JP2003076957A - Modulation circuit and contactless IC card reader / writer - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To suppress an increase in power consumption of a device when a foreign object approaches an antenna of a non-contact IC card reader / writer device, and to reduce the power consumption of the device. SOLUTION: The amplitude of a carrier signal a is set to transmission data SD.
And an antenna coil L1 for transmitting the amplitude-modulated signal of the transmission driver to the IC card 3 as a radio signal.
IC card reader / writer 1 equipped with a constant current circuit 1
2C, the transmission driver DRV from the constant current circuit 12C.
1 by supplying a constant current to the transmission driver DR.
The output current from V1 to the antenna coil L1 side is limited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulation circuit for modulating the amplitude of a carrier signal according to an input signal and a non-contact IC card reader / writer equipped with the modulation circuit.

[0002]

2. Description of the Related Art A modulation circuit of this kind shifts the amplitude of a carrier signal in accordance with binary information of an input signal. Such amplitude shift keying modulation is performed by ASK (amplit).
Udeshift keying), which is a type A modulation for shifting the amplitude of the carrier signal to a modulation degree of 100% according to "1" or "0" indicating binary information of the input signal and the value of the input signal " There is a type B modulation in which the amplitude of the carrier signal is shifted to a modulation degree of 10% according to "1" or "0".

FIG. 4 is a waveform diagram for explaining the principle of amplitude shift keying. The modulation factor of the amplitude shift keying modulation can be expressed as: modulation factor = {(x−y) / (x + y)} × 100 (%). Note that x is an input signal 2
For example, the amplitude voltage of the carrier signal when the value of the value signal is "1" is shown, and the y is the amplitude voltage of the carrier signal when the value of the binary signal is "0".

Here, when the carrier signal amplitude voltage y when the value of the binary signal is "0" is set to 0V with respect to the carrier signal amplitude voltage x when the value of the binary signal is "1", the modulation factor is obtained from the above equation. = {(X-0) / (x + 0)} * 100 = 100
(%), And as shown in FIG. 4B, a type A modulation signal that shifts the amplitude of the carrier signal to a modulation degree of 100% is generated. Further, the ratios of the carrier signal amplitude voltages x and y when the binary signal values are “1” and “0” are 100 and 8 respectively.
Assuming 1.8, the modulation factor = {(100-81.8) / (100 + 81.8)} × 100≈
10 (%), and as shown in FIG. 4 (c), a type-B modulated signal that shifts the amplitude of the carrier signal to a modulation factor of 10% is generated.

The above type A and type B modulation circuits are used in a non-contact IC card reader / writer for reading / writing data from / to an IC card. The non-contact IC card reader / writer writes the data to the IC card and reads the data from the IC card by transmitting the modulated signal of the modulation circuit as a radio wave signal from the antenna to the IC card.

[0006]

In the conventional IC card reader / writer device, a metal object or an IC card of a different type that cannot be used in this card reader / writer device (hereinafter collectively referred to as a foreign substance) is used for the antenna. When approaching, there is a possibility that excessive power will be supplied to the foreign matter due to the coupling of the foreign matter and the antenna.If such excessive power is supplied to the foreign matter, the transmission current of the antenna will increase and the power consumption of the device will decrease. There is a growing problem. Also,
In recent years, low power consumption of IC card reader / writer devices has been demanded. Therefore, it is an object of the present invention to suppress an increase in power consumption of the IC card reader / writer device when a foreign object approaches the antenna of the IC card reader / writer device, and to reduce the power consumption of the device.

[0007]

In order to solve such a problem, the present invention outputs an amplitude shift keying signal by shifting the amplitude voltage of a high frequency carrier signal according to the value of a binary signal. In the modulation circuit, when a carrier signal and a binary signal are input, a transmission driver that shifts the amplitude voltage of the input carrier signal according to the value of the input binary signal and outputs the amplitude-shift modulation signal, and a transmission driver And a constant current circuit connected to the power supply terminal for limiting the power supply current of the transmission driver to a fixed value or less. In this case, the constant current value output from the constant current circuit is set to a predetermined current consumption value of the transmission driver. Further, in a non-contact IC card reader / writer that includes a transmission driver that performs amplitude shift modulation of a carrier signal and an antenna that transmits the amplitude shift modulation signal output from the transmission driver to the IC card as a radio signal, a transmission driver And a constant current circuit connected to the power supply terminal for limiting the power supply current of the transmission driver to a fixed value or less. In this case, the value of the constant current output from the constant current circuit is set to be equal to or less than the value of the abnormal current output from the transmission driver to the antenna side when a foreign object approaches the antenna.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 2 is a block diagram showing the configuration of the non-contact IC card reader / writer according to the present invention. In FIG. 2, a non-contact IC card reader / writer (hereinafter referred to as IC card reader) 1 includes a control unit 11 that controls the entire IC card reader 1, a modulation circuit 12, a demodulation circuit 13, and a matching circuit 14. The resonant circuit 15 and the power supply unit 16 that supplies power to the above-mentioned units are configured.

Here, the modulation circuit 12 comprises an oscillator 12A and a modulation section 12B, and the modulation section 12B modulates the carrier signal a generated from the oscillator 12A according to the value of the data signal SD output from the control section 11. It is output to the matching circuit 14. The matching circuit 14 inputs the output signal from the modulation circuit 12 and outputs it to the resonance circuit 15, and
The impedance is matched with the resonance circuit 15 so that this output signal can be efficiently transmitted from the resonance circuit 15 to an IC card described later. The resonance circuit 15 is configured such that the antenna coil L1 which is an inductive element and the capacitive element C1 are connected in parallel, and when a modulation signal is applied from the modulation circuit 12 through the matching circuit 14, the resonance circuit 15 starts resonance and the antenna coil L1.
The radio wave signal is generated from and is transmitted to an IC card, which will be described later, electromagnetically coupled to the antenna coil L1. When the demodulation circuit 13 receives a modulation signal that is returned from an IC card, which will be described later, in response to the transmission of the radio wave signal, the demodulation circuit 13 demodulates the modulation signal, extracts data, and outputs the data to the control unit 11. Is.

The control unit 11 in the IC card reader 1 is connected to the host device 2 via a serial I / F (serial interface). The control unit 11 performs data communication with the higher-level device 2 via the serial I / F to record the received data from the higher-level device 2 in the later-described IC card, and stores the data read from the later-described IC card in the higher-order mode. Send to device 2.

FIG. 3 is a block diagram showing the configuration of the IC card electromagnetically coupled to the IC card reader 1. Figure 3
In, the IC card 3 has an antenna coil L2, a wireless interface 31, a control circuit 32, and a memory 33 in which an ID and information unique to the IC card are stored.

The wireless interface 31 of the IC card 3
When the antenna coil L1 of the IC card reader 1 and the antenna coil L2 of the IC card 3 are electromagnetically coupled,
Electric power POWER is generated by the radio wave signal from the card reader 1 and supplied to the control circuit 32 and the memory 33, and a clock signal CLK is generated and given to the control circuit 32.
In addition, the data DATA is extracted from the radio signal to control circuit 3
2 is output. Control circuit 32 of IC card 3
When the power POWER is supplied from the wireless interface 31, the data DATA is read from the wireless interface 31 in synchronization with the clock signal CLK. If this data DATA is write data to the memory 33, the memory 33
On the other hand, when the data DATA is command data for reading the data in the memory 33, the data in the memory 33 is read and output to the wireless interface 31.

The operation of the IC card reader 1 and the IC card 3 will be described with reference to FIGS. IC card reader 1
Generates a carrier signal a having a predetermined frequency from an oscillator 12A provided in the modulation circuit 12 and transmits the carrier signal a as a radio wave signal via the matching circuit 14 and the resonance circuit 15. here,
When the IC card 3 is placed in a predetermined place on the IC card reader 1, the antenna coil L2 of the IC card 3 and the antenna coil L1 of the IC card reader 1 are electromagnetically coupled to each other, and I
As described above, the C card 3 is supplied with electric power POWER by the radio signal of the IC card reader 1.

The control unit 11 of the IC card reader 1 transfers data from the host device 2 to the IC card 3 to the serial I / O.
When received via F, this data SD is sent to the modulation circuit 12
Output to. Upon receiving the data from the control unit 11, the modulation circuit 12 amplitude-shift-shift modulates the carrier signal a according to the value of the input data signal SD and transmits it as a radio wave signal via the matching circuit 14 and the resonance circuit 15. Let

When the radio interface 31 of the IC card 3 receives the radio signal from the IC card reader 1 via the antenna coil L2, the data DATA (that is, the data SD) is extracted from the radio signal as described above. Then
Output to the control circuit 32. Here, the control circuit 32 is
If the received data DATA from the C card reader 1 is write data for the memory 33, it is recorded in the memory 33.
When the data DATA is command data for reading the data in the memory 33, the data in the memory 33 is sequentially read in synchronization with the clock signal CLK and is output to the wireless interface 31 side.

The data sequentially read from the memory 33 of the IC card 3 is transmitted as a radio signal to the IC card reader 1 side via the wireless interface 31 and the antenna coil L2. Resonant circuit 15 of IC card reader 1
When receiving the radio wave signal from the IC card 3, outputs the radio wave signal to the demodulation circuit 13 as a modulation signal. The demodulation circuit 13 demodulates the modulated signal to extract data from the modulated signal, and outputs the extracted data in the memory 33 of the IC card 3 to the control unit 11. Control unit 11
When receiving the data from the demodulation circuit 13, transmits the data to the higher-level device 2. In this way, the IC card reader 1 can write data in the IC card 3 and read the data written in the IC card 3.

FIG. 1 is a block diagram showing a main configuration of the IC card reader 1. As shown in FIG. 1, the modulation circuit 12 includes an oscillator 12A that generates the carrier signal a having the above-described predetermined frequency, a modulation unit 12B that includes a transmission driver DRV1, a transmission driver that includes an N-type field effect transistor Q1 and a resistor R1. The constant current circuit 12C that limits the power supply current supplied to the DRV1.

A transmission driver DR which constitutes the modulator 12B
V1 is composed of a NAND gate. Therefore, the transmission driver DRV1 receives the transmission data SD of “H” level (data value “1”) from the control unit 11 and then the oscillator 1
The carrier signal a from 2A is inverted and sent to the resonance circuit 15 via the matching circuit 14. Also, the transmission driver DRV
1 is “L” level (data value “0”) from the control unit 11.
When the transmission data SD of is input, the transmission of the carrier signal a is stopped, and the amplitude voltage of the carrier signal a is "0" at this time.
Is sent to the matching circuit 14 side. That is, the modulator 12B changes the amplitude of the carrier signal a to a predetermined amplitude voltage according to the values “1” and “0” of the transmission data SD (that is, “H” level and “L” level of the transmission data SD). From 0
It outputs a modulated signal that causes 100% deviation to V.

The constant current circuit 12C receives the power source current from the power source VCC, and the transistor Q1, which is always conducting,
Also, the power is supplied to the power supply terminal VDD1 of the transmission driver DRV1 in the modulator 12B via the resistor R1 connected in series with the transistor Q1. Here, the power supply current of the transmission driver DRV1 of the modulator 12B is limited to a constant current by the constant current circuit 12C as described above.

Next, the operation of the constant current circuit 12C will be described in detail. FIG. 5 is a diagram showing a potential difference between the constant current circuit 12C and the transmission driver DRV1 in the modulator 12B in the steady state. In FIG. 5, V1 is a transistor Q1
Potential difference between the drain and source of the resistor, V2 is the potential difference across the resistor R1 (ie, Vr1), and V3 is the transmission driver DR.
The potential applied to V1 is shown. Since one end of the resistor R1 is connected to the source terminal of the transistor Q1 and the other end is connected to the gate terminal of the transistor Q1, the potential difference Vr1 across the resistor R1 is inevitably applied between the gate and source of the transistor Q1, It becomes the gate-source voltage Vgs of Q1. Furthermore, since the power supply VCC has a positive potential, the gate potential of the transistor Q1 is lower than the source potential. That is, the transistor Q1
Has a negative potential with respect to the source potential.

FIG. 6 shows a characteristic example of the gate potential Vgs vs. the drain current Id with the source potential of the transistor Q1 as a reference. The potential difference Vr across the resistor R1 shown in FIG.
Since 1 is directly applied as Vgs1 in FIG. 6 between the gate and source terminals of the transistor Q1, the drain current Id1 shown in the characteristic diagram of FIG. 6 flows.
This drain current Id1 is the current to be controlled supplied to the transmission driver DRV1.

When the load of the transmission driver DRV1 increases and the consumption current increases, the drain current Id1 of the transistor Q1 increases and the voltage Vr1 across the resistor R1 increases. The increase in the voltage Vr1 across the resistor R1 is shown in FIG.
In the characteristic diagram of the above, the gate-source voltage Vgs of the transistor Q1 is moved in the negative direction (increase direction shown in FIG. 6). As a result, the drain current Id of the transistor Q1 in the characteristic diagram of FIG. 6 decreases. vice versa,
When the load of the transmission driver DRV1 decreases and the current consumption decreases, the drain current Id1 of the transistor Q1 decreases and the voltage Vr1 across the resistor R1 also decreases. As a result, in the characteristic diagram of FIG. 6, the gate of the transistor Q1
The source-source voltage Vgs is moved in the positive direction (decreasing direction shown in FIG. 6) to increase the drain current Id1 of the transistor Q1. In this way, the constant current circuit 12C always supplies a constant current to the transmission driver DRV1.

When a foreign matter approaches the antenna coil L1 of the resonance circuit 15 of the IC card reader 1, the foreign matter and the antenna coil L1 may be coupled with each other to cause excessive power supply to the foreign matter. At this time, the current flowing from the transmission driver DRV1 to the antenna coil L1 via the matching circuit 14 increases, and the current consumption of the IC card reader 1 increases. In this case, the power supply current supplied from the constant current circuit 12C to the transmission driver DRV1 also increases. However, in the constant current circuit 12C, the transistor Q1 has the transmission driver DRV1 as described above.
Since it works to reduce the power supply current to 1,
An increase in output current from the transmission driver DRV1 to the antenna coil L1 side is suppressed. As a result, it is possible to suppress an increase in the current consumption of the IC card reader 1 due to the approach of the foreign matter to the antenna coil L1.

Here, in the present embodiment, the constant current circuit 12
The output current value of C is set to be equal to or less than the output current value of the transmission driver DRV1 when a foreign object approaches the antenna coil. In this way, by setting the output current value of the constant current circuit 12C, when the foreign matter approaches the antenna coil L1, the constant current circuit 12C does not supply sufficient power supply current to the transmission driver DRV1.
RV1 is protected. Further, when the regular IC card 3 approaches the antenna coil L1 of the IC card reader 1, the transmission driver DRV1 operates reliably with the power supply current within the output current value from the constant current circuit 12C, and the antenna coil L1.
An accurate current can be output to the 1 side. As a result, appropriate power is supplied to the IC card side, and as a result, the IC card reader 1 can read / write accurate data from / to the IC card.

As described above, the constant current circuit 12C is provided, and the current supplied to the transmission driver DRV1 by the constant current circuit 12C is limited to a constant current.
The current from 1 to the antenna coil L1 side is less than or equal to the limit current, and even when a foreign object approaches the antenna coil L1 of the IC card reader 1, an excessive current does not flow from the transmission driver DRV1 to the antenna coil L1 side. Therefore, the power consumption of the IC card reader 1 can be limited to a certain value or less, and as a result, the power consumption of the IC card reader 1 can be reduced. In order to reduce the power consumption of the IC card reader 1, the entire current consumption of the IC card reader 1 is not limited, but only the output current of the transmission driver DRV1 whose current consumption fluctuates due to the approach of a foreign object or the like is limited. Therefore, the power consumption of the IC card reader 1 can be limited with high accuracy. Further, if the total current consumption of the IC card reader 1 is limited, the entire IC card reader becomes inoperable when an excessive current flows, whereas only the output current of the transmission driver DRV1 is limited as in the present embodiment. Then, only the transmission driver DRV1 becomes inoperable, and therefore the IC
There is an advantage that only communication with the card 3 is disabled.

(Other Embodiments) FIGS. 7 to 11 are circuit diagrams showing other embodiments of the present invention. In the first configuration shown in FIG. 1 described above, one constant current circuit is configured from one N-type field effect transistor Q1 and the resistor R1, and the constant current from this one constant current circuit is transmitted to one transmission driver D.
Although it is supplied as the power supply current of RV1, the second configuration shown in FIG. 7 is such that a plurality of constant current circuits are provided and the constant current from each constant current circuit is supplied to a plurality of transmission drivers. .

That is, as shown in FIG. 7, a plurality of constant current circuits 12C1 to 12Cn each including an N-type field effect transistor Q and a resistor R are provided, and each constant current circuit 12C is provided.
The constant current I1 output from each of 1 to 12 Cn is supplied to each of the transmission drivers DRV1 to DRVn. Thereby, a plurality of current values can be set.

Further, the constant current circuit 12C and the transmission driver DRV can also be configured in a combination pattern as shown in FIG. That is, a plurality of N-type field effect transistors Q1 to Qn are provided as the constant current circuit 12C, and the drains, the sources, and the gates of the N-type field effect transistors Q1 to Qn are commonly connected. In addition, one resistor R1 is provided between the source and the gate of each of the N-type field effect transistors Q1 to Qn that are commonly connected.
The constant current circuit 12C according to the first pattern connected by
Make up. Further, as the transmission driver DRV supplied with the constant current from the constant current circuit 12C configured by the first pattern, a plurality of transmission drivers DRV1 to DRVm are provided.
And each transmission driver DRV1 to DRVm
The transmission driver DRV is configured by a first pattern in which the input side and the output side of each are commonly connected.

Further, the constant current circuit 12C and the transmission driver DRV can be configured in combination patterns as shown in FIG. That is, the constant current circuit 12C is configured by the above-mentioned first pattern shown in FIG. A plurality of transmission drivers DRV1 to DRVm are provided as the transmission drivers DRV supplied with the constant current from the constant current circuit 12C configured by the first pattern, and the input sides of the transmission drivers DRV1 to DRVm are independent of each other. To the transmission driver DRV according to the second pattern in which the input can be input to the output side and the output side can also output independently.
Make up.

Further, the constant current circuit 12C and the transmission driver DRV can also be configured in a combination pattern as shown in FIG. That is, as the constant current circuit 12C, a plurality of constant current circuits 12C1 to 12Cn each including an N-type field effect transistor Q and a resistor R are provided, and the gates of the N-type field effect transistors Q1 to Qn are commonly connected. The constant current circuit 12C is configured by the second pattern. Then, the transmission driver DRV supplied with the constant current from the constant current circuit 12C configured by the second pattern is configured by the first pattern shown in FIG. 8 described above. In the combination pattern shown in FIG. 10, the constant current circuits 12C1 to 12C that configure the constant current circuit 12C.
Since each Cn is composed of the N-type field effect transistor Q and the resistor R, the accuracy of the current value output from the constant current circuit 12C is improved.

Further, the constant current circuit 12C and the transmission driver DRV can also be configured in a combination pattern as shown in FIG. That is, the constant current circuit 12C is configured by the second pattern of FIG. 10 described above, and the transmission driver DRV is configured by the second pattern of FIG. 9 described above.
In the combination pattern shown in FIG. 11, as in FIG.
Since the constant current circuit 12C is configured with the second pattern, the accuracy of the current value output from the constant current circuit 12C is similarly improved.

The constant current circuit 12C may be composed of a constant current diode. Furthermore, the constant current circuit 1
2C can also be formed by a bipolar transistor.
Further, the resistance in the constant current circuit 12C can be a variable resistance, and the power supply current from the constant current circuit 12C can be set to an arbitrary constant value. Further, although the present embodiment has described the modulation circuit that performs amplitude shift keying with a modulation degree of 100%, the present invention can also be applied to a modulation degree other than the above. Furthermore, in the present embodiment, the example in which the modulation circuit is applied to the IC card reader has been described, but the present invention can be similarly applied to all devices having the modulation circuit.

[0033]

As described above, according to the present invention, when a carrier signal and a binary signal are input, the amplitude voltage of the input carrier signal is shifted according to the value of the binary signal, and an amplitude shift keying signal is obtained. Since the modulation circuit is configured by the transmission driver that outputs as a signal and a constant current circuit that is connected to the power supply terminal of the transmission driver and limits the power supply current of the transmission driver to a constant current or less, the output current of the transmission driver can be limited. . Therefore, if this modulation circuit is mounted on an IC card reader / writer and connected to the antenna of the IC card reader / writer, the current from the transmission driver to the antenna is limited to a fixed current or less, and even if a foreign object approaches the antenna. The excessive current does not flow to the antenna side and the power consumption of the IC card reader / writer is secured at a constant value. As a result,
An abnormal increase in power consumption of the IC card reader / writer can be suppressed. Further, since the value of the constant current output from the constant current circuit is set to a predetermined current consumption value of the transmission driver, this IC card can be used when a regular IC card approaches the antenna of the IC card reader / writer. An appropriate current can be supplied to the IC card, and therefore accurate reading / writing of data from / to the IC card becomes possible.

Further, in an IC card reader / writer equipped with a transmission driver for performing amplitude shift modulation of a carrier signal and an antenna for transmitting the amplitude shift modulation signal output from the transmission driver as a radio signal to an IC card, Since the constant current circuit is connected to the power supply terminal of the driver, the current from the transmission driver to the antenna is limited to a constant current. Therefore, even when a foreign object approaches the antenna, the power consumption of the IC card reader / writer is also the same. Can be secured at a constant value, and therefore an abnormal increase in power consumption of the IC card reader / writer can be suppressed. Further, the value of the constant current output from the constant current circuit is set to be equal to or less than the value of the abnormal current output from the transmission driver to the antenna side when a foreign substance other than a readable IC card approaches the antenna. Therefore, when a regular IC card approaches the antenna of the IC card reader / writer, it becomes possible to read / write accurate data to / from this IC card as well.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of a non-contact IC card reader / writer according to the present invention.

FIG. 2 is a block diagram showing an overall configuration of a non-contact IC card reader / writer.

FIG. 3 is a block diagram of an IC card in which data is read / written by a non-contact IC card reader / writer.

FIG. 4 is an explanatory diagram of amplitude shift keying.

FIG. 5 is a diagram showing a potential difference between a constant current circuit and a transmission driver in a steady state.

FIG. 6 is a diagram showing characteristics of transistors in a constant current circuit.

FIG. 7 is a circuit diagram showing a second configuration of a constant current circuit and a transmission driver.

FIG. 8 is a circuit diagram showing a third configuration of a constant current circuit and a transmission driver.

FIG. 9 is a circuit diagram showing a fourth configuration of a constant current circuit and a transmission driver.

FIG. 10 is a circuit diagram showing a fifth configuration of a constant current circuit and a transmission driver.

FIG. 11 is a circuit diagram showing a sixth configuration of the constant current circuit and the transmission driver.

[Explanation of symbols]

1 ... Non-contact IC card reader / writer, 2 ... Host device, 3
... IC card, 11 ... Control unit, 12 ... Modulation circuit, 12A
... Oscillator, 12B ... Modulator, 12C, 12C1 to 12C
n ... Constant current circuit, 13 ... Demodulation circuit, 14 ... Matching circuit, 1
5 ... Resonance circuit, 31 ... Wireless interface, 32 ... Control circuit, 33 ... Memory, Q1-Qn ... N-type field effect transistor, R1-Rn ... Resistor, DRV, DRV1-DRV
n ... Transmission driver, C1 ... Capacitance element, L1, L2 ... Antenna coil.

   ─────────────────────────────────────────────────── ─── Continued front page    (71) Applicant 000134257             NR Sheetkin Co., Ltd.             6-7-1 Koriyama, Taihaku-ku, Sendai City, Miyagi Prefecture (71) Applicant 000005108             Hitachi, Ltd.             4-6 Kanda Surugadai, Chiyoda-ku, Tokyo (72) Inventor Toyohiko Akatsuka             2-3 2-3 Shimo-Meguro, Meguro-ku, Tokyo Stocks             Company Tamura Electric Works (72) Takashi Nishikawa, the inventor             2-3 2-3 Shimo-Meguro, Meguro-ku, Tokyo Stocks             Company Tamura Electric Works (72) Inventor Akiyasu Sada             2-3 2-3 Shimo-Meguro, Meguro-ku, Tokyo Stocks             Company Tamura Electric Works (72) Inventor, Ichiko Okutsu             2-3 2-3 Shimo-Meguro, Meguro-ku, Tokyo Stocks             Company Tamura Electric Works (72) Inventor Akira Yamaguchi             3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Tohnichi             Inside Telegraph and Telephone Corporation (72) Inventor Hideaki Yamamoto             3-15 Babacho, Chuo-ku, Osaka-shi, Osaka             Inside Telegraph and Telephone Corporation (72) Inventor Hiromi Sato             6-7-1, Koriyama, Taihaku-ku, Sendai City, Miyagi Prefecture             Tokin Co., Ltd. (72) Inventor Tsuru Miura             6-7-1, Koriyama, Taihaku-ku, Sendai City, Miyagi Prefecture             Tokin Co., Ltd. (72) Inventor Miaki Hotta             1 Horiyamashita, Hadano City, Kanagawa Japan             Tate Seisakusho Enterprise Server Division F term (reference) 5B058 CA17 CA22 KA02 KA04 KA40                       YA20

Claims (4)

[Claims] 1. A modulation circuit that outputs an amplitude shift keying signal by shifting the amplitude voltage of a carrier signal according to the value of a binary signal, when the carrier signal and the binary signal are input, the input carrier is input. A transmission driver that shifts an amplitude voltage of a signal according to a value of an input binary signal and outputs the amplitude-shift modulation signal, and a power supply terminal of the transmission driver, which is connected to a power supply current to the transmission driver to a constant value. A modulation circuit having a constant current circuit for limiting the value to a value or less. 2. The modulation circuit according to claim 1, wherein the value of the constant current output from the constant current circuit is set to a predetermined current consumption value of the transmission driver. 3. A modulation circuit including a transmission driver that outputs an amplitude shift modulation signal by shifting the amplitude voltage of a carrier signal according to the value of a binary signal, and is connected to the output side of the transmission driver. A non-contact IC card reader / writer equipped with an antenna for transmitting the amplitude shift keying signal output from the transmission driver to an IC card as a radio wave signal, the power source of the transmission driver being connected to a power supply terminal of the transmission driver. A non-contact IC card reader / writer, comprising a constant current circuit for limiting a current to a certain value or less. 4. The value of the constant current output from the constant current circuit according to claim 3, is output from the transmission driver to the antenna side when a foreign substance other than a readable IC card approaches the antenna. A non-contact IC card reader / writer characterized by being set to a value equal to or less than an abnormal current value.