JP5761251B2 - Antenna device and communication device - Google Patents

Description

  The present invention relates to an antenna device that changes a resonance frequency of an antenna circuit using a variable capacitance element, and a communication device including the antenna device.

  In recent years, electronic devices such as mobile phones have a short-range wireless communication function such as NFC (Near Field Communication). Such an electronic device generally operates in a mode in which data is passively transmitted / received in response to a request from a reader / writer device of a communication partner (hereinafter referred to as a card mode), and operates as a reader / writer device. There is a mode in which data is actively transmitted to a communication partner (hereinafter referred to as a reader / writer mode).

  In the case of having these two modes, it is common to realize both modes with one antenna coil in order to reduce the size of the electronic device. In this case, the control IC to which the antenna coil is connected performs control to switch the internal impedance of the transmission terminal (Tx terminal) depending on the mode. For example, the control IC opens the Tx terminal in the card mode, and shorts the Tx terminal in the reader / writer mode. For this reason, the circuit impedance connected to the antenna coil differs between the card mode and the reader / writer mode, and the same resonance frequency cannot be obtained in each mode.

  Patent Document 1 discloses an invention in which a variable capacitance element is connected to an antenna coil, a capacitance value of the variable capacitance element is changed by a controller, and a resonance frequency of the antenna coil is adjusted. According to Patent Document 1, the same resonance frequency can be set even in the different modes described above.

Special table 2009-543442

  However, in Patent Document 1, a controller for changing the capacitance value of the variable capacitance element is required, and it is necessary to incorporate a control program into the controller. For this reason, in Patent Document 1, labor and cost are required for mounting the controller.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an antenna device and a communication device including the antenna device that can prevent the deterioration of communication characteristics without requiring labor and cost.

  The antenna device according to the present invention includes an input / output terminal for a signal, and an input mode in which a signal is input from the input / output terminal and an output mode in which a signal is output from the input / output terminal An impedance switching circuit that switches internal impedance; an antenna circuit connected to the input / output terminal; a variable capacitance element that changes a capacitance when a control voltage is applied and changes a resonance frequency of the antenna circuit; and the input / output terminal And a matching circuit for impedance matching between the input / output terminal and the antenna circuit, and a control voltage corresponding to a voltage generated at the input / output terminal is applied to the variable capacitance element. A control voltage application circuit, and when the control voltage application circuit outputs a signal from the input / output terminal, the resonance frequency of the antenna circuit Order to the resonant frequency corresponding to the output mode, and outputs a control voltage applied to the variable capacitance element.

  In the output mode, the resonant frequency of the antenna circuit may change due to the addition of the matching circuit. However, with the above configuration, the resonant frequency of the antenna circuit is set to the resonant frequency corresponding to the output mode. However, efficient communication can be performed. The control voltage applied to the variable capacitance element for adjusting the resonance frequency of the antenna circuit is generated from the voltage generated at the input / output terminal of the impedance switching circuit. For this reason, a controller for applying the control voltage is not required, and the time for circuit formation can be saved.

  The control voltage application circuit is preferably a rectifier circuit that rectifies a voltage generated at the input / output terminal.

  In this configuration, the capacitance value of the variable capacitance element can be controlled by a simple circuit.

  According to the present invention, efficient communication can be performed in any of the input mode and the output mode without requiring time and cost of mounting a controller for controlling the capacitance change of the variable capacitance element.

Circuit diagram of antenna apparatus according to embodiment 1 Circuit diagram of variable capacitance element Diagram showing frequency characteristics of magnetic field strength in reader / writer mode and card mode The figure which shows the structure inside the housing | casing of the radio | wireless communication apparatus to which the antenna apparatus which concerns on Embodiment 1 is applied. Circuit diagram of antenna apparatus according to embodiment 2 Circuit diagram of antenna apparatus according to embodiment 3

(Embodiment 1)
FIG. 1 is a circuit diagram of an antenna device according to the first embodiment. The antenna device 1 described below is an example of an NFC module, and operates in two modes: a card mode (input mode according to the present invention) and a reader / writer mode (output mode according to the present invention). The card mode is a mode in which the antenna device 1 operates passively, receives data from the reader / writer device of the communication partner, and receives data from the communication partner and transmits data. The reader / writer mode is a mode in which the antenna device 1 performs an active operation, becomes itself a reader / writer device, transmits data to the communication partner, and receives data from the communication partner.

  The antenna device 1 includes an RFIC 10. The RFIC 10 is an integrated circuit that processes radio frequency (RF) signals. The RFIC 10 has Tx1 and Tx2 terminals that are transmission terminals, and an antenna circuit 12 is connected to the Tx1 and Tx2 terminals via a matching circuit 11. The RFIC 10 corresponds to the impedance switching circuit according to the present invention, and the Tx1 and Tx2 terminals correspond to the input / output terminals according to the present invention.

  The antenna circuit 12 is a resonance circuit including a coil antenna 12A, a capacitor C21, and a variable capacitance element C22. The variable capacitance element C22 is a capacitance element whose capacitance value is determined according to a control voltage (bias voltage), and sets the resonance frequency of the antenna circuit 12 to a predetermined frequency. The antenna coil 12A performs electromagnetic field coupling with a communication partner antenna to perform transmission and reception for near field communication. For example, when the HF band having a center frequency of 13.56 MHz is used in NFC communication, the resonance frequency of the antenna circuit 12 is set to 13.56 MHz.

  Note that when the frequency adjustment is possible with the variable capacitance element C22 and the antenna coil 12A, the antenna circuit 12 may not include the capacitor C21.

  FIG. 2 is a circuit diagram of the variable capacitor C22. The variable capacitance element C22 has ports P11 and P12 through which RF signals are input / output, a port 13 to which a control voltage is applied, and a port P14 connected to the ground. The variable capacitance element C22 includes capacitance elements C2a to C2f and resistance elements R11 to R19 connected to the ports P11 to P12.

  The capacitive elements C2a to C2f are ferroelectric capacitors in which a ferroelectric film is sandwiched between opposing electrodes. Since the amount of polarization of the ferroelectric film changes according to the strength of the applied electric field and the apparent dielectric constant changes, the capacitance value can be determined by the control voltage.

  The resistance values of the resistance elements R11 to R19 are equal. These resistance elements R11 to R19 apply a control voltage to the capacitive elements C2a to C2f, and suppress leakage of an RF signal applied between the ports P11 and P12 to the ports P13 and P14.

  The matching circuit 11 includes inductors L11 and L12 and capacitors C11 and C12 constituting an EMC filter connected to the Tx1 and Tx2 terminals, respectively. The matching circuit 11 is connected to an EMC filter, and includes capacitors C13 and C14 as elements for impedance matching between the RFIC 10 and the antenna circuit 12. The EMC filter of the matching circuit 11 removes harmonics of the signal transmitted from the antenna circuit 12 when the antenna device 1 is in the reader / writer mode. The cutoff frequency of the EMC filter is set, for example, in the vicinity of 16 MHz when the resonance frequency of the antenna circuit is set to 13.56 MHz.

  A rectifier circuit 13 is connected to the Tx1 terminal of the RFIC 10. The rectifier circuit 13 corresponds to a control voltage application circuit according to the present invention. The rectifier circuit 13 includes a diode D1, a capacitor C3, and a resistor R1. The rectifier circuit 13 rectifies the voltage generated at the Tx1 terminal of the RFIC 10 and applies it to the port P13 of the variable capacitance element C22 as a control voltage.

  When the RFIC 10 is in the reader / writer mode, the rectifier circuit 13 that outputs the control voltage and the variable capacitor C22 to which the control voltage is applied are arranged so that the resonance frequency of the antenna circuit 12 is 13.56 MHz. The capacitance value of C22 is changed. Therefore, a separate controller for applying the control voltage and changing the capacitance value of the variable capacitance element C22 is not required. Further, since the RFIC 10 has a control voltage application control function, it is not necessary to incorporate a control program in the RFIC 10. As a result, it is possible to change the capacitance value by applying the control voltage to the variable capacitance element C22 without labor and cost.

  The RFIC 10 has an Rx terminal that is a receiving terminal, and the antenna circuit 12 is connected to the Rx terminal via a series circuit of a resistor R2 and a capacitor C4. The RFIC 10 reads data information received by the antenna circuit 12 from high and low voltages input to the Rx terminal.

  Further, the RFIC 10 has a Tb terminal that outputs a DC bias, and a capacitor C5 and a resistor R3 are connected to the Tb terminal. When the Tb terminal outputs a DC bias, the DC bias is superimposed on the voltage input to the Rx terminal, so that the signal processing in the RFIC 10 that reads the data information received by the antenna circuit 12 is facilitated. The resistor R3 is connected to the capacitor C4, so that the capacitor C4 cuts the DC bias voltage and prevents the antenna circuit 12 from being affected by the DC bias.

  In the antenna device 1 configured as described above, the RFIC 10 performs a switching process for switching the internal impedance between the Tx1 and Tx2 terminals by opening or shorting between the Tx1 and Tx2 terminals in the reader / writer mode and the card mode. Do. Specifically, when the antenna device 1 operates in the reader / writer mode, the RFIC 10 shorts between the Tx1 and Tx2 terminals and outputs high-frequency power from each of the Tx1 and Tx2 terminals. On the other hand, when the antenna device 1 operates in the card mode, the RFIC 10 opens the terminals between the Tx1 and Tx2 terminals, in other words, performs load modulation by connecting a load between the Tx1 and Tx2 terminals.

  FIG. 3 is a diagram showing the frequency characteristics of the magnetic field strength in the reader / writer mode and the card mode.

  The characteristic indicated by the one-dot chain line in FIG. 3 indicates the frequency characteristic of the magnetic field strength in the card mode. As described above, the matching circuit 11 has an EMC filter. This EMC filter is set to remove 16 MHz harmonics. Therefore, in the card mode, the resonance frequency of the antenna device 1 is the resonance frequency f1 (13.56 MHz) of the antenna circuit 12 and the cutoff frequency f2 (16 MHz) of the EMC filter.

  The characteristic indicated by the solid line in FIG. 3 indicates the frequency characteristic of the magnetic field strength in the reader / writer mode. 3 is a frequency characteristic of the magnetic field strength in the reader / writer mode when the rectifier circuit 13 is not provided. The EMC filter of the matching circuit 11 is set to remove harmonics. For this reason, in the reader / writer mode, the EMC filter affects the resonance frequency f1 of the antenna circuit 12. As a result, in the reader / writer mode, as shown by the broken line in FIG. 3, the resonance frequency of the antenna device 1 is f0 (14.7 MHz), which deviates from the communication signal frequency band centered on 13.56 MHz in the reader / writer mode. As a result, communication characteristics deteriorate. In the present invention, the resonance frequency of the antenna device 1 is adjusted to f1 (13.56 MHz) by providing the rectifier circuit 13 and changing the capacitance value of the variable capacitance element C22, as shown by the solid line in FIG. (shift.

  As described above, since the resonance frequency of the antenna device 1 is the resonance frequency f1 (13.56 MHz) of the antenna circuit 12 in both the reader / writer mode and the card mode, communication can be performed efficiently. Further, since the control voltage applied to the variable capacitance element C22 for adjusting the resonance frequency is obtained by rectifying the voltage generated at the Tx1 terminal of the RFIC 10, there is no need to provide a controller for applying the control voltage.

  In this embodiment, the resonance frequency of the antenna device 1 in the reader / writer mode is made to coincide with the resonance frequency f1 (13.56 MHz) of the antenna circuit 12. It suffices if the resonance frequency of the antenna device 1 is close to f1 (13.56 MHz) as long as the influence is not exerted.

  When a communication partner approaches the antenna device 1 in the card mode, a voltage is input from the antenna circuit 12 to the rectifier circuit 13 through a path passing through the capacitor C13 and the inductor L11. The capacitance value may change. In this case, although the resonance frequency of the antenna circuit 12 deviates from 13.56 MHz, since the power received by the antenna device 1 from the communication partner is large, power sufficient to read data is input to the RFIC 10.

  FIG. 4 is a diagram showing the internal structure of a wireless communication device to which the antenna device 1 according to this embodiment is applied. FIG. 4 is a plan view of the wireless communication apparatus 100 with the upper housing 91 and the lower housing 92 separated and the inside exposed.

  Circuit boards 71 and 81, a battery pack 83, and the like are housed inside the upper housing 91. On the circuit board 71, the RFIC 10, the matching circuit 11, the rectifier circuit 13, and the like are mounted. A UHF band antenna 72, a camera module 76, and the like are also mounted on the circuit board 71. In addition, a UHF band antenna 82 and the like are mounted on the circuit board 81. The circuit board 71 and the circuit board 81 are connected via a coaxial cable 84.

  In the lower housing 92, an opening 92A through which the lens of the camera module 76 is optically exposed is formed. Furthermore, the lower casing 92 is provided with an antenna coil 12A, which is a planar conductor, so as to surround the opening 92A. An RFIC 10 or the like is connected to the antenna coil 12A. The antenna device 12 according to this embodiment is configured by the antenna coil 12A, the RFIC 10, and the like.

(Embodiment 2)
FIG. 5 is a circuit diagram of the antenna device according to the second embodiment. In this example, the antenna device 2 includes an RFIC 10, a matching circuit 11, an antenna circuit 12, and the like, as in the first embodiment. The antenna device 2 includes a rectifier circuit 15 having diodes D1 and D2, a capacitor C3, and a resistor R1.

  The rectifier circuit 15 rectifies voltages generated at the Tx1 terminal and the Tx2 terminal of the RFIC 10 and applies them to the variable capacitor C22 as a control voltage. The variable capacitor C22 has the same configuration as that of the first embodiment. That is, the rectifier circuit 15 performs full-wave rectification on the AC voltage output from the RFIC 10. As a result, a control voltage with reduced ripple is applied to the variable capacitance element C22, and stable capacitance control of the variable capacitance element C22 can be performed.

(Embodiment 3)
FIG. 6 is a circuit diagram of the antenna device according to the third embodiment. In this example, the antenna circuit 17 is connected to the Tx terminal of the RFIC 10 included in the antenna device 3 via the matching circuit 16.

  The antenna circuit 17 is a resonance circuit in which a coil antenna 12A, one end of which is connected to the ground, a capacitor C21, and a variable capacitance element C22 are connected in parallel. About each element, since it is the same as that of Embodiment 1, description is abbreviate | omitted.

  The matching circuit 16 includes an inductor L11 and a capacitor C11 that form an EMC filter and are connected to the Tx terminal. The matching circuit 16 is connected to an EMC filter, and includes a capacitor C13 as an element for impedance matching between the RFIC 10 and the antenna circuit 17. Thus, the present invention can be similarly applied to a non-balanced circuit.

1,2,3-antenna device 10-RFIC (impedance switching circuit)
11, 16-matching circuit 12, 17-antenna circuit 12A-antenna coil 13, 15-rectifier circuit (voltage application circuit)
71, 81-Circuit board 76-Camera module 82-UHF band antenna 83-Battery pack 84-Coaxial cable 91-Upper casing 92-Lower casing 92A-Opening 100-Wireless communication devices C3, C4, C5, C11, C12 , C13, C14, C21-capacitor C22-variable capacitance elements L11, L12-inductors D1, D2-diodes R1, R2, R3-resistance

Claims (3)

An impedance switching circuit having an input / output terminal for a signal, and switching an internal impedance of the input / output terminal between an input mode in which the signal is input from the input / output terminal and an output mode in which the signal is output from the input / output terminal; ,
An antenna circuit connected to the input / output terminal;
A variable capacitance element that changes a capacitance when a control voltage is applied, and changes a resonance frequency of the antenna circuit;
A matching circuit that is connected between the input / output terminal and the antenna circuit and performs impedance matching between the input / output terminal and the antenna circuit;
A control voltage application circuit for applying a control voltage corresponding to the voltage generated at the input / output terminal to the variable capacitance element;
With
The control voltage application circuit includes:
When a signal is output from the input / output terminal, a control voltage to be applied to the variable capacitance element is output so that the resonance frequency of the antenna circuit is a resonance frequency corresponding to the output mode.
Antenna device.   The antenna device according to claim 1, wherein the control voltage application circuit is a rectifier circuit that rectifies a voltage generated at the input / output terminal.   A communication device comprising the antenna device according to claim 1.

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