Impedance conversion circuit and antenna device

1. Field of the Invention

The present invention relates to impedance conversion circuits and antenna devices including impedance conversion circuits.

2. Description of the Related Art

In recent years, it is desired that communication terminal devices such as cellular phones are compatible with a wide variety of communication systems. In such communication terminal devices, antenna devices need to cover a wide range of frequency bands, such as, for example, from 800 MHz to 2.4 GHz.

For example, Japanese Patent No. 4935956 discloses such an antenna device compatible with a wide frequency band. FIG. 6 is one example of a circuit diagram of the antenna device disclosed in the Japanese Patent No. 4935956. An impedance conversion circuit 25 of this antenna device includes a primary coil composed of a coil element L 1 a and a coil element L 1 b and a secondary coil composed of a coil element L 2 a and a coil element L 2 b . A first end portion of the coil element L 1 a is connected to a feeding circuit 30 , a second end portion of the coil element L 1 b is connected to an antenna element 11 , a first end portion of the coil element L 2 b is connected to the antenna element 11 , and a second end portion of the coil element L 2 a is connected to ground. FIG. 7 is a diagram illustrating an example of conductor patterns at respective layers in a case where the impedance conversion circuit illustrated in FIG. 6 is formed as a multilayer substrate. Conductor patterns that form the coil element L 1 a , the coil element L 1 b , the coil element L 2 a , and the coil element L 2 b are formed at a plurality of dielectric layers, and those conductor patterns are connected across the layers with a number of via conductors. Further, the coil element L 1 a is connected in series to the coil element L 1 b , and similarly the coil element L 2 a is connected in series to the coil element L 2 b.

In the antenna devices including impedance conversion circuits, downsizing of the impedance conversion circuit itself is desirable since electronic devices such as cellular phones and the like, in which the antenna devices are incorporated, are being downsized, and the available space for the antenna device is limited. However, as illustrated in FIG. 7 , the impedance conversion circuit, in which the conductor patterns are formed in such a way that the coil elements are sequentially connected in series across a plurality of layers, has a larger thickness in a stacking direction of the dielectric layers.

To downsize the impedance conversion circuit itself, it is preferable that the smallest possible number of turns is used in the coils, and the smallest possible number of layers is used in the dielectric layers. However, the impedance conversion ratio of the impedance conversion circuit is determined basically by the number of turns in the primary coil and the secondary coil. Thus, in the case where the number of turns is smaller, it may be difficult to obtain a desired transformer ratio. Further, as the number of turns is reduced, the percentages of inductance at input and output portions, which does not contribute to the coupling of transformer, become larger. This creates another issue in obtaining a coupling factor between the primary coil and the secondary coil.

To obtain a predetermined (larger) coupling factor with even smaller coils, it is desirable to use an identical shape (nearly congruent shape) for the primary coil and the secondary coil and arrange them to overlap on top of each other. In the case where the identical shape is used for the primary coil and the secondary coil, however, it may be very difficult to obtain desired inductance values for the primary coil and the secondary coil.

Therefore, preferred embodiments of the present disclosure provide an impedance conversion circuit that achieves a larger coupling factor with even smaller coils and a desired impedance conversion ratio, and provide an antenna device including the impedance conversion circuit.

An impedance conversion circuit according to a preferred embodiment of the present invention includes a primary coil L 1 and a secondary coil L 2 that are electromagnetically coupled to each other, wherein the primary coil L 1 includes a first coil element L 1 a and a second coil element L 1 b that are connected in parallel, the first coil element L 1 a and the second coil element L 1 b being circularly arranged on a single plane so as to allow currents flowing through the first coil element L 1 a and the second coil element L 1 b to flow in directions opposite to each other; the secondary coil L 2 includes a third coil element L 2 a and a fourth coil element L 2 b that are connected in series, the third coil element L 2 a and the fourth coil element L 2 b being arranged in a figure-of-eight pattern on a single plane so as to allow currents flowing through the third coil element L 2 a and the fourth coil element L 2 b to flow in directions opposite to each other; the first coil element L 1 a and the third coil element L 2 a are arranged coaxially adjacent to each other, the first coil element L 1 a is mainly magnetically coupled with the third coil element L 2 a ; and the second coil element L 1 b and the fourth coil element L 2 b are arranged coaxially adjacent to each other, the second coil element L 1 b is mainly magnetically coupled with the fourth coil element L 2 b.

The foregoing configuration provides an impedance conversion circuit that has a large transformer ratio of the primary coil and the secondary coil even with a smaller number of turns and a smaller number of layers.

Further, the foregoing configuration improves the coupling factor of the primary coil L 1 and the secondary coil L 2 .

Preferably, conductor patterns defining the third coil element L 2 a and the fourth coil element L 2 b have line widths wider than conductor patterns defining the first coil element L 1 a and the second coil element L 1 b . This configuration achieves a stable coupling factor even if there is a misalignment between the conductor patterns of the third coil element L 2 a and the fourth coil element L 2 b and the conductor patterns of the first coil element L 1 a and the second coil element L 1 b.

Preferably, the first coil element L 1 a overlaps the third coil element L 2 a in a planar view, and the second coil element L 1 b overlaps the fourth coil element L 2 b in planar view. This configuration allows the coil elements arranged adjacent to each other in a layering direction to couple more strongly, and achieves a higher coupling factor.

Preferably, coil openings of the first coil element L 1 a and the third coil element L 2 a are smaller than coil openings of the second coil element L 1 b and the fourth coil element L 2 b . This configuration allows the ratio between the inductance of the primary coil and the inductance of the secondary coil to be even larger without decreasing the coupling factor of the primary coil and the secondary coil.

An antenna device according to a preferred embodiment of the present invention includes the impedance conversion circuit having the foregoing configuration and an antenna element including an input portion to which the impedance conversion circuit is connected. This configuration provides an antenna device that matches the impedance of a feeding circuit despite of having the antenna element that generally has a lower impedance.

Preferably, the primary coil L 1 is connected to a feeding circuit at one end portion and connected to the antenna element at the other end portion, and the secondary coil L 2 is connected to ground at one end portion and connected to the antenna element at the other end portion. This configuration allows a negative inductance due to the mutual inductance between the primary coil and the secondary coil to operate in such a manner that the negative inductance cancels out an inductance component of the antenna so as to achieve wide band antenna characteristics.

According to various preferred embodiments of the present invention, there are provided an impedance conversion circuit and an antenna device including the impedance conversion circuit, which has a predetermined coupling factor and a predetermined transformer ratio even with smaller coils having a smaller number of turns and a smaller number of layers.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

FIG. 1A is a circuit diagram of an antenna device including an impedance conversion circuit according to a first preferred embodiment of the present invention, and FIG. 1B is an equivalent circuit diagram thereof.

FIG. 2 is a perspective view of various conductor patterns of the impedance conversion circuit according to the first preferred embodiment of the present invention.

FIG. 3 is a perspective view of the impedance conversion circuit.

FIG. 4 is a diagram illustrating conductor patterns formed at respective dielectric layers of the impedance conversion circuit.

FIG. 5 is a diagram illustrating conductor patterns formed at respective dielectric layers of an impedance conversion circuit according to a second preferred embodiment of the present invention.

FIG. 6 is one example of a circuit diagram of an antenna device illustrated in Japanese Patent No. 4935956.

FIG. 7 is a diagram illustrating an example of conductor patterns at respective layers in a case where the impedance conversion circuit illustrated in FIG. 6 is configured as a multilayer substrate.

First Preferred Embodiment

FIG. 1A is a circuit diagram of an antenna device 101 including an impedance conversion circuit 10 according to a first preferred embodiment of the present invention, and FIG. 1B is an equivalent circuit diagram thereof.

As illustrated in FIG. 1A , the antenna device 101 includes an antenna element 11 and the impedance conversion circuit 10 connected to the antenna element 11 . A feeding end portion of the antenna element 11 is connected to the impedance conversion circuit 10 . In the impedance conversion circuit 10 , a primary coil L 1 is provided between the antenna element 11 and a feeding circuit 30 . The feeding circuit 30 is a feeding circuit configured to supply a high frequency signal to the antenna element 11 , and generate and process the high frequency signal. The feeding circuit 30 may additionally include circuitry configured to multiplex or demultiplex the high frequency signal.

The impedance conversion circuit 10 includes the primary coil L 1 connected to the feeding circuit 30 and a secondary coil L 2 coupled to the primary coil L 1 . Specifically, a first end portion of the primary coil L 1 is connected to the feeding circuit 30 , and a second end portion thereof is connected to the antenna element 11 . A first end portion of the secondary coil L 2 is connected to the antenna element 11 , and a second end portion thereof is connected to ground.

The impedance conversion circuit 10 includes a transformer circuit in which the primary coil L 1 and the secondary coil L 2 are coupled to each other via mutual inductance M 1 and mutual inductance M 2 . As illustrated in FIG. 1B , the transformer circuit may be equivalently transformed to a T-shape circuit including three inductance elements Z 1 , Z 2 , and Z 3 . In other words, this T-shape circuit includes a first port P 1 to be connected to the feeding circuit 30 , a second port P 2 to be connected to the antenna element 11 , a third port P 3 to be connected to ground, the inductance element Z 1 connected between the first port P 1 and a fork A, the inductance element Z 2 connected between the second port P 2 and the fork A, and the inductance element Z 3 connected between the third port P 3 and the fork A.

When the inductance of the primary coil L 1 illustrated in FIG. 1A is expressed as L 1 , the inductance of the secondary coil L 2 illustrated in FIG. 1A is expressed as L 2 , and the mutual inductance is expressed as M, the inductance of the inductance element Z 1 illustrated in FIG. 1B is L 1 +M, the inductance of the inductance element Z 2 illustrated in FIG. 1B is −M, and the inductance of the inductance element Z 3 illustrated in FIG. 1B is L 2 +M.

In the T-shape circuit illustrated in FIG. 1B , a portion (Z 1 and Z 3 ) between the port P 1 , which is to be connected to the feeding circuit 30 , and the port P 3 , which is to be connected to ground, is configured to contribute to the impedance conversion due to the transformer ratio. In other words, the impedance conversion ratio of the impedance conversion circuit 10 is (L 1 +L 2 +2M):L 2 .

FIG. 2 is a perspective view of various conductor patterns of the impedance conversion circuit 10 according to the first preferred embodiment. In the figure, dielectric layers on which those conductor patterns are provided are omitted. Further, FIG. 3 is a perspective view of the impedance conversion circuit 10 . Still further, FIG. 4 is a diagram illustrating the conductor patterns provided at respective dielectric layers of the impedance conversion circuit 10 .

As illustrated in FIG. 4 , a first coil element L 1 a and a second coil element L 1 b are provided on a dielectric layer 21 . The second coil element L 1 b includes a bended portion L 1 bc as a portion thereof (a function of the bended portion L 1 bc will be described below). A first end portion of the first coil element L 1 a is connected to the first port (feeding port) P 1 , and a second end portion of the first coil element L 1 a is connected to the second port (antenna port) P 2 . A first end portion of the second coil element L 1 b is connected to the first port (feeding port) P 1 , and a second end portion of the second coil element L 1 b is connected to the second port (antenna port) P 2 . In other words, the first coil element L 1 a is connected in parallel to the second coil element L 1 b , and both coil elements are connected between the first port P 1 and the second port P 2 . The first coil element L 1 a and the second coil element L 1 b define the primary coil.

A third coil element L 2 a and a fourth coil element L 2 b are provided on a dielectric layer 22 . The fourth coil element L 2 b includes a bended portion L 2 bc as a portion thereof (a function of the bended portion L 2 bc will be described below). A first end portion of the third coil element L 2 a is connected to the second port (antenna port) P 2 via a via-conductor VIA. A first end portion of the fourth coil element L 2 b is connected to the third port (ground port) P 3 . A second end portion of the third coil element L 2 a is connected to a second end portion of the fourth coil element L 2 b . In other words, the third coil element L 2 a is connected in series to the fourth coil element L 2 b , and both coil elements are connected between the second port P 2 and the third port P 3 . The third coil element L 2 a and the fourth coil element L 2 b define the secondary coil.

As illustrated in FIG. 2 to FIG. 4 , the first coil element L 1 a and the third coil element L 2 a are arranged coaxially adjacent to each other, and the second coil element L 1 b and the fourth coil element L 2 b are arranged coaxially adjacent to each other. Further, the first coil element L 1 a is mainly magnetically coupled with the third coil element L 2 a , and the second coil element L 1 b is mainly magnetically coupled with the fourth coil element L 2 b . Dashed-dotted lines in FIG. 2 represent how the magnetic flux flows through openings of respective coil elements.

As described above, the first coil element L 1 a and the third coil element L 2 a are arranged coaxially adjacent to each other, and the second coil element L 1 b and the fourth coil element L 2 b are arranged coaxially adjacent to each other. Thus, the first coil element L 1 a and the third coil element L 2 a are in close proximity in a layering direction, and the second coil element L 1 b and the fourth coil element L 2 b are in close proximity in the layering direction. Accordingly, a larger coupling factor is obtained despite of a smaller number of coil turns.

Further, the first coil element L 1 a and the second coil element L 1 b are arranged on the same plane, and the third coil element L 2 a and the fourth coil element L 2 b are arranged on the same plane. Thus, the impedance conversion circuit includes a lower number of layers, and the thickness dimension in the stacking direction may be reduced.

When the inductances of the first coil element L 1 a , the second coil element L 1 b , the third coil element L 2 a , and the fourth coil element L 2 b are expressed as L 1 a , L 1 b , L 2 a , and L 2 b , the inductance of the primary coil L 1 is expressed as L 1 , and the inductance of the secondary coil is expressed as L 2 , the following equations are satisfied:

L 1=( L 1 a*L 1 b )/( L 1 a+L 1 b )

L 2 =L 2 a+L 2 b

Suppose that the inductances L 1 a , L 1 b , L 2 a , and L 2 b are all the same value, and that value is expressed as Lu, then the following relationships are satisfied:

L 1 =Lu/ 2

L2=2Lu

Thus, the ratio of inductance values of the primary coil L 1 and the secondary coil L 2 becomes as follows:

L1:L2=1:4

As illustrated in FIG. 4 , the second coil element L 1 b includes, as a portion thereof, the bended portion L 1 bc that bends toward the inside of a coil element formation region. The fourth coil element L 2 b includes, as a portion thereof, the bended portion L 2 bc that faces the bended portion L 1 bc in the layering direction. This is adopted to facilitate passing of magnetic flux in each loop of the coil elements as illustrated in FIG. 2 . In other words, if the first coil element L 1 a and the second coil element L 1 b were configured in a simple rectangle loop without the bended portion L 1 bc , the loop functions as a metal plate equivalent. This makes the passing of magnetic flux somewhat difficult. In the case where the bended portion L 1 bc is provided, currents (i 1 , i 6 ), (i 2 , i 5 ), and (i 3 , i 4 ), which are denoted by arrows, flow in opposite directions to each other in each current pair when the currents i 1 to i 6 are flowing in the first coil element L 1 a and the second coil element L 1 b . This facilitates the passing of magnetic flux through loops of the first coil element L 1 a and the second coil element L 1 b.

Further, as illustrated in FIG. 4 , the conductor patterns defining the third coil element L 2 a and the fourth coil element L 2 b have line widths wider than the conductor patterns forming the first coil element L 1 a and the second coil element L 1 b . Therefore, when dielectric sheets on which conductor patterns are provided are being stacked, nearly no effect is produced even if there is a misalignment of the conductor patterns due to the influence of printing position accuracy of the conductor patterns or stacking position accuracy of the dielectric sheets. In other words, in planar view of these conductor patterns, the third coil element L 2 a and the fourth coil element L 2 b overlap and cover the whole of the first coil element L 1 a and the second coil element L 1 b . Thus, a stable coupling factor may be obtained.

Further, as illustrated in FIG. 4 , the conductor patterns of the third coil element L 2 a and the fourth coil element L 2 b that are connected in series are made thicker than the conductor patterns of the first coil element L 1 a and the second coil element L 1 b that are connected in parallel. This reduces the total conductor loss of the impedance conversion circuit and improves the Q value. In other words, compared to the case where the conductor patterns of the first coil element L 1 a and the second coil element L 1 b that are connected in parallel are made thicker, the reduction effect of the conductor loss increases in the case where the conductor patterns of the third coil element L 2 a and the fourth coil element L 2 b that are connected in series are made thicker.

Second Preferred Embodiment

FIG. 5 is a diagram illustrating conductor patterns provided at respective dielectric layers of an impedance conversion circuit according to a second preferred embodiment of the present invention.

In FIG. 5 , a first coil element L 1 a and a second coil element L 1 b , which are provided on a dielectric layer 21 , define a primary coil L 1 . Further, a third coil element L 2 a and a fourth coil element L 2 b , which are provided on a dielectric layer 22 , define a secondary coil L 2 . Basic patterns of the first coil element L 1 a , the second coil element L 1 b , the third coil element L 2 a , and the fourth coil element L 2 b are the same as those illustrated in FIG. 4 . In the example illustrated in FIG. 5 , the inductance of the first coil element L 1 a is different from that of the second coil element L 1 b , the inductance of the first coil element L 1 a is the same or substantially the same as that of the third coil element L 2 a , and the inductance of the second coil element L 1 b is the same or substantially the same as that of the fourth coil element L 2 b . In the example of FIG. 5 , the path length of the first coil element L 1 a is preferably relatively shorter than that of the second coil element L 1 b , and correspondingly the path length of the third coil element L 2 a is preferably relatively shorter than that of the fourth coil element L 2 b . When the inductances of the first coil element L 1 a , the second coil element L 1 b , the third coil element L 2 a , and the fourth coil element L 2 b are expressed as L 1 a , L 1 b , L 2 a , and L 2 b , the following relationships are satisfied:

L1a≠L1b

L2a≠L2b

L1a=L2a

L1b=L2b

This configuration further increases the ratio between the inductance of the primary coil L 1 , ((L 1 a *L 1 b )/(L 1 a +L 1 b )), and the inductance of the secondary coil L 2 , (L 2 a +L 2 b ), without reducing the coupling factor of the primary coil L 1 and the secondary coil L 2 . In other words, the inductance of the primary coil L 1 decreases as the inductance L 1 a decreases. Here, of the first coil element L 1 a and the second coil element L 1 b that are connected in parallel, the inductance L 1 a is the inductance of one that has the smaller inductance. On the other hand, the inductance of the secondary coil L 2 may be kept constant since the inductance of the secondary coil L 2 is the sum of the inductances of the third coil element L 2 a and the fourth coil element L 2 b that are connected in series. Accordingly, the ratio between the primary coil L 1 and the secondary coil L 2 is easily made larger.

With regard to materials of substrates forming the impedance conversion circuits according to the preferred embodiments of the present invention, rigid substrates such as glass epoxy substrates, polytetrafluoroethylene (PTFE) substrates, and the like may be used, or flexible substrates such as polyethylene terephthalate (PET) films, polyimide (PI), liquid crystal polymer (LCP), and the like may be alternatively used. The flexible substrate is thin and flexible, provides higher flexibility when installing inside devices, and is suitable for use in small electronic devices and the like. Particularly, the liquid crystal polymer is a suitable substrate material for high frequency circuits since the liquid crystal polymer has a smaller loss (smaller passing loss) and a smaller relative permittivity (higher self-resonant frequency) compared to other flexible substrate materials.

In the foregoing examples, one end portion of the primary coil L 1 is connected to the feeding circuit, the other end portion of the primary coil L 1 is connected to the antenna element, one end portion of the secondary coil L 2 is connected to ground, and the other end portion of the secondary coil L 2 is connected to the antenna element. However, the preferred embodiments of the present invention are not limited thereto. Alternatively, for example, the impedance conversion circuit may be provided between the feeding circuit and the antenna element by connecting one end portion of the primary coil L 1 to the feeding circuit, connecting the other end portion of the primary coil L 1 to ground, connecting one end portion of the secondary coil L 2 to the antenna element, and connecting the other end portion of the secondary coil L 2 to ground.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.