JP2009044561A - Antenna matching circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve Q value of an antenna matching circuit and to extend a frequency variable range. <P>SOLUTION: The antenna matching circuit 10 is provided between an input terminal 11 of an RF circuit which is a feed end of an antenna element and a circuit side end part 12 of the antenna element (radiation conductor). The antenna matching circuit 10 is constituted by connecting two chip inductors 13, 14 in series between the input terminal 11 of the RF circuit and the circuit side end part 12 of the antenna element. A first varactor diode 15 is connected between the two chip inductors 13, 14 and a second varactor diode 16 is connected between a chip inductor 14 at an antenna element side and the circuit side end part 12 of the antenna element. The circuit is constituted so that a tuning voltage Vt for controlling a tuning band is applied to each cathode of first and second varactor diodes 15, 16. An inductance element 17 is connected between one end part of the RF circuit of the first chip inductor 13 and a ground thereof. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an antenna matching circuit applicable to a tunable antenna for receiving digital television broadcasts.

  There has been proposed a tunable antenna that can receive a broadband radio wave such as a digital television broadcast signal by a mobile device such as a mobile phone (for example, see Patent Document 1). In the antenna matching circuit provided for the tunable antenna, the variable capacitance element and the inductance element are combined to make the frequency variable in the frequency band (470 MHz to 770 MHz) of the digital television broadcast signal and match with the subsequent circuit to reduce the signal loss. Suppressed.

  FIG. 7 is a circuit configuration diagram of the antenna matching circuit described in Patent Document 1. In FIG. An antenna matching circuit 3 is provided between the antenna element 1 and the transmission line 2. The antenna matching circuit 3 includes a first variable capacitance element 4 connected in series with the antenna element 1, a second variable capacitance element 5 connected in parallel with the antenna element 1, and a first variable connected in series with the antenna element 1. It has an inductor 6 and a second inductor 7 connected in parallel with the antenna element 1.

In the antenna matching circuit configured as described above, the first variable capacitance element 4 makes the Q value of the antenna matching circuit 3 substantially constant regardless of the frequency, and the variable range of the resonance frequency by the second variable capacitance element 5 is increased. Expand. The second variable capacitance element 5 varies the resonance frequency of the antenna matching circuit 3. The first inductor 6 cancels the capacitive reactance and matches the load resistance of the transmission line 2 by a tap-down effect with the second inductor 7.
JP 2007-159083 A

  By the way, the antenna characteristics of the tunable antenna are affected by the antenna gain and the variable frequency range. However, since the circuit loss greatly affects the antenna gain, it is desired to improve the Q value of the antenna matching circuit. Further, regarding the frequency variable range, it is desired to extend the variable range in the antenna matching circuit without changing the capacitance variable range and the inductance value.

  The present invention has been made in view of this point, and an object of the present invention is to provide an antenna matching circuit that can improve the Q value of the circuit and expand the frequency variable range.

  An antenna matching circuit of the present invention is an antenna matching circuit having a series circuit that is provided between an antenna element and a feeding point to the antenna element and in which a variable capacitance element and a first inductance element are connected in series. The first inductance element is composed of a plurality of chip inductors.

  According to this configuration, the first inductance element that forms the series circuit with the variable capacitance element is configured by a plurality of chip inductors, so that each self-resonant frequency can be kept away from the operating frequency, thereby reducing circuit loss. In addition, the tuning band can be extended to the high band side.

  In the antenna matching circuit, a second inductance element is connected between the feeding point and the ground. As a result, it is possible to adjust the matching with the subsequent circuit by adjusting the inductance value of the second inductance element.

  According to the present invention, the Q value of the antenna matching circuit can be improved, the frequency variable range can be expanded, and the antenna characteristics of the tunable antenna can be improved.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of an antenna matching circuit according to an embodiment of the present invention. The antenna matching circuit 10 of the present embodiment is provided between an input terminal 11 of an RF circuit serving as a feeding end of the antenna element and a circuit side end portion 12 of the antenna element (radiation conductor). In the antenna matching circuit 10, two chip inductors 13 and 14 constituting a first inductance element are connected in series between an input terminal 11 of the RF circuit and a circuit side end 12 of the antenna element. A first varactor diode 15 as a variable capacitance element is connected between the two chip inductors 13 and 14, and a first variable capacitance element is formed between the chip inductor 14 on the antenna element side and the circuit side end 12 of the antenna element. Two varactor diodes 16 are connected. A tuning voltage Vt for controlling the tuning band is applied to each cathode of the first and second varactor diodes 15 and 16 via a resistor R1. An inductance element 17 as a second inductance element is connected between one end of the RF circuit of the chip inductor 13 and the ground. A resistor 18 is connected between the circuit element side end 12 of the antenna element and the ground, and a DC cut capacitor 19 is provided between the input terminal 11 of the RF circuit and one end of the chip inductor 13. .

  FIG. 2 is a schematic diagram of a tunable antenna provided with the antenna matching circuit 10. The tunable antenna 20 includes an antenna unit 21 and a circuit unit 22. The antenna unit 21 forms an antenna element by winding a radiating conductor 24 around the outer periphery of a base 23 made of a dielectric or magnetic material. The circuit unit 22 is provided on one surface of the base 23 and includes the antenna matching circuit 10.

  In the antenna matching circuit 10 configured as described above, the resonance is determined by the inductance values of the plurality of chip inductors 13 and 14 constituting the first inductance element and the variable capacitances of the first and second varactor diodes 15 and 16. A band pass filter having a frequency as a center frequency is configured to pass a desired frequency. The resonance frequency is set to a desired frequency by controlling the tuning voltage Vt applied to the cathodes of the first and second varactor diodes 15 and 16 to control the capacitance of the first and second varactor diodes 15 and 16. be able to. Further, matching adjustment with the receiving circuit of the subsequent stage is performed by the plurality of chip inductors 13 and 14 and the inductance element 17 connected in parallel, and the signal is propagated to the receiving circuit with a small circuit loss.

  Further, in the present embodiment, the first inductance element connected in series between the input terminal 11 of the RF circuit and the circuit side end 12 of the antenna element is replaced with a plurality of chip inductors (in this example, two chip inductors 13, By dividing into 14), the following effects can be obtained.

  The operational effects of the present embodiment will be described in comparison with the antenna matching circuit 30 shown in FIG. The antenna matching circuit 30 uses one chip inductor 31 as a first inductance element inserted in series between the input terminal 11 of the RF circuit and the circuit side end 12 of the antenna element. The chip inductor 31 is assumed to have an inductance value necessary for matching and tuning in the antenna matching circuit 30. Therefore, one chip inductor 31 has substantially the same inductance value as the sum of the inductance values of the two chip inductors 13 and 14.

  Based on the circuit model shown in FIGS. 4A and 4B, the frequency characteristics related to the Q value of the inductance element were simulated. The relationship between one chip inductor L1 and two chip inductors L2 and L3 is such that the inductance value of L1 and the inductance value of L2 + L3 are substantially the same.

  FIG. 5 is a diagram showing a simulation result of frequency characteristics of the inductance element. It is assumed that 90 nH is required as an inductance inserted in series in the antenna matching circuit. The frequency characteristic when configured with one chip inductor L1 (90 nH) is M1, and the self-resonant frequency is in the vicinity of 2.8 GHz. On the other hand, when the required inductance is divided into two chip inductors L2 (47nH) and L3 (43nH), the frequency characteristic is M2, and the self-resonant frequency is in the vicinity of 3.75 GHz. Thus, by separating the self-resonant frequency from the operating frequency, good characteristics can be obtained in the circuit loss and the tuning band.

  In the simulation result of FIG. 5, when the use frequency is 1 GHz, one chip inductor L1 has a Q value = 80.23, whereas when divided into two chip inductors L2 and L3, the Q value is Q. The value is 86.44, and the Q value is improved by 6. Therefore, the antenna matching circuit 10 (FIG. 1) in which the two chip inductors 13 and 14 having an excellent Q value are inserted has a Q value compared to the antenna matching circuit 30 (FIG. 3) in which one chip inductor 31 is connected in series. As a result, circuit loss in the antenna matching circuit 10 can be reduced.

FIG. 6 is a characteristic diagram showing the frequency characteristics of the inductance. The frequency characteristic due to one chip inductor is represented by T1, and the frequency characteristic due to two chip inductors is denoted by T2. For example, when the tuning band of the tunable antenna is changed from 470 MHz to 750 MHz, even if both inductance values are the same at the lowest frequency of the tuning band (470 MHz), the inductance values open to both as the frequency increases. The inductance value of the two chip inductors is smaller. The resonance frequency f in the antenna matching circuit 10 is determined by the following equation, where L is the inductance value by the chip inductors 13 and 14 and C is the capacitance by the varactor diodes 15 and 16.
f = 1 / (2π (LC) ^1/2 )
Therefore, if the variable capacitance ranges of the varactor diodes 15 and 16 are the same, the higher frequency tuning band is expanded by the frequency characteristic T2 as in the present embodiment.

  Thus, according to the present embodiment, since the inductance element inserted between the input terminal 11 of the RF circuit and the circuit side end 12 of the antenna element is divided into the two chip inductors 13 and 14, The self-resonant frequency can be kept away from the operating frequency, the circuit loss can be reduced, and the tuning band can be extended to the high frequency side.

  In the above description, the first varactor diode 15 is connected between the two chip inductors 13 and 14, but it is not always necessary to connect the variable capacitance element between the two chip inductors 13 and 14. Further, the inductance element inserted between the input terminal 11 of the RF circuit and the circuit side end 12 of the antenna element can be divided into three or more.

  The present invention is applicable to an antenna matching circuit of a tunable antenna.

1 is a configuration diagram of an antenna matching circuit according to an embodiment of the present invention. Schematic of tunable antenna with antenna matching circuit Configuration diagram of comparative example of antenna matching circuit (A) Circuit diagram of simulation model of one chip inductor, (b) Circuit diagram of simulation model of two chip inductors The figure which shows the frequency characteristic of Q value in the circuit model shown to Fig.4 (a) (b) The figure which shows the frequency characteristic of the inductance value in the antenna matching circuit by one chip inductor and two chip inductors Configuration of conventional antenna matching circuit

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Antenna element 2 ... Transmission line 3 ... Antenna matching circuit 4 ... 1st variable capacitance element 5 ... 2nd variable capacitance element 6 ... 1st inductor 7 ... 2nd inductor 10 ... Antenna matching circuit 11 ... Input terminal 12 ... Circuit side End portions 13 and 14 Chip inductor 15 First varactor diode 16 Second varactor diode 17 Inductance element (second inductance element)
DESCRIPTION OF SYMBOLS 18 ... Resistance 19 ... DC cut capacitor 20 ... Tunable antenna 21 ... Antenna part 22 ... Circuit part 23 ... Base | substrate 24 ... Radiation conductor

Claims (2)

  An antenna matching circuit provided between an antenna element and a feeding point to the antenna element and having a series circuit in which a variable capacitance element and a first inductance element are connected in series, wherein the first inductance element is An antenna matching circuit comprising a plurality of chip inductors. The antenna matching circuit according to claim 1, wherein a second inductance element is connected between the feeding point and the ground.

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