Nonreciprocal circuit device comprising first and second ferromagnetic bodies having different saturation magnetizations

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonreciprocal circuit device for use in a mobile communications apparatus such as a portable telephone, etc., and particularly to a nonreciprocal circuit device used as a circulator or an isolator in a high frequency band such as the microwave band or the like.
2. Description of the Related Art
Recently in mobile communications, high frequency apparatus has increasingly been miniaturized and generalized, and it has also been strongly demanded to decrease the size and cost of a nonreciprocal circuit device used in such apparatus.
A known nonreciprocal circuit device is, for example, a device comprising a plurality of central electrodes arranged to cross each other in an electrically isolated state, microwave magnetic materials provided above and below the plurality of central electrodes, and a permanent magnet for applying a DC magnetic field to the plurality of central electrodes, i.e., a lumped-parameter nonreciprocal circuit device. Such a lumped-parameter nonreciprocal circuit device is used as, for example, a circulator or an isolator.
FIG. 1 is an exploded perspective view showing an example of a conventional circulator. In this circulator, in order to decrease the size thereof, a plurality of central conductors are arranged in a ferromagnetic body 1 to cross each other in an electrically isolated state. Namely, as shown in an exploded perspective view of FIG. 2, the ferromagnetic body 1 has a laminated structure comprising a plurality of ferromagnetic material layers 1a to 1e. On the upper sides of the ferromagnetic material layers 1b, 1c and 1d are formed central conductors 2a and 2b, 2c and 2d and 2e and 2f, respectively. In other words, on the upper side of each of the ferromagnetic material layers 1b to 1d are arranged a pair of central conductors.
The central conductors 2a and 2b, the central conductors 2c and 2d, and the central conductors 2e and 2f are arranged to cross each other in a laminated state, and are electrically isolated by the ferromagnetic material layers 1c and 1d.
On the ferromagnetic material layers 1a and 1e are formed earth electrodes 3a and 3b, respectively.
In the ferromagnetic body 1 shown in FIG. 1, external electrodes 4a, 4b and 4c are formed on the side thereof so as to be connected to the earth electrodes 3a and 3b in common, and each external electrode being electrically connected to the ends of one of the pairs of central conductors 2a-2f.
In FIG. 1, to the upper side of the ferromagnetic body 1 is fixed a dielectric body 5. The dielectric body 5 comprises dielectric ceramic and has a capacitor provided therein for forming a matching circuit. Namely, as shown in an exploded view of FIG. 3, the dielectric body 5 has a laminated structure comprising dielectric layers 5a and 5b. On the upper side of the dielectric layer 5a are formed capacity electrodes 6a, 6b and 6c. On the upper side of the dielectric layer 5b is formed an earth electrode 7. Therefore, in each portion where the capacity electrodes 6a to 6c are overlapped with the earth electrode 7 and the earth electrode 3b through the dielectric layers 5b and 5a, respectively, a capacitor is formed.
In FIG. 1, external electrodes 8a, 8b and 8c are formed on the side of the dielectric body 5. Each of these external electrodes 8a to 8c is electrically connected to one of the capacity electrodes or one of the earth electrodes.
On the other hand, the ferromagnetic body 1 and the dielectric body 5 are contained in a terminal plate 9 having a cylindrical concave 9a at the center thereof. In the terminal plate 9 are formed conductor patterns 10a to 10c which constitute input/output terminals, and conductive patterns 10d, 10e and 10f which are connected to the earth potential.
The external electrodes 4a to 4c formed on the side of the ferromagnetic body 1 and the external electrodes 8a to 8c formed on the side of the dielectric body 5 are contained in the concave 9a of the terminal plate 9 and appropriately connected to the conductive patterns 10a to 10f.
In FIG. 1, a permanent magnet 11 is provided for applying a magnetic field to a portion in the ferromagnetic body 1 where the central conductors cross each other. The nonreciprocal circuit device shown in FIG. 1 further comprises metallic yokes 12 and 13. The terminal plate 9 and the magnet 11 are held between the yokes 12 and 13. The yokes 12 and 13 constitute a magnetic circuit for applying a magnetic field together with the magnet 11.
In the nonreciprocal circuit device shown in FIGS. 1 to 3, since the portion where the plurality of central conductors 2a, 2b to 2e and 2f cross each other in an electrically isolated state is integrally formed by using the ferromagnetic body 1, the nonreciprocal circuit device can easily be manufactured, and miniaturized.
However, since the ferromagnetic body 1 and the dielectric body 5 are separately fired and then joined together, the external electrodes 4a to 4c and the external electrodes 8a to 8c on the sides thereof must be electrically connected by soldering or the like. Therefore, the number of the connection points is increased, and thus, a problem occurs with respect to insufficient reliability. Also, since the ferromagnetic body 1 and the dielectric body 5 are fired separately, a plurality of firing steps must be carried out, and a troublesome assembly process is required, thereby making it difficult to reduce the manufacturing cost.
Therefore, the above problems can be possibly solved by simultaneously firing the ferromagnetic body 1 and the dielectric body 5. Namely, the above problems can be possibly solved by a method in which a green sheet for forming the ferromagnetic body 1 and a green sheet for forming the dielectric body 5 are laminated and simultaneously fired.
However, firing conditions for the ferromagnetic body 1 and the dielectric body 5 are different, and thus firing under conditions suitable for one of the bodies causes the possibility that firing of the other does not sufficiently proceed. Also firing under conditions intermediate between the conditions for both bodies causes a problem in that both the ferromagnetic body 1 and the dielectric body 5 may not be properly fired.
In addition, even if the ferromagnetic body 1 and the dielectric body 5 can be simultaneously fired, it is still not possible to use the same line in the step of preparing raw materials, thereby causing difficulties in decreasing the manufacturing cost.
Therefore, as a method of solving the above problems, a method has been proposed in which a central conductor arrangement portion and a capacitor formation portion for forming a matching circuit are formed in the same ferromagnetic body. This method will be described with reference to FIGS. 4 and 5.
FIG. 4 is an exploded perspective view illustrating another example of a conventional nonreciprocal circuit device. In the ferromagnetic body 15 shown in FIG. 4, a plurality of central conductors and a matching circuit are arranged. The electrode structure in the ferromagnetic body 15 is shown in an exploded perspective view of FIG. 5.
In the ferromagnetic body 15, ferromagnetic layers 15a to 15e are laminated. On the upper sides of the ferromagnetic layers 15b to 15d are formed a plurality of central conductors 16a, 16b to 16e and 16f, like the case of the ferromagnetic body 1 shown in FIG. 2. In this structure, ends of the central conductors 16a and 16b on the upper surface of the ferromagnetic layer 15b are electrically connected to a capacity electrode 17a. Similarly, ends of the central conductors 16c and 16d on the upper side of the ferromagnetic layer 15c are connected to a capacity electrode 17b formed thereon, and ends of the central conductors 16e and 16f on upper side of the ferromagnetic layer 15d are electrically connected to a capacity electrode 17c formed thereon.
On the upper sides of the ferromagnetic layers 15a and 15e are formed earth electrodes 18a and 18b, respectively. Therefore, in the ferromagnetic material 15 formed by laminating the ferromagnetic layers 15a to 15e and integrally firing the layers, not only the plurality of central conductors 16a to 16f are arranged, but also the capacity electrodes 17a to 17c for forming a matching circuit are arranged. The capacity electrodes 17a to 17c are overlapped with the earth electrodes 18a and 18b to form capacitors.
Referring to FIG. 4, the ferromagnetic body 15 is inserted into a concave 9a of a terminal plate 9 with a permanent magnet 11 disposed thereon, and held between metallic yokes 12 and 13 to form a nonreciprocal circuit device.
The nonreciprocal circuit device shown in FIG. 4 comprises the portion where the plurality of central conductors are arranged by using the ferromagnetic body 15, and the matching circuit. Therefore, the assembly step can be simplified, and the manufacturing cost can be decreased because a plurality of lines need not be used in the raw material preparing step. Also, since there is no need for joining the central conductors and the matching circuit by soldering or the like, the reliability can be improved.
However, since the capacitors for forming the matching circuit are formed by the ferromagnetic body 15, it is possible that a loss in the matching circuit may be increased due to the magnetic loss of the ferromagnetic body, whereby the insertion loss of the nonreciprocal circuit device may be increased.
SUMMARY OF THE INVENTION
The present invention is able to remove the above faults of the prior art, and provide a nonreciprocal circuit device with excellent reliability which permits reduction in its size and simplification of its manufacturing process without deterioration in its properties, such as insertion loss.
In accordance with a broad aspect of the present invention, there is provided a nonreciprocal circuit device comprising a first ferromagnetic body, a plurality of central conductors which are formed in the first ferromagnetic body and which are arranged to cross each other in an electrically isolated state, a second ferromagnetic body fixed to the first ferromagnetic body, and a matching circuit formed in the second ferromagnetic body and electrically connected to the plurality of central conductors, wherein the first and second ferromagnetic bodies have different saturation magnetizations.
In this nonreciprocal circuit device, the first ferromagnetic body and the second ferromagnetic body are integrally formed, the plurality of central conductors are arranged in the first ferromagnetic body, and the matching circuit is provided in the second ferromagnetic body. Thus, the portion where the plurality of central conductors are arranged and the portion where the matching circuit is formed both comprise ferromagnetic bodies, and thus the same line can be used in the raw material preparing step. In addition, since the first and second ferromagnetic bodies have different saturation magnetizations, for example, the magnetic loss can be decreased both when the saturation magnetization of the second ferromagnetic body is smaller than that of the first ferromagnetic body, and when the saturation magnetization of the second ferromagnetic body is larger than that of the first ferromagnetic body.
Further, a magnetic circuit is preferably provided on the first ferromagnetic body in order to apply a DC magnetic field. In this case, since the magnetic circuit for applying a DC magnetic field is integrated with the portion where the central conductors are arranged, the assembly step can further be simplified.
In accordance with a more specific aspect of the present invention, the matching circuit comprises plural pairs of capacity electrodes and earth electrodes which are formed to hold the magnetic layers of the second ferromagnetic body therebetween. In this case, the pairs of capacity electrodes respectively form the capacitors each of which is electrically connected to any one of the plurality of central conductors.
Also the first and second ferromagnetic bodies are preferably integrated by simultaneous firing. Therefore, the step of fixing the first and second ferromagnetic bodies can be omitted, thereby increasing the reliability of the electric connection between the central conductors and the matching circuit.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view illustrating an example of a conventional nonreciprocal circuit device;
FIG. 2 is an exploded perspective view of the ferromagnetic body shown in FIG. 1;
FIG. 3 is an exploded perspective view illustrating the internal structure of the dielectric body shown in FIG. 1;
FIG. 4 is an exploded perspective view illustrating another example of a conventional nonreciprocal circuit device;
FIG. 5 is an exploded perspective view illustrating the internal structure of the ferromagnetic body in the conventional nonreciprocal circuit device shown in FIG. 4;
FIG. 6 is an exploded perspective view illustrating a nonreciprocal circuit device in accordance with an embodiment of the present invention;
FIG. 7 is an exploded perspective view illustrating the ferromagnetic body used in the embodiment;
FIG. 8 is a perspective view illustrating the appearance of the ferromagnetic body used in the embodiment; and
FIG. 9 is a drawing showing the relation between an external magnetic field and the imaginary part .mu.+" of the permeability for a positive circularly polarized wave, for illustrating the reason why the magnetic loss is decreased in the nonreciprocal circuit device of the embodiment.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
An example of the structure of a nonreciprocal circuit device of the present invention will be described below.
FIG. 6 is an exploded perspective view of a nonreciprocal circuit device in accordance with an embodiment of the present invention. The construction shown in FIG. 6 is the same as the conventional nonreciprocal circuit device shown in FIG. 4 except for the structure of a ferromagnetic body 25.
The central conductors and matching circuit formed in the ferromagnetic body 25 are described with reference to FIG. 7. FIG. 7 is an exploded perspective view of the ferromagnetic body 25.
The ferromagnetic body 25 has a structure in which ferromagnetic layers 25a to 25g are laminated and integrally fired. The ferromagnetic layers 25a to 25d constitute a first ferromagnetic body 25A, and the ferromagnetic layers 25e to 25g constitute a second ferromagnetic body 25B.
On the upper side of the magnetic layer 25b are formed central conductors 26a and 26b. Also, on the upper sides of the magnetic layers 25c and 25d are formed central conductors 26c and 26d and central conductors 26e and 26f, respectively. The central conductors 26a and 26b are parallel with each other. Similarly, the central conductors 26c and 26d are formed in parallel to each other, and the central conductors 26e and 26f are also formed in parallel to each other. Namely, in this embodiment, each central conductor extending in a given direction comprises a pair of central conductors, as described above.
The central conductors 26a and 26b, the central conductors 26c and 26d, and the central conductors 26e and 26f are arranged so as to cross each other in the vicinity of the center. Also the central conductors 26a and 26b, the central conductors 26c and 26d, and the central conductors 26e and 26f are arranged with the ferromagnetic layers 25c and 25d, respectively, therebetween, and thus electrically isolated.
On the upper surfaces of the ferromagnetic layers 25a, 25e and 25g are formed earth electrodes 27a, 27b and 27c, respectively. On the upper surface of the ferromagnetic layer 25f are formed three capacity electrodes 28a, 28b and 28c. The capacity electrodes 28a to 28c are opposed to the earth electrodes 27b and 27c through the ferromagnetic layers 25f and 25g, respectively, to form three capacitors.
The central conductors 26a to 26f, the earth electrodes 27a to 27c and the capacity electrodes 28a to 28c are formed by coating conductive paste on the upper surfaces of magnetic green sheets, laminating the green sheets, and then integrally firing the green sheets. Namely, the ferromagnetic body 25 comprises an integrally sintered compact.
As shown in FIG. 8, on the side of the ferromagnetic body 25 are formed external electrodes 29a to 29f. The external electrodes 29a, 29c and 29e are connected to the earth electrodes 27a, 27b and 27c. The external electrode 29a is also connected to ends of the central conductors 26a and 26b. The external electrode 29c is connected to ends of the central conductors 26c and 26d. The external electrode 29e is electrically connected to ends of the central conductors 26e and 26f, i.e., the ends to be connected to the earth potential.
On the other hand, the external electrode 29b is connected to the other ends of the central conductors 26e and 26f. The external electrodes 29d and 29f are connected to the other ends of the central conductors 26a and 26b and the central conductors 26c and 26d, respectively.
Further, the external electrodes 29b, 29d and 29f are electrically connected to the capacity electrodes 28c, 28b and 28a, respectively.
Therefore, in the ferromagnetic body 25, the external electrodes 29b, 29d and 29f constitute portions to be connected to an input/output terminal, and the external electrodes 29a, 29c and 29e constitute connecting ends to be connected to the earth electrodes.
The external electrodes 29a to 29f are formed by coating conductive paste on the ferromagnetic body 25 obtained by integral firing, and then hardening or baking the conductive paste. Alternatively, the external electrodes 29a to 29f may be completed by laminating magnetic green sheets before firing, coating conductive paste on the side of the laminate and then integrally firing it to fire the magnetic material and bake the external electrodes 29a to 29f.
As described above, the ferromagnetic body 25 of this embodiment contains not only the plurality of central conductors 26a to 26f and the earth electrodes 27a and 27b, but also the capacity electrodes 28a to 28c and the earth electrode 27c for forming a matching circuit. Thus, there is no need for carrying out the troublesome work of joining the portion where the central conductors are arranged and the portion which the matching circuit is formed. Also the number of joints is decreased, and thus reliability is improved.
In addition, since the portion where the matching circuit is formed also comprises the ferromagnetic material, raw materials can be prepared by using the same line, and thus the matching circuit cost can be reduced.
Further, the saturation magnetization of the second ferromagnetic body 25B comprising the magnetic layers 25e to 25g is lower than the saturation magnetization of the first ferromagnetic body 25A comprising the magnetic layers of the portion where the central conductors are arranged, i.e., the magnetic layers 25a to 25d. Therefore, it is possible to reduce the magnetic material loss of the matching circuit.
This will be described with reference to FIG. 9. FIG. 9 shows the characteristics of the imaginary part (.mu.+") of the permeability for a positive circularly polarized wave to an external magnetic field. In FIG. 9, a solid line shows .mu.+" of the magnetic body which constitutes the central conductors, and a broken line shows .mu.+" of the ferromagnetic body with lower saturation magnetization which constitutes the matching circuit. Since the imaginary part of the permeability for a negative circularly polarized wave is close to zero, the magnetic material loss of the ferromagnetic material is proportional to the intensity of the imaginary part .mu.+" of permeability for a positive circularly polarized wave.
On the other hand, a nonreciprocal circuit device is generally formed so as to operate in the region A shown in FIG. 9. Therefore, it can be found that since the saturation magnetization of the second ferromagnetic body which forms the matching circuit is made lower than the saturation magnetization of the first ferromagnetic body where the central conductors are arranged, the magnetic material loss of the matching circuit can be reduced.
In this embodiment, each of the first and second ferromagnetic bodies 25A and 25B comprises, for example, microwave ferrite such as yttrium iron garnet or calcium-vanadium garnet, represented by Y.sub.3 Fe.sub.5-z Al.sub.z O.sub.12 or {Ca.sub.3-y Y.sub.y } �Fe.sub.2 ! (Fe.sub.1.5+0.5y-z Al.sub.z V.sub.1.5-5y)O.sub.12 (0<Z<1.0, 0<Y<3.0). The saturation magnetization of the second ferromagnetic body 25B can be decreased by relatively increasing the amount of Al (the Z value) in the microwave ferrite.
Although, in this embodiment, the saturation magnetization of the second ferromagnetic body is lower than the saturation magnetization of the first ferromagnetic body, the saturation magnetization of the second ferromagnetic body may be larger than that of the first ferromagnetic body. This also permits reduction in the magnetic material loss. Namely, although a nonreciprocal circuit device is generally operated in the region A shown in FIG. 9, it can also be operated in the region B. In this case, the magnetic material loss can be decreased in a similar manner to the above embodiment, i.e., the magnetic material loss of the matching circuit can be decreased by making the saturation magnetization of the ferromagnetic body which forms the matching circuit larger than that of the first ferromagnetic body. It can thus be found that the .mu.+" of the second ferromagnetic body which forms the matching circuit is as shown by a one-dot chain line in FIG. 9, and the magnetic material loss is decreased in the region B.
As described above, the saturation magnetization of either of the first and second ferromagnetic bodies may be increased. In both cases, the magnetic material loss of the matching circuit can be effectively reduced.
Returning to FIG. 6, the nonreciprocal circuit device of this embodiment comprises the ferromagnetic body 25 which is contained in the concave 9a of the terminal plate 9 and electrically connected to conductive patterns 10a to 10f formed on the terminal plate 9. Since the terminal plate 9 of this embodiment is the same as the terminal plate 9 shown in FIG. 1, the corresponding portions are denoted by corresponding reference numerals, and the description thereof is omitted.
On the ferromagnetic body 25 is fixed a permanent magnet 11. As the permanent magnet 11, an appropriate permanent magnet comprising ferrite or the like can be used.
Also metallic yokes 12 and 13, together with the permanent magnet, apply a magnetic field to the portion where the central conductors cross each other.
Although not shown in the drawings, the ferromagnetic body 25 may further comprise a magnetic circuit for applying a magnetic field to the portion where the central conductors cross each other. For example, a coiled conductive pattern may be integrally formed in the ferromagnetic body so that a magnetic field is generated by supplying the coiled conductive pattern with electricity.
Further, although, in this embodiment, each central conductor formed at a given level comprises a pair of central conductors, e.g., the central conductors 26a and 26b, as shown in FIG. 7, a single central conductor extending in a given direction at a given level may be provided.
Since the plurality of central conductors are formed in the first ferromagnetic body, and the matching circuit is formed in the second ferromagnetic body, dielectric materials which are fired under completely different firing conditions need not be used, and thus the firing conditions need not be greatly changed even when the first and second ferromagnetic bodies are fired separately. Also the number of the raw materials prepared can be reduced, and the manufacturing cost can thus be reduced.
Also, since the first and second ferromagnetic bodies have different saturation magnetizations, i.e., the saturation magnetization of the second ferromagnetic body is relatively decreased or relatively increased, the magnetic material loss of the matching circuit can be reduced. It is thus possible to provide a small, low-priced nonreciprocal circuit device having low insertion loss.
Further, when a magnetic circuit is integrally formed for applying a DC magnetic field to the first ferromagnetic body, means for applying a magnetic field to the central conductors can also be integrally provided, and a smaller nonreciprocal circuit device with excellent reliability can thus be provided.
Further, in a structure where plural pairs of capacity electrodes are provided to form the matching circuit, the capacity electrodes are formed on magnetic material green sheets and then fired. It is thus possible to easily form each of the capacitors for forming the matching circuit in the second ferromagnetic body by a ceramic firing technique.
Further, in a structure where the first and second ferromagnetic bodies are integrated by simultaneous firing, there is no need for the work of fixing the first and second ferromagnetic bodies, and thus a nonreciprocal circuit device having more excellent reliability can be obtained. Also, since the first and second ferromagnetic bodies need not be fired separately, the manufacturing process can significantly be simplified, and the same line can be used in the raw material preparing step, thereby significantly decreasing the manufacturing cost.

Number	Name	Date
4496915	Mathew et al.	Jan 1985
5498999	Marusawa et al.	Mar 1996
5786736	Dejima et al.	Jul 1998

Number	Date	Country
0664573	Jul 1995	EPX
7137534	Jun 1972	FRX
1359175	Jul 1974	GBX

Nonreciprocal circuit device comprising first and second ferromagnetic bodies having different saturation magnetizations

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US Referenced Citations (3)

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Non-Patent Literature Citations (5)

Entry
Patent Abstracts of Japan, vol. 6, No. 144 (E-122), Aug. 3, 1982 & JP 57 067308 A (Hitachi Metals Ltd), Apr. 23, 1982, abstract.
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Webb D C: "Design and Fabrication of Low-Cost Ferrite Circulators" Proceedings of the 25th European Microwave Conference 1995, Bologna, Sep. 4-7, 1995, vol. vol. 2, No. Conf. 25, Sep. 4, 1995, European Microwave Conference Committee, pp. 1191-1200, XP000681870 *p. 1195, left-hand column, line 10-line 22, figure 6.
C.P. Hartwig et al.: "Ferrite Film Circulator" Journal of Applied Physics, vol. 41, No. 3, Mar. 1, 1970, New York, U.S., pp. 1351-1352, XP002055955, whole document.
European Search Report dated Mar. 3, 1998.