This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-169221 filed in Japan on Jul. 17, 2009; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a contactless IC card and a wireless system, and in particular, to a contactless IC card and a wireless system which enable carrier extraction and clock generation to be reliably performed with a contactless IC card or the like to be mounted on a mobile device that performs contactless communication via an electromagnetic field or radio waves.
2. Description of Related Art
IC cards and RF tags are used in various fields as wireless communication means for exchanging information by near-field wireless communication using an electromagnetic field or radio waves (hereinafter, simply referred to as an electromagnetic field).
Contactless IC cards and RF tags which communicate via an electromagnetic field can be divided into passive IC cards and RF tags which obtain power from the electromagnetic field and active IC cards and RF tags which are supplied power from a battery or the like.
Hereinafter, while a contactless IC card among contactless IC cards and RF tags will be described for the sake of simplicity, similar configurations, effects, and advantages can also be achieved by using a contactless RF tag instead of a contactless IC card.
Prior art regarding conventional contactless IC cards include those disclosed in Japanese Patent Application Laid-Open Publication No. 2006-31473, Japanese Patent Application Laid-Open Publication No. 2007-148957, and Japanese Patent Application Laid-Open Publication No. 5-128319.
According to Japanese Patent Application Laid-Open Publication No. 2006-31473, an RFID tag is configured such that a resistor can be inserted between an antenna and an RFID tag IC so as to enable adjustment of receiver sensitivity.
According to Japanese Patent Application Laid-Open Publication No. 2007-148957, a wireless tag information reading apparatus (reader-writer) is arranged such that a threshold of a demodulating circuit can be varied according to command types and noise levels so as to enable adjustment of receiver sensitivity.
According to Japanese Patent Application Laid-Open Publication No. 5-128319, a contactless IC card is arranged such that a capacity of a tuning capacitor inside the contactless IC card can be varied according to an RF level received from a reader-writer so as to enable adjustment of receiver sensitivity.
However, while all three patent documents described above are intended to improve data demodulation by enabling adjustment of receiver sensitivity, the patent documents are not designed to adjust carrier extraction sensitivity in order to improve carrier extraction and clock generation.
A contactless IC card according to an aspect of the present invention includes: an antenna configured to receive an electromagnetic wave and to induce an AC signal; a rectifier circuit configured to rectify an AC signal from the antenna; a demodulating circuit configured to demodulate received data from a rectified signal from the rectifier circuit; a carrier extraction circuit configured to extract a carrier from the AC signal or the rectified signal and to generate an operation clock; a return data generating unit configured to operate at the operation clock from the carrier extraction circuit and, after receiving received data from the demodulating circuit, generate and output return data to a reader-writer; a modulating unit configured to load-modulate the carrier of the AC signal with the return data; and a sensitivity control unit configured to operate at the operation clock from the carrier extraction circuit and, during a return period to the reader-writer, output a sensitivity control signal and perform control so as to increase a carrier extraction sensitivity of the carrier extraction circuit.
A contactless IC card according to another aspect of the present invention includes: an antenna configured to receive an electromagnetic wave and to induce an AC signal; a rectifier circuit configured to rectify an AC signal from the antenna; a demodulating circuit configured to demodulate received data from a rectified signal from the rectifier circuit; a carrier extraction circuit configured to extract a carrier from the AC signal or the rectified signal and generate an operation clock; a return data generating unit configured to operate at the operation clock from the carrier extraction circuit and, after receiving received data from the demodulating circuit, output return data to a reader-writer; and a modulating unit configured to load-modulate the carrier of the AC signal with the return data, wherein a carrier extraction sensitivity of the carrier extraction circuit is increased during a return period to the reader-writer by supplying return data from the return data generating unit as a sensitivity control signal to the carrier extraction circuit.
A wireless system according to yet another aspect of the present invention includes: a contactless IC card having an antenna configured to receive an electromagnetic wave and induce an AC signal, a rectifier circuit configured to rectify an AC signal from the antenna, a demodulating circuit configured to demodulate received data from a rectified signal from the rectifier circuit, a carrier extraction circuit configured to extract a carrier from the AC signal or the rectified signal and generate an operation clock, a return data generating unit configured to operate at the operation clock from the carrier extraction circuit and, after receiving received data from the demodulating circuit, generate and output return data to a reader-writer, a modulating unit configured to load-modulate the carrier of the AC signal with the return data, and a sensitivity control unit configured to operate at the operation clock from the carrier extraction circuit and, during a return period to the reader-writer, output a sensitivity control signal and perform control so as to increase a carrier extraction sensitivity of the carrier extraction circuit; and a reader-writer having a second antenna and which is configured to read load-modulated return data from the contactless IC card contained in the AC signal.
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
Before describing the embodiments of the present invention with reference to
In
With the passive contactless IC card illustrated in
On the other hand, there is a recent trend towards mounting functions of a contactless IC card on mobile devices such as mobile phones. In such device-mounted specifications, active IC cards are employed. This is because in addition to a mobile device including a power supply in the first place, with a passive IC card, metallic portions inside the device affect an electromagnetic field and makes it difficult to obtain power.
In
With the active contactless IC card 10 illustrated in
On the other hand, with active contactless IC cards, regardless of how much an IC card's sensitivity is enhanced, communication is not established and communication characteristics cannot be improved unless a reply can be made to a reader-writer. Therefore, it is necessary to enhance the IC card's data demodulation and carrier extraction sensitivity while also improving replying capability.
An IC card uses a load modulation method to reply to a reader-writer. The method involves, in a state where the loop antenna 20-1 of the reader-writer and the loop antenna 10-1 of the IC card are coupled to each other by mutual inductance, varying an impedance of the IC card so as to vary a carrier voltage amplitude generated at the loop antenna 20-1 of the reader-writer. Generally, ASK modulation is adopted in which a modulation of the reader-writer and a load modulation of the IC card both binarily vary carrier amplitude.
Therefore, when having the IC card reply at a more intense level, a greater impedance variation is to take place in correspondence to return data. This means that in a state where impedance drops, a carrier voltage amplitude generated at the IC card loop antenna 10-1 is suppressed and becomes significantly small. Accordingly, as a consequence of a period being generated in which the carrier voltage amplitude is suppressed to a significantly small level for the purpose of further increasing the return level of the IC card, carrier extraction becomes difficult during the return period (i.e., load modulation period). Therefore, it is required that a carrier extraction sensitivity with respect to a carrier voltage amplitude generated at the IC card's loop antenna 10-1 be dramatically improved at least during a load modulation period (return period).
In the following embodiment, an active contactless IC card to be mounted on a mobile device that performs contactless communication via, for example, an electromagnetic field and a wireless system using the active contactless IC card will be described. As described earlier, the embodiment can be similarly applied to an RF tag and a wireless system using the RF tag.
Unlike the contactless IC card 10 illustrated in
Therefore, the carrier extraction sensitivity of the carrier extraction circuit 13A is raised or lowered (controlled) by a sensitivity control signal from the processing and control unit 15A. Accordingly, for example, even when carrier amplitude drops during load modulation in an active contactless IC card, carrier extraction can be reliably performed by raising carrier extraction sensitivity and clock generation can always be performed reliably.
The wireless system illustrated in
The reader-writer 20 exchanges data with the contactless IC card 10A. The data exchange between the reader-writer 20 and the contactless IC card 10A is performed in a contactless state respectively via loop antennas 20-1 and 10-1.
The contactless IC card 10A includes: a rectifier circuit 11 connected to the secondary antenna 10-1; a demodulating circuit 12 to which rectifier output is supplied from the rectifier circuit 11; a carrier extraction circuit 13A to which is supplied a same signal as the input of the rectifier circuit 11; a modulating unit 14 constituted by a load-modulating transistor; a processing and control unit 15A having a logic, a memory, and a microcomputer; a constant-voltage circuit 16; and external power-supply terminals 10-2 and 10-3 to which respective positive and negative electrodes of a battery, not shown, are connected. A power-supply voltage from an external power supply such as a battery is supplied to the constant-voltage circuit 16.
The secondary antenna 10-1 receives an electromagnetic wave transmitted from the primary antenna 20-1 connected to the reader-writer 20, whereby an AC signal (for example, an RF signal of 13.56 MHz) is induced and supplied to the rectifier circuit 11. In this case, an AC signal refers to a carrier signal containing data or, in other words, a signal of a carrier modulated by a modulation signal of transmission data of the reader-writer or return data of the IC card, or a signal solely of a non-modulated carrier.
The contactless IC card 10A is not limited to a card-like shape, and box-like, cylindrical, disk-like, stick-like, label-like shapes and the like can be adopted. When mounting on a substrate, for example, the antenna 10-1 is formed so as to enclose the IC card 10A.
The rectifier circuit 11 is constituted by, for example, a full-wave rectifier circuit including a diode bridge, and rectifies an AC signal from the antenna 10-1 and outputs the rectified signal to the demodulating circuit 12.
The carrier extraction circuit 13A extracts a carrier component of an electromagnetic wave transmitted from the reader-writer 20 and generates a clock CLK to be used as an operation clock of the processing and control unit 15A.
An example of operations for generating a clock CLK with the carrier extraction circuit 13A is described in Japanese Patent Application Laid-Open Publication No. 2007-142873, a Japanese Patent Application made by the present applicant.
The constant-voltage circuit 16 voltage-regulates a DC voltage from the external power-supply terminals 10-2 and 10-3 to generate an internal power supply voltage VDD. The generated internal power supply voltage VDD is to be used as an operating voltage of the demodulating circuit 12, the carrier extraction circuit 13A, and the processing and control unit 15A.
The demodulating circuit 12 demodulates received data from an envelope detection voltage of the rectified signal outputted from the rectifier circuit 11 and supplies the demodulated received data to the processing and control unit 15A.
The processing and control unit 15A includes a microprocessor (MPU), a logic circuit (logic), and ROM and RAM memories. The ROM stores a program for executing processing, controls, and the like to be executed at the logic circuit and the MPU. The RAM is used as a storage area and work area of data and the like to be used during program execution processing and control at the logic circuit and the MPU. As described earlier, function-wise, the processing and control unit 15A includes: a return data generating unit 151 that operates at the operation clock from the carrier extraction circuit 13A and outputs return data to a reader-writer; and a sensitivity control unit 152 that operates at the operation clock from the carrier extraction circuit 13A and, during a return period, outputs a sensitivity control signal that performs control so as to increase carrier extraction sensitivity of the carrier extraction circuit 13A.
A load modulation (load switching) method is used when transmitting data (replying) to the reader-writer 20. The load modulation method involves varying the impedance of the loop antenna 10-1 of the IC card 10A in order to increase/decrease the load on the loop antenna 20-1 of the reader-writer 20. Consequently, a variance of a carrier amplitude generated at the loop antenna 20-1 of the reader-writer 20 is to be detected as return data from the IC card 10A.
The modulating unit 14 is constituted by, for example, an N-channel MOS transistor that is a load-modulating transistor. A drain of the MOS transistor is connected to one of the lines of an input terminal of the rectifier circuit 11, and a source of the MOS transistor is connected to the other line of the input terminal of the rectifier circuit 11. Transmission data that is to be transmitted from the processing and control unit 15A as return data during a return period to the reader-writer 20 is inputted to a gate of the MOS transistor. Accordingly, when a modulation signal (refer to
Hereinafter, operations depicted in
In
The period indicated by reference character A in
While carrier extraction by a contactless IC card must be constantly performed during all periods, the embodiments of the present invention are arranged so that carrier extraction can be reliably performed through all periods including a load modulation period by raising carrier extraction sensitivity during a load modulation period (return period) in which carrier amplitude may sometimes become suppressed and reduced.
An H level of the modulation signal illustrated in
While the timing charts in
The antenna 10-1 is constituted by, for example, a parallel resonance circuit including a coil L and a capacitor C. The rectifier circuit 11 is made of, for example, a full-wave rectifier circuit including a diode bridge constituted by first to fourth diodes D1 to D4. A full-wave rectifier output of the rectifier circuit 11 is smoothed by a smoothing capacitor C1 and becomes an envelope detection voltage that is then inputted to the demodulating circuit 12. Received data digitally demodulated at the demodulating circuit 12 is supplied to the processing and control unit 15A.
A half-wave rectified signal outputted from the rectifier circuit 11 is supplied to the carrier extraction circuit 13A. The carrier extraction circuit 13A generates a clock CLK with a same frequency as the carrier frequency by binarizing the rectified signal using a threshold.
The carrier extraction circuit 13A includes: a comparator COM that compares the rectified voltage inputted from the rectifier circuit 11 with a threshold voltage and generates an operation clock with a same frequency as a carrier and; means that switches the threshold according to a sensitivity control signal from the sensitivity control unit 152 in the processing and control unit 15A.
The carrier extraction circuit 13A is constituted by a comparator circuit. The comparator circuit includes: a comparator COM with a non-inverting input terminal (+) into which is inputted a rectified voltage from the rectifier circuit 11 and an inverting input terminal (−) into which is inputted a threshold voltage; a voltage-dividing circuit connected to the inverting input terminal (−) of the comparator COM and which applies a voltage obtained by dividing a voltage r of the voltage supply with resistors R1 and R2 as a threshold to the inverting input terminal (−); and a switch SW connected in parallel to the resistor R2 on a reference potential point GND-side of the voltage-dividing circuit and which opens or short-circuits both ends of the resistor R2. Switching on/off of the switch SW is controlled according to a sensitivity control signal from the processing and control unit 15A. Alternatively, a small resistor may be serially connected to the switch SW.
The processing and control unit 15A is operated at the clock CLK from the carrier extraction circuit 13A, and after bringing the IC card 10A into close proximity of the reader-writer 20 and receiving transmission data from the reader-writer 20 with the antenna 10-1, the processing and control unit 15A functions to transmit transmission data as the return data to the modulating unit 14 and to supply a sensitivity control signal CTL to the carrier extraction circuit 13A.
Hereinafter, operations depicted in
In
An electromagnetic wave from the reader-writer 20 is received by the antenna 10-1 and rectified by the rectifier circuit 11, and received data is demodulated by the demodulating circuit 12. At the same time, the rectified signal rectified by the rectifier circuit 11 is sent to the carrier extraction circuit 13A. When the switch SW is in open position, the comparator circuit constituted by the comparator COM compares the rectified signal with the threshold Vth1 and generates a clock CLK1 that sets a rectified signal period exceeding the threshold Vth1 as an H level. The clock CLK1 is a clock whose frequency is the same as the carrier frequency. The demodulated data and the clock CLK1 are sent to the processing and control unit 15A. The processing and control unit 15A judges the demodulated data from the demodulating circuit 12 using the clock CLK1 as the operation clock. After judging that the demodulated data is received data from the reader-writer 20, the processing and control unit 15A generates a sensitivity control signal CTL and supplies the same to the comparator circuit that constitutes the carrier extraction circuit 13A and, at the same time, sends return data prepared in advance in the memory of the processing and control unit 15A to the modulating unit 14 as transmission data.
The sensitivity control signal CTL (refer to
Moreover, at the one of the terminals of the antenna (the connection point of D3 and D4), a carrier waveform denoted by reference character W1 in
The antenna 10-1 is constituted by, for example, a parallel resonance circuit including a coil Land a capacitor C. The rectifier circuit 11 is made of, for example, a full-wave rectifier circuit including a diode bridge constituted by first to fourth diodes D1 to D4. A full-wave rectifier output of the rectifier circuit 11 is smoothed by a smoothing capacitor C1 and becomes an envelope detection voltage that is then inputted to the demodulating circuit 12. Received data digitally demodulated by the demodulating circuit 12 is supplied to the processing and control unit 15A.
A half-wave rectified signal outputted from the rectifier circuit 11 is supplied to the carrier extraction circuit 13A via a DC current cutoff capacitor C2. The carrier extraction circuit 13A is constituted by a Schmitt trigger circuit and compares an inputted half-wave rectified signal with two thresholds that regulate a hysteresis width and binarizes the signal to generate a clock CLK with the same frequency as the carrier frequency.
The carrier extraction circuit 13A includes: a Schmitt trigger circuit having a comparator COM that compares a rectified signal inputted from the rectifier circuit 11 via the DC current cutoff capacitor C2 with two thresholds that regulate a hysteresis width and generates an operation clock with a same frequency as a carrier; and means that switches the hysteresis width corresponding to the difference between the two thresholds according to a sensitivity control signal from the sensitivity control unit 152.
The Schmitt trigger circuit has a characteristic (hysteresis) in which thresholds of the comparator COM differ between when an inputted rectified signal changes from a low potential side to a high potential side (during rising) and when changing from a high potential side to a low potential side (during falling), and enables a hysteresis width (range) corresponding to a difference between the two thresholds to be varied according to an H level and an L level of a sensitivity control signal CTL from the sensitivity control unit 152 in the processing and control unit 15A.
As illustrated in
In this configuration, if ER2 denotes a voltage generated between both ends of the resistor R12, then the two thresholds VH1 and VL1 of the Schmitt trigger circuit may be expressed as VH1=(V+/2)+ER2, VL1=(V+/2)−ER2 and the hysteresis width (=VH1−VL1) may be expressed as double the voltage ER2. Therefore, in order to vary hysteresis width (i.e., vary carrier extraction sensitivity), varying the value of the resistor R12 shall suffice. Since an intermediary position of the amplitude of a AC signal with a median potential V+/2 as its center is considered to be a midpoint potential V+/2, even when the carrier amplitude is suppressed from W11 to W12 during load modulation as illustrated in
Consequently, with a configuration in which a small resistance-added switch SW to which is serially connected a small resistor R13 is connected in parallel to both ends of the resistor R12 in the Schmitt trigger circuit and both ends of the switch SW are opened or short-circuited, two thresholds (V+/2)+ER2 and (V+/2)−ER2 exist when both ends of the switch SW are opened and hysteresis has a somewhat broad width expressible as 2×ER2. In addition, when both ends of the switch SW are short-circuited, since the small resistor R13 is connected in parallel, the value of ER2 decreases or, in other words, hysteresis width significantly decreases. Otherwise, if the resistor R13 is near 0, hysteresis width approximates 0 during a short-circuit of the switch SW and a state is entered in which the two thresholds approximate to one threshold that equals the median potential V+/2.
Hereinafter, operations depicted in
An electromagnetic wave from the reader-writer 20 is received by the antenna 10-1 and full-wave rectified by the rectifier circuit 11, and received data is digitally demodulated by the demodulating circuit 12. At the same time, a rectified half-wave rectified signal from the connection point of the diodes D3 and D4 of the rectifier circuit 11 is sent to the carrier extraction circuit 13A. Although the carrier extraction circuit 13A constituted by the Schmitt trigger circuit is set to a broad hysteresis width when the switch SW is open and assumed to be in a noise-tolerant (less affected by noise) state, carrier extraction sensitivity is not favorable due to the broad hysteresis width. However, the rectified signal is compared with two different thresholds VH1 and VL1, and a clock CLKa (refer to
The aforementioned demodulated data and the clock CLKa are sent to the processing and control unit 15A. The processing and control unit 15A judges the demodulated data using the clock CLKa. After judging that the demodulated data is received data from the reader-writer 20, the processing and control unit 15A arrives at a return period to the reader-writer and generates a sensitivity control signal CTL, and supplies the same to the carrier extraction circuit 13A and sends return data prepared in advance in the memory to the modulating unit 14 as transmission data. By adding the return data as a modulation signal (refer to
Besides a comparator, the carrier extraction circuit may be constituted by a buffer amplifier or an inverter, whereby carrier extraction sensitivity can be controlled by varying the threshold of the buffer amplifier or the inverter.
The first embodiment described above is an active contactless IC card to which power is supplied from a power supply such as a battery, the active contactless IC card including a carrier extraction circuit 13A that extracts a carrier from an AC voltage induced at an antenna terminal or a rectified voltage thereof and generates an operation clock, wherein the active contactless IC card is capable of improving difficulties in carrier extraction that occur when the IC card performs transmission by load modulation (load switching), thereby improving the sensitivity of the carrier extraction circuit and ensuring that carrier extraction and clock generation are to be performed. In other words, timings at which a contactless IC card performs load modulation is known by the IC card itself, thereby enabling the IC card to readily perform control in which carrier extraction sensitivity is enhanced only during a load modulation period and is reduced during other periods. Moreover, the timings at which a contactless IC card performs load modulation are predetermined periods of the data transmission idle periods from the reader-writer as illustrated in
However, when performing transmission by load modulation by a contactless IC card in this manner, generally, the following issues may conceivably exist as carrier extraction sensitivity is improved. Firstly, electromagnetic field noises induced by the antenna are inadvertently extracted in addition to a carrier. However, since a loop antenna is used in synchronization with a carrier, a filter effect thereof normally prevents electromagnetic field noise from becoming an issue. Secondly, a transient voltage fluctuation that accompanies logic operations penetrates into a rectifier circuit via a ground GND and appears as noise of an antenna terminal. However, since a carrier voltage amplitude is normally greater than noise, this does not particularly pose a problem. Therefore, even when a return level by load modulation is raised, noise is similarly suppressed during carrier amplitude suppression and erroneous extraction of a carrier due to enhanced sensitivity of the carrier extraction circuit is unlikely to occur. As a result, communication quality can be improved.
According to the first embodiment, when replying to a reader-writer from a contactless IC card, even when raising a return level causes carrier amplitude to drop, carrier extraction sensitivity can be automatically increased so as to ensure that carrier extraction and clock generation are to be performed. This is particularly useful when used in an active contactless IC card or an active contactless RF tag and a wireless system using the same.
In a contactless IC card 10B illustrated in
Since return data includes a period (e.g., H-level period) in which modulation is applied to each transmission symbol such as the modulation signal illustrated in
According to the second embodiment, when replying to a reader-writer from an IC card, even when raising a return level causes carrier amplitude to drop, a contactless IC card and a wireless system capable of reliably performing carrier extraction and clock generation can be realized.
According to the embodiments described above, since an erroneous extraction of a carrier due to enhanced sensitivity of the carrier extraction circuit is unlikely to occur even when a return level by load modulation is raised, communication quality can be improved.
Having described the embodiments of the invention referring to the accompanying drawings, it should be understood that the present invention is not limited to those precise embodiments and various changes and modifications thereof could be made by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
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2009-169221 | Jul 2009 | JP | national |