JP2012129639A - Signal transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a signal transmitter capable of transmitting a plurality of trigger signals with different characteristics without degradation in performance required for a signal transmitter transmitting trigger signals.SOLUTION: A signal transmitter that transmits trigger signals to activate wireless IC tags via antennas to the wireless IC tags comprises a plurality of connection units connecting a plurality of antennas so as to correspond one-to-one, and a selection unit selecting one of the plurality of antennas connected to the plurality of connection units. The signal transmitter is configured such that a plurality of trigger signals with different characteristics can be transmitted via the plurality of antennas connected to the plurality of connection units, and the voltage across the antenna other than the antenna selected by the selection unit is zero after power-on.

Description

  The present invention relates to a signal transmitter, and more particularly, to a technique preferably applied to, for example, a moving object management system that manages a trend of a person holding a wireless IC tag.

  2. Description of the Related Art Conventionally, a security system that manages a product by attaching a wireless IC tag to a product in a store or the like, and a moving body management system that manages a trend of a person holding the wireless IC tag are known. Wireless IC tags (RFID tags) are roughly classified into active tags having a power supply unit such as a battery and passive tags not having a power supply unit. Active tags receive several tens of power supplies from the power supply unit. It is possible to communicate in a range of about m, and it is possible to transmit signals over a wide range compared to passive tags. For this reason, active tags are often used in the above systems.

  In a security system and a moving body management system using an active tag, a trigger transmitter that transmits a trigger signal for starting the tag to the tag is used. The trigger transmitter sends an electric current of about 10 Ap-p to an external trigger coil of about several tens of μH to 100 μH at a frequency of about 100 kHz modulated by ASK (Amplitude Shift Keying) with position information etc. Activate the tag.

  For example, Patent Document 1 discloses a management system capable of finely managing wireless IC tags, and a trigger transmitter is also used in the management system. As shown in FIG. 5, the trigger transmitter used in the management system includes one external trigger coil as an antenna (one channel trigger transmitter), and a plurality of different characteristics from one transmitter. The trigger signal cannot be transmitted.

JP 2010-41475 A

  The moving body management system and security system using wireless IC tags install trigger transmitters at the entrances and passages of each floor in the building where they are installed. This increases the installation cost. In order to reduce the installation cost, it is effective to be able to transmit a plurality of trigger signals having different characteristics from one trigger transmitter. For example, if two external trigger coils are mounted on one trigger transmitter, the number required for installation can be halved.

  However, when two external trigger coils are mounted on one trigger transmitter as described above, the leakage of the trigger signal to the counterpart coil due to electrical interference between the trigger coils or between the connectors connecting the respective coils is a problem. Become. This trigger signal leak causes a trigger signal to be transmitted from an unscheduled external trigger coil, resulting in a system malfunction that activates a tag with an ID not assigned to that channel.

  Therefore, in view of the above-described circumstances, the present invention realizes a signal transmitter capable of transmitting a plurality of trigger signals having different characteristics without degrading performance required for a signal transmitter that transmits a trigger signal. For the purpose.

  The signal transmitter according to the present invention is a signal transmitter that transmits a trigger signal for starting a wireless IC tag to a wireless IC tag via an antenna, and corresponds to a plurality of antennas on a one-to-one basis. A plurality of connecting portions connected to each other and a selection portion for selecting one of the plurality of antennas connected to the plurality of connecting portions, and the characteristics via the plurality of antennas connected to the plurality of connecting portions Are configured such that a plurality of trigger signals can be transmitted, and the voltage at both ends of the antenna other than the antenna selected by the selection unit becomes 0 after the power is turned on.

  In the above trigger transmitter, the current detector that detects the current flowing through the antenna selected by the selector and outputs it as a voltage value, and the voltage selected by the selector is selected based on the voltage value output by the current detector. And a control unit for determining a peak point of a resonance voltage generated in the antenna.

  Further, in the trigger transmitter described above, a resonance capacitor connected to the control unit and the selection unit in a circuit incorporated so that a plurality of capacitors are turned on / off according to a set value to form a plurality of types of resonance capacitors. The current detection unit detects the voltage value of the current flowing through the antenna and the resonance capacitor circuit selected by the selection unit, and the control unit outputs a set value to the resonance capacitor circuit at a predetermined interval to set The voltage value for each value is acquired from the current detection unit, and the setting value corresponding to the maximum voltage value among the acquired voltage values is stored as the optimal setting value of the resonant capacitor with the selected antenna. May be.

  In the trigger transmitter, the first connection portion connected to the first antenna has one terminal connected to the ground, the other terminal connected to the ground via the first switch, The second connection unit connected to the second antenna has one terminal connected to the ground, the other terminal connected to the ground via the second switch, and the control unit configured to select the first antenna by the selection unit. Is selected, the first switch is turned off and the second switch is turned on. When the second antenna is selected by the selection unit, the first switch is turned on and the second switch is turned off. You may do.

  In the trigger transmitter described above, one terminal of the first connection portion is connected to the ground via the first capacitor, and one terminal of the first connection portion is connected to the ground via the second capacitor. It may be connected to the ground.

  In the trigger transmitter, the first capacitor operates as a part of the resonance circuit when the first antenna is selected by the selection unit, and the second capacitor is the second capacitor by the selection unit. When the antenna is selected, it may operate as a part of the resonance circuit.

  In the above trigger transmitter, the first capacitor removes the voltage of the second antenna leaking into the terminal to which the first capacitor is connected when the second antenna is selected by the selection unit. The second capacitor is a filter that removes the voltage of the second antenna leaking into the terminal to which the second capacitor is connected when the first antenna is selected by the selection unit. It may operate.

  In the above-described trigger transmitter, the first connection portion has the other terminal connected to the ground via the third capacitor and the first switch, and the second connection portion has the other terminal connected to the fourth terminal. When the second antenna is selected by the selection unit, the third capacitor leaks to a terminal for selecting the first antenna in the selection unit. The fourth capacitor acts as a terminal for selecting the second antenna in the selection unit when the selection unit selects the first antenna. It may operate as a filter that removes the leaked voltage of the first antenna.

  In the above trigger transmitter, the selection unit may be constituted by a power MOSFET switch.

  According to the present invention, it is possible to realize a trigger transmitter that can transmit a plurality of trigger signals having different characteristics without preventing trigger signal leakage and without causing malfunction on the system.

It is the block diagram which showed the internal structure of the 2 channel trigger transmitter which concerns on embodiment of this invention. It is the block diagram which showed the internal structure of the N bit resonant capacitor in the 2 channel trigger transmitter which concerns on embodiment of this invention. It is the block diagram which showed the internal structure of 3rd SW (switch) in the 2-channel trigger transmitter which concerns on embodiment of this invention. It is the figure which showed the relationship between the output from CPU and the output of each AND gate and each SW (switch) in embodiment of this invention. It is the block diagram which showed the internal structure of the conventional 1 channel trigger transmitter.

  The present invention embodies a two-channel transmitter by using a common circuit block and a new circuit without lowering the specifications required for the trigger transmitter. Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a block diagram showing an internal configuration of a 2-channel trigger transmitter according to an embodiment of the present invention, and FIG. 5 is a block diagram showing an internal configuration of a conventional 1-channel trigger transmitter. The two-channel trigger transmitter of the present embodiment is configured so that two external trigger coils can be connected based on a conventional one-channel trigger transmitter.

  As an obstacle to making the conventional one-channel trigger transmitter into two channels, there has been a trigger signal leak to the partner channel due to electrical interference between the channel 1 connector connected to the external trigger coil and the channel 2 connector. In some cases, a current of 20 Ap-p or more flows through the external trigger coil via the channel 1 connector and the channel 2 connector during normal operation. Depending on the inductance value of the external trigger coil, the current may be as high as 600 Vp-p or higher. Since a voltage is generated, leakage between channels occurs. Further, there is a market demand for miniaturization of equipment, and it is necessary to eliminate the trigger signal leak in a state where the connector for channel 1 and the connector for channel 2 are arranged at a distance of about several centimeters. This is the first problem.

  In addition, a switch circuit (for example, a power MOSFET switch) connected to an N-bit resonant capacitor switches between channel 1 output and channel 2 output, but also to an unselected channel on the other channel side via the impedance when the switch circuit is off. A voltage of about several volts is induced in the external trigger coil. Then, a malfunction in the system that a tag is activated by a trigger signal that is not assigned to the unselected channel occurs. This is the second problem.

  As a method for solving the first and second problems, in the present invention, one terminal of the connector for the external trigger coil is connected to the ground via a capacitor, and the other terminal is connected to the ground via a capacitor and a switch circuit. It is configured to connect with. The switch circuit is switched on when the connected connector is not selected by the switch circuit connected to the N-bit resonant capacitor (the other connector is selected). That is, when the voltage across the external trigger coil connected to the unselected connector is set to 0, the trigger signal leaks due to the short distance of the connector, or the switch circuit connected to the N-bit resonant capacitor is turned off. Prevents trigger signal leakage through impedance.

  Conventionally, as a means for the CPU to determine the optimum combination of the N-bit capacitor that resonates with the external trigger coil, the resonance voltage generated in the external trigger coil is determined by the peak hold circuit. An erroneous peak point may be determined while maintaining a transient, and the optimum combination of the external trigger coil and the N-bit capacitor cannot be determined. Further, since the resonance voltage increases in proportion to the inductance value of the external trigger coil, it is difficult to determine the strength of the magnetic field generated from the external trigger coil from the A / D value taken into the CPU. This is the third problem.

  As a solution to the third problem, in the present invention, a resonance peak point is detected by detecting a current flowing through an external trigger coil connected to the selected connector side or a resonance drive that outputs a trigger signal to the coil. By grasping (the maximum point of the resonance voltage), the optimum combination of the N-bit capacitors that resonate with the external trigger coil is determined.

  The configuration of the two-channel trigger transmitter of this embodiment will be described in further detail. The two-channel trigger transmitter 1 of this embodiment includes a power supply unit 11, an external I / F 12, a CPU 13, an N-bit resonant capacitor 14, a third SW 15, a first current detection unit 21, a first AND gate 22, a first resonant drive 23, The first SW 24, the capacitor a 25, the capacitor b 26, the second current detector 31, the second AND gate 32, the second resonance drive 33, the second SW 34, the capacitor c 35, and the capacitor d 36 are configured.

  The power supply unit 11 supplies a DC voltage to each unit of the 2-channel trigger transmitter 1.

  The external I / F 12 is an interface for performing communication with an external device such as a PC, and in the present embodiment, is configured using a serial communication standard RS-485. An external device such as a PC can perform various settings such as trigger transmission interval, trigger ID, and output adjustment by changing the duty of the trigger signal, and disconnection of the external trigger coil via the external I / F 12. Detection can be performed.

  The CPU 13 is a circuit that controls the overall operation of the two-channel trigger transmitter 1. The CPU 13 outputs a trigger signal and a control signal toward each resonance drive and each SW, inputs a detection result from each current detection unit, and sets an N-bit set value as a control signal based on the detection result. Output to the resonant capacitor 14. Further, the CPU 13 generates a trigger signal for each external trigger coil connected to each connection unit based on the trigger ID setting from an external device such as a PC.

  The N-bit resonant capacitor 14 is a circuit that has N capacitors and selects a predetermined number of capacitors to flow current based on a control signal (N-bit set value) output from the CPU 13. The N-bit resonant capacitor 14 has a large number of parts depending on the number of bits (usually 8 bits), and if the current flowing through the external trigger coil is set to 10 Ap-p or more, it is necessary to attach a radiator to the resonance capacitor switching switch (power MOSFET). Therefore, a common circuit is used for cost reduction and size reduction. A specific configuration is shown in FIG. Although FIG. 2 has been described as an 8-bit resonant capacitor, it is not limited to this and may be 6 bits or 10 bits.

  As shown in FIG. 2, the N-bit resonant capacitor 14 is configured by connecting eight sets of capacitors and switching elements in parallel from N1 capacitors 1411 and N1SW1421 to N8 capacitors 1418 and N8SW1428. Each switching element is connected to the ground and also connected to a capacitor. The switching element is turned on / off based on the N-bit set value from the CPU 13, and a current flows through the capacitor when switched on. A capacitor connected to the switching element switched to ON and an external trigger coil connected to the connecting portion mainly constitute a resonance circuit (more precisely, as will be described later, the capacitor a25 and the capacitor c35 also resonate). Included in the circuit).

  In this embodiment, a power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is used as a switching element, but the present invention is not limited to this, and an IGBT (Insulated Gate Bipolar Transistor) may be used. The same applies to the other switching elements (first SW, second SW, SWa151 (FIG. 3), SWb152 (FIG. 3)).

  The third SW 15 is a circuit that selects one of the channels (the connection unit and the external trigger coil connected thereto) based on the control signal output from the CPU 13 to flow current. A specific configuration is shown in FIG.

  As shown in FIG. 3, the third SW 15 includes a switching element SWa 151 that performs switching for selecting the channel 1 (the first connection unit 27 and the external trigger coil connected thereto) and the channel 2 (the second connection). SWb152 which is a switching element for performing switching for selecting the unit 37 and an external trigger coil connected thereto.

  The first current detection unit 21 is a circuit that detects a current flowing through an external trigger coil connected to the first connection unit 27, a capacitor selected by the N-bit resonant capacitor 14, and the like, and a voltage value obtained by current detection. To the port E of the CPU 13. The second current detection unit 31 is a circuit that detects a current flowing through an external trigger coil connected to the second connection unit 37, a capacitor selected by the N-bit resonant capacitor 14, and the like, and a voltage value obtained by current detection. Is output to the port F of the CPU 13.

  The first AND gate 22 is a circuit that outputs a trigger signal to the first resonance drive 23 based on the trigger signal and control signal output from the CPU 13. The second AND gate 32 is a circuit that outputs a trigger signal to the second resonance drive 33 based on the trigger signal and control signal output from the CPU 13. Both gates are AND gates, and when a control signal of H is input from two ports, a trigger signal is output to each resonance drive.

  The first resonance drive 23 receives a trigger signal generated by the CPU 13 (assuming transmission from an external trigger coil connected to the first connection unit 27) from the first AND gate 22, and receives the first connection unit. To 27. The second resonance drive 33 receives the trigger signal generated by the CPU 13 (assuming transmission from an external trigger coil connected to the second connection portion 37) from the second AND gate 32, and receives the second connection portion. Output to 37.

  The first SW 24 is a switching element that switches the connection between the capacitor b26 and the ground. The first SW 24 is turned on / off based on the control signal from the port B of the CPU 13, and when switched on, the capacitor b26 is connected to the ground, and the current leaked when the SWa 151 of the third SW 15 is switched off. Run to ground. The second SW 34 is a switching element that switches connection between the capacitor d36 and the ground. The second SW 34 is turned on / off based on a control signal from the port C of the CPU 13, and when switched on, the capacitor d36 is connected to the ground, and the current leaked when the SWb 152 of the third SW 15 is switched off. Run to ground.

  As described in the problem, when two external trigger coils are mounted on one trigger transmitter, the leakage of the trigger signal to the counterpart coil becomes a problem. In addition, when a voltage significantly higher than usual is applied, the problem of signal leakage becomes more prominent. In addition, some users may request more accurate signal transmission (complete system operation without causing malfunction). In view of these circumstances, in the present invention, the voltage across the coil on the side not performing signal transmission is configured to be 0, and the first SW 24 is independent of grounding in each circuit (module). The second SW 34 is connected to the ground. The ground connection in the module and the ground connection between the first SW 24 and the second SW 34 have different meanings and purposes, and in the latter case, a ground connection that is not usually required is intentionally provided to prevent signal leakage.

  Although not shown in FIG. 1, one inverter is provided between the port B and the first SW 24 of the CPU 13 and between the port C and the second SW 34, and the control signal from each port is inverted. Input to each SW. Further, two inverters are provided between the port B and the third SW 15 of the CPU 13 and between the port C and the third SW 15, and the control signal from each port is input to each SW as it is through two inversions.

  The capacitor a25 has one terminal connected to a terminal different from the terminal to which the capacitor b26 is connected in the first connection portion 27, and the other end connected to the ground. The capacitor c35 has one terminal connected to a terminal different from the terminal to which the capacitor d36 is connected in the second connection portion 37, and the other end connected to the ground.

  The first connecting portion 27 is a connector for connecting one external trigger coil, and is a channel 1 connector. The second connection portion 37 is a connector for connecting the other external trigger coil, and is a channel 2 connector.

  Next, the operation of the two-channel trigger transmitter of the present invention will be described. First, after the power is turned on, a DC voltage is supplied to each unit by the power supply unit 11, and the CPU 13 generates a trigger signal based on a trigger ID setting by an external device (PC or the like).

  Subsequently, the CPU 13 outputs a trigger signal from the port A, a control signal from the port B to H, and a control signal from the port C to L. The signals are the first AND gate 22, the second AND gate 32, the first SW 24, and the second SW 34, respectively. , Input to the third SW 15. The trigger signal output from the port A is input to the first AND gate 22 and the second AND gate 32. The H control signal output from the port B is input to the first AND gate 22 in the H state, inverted to L via one inverter, input to the first SW 24, and then passed through the two inverters. In the H state, the signal is input to the SWa 151 in the third SW 15. The L control signal output from the port C is input to the second AND gate 32 in the L state, is inverted to H via one inverter, is input to the second SW 34, and passes through the two inverters. In the H state, the signal is input to the SWb 152 in the third SW 15.

  The first AND gate 22 opens the gate and outputs the trigger signal to the first resonance drive 23 in order to input the trigger signal from the port A and the H control signal from the port B. Since the second AND gate 32 receives the trigger signal from the port A and the L control signal from the port C, the second AND gate 32 does not open the gate and does not output the trigger signal to the second resonance drive 33. Since the first SW 24 receives the L control signal inverted by one inverter, the first SW 24 is switched OFF to disconnect the capacitor b26 from the ground. The second SW 34 is turned on to connect the capacitor d36 and the ground in order to input the H control signal inverted by one inverter. The SWa 151 in the third SW 15 inputs the control signal in the H state via the two inverters, and is switched to ON to connect the external trigger coil connected to the first connection unit 27 and the N-bit resonant capacitor 14. . The SWb 152 in the third SW 15 inputs the control signal in the L state via the two inverters, so it is switched off and the external trigger coil connected to the second connection unit 37 and the N-bit resonant capacitor 14 are disconnected. And FIG. 4 shows the relationship between the output from the CPU 13 and the output of each AND gate and each SW.

  Further, the CPU 13 outputs a control signal (N-bit set value) from the port N to the N-bit resonant capacitor 14 at a predetermined interval (for example, 4 ms), and the external trigger coil of the channel 1 selected by the third SW 15 and the N-bit. A scan for N bits for automatically tuning the resonant capacitor 14 is performed. In the automatic tuning, the CPU 13 inputs the output result (voltage value) from the first current detection unit 21 connected to the first resonance drive 23 from the port E, and the set value of N bits at which the input voltage value becomes the maximum. It is done by memorizing.

  Next, the CPU 13 outputs a trigger signal from the port A, a control signal of L from the port B, and a control signal of H from the port C, and automatically tunes the external trigger coil of the channel 2 and the N-bit resonant capacitor 14 in the same manner as described above. The N-bit set value for which the resonance voltage is maximized is stored by performing a scan for N bits.

  In this embodiment, the CPU makes a determination by performing current detection on the optimal combination of the N-bit resonant capacitor that resonates with the external trigger coil, but by configuring in this way, the resonance point can be accurately tuned. Can now. Further, since the current flowing through the external trigger coil can be measured, it is possible to grasp the strength of the magnetic field generated from the coil. Furthermore, since the current flowing through the resonant drive circuit is also added, it can be applied to detection of abnormal operation of the entire resonant circuit.

  Trigger signals are alternately supplied to channels 1 (first connection) and channel 2 (second connection) at an arbitrary transmission interval (for example, 15 to 100 ms) set by the CPU 13 and triggered via an external trigger coil. A signal is transmitted to activate the wireless IC tag. When supplying the trigger signal to the channel 1, the CPU 13 outputs the N-bit set value obtained by the above automatic tuning from the port N to the N-bit resonant capacitor 14, and then the trigger signal from the ports A, B, and C. A control signal is output to drive the trigger signal to channel 1. When supplying the trigger signal to the channel 2, the CPU 13 first outputs the N-bit set value from the port N, and then outputs the trigger signal and the control signal from the ports A, B, and C as described above. Supply trigger signal to channel 1.

  As shown in FIG. 4, the first SW 24 and the second SW 34 are switched ON when the other channel is selected, and connect the capacitor and the ground. That is, when channel 1 is selected, the first SW 24 is OFF and the second SW 34 is ON, and when channel 2 is selected, the first SW 24 is ON and the second SW 34 is OFF.

  For example, when channel 1 (the first connecting portion 27 and the external trigger coil connected thereto) is selected, the second SW 34 is turned on, the capacitor d36 is connected to the ground, and the capacitor c35 is also connected to the ground. The voltage across the external trigger coil in channel 2 is zero. Further, in this case, the capacitor c35 functions as a filter for removing a minute voltage leaked to the Y point (FIG. 1) at the time of output to the channel 1, and the capacitor d36 at the Q point (FIG. 4) at the time of output to the channel 1. It functions as a filter that removes the minute voltage that leaks into ().

  When channel 2 (second connecting portion 37 and external trigger coil connected thereto) is selected, the voltage at both ends of the external trigger coil in channel 1 is 0, and in this case, capacitor a25 is connected to channel a. 2 functions as a filter that removes the minute voltage leaked to the W point (FIG. 1) when output to the channel 2, and the capacitor b26 removes the minute voltage leaked to the Q point (FIG. 4) when output to the channel 2. Functions as a filter.

  Since the external trigger coils in channel 1 and channel 2 are installed in independent areas and have different trigger IDs, the isolation between channel 1 and channel 2 is extremely important in operating the system. In the present embodiment, by configuring as described above, there is no operation at both ends (W point, X point) of the external trigger coil in channel 1 and at both ends (Y point, Z point) of the external trigger coil in channel 2. When the trigger signal is not driven, the channel on the operation (trigger signal drive) side is prevented from being generated even if it is very small.

  In addition, capacitors (capacitors a25 and c35) different from the capacitors connected to the SW at each connection portion function as a part of the resonance circuit when the channel on the side to which the capacitors are connected is selected. That is, the resonant circuit is configured with the N-bit resonant capacitor 14 and the external trigger coil connected to the connection portion.

  In the present invention, the circuit block used in the conventional one-channel trigger transmitter is shared in order to realize two channels. However, in addition to the embodiment described above, the current detection unit is configured to be shared. May be. By sharing the circuit blocks as much as possible, it is possible to reduce costs and reduce the size of the equipment.

  The above-described embodiment is a preferred embodiment of the present invention, and the scope of the present invention is not limited to the above-described embodiment alone, and various modifications are made without departing from the gist of the present invention. Implementation is possible.

1 2ch trigger transmitter 5 1ch trigger transmitter 11, 51 Power supply unit 12, 52 External I / F
13,53 CPU
14, 54 N-bit resonant capacitor 15 3rd SW (switch)
21 1st current detection part 22 1st AND gate 23 1st resonance drive 24 1st SW (switch)
25 Capacitor a
26 Capacitor b
27 1st connection part 31 2nd electric current detection part 32 2nd AND gate 33 2nd resonance drive 34 2nd SW (switch)
35 capacitor c
36 capacitor d
37 Second connection portion 55 Resonant drive 56 Connection portion 151 SW (switch) a
152 SW (Switch) b
1411 to 1418 N1 capacitor to N8 capacitor 1421 to 1428 N1SW (switch) to N8SW (switch)

Claims (9)

A signal transmitter for transmitting a trigger signal for activating the wireless IC tag to the wireless IC tag via an antenna,
A plurality of connection portions for connecting the plurality of antennas in a one-to-one correspondence;
A selection unit that selects one of a plurality of antennas connected to the plurality of connection units;
Have
The voltage at both ends of the antenna other than the antenna selected by the selection unit after the power is turned on so that a plurality of trigger signals having different characteristics can be transmitted through the plurality of antennas connected to the plurality of connection units. The signal transmitter according to claim 4, wherein the signal transmitter is configured to be zero. A current detection unit that detects a current flowing through the antenna selected by the selection unit and outputs it as a voltage value;
A control unit for determining a peak point of a resonance voltage generated in the antenna selected by the selection unit based on the voltage value output by the current detection unit;
The signal transmitter according to claim 1, comprising: A circuit in which a plurality of capacitors are turned on / off according to a set value to form a plurality of types of resonant capacitors, and has a resonant capacitor circuit connected to the control unit and the selection unit,
The current detection unit detects a voltage value of a current flowing through the antenna and the resonant capacitor circuit selected by the selection unit;
The control unit outputs a set value to the resonant capacitor circuit at a predetermined interval, acquires a voltage value for each set value from the current detection unit, and corresponds to a maximum voltage value among the acquired voltage values. The signal transmitter according to claim 2, wherein the set value is stored as an optimum set value of a resonance capacitor with the selected antenna. The first connection portion connected to the first antenna has one terminal connected to the ground, the other terminal connected to the ground via the first switch,
The second connection portion connected to the second antenna has one terminal connected to the ground, the other terminal connected to the ground via the second switch,
The control unit turns off the first switch and turns on the second switch when the selection unit selects the first antenna, and the selection unit selects the second antenna. The signal transmitter according to any one of claims 1 to 3, wherein the first switch is turned on and the second switch is turned off. In the first connection portion, the one terminal is connected to the ground via a first capacitor,
5. The signal transmitter according to claim 4, wherein the one second terminal of the second connection portion is connected to the ground via a second capacitor.   The first capacitor operates as a part of a resonance circuit when the selection unit selects the first antenna, and the second capacitor has the second antenna selected by the selection unit The signal transmitter according to claim 5, wherein the signal transmitter operates as a part of the resonance circuit.   The first capacitor operates as a filter that removes the voltage of the second antenna that has leaked into a terminal to which the first capacitor is connected when the second antenna is selected by the selection unit. The second capacitor operates as a filter that removes the voltage of the second antenna that has leaked into a terminal to which the second capacitor is connected when the selection unit selects the first antenna. The signal transmitter according to claim 5 or 6, characterized in that: In the first connection portion, the other terminal is connected to the ground via a third capacitor and the first switch,
In the second connection portion, the other terminal is connected to the ground via a fourth capacitor and the second switch,
The third capacitor removes the voltage of the second antenna leaking into a terminal for selecting the first antenna in the selection unit when the second antenna is selected by the selection unit. The fourth capacitor leaks into a terminal for selecting the second antenna in the selection unit when the first antenna is selected by the selection unit. The signal transmitter according to claim 4, wherein the signal transmitter operates as a filter that removes the voltage of the antenna.   The signal transmitter according to any one of claims 1 to 8, wherein the selection unit includes a power MOSFET switch.

Images