D1and D2or the like) and a capacitor C (C1and C2or the like) are connected in N stages is used. In addition, the rectification circuit13may a circuit that can output a voltage for the load25and a voltage for circuits other than the load25.

The load25is an electronic circuit (a sensor, an LED, an IC, a microcomputer, or the like) used by the host device90through the reader/writer (R/W)85. A power supply voltage VOUT is supplied to the load25through a first switch22of a normally-on type that can be controlled such that it is turned on/off by the control unit18.

The variable load21is a circuit of which the power consumption amount can be changed. The power supply voltage VOUT is supplied to the variable load21through a second switch23of a normally-off type that can be controlled such that it is turned on/off by the control unit18. As the variable load21, for example, a variable resistance circuit of which the resistance value can be changed (designated) using a control signal is used.

The limiter16(FIG. 4) is a circuit that limits the output voltage of the rectification circuit13to a predetermined voltage (hereinafter, also represented as a limiter voltage VLIM or a saturation voltage) or less. The ADC17is an AD converter that digitalizes the output voltage of the rectification circuit13.

The impedance adjustment control circuit15is a circuit that outputs a control signal (in this embodiment, a control signal designating the capacitance of the variable-capacitance capacitor53(FIG. 5B)) designating the impedance of the matching circuit11. In the RF tag10according to this embodiment, the impedance adjustment control circuit15having the configuration illustrated inFIG. 6is used.

Although the overall operation of the impedance adjustment control circuit15will be described later, an up counter61is a counter that clears a count value to “0” when a Reset pulse is input and counts up when an up pulse is input. The counter value of the up counter61is used as a control signal designating the impedance (the capacitance of the variable-capacitance capacitor53(FIG. 4B)) of the matching circuit11.

A comparator62is a circuit that outputs a result of comparison between the power supply voltage (the output voltage of the rectification circuit13) VOUT and the voltage of a capacitor63. The output of the comparator62is input to the control unit18through a CMP_OUT signal line. The switch64is a switch that is controlled to be turned on/off by the control unit18through a Ctrl signal line.

The control unit18is a unit, which is configured of a processor (a CPU or the like), responding to a command extracted by the demodulation circuit12by controlling the demodulation circuit14and the like.

The control unit18is configured (programmed) such that it can execute an impedance optimization process of a flow illustrated inFIG. 8. An execution timing of the impedance optimization process is not particularly limited. Accordingly, for example, the control unit18may be configured such that it executes the impedance optimization process when the power supply voltage becomes a threshold value or less, or the control unit18may be configured such that it executes the impedance optimization process when the execution of the impedance optimization process is directed from the reader/writer85.

As illustrated inFIG. 8, when the impedance optimization process is started, the control unit18, first, performs a process of turning off the first switch22and turning on the second switch23(Step S101). In other words, the control unit18performs a process for stopping supply of power to the load25and starting supply of power to the variable load21.

Next, the control unit18measures the power supply voltage VOUT by controlling the ADC17(Step S102). Thereafter, the control unit18determines whether or not the power supply voltage VOUT is equal to or less than a voltage VREF (Step S103). Here, the voltage VREF is a voltage that is less than the limiter voltage VLIM set in advance.

In a case in which the power supply voltage VOUT is not the voltage VREF or less (Step S103: No), the control unit18performs a process of changing the amount of power consumption of the variable load21in a direction in which the power supply voltage VOUT reduces (Step S107). In addition, the limiter16is disposed in a rear stage of the rectification circuit13. Thus, there are also cases in which the power supply voltage VOUT does not decrease even when the process of Step S107is performed.

The control unit18that has completed the process of Step S107executes the process of Step S102and subsequent steps again. Then, as a result of repeating the process of Steps S102, S103, and S107, the control unit18executes the impedance adjusting process (Step S104) in a case in which the power supply voltage VOUT becomes the voltage VREF or less (Step S103: Yes).

The impedance adjusting process is a process of a flow illustrated inFIG. 9.

In other words, the control unit18that has started the impedance adjusting process, first, outputs a Reset pulse (Step S201). Accordingly, the count value of the up counter61disposed inside the impedance adjustment control circuit15(seeFIG. 7) is Reset to “0”, and the capacitance of the variable-capacitance capacitor53(FIG. 5A) disposed inside the matching circuit11is adjusted to the lowest capacitance C0.

Next, the control unit18outputs a Ctrl pulse (Step S202). In other words, the control unit18causes the voltage (an input voltage input to the “−” terminal of the comparator62) of the capacitor63to coincide with the power supply voltage VOUT at that time point by turning on the switch64and then holds the voltage of the capacitor63by turning off the switch64.

Thereafter, the control unit18outputs an up pulse (Step S203) and then determines whether or not the output CMP_OUT of the comparator62is low (Step S204).

When the up pulse is input, the count value of the up counter61is counted up, and accordingly, the capacitance of the variable-capacitance capacitor53disposed inside the matching circuit11increases. When the capacitance of the variable-capacitance capacitor53increases, the power supply voltage increases in a case in which the impedance of the matching circuit11does not have an appropriate value and the power supply voltage is not limited by the limiter16. On the other hand, in a case in which the impedance of the matching circuit11has an appropriate value and the power supply voltage is limited by the limiter16, even when the capacitance of the variable-capacitance capacitor53increases, the power supply voltage hardly changes. Thus, in a case in which the output CMP_OUT is low, the adjustment of the impedance of the matching circuit11is completed. However, in a case in which the output CMP_OUT is high, the adjustment of the impedance of the matching circuit11is not completed.

For this reason, in a case in which the output CMP_OUT of the comparator62is high (Step S204: No), the control unit18returns the process to Step S203and outputs the up pulse again. Then, when the output CMP_OUT of the comparator62becomes low (Step S204: Yes), the control unit18ends the impedance adjusting process (the process illustrated inFIG. 9).

The description of the impedance optimization process will be continued with reference back toFIG. 8.

The control unit18that has ended impedance adjusting process (Step S104) measures the power supply voltage VOUT (Step S105) and determines whether or not the power supply voltage VOUT is less than the limiter voltage VLIM (Step S106). In a case in which the power supply voltage VOUT is not less than the limiter voltage VLIM (Step S106: No), the control unit18re-executes the process of Step S102and subsequent steps.

Then, when the power supply voltage VOUT becomes less than the limiter voltage VLIM during the repetition of the process described above (Step S106: Yes), the control unit18performs a process of turning off the second switch23and turning on the first switch22(Step S108) and then ends this impedance optimization process (the process illustrated inFIG. 8). Although not illustrated in the flowchart (FIG. 8), in a case in which the power supply voltage VOUT is not less than the limiter voltage VLIM, the control unit18determines whether or not the amount of power consumption of the variable load21at that time point is an upper limit amount. Then, in a case in which the amount of power consumption of the variable load21is not the upper limit amount (in other words, in a case in which the amount of power consumption of the variable load21can be increased), the control unit18causes the process to proceed to Step S107and increase the amount of power consumption of the variable load21. On the other hand, in a case in which the amount of power consumption of the variable load21is the upper limit amount, the control unit18performs the process of Step S108and then ends this impedance optimization process.

The control unit18is configured such that it performs the impedance optimization process of the sequence described above. Thus, in a case in which the impedance optimization process is started in a state in which the power supply voltage VOUT is less than the limiter voltage VLIM, as illustrated inFIG. 10, first, the first switch22and the second switch23are controlled such that a state is formed in which power is not supplied to the load25, and power is supplied to the variable load21. Next, in accordance with adjustment of the amount of power consumption of the variable load21, the power supply voltage VOUT is reduced to the voltage VREF. Then, after the impedance adjusting process is performed, it is determined whether or not the power supply voltage VOUT after the impedance adjusting process is less than the limiter voltage VLIM.

In a case in which the power supply voltage VOUT after the impedance adjusting process is not less than the limiter voltage VLIM, as illustrated inFIG. 10, again, the power supply voltage VOUT is reduced to the voltage VREF in accordance with adjustment of the amount of power consumption of the variable load21. Thereafter, the impedance adjusting process is performed. Then, it is determined whether or not the power supply voltage VOUT after the impedance adjusting process is less than the limiter voltage VLIM. In a case in which the power supply voltage VOUT is less than the limiter voltage VLIM, the first switch22and the second switch23are controlled such that the state is returned to the original state in which power is not supplied to the variable load21, and power is supplied to the load25, and the impedance optimization process ends.

The power supply voltage after the impedance adjusting process being less than the limiter voltage VLIM represents that the impedance of the matching circuit11has an optimal value. More specifically, as illustrated inFIG. 11, a power supply voltage-capacitance curve (broken line) of a case in which the limiter16is not disposed represents that a peak voltage changes to a lower side in accordance with an increase in the amount of power consumption of the variable load21. The impedance adjusting process (FIG. 8) is a process of adjusting the capacitance of the variable-capacitance capacitor53to a capacitance for which the power supply voltage is a maximum. Accordingly, in a case in which the power supply voltage after the impedance adjusting process is less than the limiter voltage VLIM, the impedance of the matching circuit11has an optimal value.

As described above, the RF tag10according to this embodiment has a function of adjusting the impedance of the matching circuit11to an optimal value. Accordingly, by using the RF tag10, a radio communication system that is robust with respect to environmental changes, in other words, a radio communication system in which it is difficult for an obstacle to occur in communication with the reader/writer85even when power input to the RF tag10is reduced in accordance with environmental changes can be built.

Various modifications can be made in the RF tag10according to the embodiment described above. For example, in a case in which there is a unit of which the amount of power consumption can be adjusted relatively finely among power supply targets in the RF tag10(the load25, the control unit18, the demodulation circuit12, the modulation circuit14, the impedance adjustment control circuit15, and the like), the RF tag10may be modified into a device in which the variable load21is not included, and the unit described above is used as the variable load21. In addition, the RF tag10may be modified into a device not including the first switch22and/or the second switch23. Furthermore, in a case in which the RF tag10is modified into a device not including the second switch23, a process of adjusting the power consumption of the variable load21to “0” (or minimum power consumption) may be performed in Step S108.

The RF tag10may be modified to a multi-level comparator comparing the power supply voltage VOUT with the voltage VREF and comparing the power supply voltage VOUT with the limiter voltage VLIM instead of the ADC17. In addition, the RF tag10may be modified into a device that includes a comparator comparing the power supply voltage VOUT with the voltage VREF and a comparator comparing the power supply voltage VOUT with the limiter voltage VLIM instead of the ADC17.

Furthermore, a function for executing a second impedance adjusting process (an impedance adjusting process in which the process of Step S101is not performed) starting adjustment of the impedance of the matching circuit11from the impedance at that time point may be included in the RF tag10, and the impedance optimization process may be changed to the process in which the second impedance adjusting process is performed in the second and subsequent Step S105.

The impedance optimization process (FIG. 8) may be modified to a process in which the process of Step S106is not performed, that is, a process in which, after the power supply voltage VOUT is reduced to a voltage less than the limiter voltage VLIM (a voltage that is VREF or less), the impedance adjusting process is performed only once. The condition for ending the impedance optimization process may be changed to a condition other than “VOUT<VLIM”, for example, “the number of times of execution of the impedance adjusting process<a predetermined number of times” or “an execution time of the impedance optimization process (an elapsed time after the start of the impedance optimization process)<a predetermined number of times”. In addition, in a case in which a condition of not using the power supply voltage VOUT is used as the condition for ending the impedance optimization process, a comparator comparing the power supply voltage VOUT with the voltage VREF may be used instead of the ADC17.

When the condition for ending the impedance optimization process is changed to the condition described above, the impedance of the matching circuit11may not be adjusted to the optimal value in some cases. Even in a case in which the condition for ending the impedance optimization process is changed to the condition described above, the impedance of the matching circuit11can be adjusted more satisfactorily than in a conventional case (a form in which a result of the adjustment of the impedance is closer to the optimal value than that of the conventional case).

While the impedance adjusting process described above is a process in which the capacitance of the variable-capacitance capacitor53for which the impedance of the matching circuit11has an optimal value (hereinafter, referred to as optimal capacitance) is retrieved through a linear search of monotonously increasing the capacitance of the variable-capacitance capacitor53(FIG. 5A), the impedance adjusting process may be a process in which an optimal capacitance of the variable-capacitance capacitor53is retrieved through a linear search monotonously reducing the capacitance of the variable-capacitance capacitor53. In addition, the impedance adjusting process may be a process of retrieving the optimal capacitance of the variable-capacitance capacitor53using another search algorithm such as a binary search or a tree search. Also in an impedance adjusting process of an RF tag10in which the matching circuit11is a circuit of which the impedance can be adjusted by changing the capacity of an inductor, similarly, a process using various search algorithms may be employed.

A function of monitoring the power supply voltage VOUT being a predetermined voltage, which is less than the voltage VLIM, or less and performing an impedance adjusting process when the power supply voltage VOUT becomes the predetermined voltage or less may be given to the RF tag10. In addition, the RF tag10may be modified into a device, which has the function described above, not performing the impedance optimization process.