Nonreciprocal circuit element, manufacturing method of the same, and communication apparatus using the same

Disclosed herein is a nonreciprocal circuit element that includes a magnetic rotator disposed between first and second ground conductors, and a permanent magnet that applies a DC magnetic field to the magnetic rotator. The magnetic rotator includes a first ferrite core having a first surface covered with the first ground conductor, a second ferrite core having a second surface covered with the second ground conductor, a first center conductor directly fixed to a third surface of the first ferrite core positioned opposite to the first surface, and a second center conductor directly fixed to a fourth surface of the second ferrite core positioned opposite to the second surface.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a nonreciprocal circuit element and a communication apparatus using the nonreciprocal circuit element and, more particularly, to an nonreciprocal circuit element such as an isolator or a circulator suitably used in microwave or millimeter-wave frequency bands and a communication apparatus using such a nonreciprocal circuit element. The present invention also relates to a manufacturing method of such a nonreciprocal circuit element.

Description of Related Art

A nonreciprocal circuit element such as an isolator or a circulator is incorporated in, e.g., a mobile communication device like a mobile phone or a communication apparatus used in a base station. As described in Japanese Patent No. 6,231,555, a general nonreciprocal circuit element is constituted of a magnetic rotator having a center conductor and a pair of ferrite cores sandwiching the center conductor and a permanent magnet applying a magnetic field to the magnetic rotator.

However, in conventional nonreciprocal circuit elements, when an unevenness or distortion is present in a center conductor, a grounding conductor, a ferrite core, or the like, a gap may exist between the center conductor and the ferrite core, or between the grounding conductor and ferrite core. The presence of such a gap reduces an effective dielectric constant between the center conductor and the grounding conductor, which poses a problem in that the operation frequency of the nonreciprocal circuit element becomes higher than a designed value.

That is, in an ideal nonreciprocal circuit element, a radiusaof the ferrite core is determined by the following expression (1).

In the above expression, Xa(θ) is a constant obtained from a contact angle θ, λ0is the free-space wavelength of a use frequency, εris the specific dielectric constant of the ferrite core, and μeff,ris an effective permeability. Assuming that the propagation speed of electric wave is v, a use frequency F0can be represented by F0=v/λ0, so that the expression (1) can be modified into the following expression (1)′.

When the expression (1)′ is solved for F0, the following expression (2) can be obtained.

As is clear from the expression (1)′, when the F0is constant, a reduction in the effective dielectric constant due to existence of the gap increases the radiusaof the ferrite core. On the other hand, as is clear from the expression (2), when the radiusaof the ferrite core is constant, a reduction in the effective dielectric constant increases the operation frequency.

SUMMARY

It is therefore an object of the present invention to provide a nonreciprocal circuit element capable of preventing a change in electrical characteristics due to a gap between the center conductor and the ferrite core and a communication apparatus using the same. Another object of the present invention is to provide a manufacturing method for such a nonreciprocal circuit element.

A nonreciprocal circuit element according to the present invention has a magnetic rotator disposed between first and second ground conductors and a permanent magnet that applies a DC magnetic field to the magnetic rotator. The magnetic rotator includes a first ferrite core whose one surface is covered with the first ground conductor, a second ferrite core whose one surface is covered with the second ground conductor, a first center conductor directly fixed to the other surface of the first ferrite core, and a second center conductor directly fixed to the other surface of the second ferrite core.

A communication apparatus according to the present invention includes the above nonreciprocal circuit element.

According to the present invention, the first and second center conductors are directly fixed respectively to the first and second ferrite cores, so that no gap is generated therebetween. Thus, it is possible to prevent a change in electrical characteristics due to a gap between the center conductor and the ferrite core.

In the present invention, the first ground conductor may directly be fixed to one surface of the first ferrite core, and the second ground conductor may directly be fixed to one surface of the second ferrite core. With this configuration, no gap is generated between the first ground conductor and first ferrite core and between the second ground conductor and the second ferrite core. Thus, it is possible to suppress a change in electrical characteristics due to a gap between the ground conductor and the ferrite core.

The nonreciprocal circuit element according to the present invention may further have a dielectric that bonds the other surface of the first ferrite core and the other surface of the second ferrite core together. The first ferrite core and the first center conductor may be fixed to each other without interposition of the dielectric, and the second ferrite core and the second center conductor may be fixed to each other without interposition of the dielectric. With this configuration, the first and second ferrite cores can be mutually fixed.

In the present invention, the first and second center conductors may contact each other. Even in this case, the nonreciprocal circuit element can operate properly.

In the present invention, the first and second center conductors may have the same planar shape. With this configuration, influence due to a capacitance component between the first center conductor and the second ground conductor and between the second center conductor and the first ground conductor can be eliminated.

A nonreciprocal circuit element manufacturing method according to the present invention includes the steps of: forming a first ground conductor directly on one surface of a first ferrite core and forming a first center conductor directly on the other surface of the first ferrite core; forming a second ground conductor directly on one surface of a second ferrite core and forming a second center conductor directly on the other surface of the second ferrite core; fixing the first and second ferrite cores such that the other surface of the first ferrite core and the other surface of the second ferrite core face each other; and disposing a permanent magnet that applies a DC magnetic field to the first and second ferrite cores.

According to the present invention, no gap is generated between the ferrite core and the ground conductor and between the ferrite core and the center conductor, so that a dielectric constant between the ground conductor and the center conductor does not change. Thus, it is possible to manufacture a nonreciprocal circuit element having stable electrical characteristics.

In the present invention, the first and second center conductors may be formed on the other surfaces of the first and second ferrite cores, respectively, by printing, plating, or diffusion bonding. This allows the ferrite core and the center conductor to be fixed without gap.

As described above, according to the present invention, it is possible to prevent a change in electrical characteristics due to a gap between the center core and the ferrite core.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a schematic perspective view illustrating the configuration of a nonreciprocal circuit element10according to a preferred embodiment of the present invention.FIG. 2is a schematic exploded perspective view of the nonreciprocal circuit element10.

The nonreciprocal circuit element10illustrated inFIGS. 1 and 2is a distributed-constant-type nonreciprocal circuit element. The nonreciprocal circuit element10is incorporated in, e.g., a mobile communication device like a mobile phone or a communication apparatus used in a base station and used as an isolator or a circulator. Although not particularly limited, the nonreciprocal circuit element10according to the present embodiment is suitably used for a communication apparatus used in a base station.

As illustrated inFIGS. 1 and 2, the nonreciprocal circuit element10according to the present embodiment is a surface-mount-type chip component having a substantially rectangular parallelepiped shape and has first and second side surfaces11and12(xz plane), third and fourth side surfaces13and14(yz plane), and a mounting surface15(xy plane) and a top surface16(xy plane). The first side surface11is provided with a first external terminal21, the second side surface12is provided with a second external terminal22, and the third side surface13is provided with a third external terminal23. Further, the first to fourth side surfaces11to14are each provided with a plurality of ground terminals20. A portion of each of the external terminals21to23and ground terminals20is tucked under the mounting surface15.

The three external terminals21to23are connected to their corresponding signal lines when the nonreciprocal circuit element10according to the present embodiment is used as a circulator. On the other hand, when the nonreciprocal circuit element10according to the present embodiment is used as an isolator, for example, the external terminals21and22are connected to their corresponding signal lines, and the external terminal23is grounded through a terminal resistor. Further, even when the external terminal21or22is grounded through a terminal resistor, the nonreciprocal circuit element10according to the present embodiment can be used as an isolator. A ground potential is given to the plurality of ground terminals20in common.

The nonreciprocal circuit element10further has permanent magnets31and32and a magnetic rotator40sandwiched between the permanent magnets31and32in the z-direction which is the lamination direction. The permanent magnets31and32apply a DC magnetic field to the magnetic rotator40. In the present invention, one of the permanent magnets31and32may be omitted or replaced with an iron plate or the like as a magnetic substrate having small coercive force; however, to perpendicularly apply a strong magnetic field to the magnetic rotator40, it is preferable to sandwich the magnetic rotator40by the two permanent magnets31and32.

The magnetic rotator40includes two ferrite cores41and42and two center conductors70A and70B sandwiched between the ferrite cores41and42in the z-direction. As the material for the ferrite cores41and42, a soft magnetic material such as yttrium/iron/garnet (YIG) is preferably used. The planar shape of each of the center conductors70A and70B is as illustrated inFIG. 2, and the center conductors70A and70B have respectively three ports71A to73A and three ports71B to73B which are radially led from the center point thereof and branch conductors74A to76A and74B to76B for adjusting electrical characteristics. The center conductor70A and the ferrite core42are directly fixed to each other without an adhesive or the like. Similarly, the center conductor70B and ferrite core41are directly fixed to each other without using an adhesive or the like between them. The ferrite cores41and42adhere to each other through a dielectric43having adhesiveness. The dielectric43may be interposed between the center conductors70A and70B. The center conductors70A and70B may partly or entirely contact each other without interposition of the dielectric43. Preferably, the center conductors70A and70B have mutually the same planar shape and accurately overlap each other as viewed in the z-direction.

The tip ends of the first ports71A and71B led respectively from the center conductors70A and70B are exposed to the first side surface11and are thus connected to the first external terminal21. The tip ends of the second ports72A and72B led respectively from the center conductors70A and70B are exposed to the second side surface12and are thus connected to the second external terminal22. The tip ends of the third ports73A and73B led respectively from the center conductors70A and70B are exposed to the third side surface13and are thus connected to the third external terminal23.

The nonreciprocal circuit element10according to the present embodiment further has a grounding conductor51sandwiched between the permanent magnet31and the magnetic rotator40in the z-direction and a grounding conductor52sandwiched between the permanent magnet32and the magnetic rotator40in the z-direction. Thus, the center conductors70A and70B are sandwiched between the two grounding conductors51and52and thus isolated from the permanent magnets31and32. The grounding conductor51has cuts51ato51cformed at portions respectively overlapping the external terminals21to23, and the grounding conductor52has cuts52ato52cformed at portions respectively overlapping the external terminals21to23, thereby preventing the grounding conductors51and52from interfering with the external terminals21to23. The remaining parts of each of the grounding conductors51and52are exposed from the first to fourth side surfaces11to14. Thus, the plurality of ground terminals20are each connected to both the grounding conductors51and52.

In the present embodiment, the grounding conductor51is printed on the lower surface of the ferrite core41, and the grounding conductor52is printed on the upper surface of the ferrite core42. Thus, the grounding conductor51and the ferrite core41closely adhere to each other with substantially no gap, and the grounding conductor52and the ferrite core42closely adhere to each other with substantially no gap. The permanent magnet31and the grounding conductor51adhere to each other through a dielectric61having adhesiveness, and the permanent magnet32and the grounding conductor52adhere to each other through a dielectric62having adhesiveness. The dielectrics61and62may be formed using the same material as the dielectric43.

FIG. 3is a partial cross-sectional view of the magnetic rotator40.

As illustrated inFIG. 3, in the present embodiment, the center conductor70A is directly fixed to a lower surface42aof the ferrite core42, and the ground conductor52is directly fixed to an upper surface42bof the ferrite core42. That is, no gap or no separate member is interposed between the lower surface42aof the ferrite core42and center conductor70A, and no gap or another member is interposed between the upper surface42bof the ferrite core42and the ground conductor52. Similarly, the center conductor70B is directly fixed to an upper surface41aof the ferrite core41, and the ground conductor51is directly fixed to a lower surface41bof the ferrite core41. That is, no gap or no separate member is interposed between the upper surface41aof the ferrite core41and the center conductor70B, and no gap or no separate member is interposed between the lower surface41bof the ferrite core41and the ground conductor51.

As a result, the dielectric constant between the center conductor70A and the ground conductor52completely coincides with the dielectric constant of the ferrite core42, and the dielectric constant between the center conductor70B and ground conductor51completely coincides with the dielectric constant of the ferrite core41. That is, there is no chance at all that effective dielectric constant will change in the presence of a gap or by the interposition of a separate member. Therefore, the nonreciprocal circuit element10according to the present embodiment can obtain extremely stable electrical characteristics. In particular, when the center conductor70A and the center conductor70B accurately overlap each other as viewed in the z-direction, no capacitance component is added between the center conductor70A and the ground conductor51, and no capacitance component is added between the center conductor70B and the ground conductor52.

In order to directly fix the center conductors70A,70B and the ferrite cores42,41, respectively, the center conductor70A may be directly formed on the lower surface42aof the ferrite core42, and the center conductor70B may be directly formed on the upper surface41aof the ferrite core41. As a concrete method, printing, plating or diffusion bonding can be used. According to these methods, the center conductors70A and70B are directly fixed respectively to the ferrite cores42and41, preventing a gap or a separate member from being interposed therebetween. The same applies to the ground conductors51and52. That is, the ground conductors51and52may be directly formed respectively on the lower surface41bof the ferrite core41and on the upper surface42bof the ferrite core42using printing, plating or diffusion bonding. Thereafter, the ferrite cores41and42are fixed through the dielectric43having adhesiveness such that the upper surface41aof the ferrite core41and the lower surface42aof the ferrite core42face each other, followed by disposition of the permanent magnets31and32, and then the ground terminal20and external terminals21to23are formed, whereby the nonreciprocal circuit element10according to the present embodiment is completed.

As described above, in the nonreciprocal circuit element10according to the present embodiment, the center conductor70A and the ground conductor52are directly fixed to the ferrite core42, and the center conductor70B and the ground conductor51are directly fixed to the ferrite core41. Thus, there is almost no chance that the dielectric constant will change, due to variations in manufacturing, between the center conductor70A and the ground conductor52and between the center conductor70B and the ground conductor51, whereby extremely stable electrical characteristics can be obtained.

FIG. 4is a block diagram illustrating the configuration of a communication apparatus80using the nonreciprocal circuit element according to the present embodiment.

The communication apparatus80illustrated inFIG. 4is provided in a base station in, e.g., a mobile communication system. The communication apparatus80includes a receiving circuit part80R and a transmitting circuit part80T, which are connected to a transmitting/receiving antenna ANT. The receiving circuit part80R includes a receiving amplifier circuit81and a receiving circuit82for processing received signals. The transmitting circuit part80T includes a transmitting circuit83for generating audio signals and video signals and a power amplifier circuit84.

In the thus configured communication apparatus80, nonreciprocal circuit elements91and92according to the present embodiment are used in a path from the antenna ANT to the receiving circuit part80R and a path from the transmitting circuit part80T to the antenna ANT, respectively. The nonreciprocal circuit element91functions as a circulator, and the nonreciprocal circuit element92functions as an isolator having a terminal resistor R0.

For example, in the above embodiment, the distributed-constant-type nonreciprocal circuit element is taken as an example; however, the present invention may be applied also to a lumped-constant-type nonreciprocal circuit element.

EXAMPLES

Samples A and B of nonreciprocal circuit elements having the same structure of the nonreciprocal circuit element illustrated inFIGS. 1 and 2were assumed, and passage losses of the respective samples A and B were evaluated by simulation. The dielectric constant of the dielectric43was set to 1 and 2.2 in the samples A and B, respectively. Further, a resonance frequency was set to 3.5 GHz in both the samples A and B. Simulation results are illustrated inFIG. 5. Reference signs A and B inFIG. 5correspond to simulation results of the samples A and B, respectively.

As illustrated inFIG. 5, in both the samples A and B, the passage loss was as very small as about −0.3 dB in a band of 3.3 GHz to 3.8 GHz. Further, no significant difference was not found between the passage losses of the samples A and B in the same band. This is probably because the center conductors70A and70B closely contact the ferrite cores42and41, respectively, the dielectric constant of the dielectric43has little influence on electrical characteristics.