Abstract:
A nonreciprocal circuit device requires less layout space when single-board capacitors are used, and meets demands for a smaller and lighter configuration. An isolator (nonreciprocal circuit device) comprises a ferrite, a permanent magnet applying a direct current magnetic field to the ferrite, a plurality of central electrodes respectively having ports disposed on the ferrite and a single-based matching capacitor with capacitor electrodes formed on both surfaces of a dielectric substrate such that the capacitor electrodes are opposed to each other and sandwich the dielectric substrate. In various embodiments, the permanent magnet and/or; the ferrite has a square shape and the capacitor electrodes of the matching capacitors are arranged at an angle of 60 to 90 degrees with respect a mounting surface and the matching capacitors are disposed so as to surround the sides of the ferrite.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nonreciprocal circuit device for use in the microwave band such as, for instance, an isolator or a circulator. 
     2. Description of the Related Art 
     Generally, a lumped constant isolator, used in mobile communication equipment such as mobile telephones, has a function which allows signals to pass only in the transmission direction while preventing transmission in the reverse direction. Furthermore, given the recent usage of mobile communication equipment, there are growing demands for smaller, lighter and less expensive devices. In the case of the isolator, there are similar demands for a smaller, lighter and cheaper device. 
     Conventionally, as shown in FIG. 6, this type of lumped constant isolator has a structure comprising top and bottom yokes  50  and  51  which contain, in sequence from the top, a permanent magnet  52 , a central electrode body  53 , a matching circuit board  54  and a ground board  55 . The central electrode body  53  comprises three central electrodes  57 , which intersect in an electrically insulated state on a disc-shaped ferrite  56 . 
     Furthermore, the matching circuit board  54  comprises a rectangular thin-board dielectric substrate  54   a , having a round hole  54   b , which the central electrode body  53  is inserted into, formed in the center thereof; and capacitor electrodes  58  . . . , which input/output ports P 1 -P 3  of the central electrodes  57  are connected to, formed around the round hole  54   b  in the dielectric substrate  54   a . Further, an end resistance film  59  is connected to the port P 3 . 
     However, since the above conventional matching circuit board  54  requires forming the round holes  54   b  in the thin-board dielectric substrate  54   a  and patterning the central electrodes  57 , there is a problem of complex processing during manufacture and assembly, increasing costs. 
     A further problem is that the parts other than the capacitor electrodes  58  unnecessarily increase the area and weight of the conventional dielectric substrate  54   a , making it more difficult to produce a smaller and lighter device. In this connection, recently there is a demand for reducing the weight of isolators to the milligram level. 
     Yet another problem of the conventional matching circuit board  54  is that, since the capacitor electrodes  58  are formed on a dielectric substrate  54   a  having high permittivity, adjacent capacitor electrodes  58  are prone to electrostatic coupling Cp, which is damaging to the attenuation properties of the isolator outside the band. 
     In another conventional isolator, a single plate capacitor, comprising opposing electrodes provided on either side of a dielectric substrate so as to completely cover the surfaces thereof, is used as each of the the capacitors in lieu of the matching circuit board. 
     This single plate capacitor can be manufactured by forming electrodes on the two main surfaces of a motherboard, which comprises a large flat board, and cutting the motherboard to predetermined dimensions. Such a single plate capacitor can therefore be mass-produced. Consequently, processing and handling are easier than when round holes and multiple capacitors are provided to a conventional dielectric substrate, and cost can be reduced. In addition, since electrodes are formed over the entire faces of the substrate, unnecessary increase of area and weight can be eliminated, thereby enabling the isolator to be made smaller and lighter by a proportionate amount. Moreover, since the capacitors are provided separately, it is possible to prevent electrostatic coupling between them and thereby avoid deterioration of attenuation properties outside the band. 
     FIG.  4  and FIG. 5 show a example of an isolator using a single plate capacitor and are not the prior art. Like members corresponding to those in FIG. 6 are designated by like reference characters. This isolator comprises a resin terminal block  60 , having a round hole  61  provided in the base wall  60   a  thereof, the central electrode body  53  being inserted into the round hole  61 ; rectangular single plate capacitors C 1 -C 3 , provided on the periphery of the round hole  61  so as to surround the central electrode body  53 ; and a single plate resistor R. 
     As shown in FIG. 5, when the single plate capacitors C 1 -C 3  are provided around the central electrode body  53 , unwanted vacant spaces  62  are created therebetween. This is an obstacle to making the device smaller and lighter, and the demand mentioned above cannot be fulfilled. 
     Moreover, although the above single plate capacitors C 1 -C 3  enable the isolator to be made smaller and lighter than the conventional device, a considerable amount of space is nevertheless taken up with respect to the whole of the isolator since the electrode area is determined by the required matching capacitance. This is a further obstacle to making the device small and light. 
     In order to reduce the size of the capacitors themselves, countermeasures such as the following have been considered and implemented: (1) use a high-permittivity material as the dielectric substrate; (2) further reduce the thickness of the dielectric substrate; (3) use laminated-chip capacitors. 
     However, in the case of (1), material having maximum permittivity of 100-120 is already being used. Material of even higher permittivity has unsuitable temperature characteristics and high-frequency characteristics decline, thus loss at the microwave band becomes considerably large. For these reasons, such material can not be employed. 
     Furthermore, in the case of (2), a substrate of approximate thickness 0.2 mm is generally used. Reducing the thickness even further would cause an extreme reduction in the strength of the substrate, worsening yield and consequently lowering productivity as well as lowering the reliability of product quality. 
     Finally, in the case of (3), laminated capacitors generally have Q of 20-100 at the microwave band. This is much lower than the single plate capacitor using dielectric material for high-frequency, which has Q of more than 200, causing further loss of the characteristics of isolator. Furthermore, although the conventional laminated capacitor has relatively small top area S of approximately 0.5 mm 2 , it is approximately 0.5 mm tall, and hence has volume V of 0.25 mm 3 . By contrast, the single plate capacitor has S of 1.2 mm 2  and V of approximately 0.24 mm 3 . Therefore, the size reduction achieved when using a laminated capacitor is hardly significant. 
     SUMMARY OF THE INVENTION 
     The present invention has been realized after consideration of the above points and aims to provide a nonreciprocal circuit device capable of reducing layout space when using single plate capacitors, and meeting demands for a smaller and lighter device. 
     The nonreciprocal circuit device of the present invention comprises a plurality of central electrodes provided to a ferrite, to which a permanent magnet applies a direct current magnetic field, ports of the central electrodes being connected to capacitors for matching; wherein the capacitors for matching comprise single plate capacitors, formed by providing electrodes on both main surfaces of a dielectric substrate such that the electrodes completely cover the main surfaces and oppose each other with the dielectric substrate disposed therebetween; and electrode surfaces of the single plate capacitors are provided at an angle of 60-90 degrees to a mounting surface. 
     A second aspect of the present invention comprises the nonreciprocal circuit device according to the first aspect, wherein at least a portion of electrodes at the cold (ground) ends of the single plate capacitors face the outside of the device. 
     A third aspect of the present invention comprises the nonreciprocal circuit device according to the first aspect, wherein at least a portion of electrodes at the hot (port) ends of the single plate capacitors face the outside of the device. 
     A fourth aspect of the present invention comprises the nonreciprocal circuit device according to any one of the first to third aspects, wherein the ferrite is square when viewed from the top and the single plate capacitors are provided so as to enclose the sides of the ferrite. 
     A fifth aspect of the present invention comprises the nonreciprocal circuit device according to any one of the first to fourth aspects, wherein the permanent magnet is square when viewed from the top. 
     A sixth aspect of the present invention comprises a nonreciprocal circuit element comprising a ferrite, a permanent magnet applying a direct current magnetic field to the ferrite, a plurality of central electrodes respectively having ports disposed on the ferrite and a matching capacitor with capacitor electrodes formed on both surfaces of a dielectric substrate such that the capacitor electrodes are opposed to each other and sandwich the dielectric substrate, wherein the ferrite has a square shape and the capacitor electrodes of the matching capacitors are inclined at an angle of 60 to 90 degrees toward a mounting surface and the matching capacitors are disposed so as to surround sides of the ferrite. 
     A seventh aspect of the nonreciprocal circuit device of the present invention comprises a plurality of central electrodes provided on a ferrite, to which a permanent magnet applies a direct current magnetic field, ports of the central electrodes being connected to capacitors for matching; wherein the capacitors for matching are single plate capacitors, comprising electrodes provided on both main surfaces of a dielectric substrate such that electrodes completely cover the main surfaces and oppose each other with the dielectric substrate disposed therebetween; the ferrite is square when viewed from the top, and the single plate capacitors are provided so as to enclose the ferrite. 
     A eighth aspect of the present invention comprises the nonreciprocal circuit device according to the seventh aspect, wherein the single plate capacitors are rectangular and extend along the sides of the ferrite. 
     A ninth aspect of the present invention comprises the nonreciprocal circuit device according to either of the seventh and eighth aspects, wherein the permanent magnet is square. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view explaining a lumped constant isolator according to an exemplary embodiment of the present invention; 
     FIG. 2 is a top view of the above isolator with the top yoke and the permanent magnet removed; 
     FIG. 3 is an exploded perspective view showing an isolator in another exemplary embodiment according to the present invention; 
     FIG. 4 is an exploded perspective view of an example of an isolator using a single plate capacitor; 
     FIG. 5 is a top view of the isolator shown in FIG. 4; 
     FIG. 6 is an exploded perspective view of a conventional isolator in general use; 
     FIG. 7 is an exploded perspective view explaining an lumped constant isolator according to another exemplary embodiment of the present invention; 
     FIG. 8 is a top view of the above isolator with the top yoke and the permanent magnet removed; 
     FIG. 9 is an exploded perspective view of an isolator according to another exemplary embodiment of the present invention; and 
     FIG. 10 is a diagram showing attenuation characteristics of the above isolator outside the band. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There will be detailed below the preferred embodiments of the present invention with reference to the accompanying drawings. 
     FIG.  1  and FIG. 2 are diagrams explaining a lumped constant isolator according to a first embodiment of the present invention, FIG. 1 showing an exploded perspective view of the isolator, and FIG. 2, a top view of the isolator when the top yoke and the permanent magnet are removed. 
     The lumped constant isolator  1  of the present embodiment comprises a resin terminal substrate  3  provided on a magnetic metallic bottom yoke  2 , having right-side and left-side walls  2   a  and  2   a  and a base wall  2   b . In addition, a central electrode assemblage  4  is provided on the terminal substrate  3 , and a box-shaped top yoke  5 , comprising the same magnetic metal as the bottom yoke  2 , is provided on top, thereby forming a magnetic closed circuit. Furthermore, a disc-shaped permanent magnet  6 , which applies a direct current magnetic field to the central electrode assemblage  4 , is affixed to the inner surface of the top yoke  5 . 
     The above isolator  1  is a parallelepiped with outer dimensions: top of less than 7.5×7.5 mm; height of less than 2.5 mm. The isolator  1  is surface-mounted on the base wall  2   b  and connected to conductive lines on a circuit board which is not shown in the diagram. 
     The central electrode assemblage  4  comprises three central electrodes 13-15, which intersect alternately every 120 degrees, provided in an electrically insulated state on the upper surface of a microwave ferrite  12 , which is square when viewed from above. Input/output ports P 1 -P 3  connected to one terminal side of each of the central electrodes  13 - 15  project outwards, and a shield  16 , which is shared by the other terminal sides of the central electrodes  13 - 15 , abuts to the lower surface of the ferrite  12 . This shield  16  is connected to the base wall  2   b  of the bottom yoke  2 . 
     The central electrodes 13-15 are arranged parallel towards the mounting surface. The input/output ports P 1 -P 3  of the central electrodes 13-15 are bent downwards at right angles towards the mounting surface. Furthermore, tips P 1   a  and P 2   a  of two of the input/output ports P 1  and P 2  are parallel with the mounting surface. 
     The terminal substrate  3  comprises a base wall  3   b , having a square hole  7  provided therein, secured in a single body to rectangular side walls  3   a . The ferrite  12  is inserted into the square hole  7  and secured in position. 
     Thus, the ground electrodes  8 , provided on the inner surfaces of the left, right and lower side walls  3   a , are connected to the ground terminals  9  and  9  provided on the outer surfaces of the left and right side walls  3   a . Furthermore, input/output ports  10  and  10  are provided at both ends of the upper edge of the base wall  3   b . These ports  10  are connected to input/output terminals  11  and  11  which are provided on the outer surfaces of the left and right side walls  3   a . The input/output terminals  11  and the ground terminals  9  are connected to conductive lines on a circuit board which is not depicted in the diagram. 
     Single plate capacitors C 1 -C 3 , which are provided on the inner surfaces of the left, right and lower side walls  3   a  of the terminal substrate  3 , fit along the sides  12   a  of the ferrite  12  so as to enclose the ferrite  12 . Furthermore, an end resistance R is provided on the lower side wall  3   a  in parallel with the single plate capacitor C 3 . The resistance R is connected to the ground terminal  9 . 
     Each of the single plate capacitors C 1 -C 3  is formed by providing capacitor electrodes on both main surfaces of a rectangular dielectric substrate in such a manner that the capacitor electrodes completely cover the main faces and oppose each other with the dielectric substrate disposed therebetween. Alternatively, the single plate capacitors C 1 -C 3  can be formed by patterning capacitor electrodes on a motherboard, comprising a large flat board, and cutting the motherboard into predetermined shapes. 
     Then, the single plate capacitors C 1 -C 3  are provided at an angle of 90 degrees, that is, perpendicular to the mounting surface. Furthermore, the electrodes at the cold ends of the single plate capacitors C 1 -C 3  are connected to the ground electrodes.  8 , and the electrodes at the hot ends are connected to the input/output ports P 1 -P 3 . Consequently, the cold end electrode sides of the single plate capacitors C 1 -C 3  are facing the outside of the isolator since the ground electrode  8  is connected to the ground terminal  9 . 
     Here the cold end of a capacitor means the electrode of the capacitor that is connected to the ground electrode. The hot end means a side of capacitor electrode connected to the port. 
     Furthermore, the tips P 1   a  and P 2   a  of the input/output ports P 1  and P 2  connect to the ports  10 . The tip P 3   a  of the remaining port P 3  is connected to the end resistance R. As above, the end resistance R is provided at an angle of 90 degrees to the mounting surface. 
     Now referring to FIGS. 7 and 8, the second embodiment of the present invention will be explained in detail. Same numerals are assigned to similar members of the first embodiment and the detailed explanation thereof is omitted. 
     As shown in FIG. 7, the terminal substrate  3  comprises a base wall  3   b , having a square hole  7  provided in the center thereof, secured in a single body to rectangular side walls  3   a . Recesses  3   c  for positioning capacitors are provided in the left, right and lower edges of the square hole  7  in the base wall  3   b , and a ground electrode  80  is provided on the bottom surface of each recess  3   c . These ground electrodes  80  are connected to ground terminals  9  and  9  provided on the outer surfaces of the left and right side walls  3   a.    
     Furthermore, input/output ports  10  and  10  are provided at the left and right upper ends of the base wall  3   b . These ports  10  are connected to input/output terminals  11  and  11  which are provided on the outer surfaces of the left and right side walls  3   a . The input/output terminals  11  and the ground terminals  9  are surface-mounted on a circuit board which is not depicted in the diagram. 
     Single plate capacitors for matching C 1 -C 3  are accommodated in the positioning recesses  3   c . The lower surfaces of the electrodes at the cold ends sides of the single plate capacitors C 1 -C 3  are connected to the ground electrodes  80 . Furthermore, an end resistance R is provided in parallel with the single plate capacitor C 3  inside the positioning recess  3   c . This end resistance R is connected to the ground terminal  9 . 
     The input/output ports Q 1 -Q 3  of the central electrodes  13 - 15  are connected to the upper surface electrodes at the hot ends, of the single plate capacitors C 1 -C 3 . Tips of two of the input/output ports Q 1  and Q 2  connect to the input/output ports  10 , and the tip of the remaining Q 3  is connected to the end resistance R. 
     Furthermore, the ferrite  12  is square and is inserted in the square hole  7  provided in the terminal substrate  3 . Consequently, the single plate capacitors C 1 -C 3  enclose the sides  12   a  of the ferrite  12  while also extending along these sides  12   a.    
     The nonreciprocal circuit device of the present invention includes that a ferrite has a circular shape and electrode surfaces of the single plate capacitors are disposed at an angle of 60 to 90 degrees to a mounting surface, although only a 90 degree angle is shown in some of the drawings. 
     Additionally the shape of the ferrite is not limited to being a square. For example, a circular shape as mentioned above or any other shapes may be employed. 
     FIG. 3 is a diagram illustrating a lumped constant isolator according to the third embodiment of the present invention. In the diagram, like members are designated by like reference characters. 
     The configuration of the lumped constant isolator  20  of the present embodiment is basically the same as the first embodiment already described, comprising single plate capacitors C 1 -C 3  provided at an angle of 90 degrees to the mounting surface. However, in the present embodiment, a square permanent magnet  21  applies the direct current magnetic field to the ferrite  12 . 
     FIG. 9 is a diagram illustrating a lumped constant isolator according to the fourth embodiment of the present invention. In the diagram, like members to those depicted in FIG. 1 are designated by like reference characters. 
     The configuration of the lumped constant isolator  20  of the present embodiment is basically the same as the second embodiment already described, comprising single plate capacitors C 1 -C 3  extending along the sides of the ferrite  12 , which is square. However, in the present embodiment, a permanent magnet  21 , which applies direct current magnetic field to the ferrite  12 , is square when viewed from the top. 
     According to the third and fourth embodiments, the ferrite  12  and the permanent magnet  21  are both square in shape. Consequently, an optimum magnetic field can be applied to the ferrite  12 , improving electrical characteristics. Furthermore, since the permanent magnet  21  is square, it can easily be manufactured by calcinating a cluster of magnetic blocks and cutting out pieces of predetermined thickness, thereby lowering costs in the same way as above. 
     Further, the above embodiments described an example of a lumped constant isolator, but the present invention can also be applied to a circulator, in addition to other nonreciprocal circuit devices used in high-frequency parts. 
     Next, the effects of the present embodiment will be explained. 
     According to the lumped constant isolator  1  of the present embodiment, since the single plate capacitors C 1 -C 3  are provided at an angle of 90 degrees to the mounting surface, the area occupied by the single plate capacitors C 1 -C 3  when viewed from the top can be greatly reduced. Therefore, the isolator can be made smaller by a proportionate amount, meeting the demand mentioned above. By providing the single plate capacitors C 1 -C 3  in a perpendicular position, the top area of the terminal substrate  3  can be reduced and the weight can be reduced by a proportionate amount. 
     It may be envisaged that providing the single plate capacitors C 1 -C 3  in a perpendicular position will increase the height of the isolator. However, the height of the single plate capacitors C 1 -C 3  can be accommodated enough by the thickness of the ferrite  12  and the gap between the ferrite  12  and the permanent magnet  6  without increasing the height of the isolator. The above gap is generally provided in order to prevent the permanent magnet from being so close to the high-frequency circuits that its electrical characteristics deteriorate. Therefore the thickness and the gap might be employed as play for accommodating the height of the single plate capacitors. 
     In the present embodiment, since the cold end electrodes of the single plate capacitors C 1 -C 3  face the outside of the isolator and the hot end electrodes face the inside, it is possible to prevent electromagnetic waves radiating from the hot ends from leaking to the outside. As a consequence, when the device is used in mobile communications equipment, unnecessary radiation inside the equipment can be reduced, contributing to stable operation. 
     According to the present embodiment, the single plate capacitors C 1 -C 3  are provided so as to enclose the sides  12   a  of the ferrite  12 , which is square. As a result, the area around the ferrite  12  can be utilized more efficiently without changing the actual area and capacity of the ferrite  12 , or the length and width of the central electrodes. Therefore, vacant space between the ferrite  12  and the single plate capacitors C 1 -C 3  can be eliminated, further contributing to making the isolator smaller and lighter. 
     Furthermore, since the ferrite  12  is square, it can easily be manufactured by calcinating a cluster of ferrite blocks and cutting out pieces of predetermined thickness, thereby lowering costs. In this connection, when manufacturing the conventional disc-shaped ferrite, there is a problem of high cost since ferrites must be formed individually from metal and then calcinated separately. 
     In the embodiment detailed above, the cold end electrodes of the single plate capacitors C 1 -C 3  face the outside of the isolator. However, according to the present invention, the hot end electrodes may face the outside. When the hot end electrodes face the outside, it is easier to send and receive signals to/from the outside. 
     Furthermore, the above embodiment described an example in which the single plate capacitors C 1 -C 3  were provided perpendicular to the mounting surface, but alternatively, although not shown, may be provided diagonal thereto. In such a case, the projected area when viewed from the top can be reduced, enabling the isolator to be made smaller. 
     According to the lumped constant isolator  1  of the present embodiment, since the single plate capacitors C 1 -C 3  are provided so as to enclose the sides  12   a  of the ferrite  12  which is square, the area around the ferrite  12  can be utilized more efficiently without changing the actual area and volume (capacity) of the ferrite, or the length and width of the central electrodes  13 - 15 . In this case, there is almost no change in the electrical characteristics of the device as compared with a case where a conventional medium size ferrite is used. Consequently, vacant space between the ferrite  12  and the single plate capacitors C 1 -C 3  can be eliminated, whereby the total size can be reduced and made lighter by a proportionate amount, fulfilling the demand mentioned above. 
     Furthermore, since the single plate capacitors C 1 -C 3  are rectangular in shape and extend along the sides  12   a  of the ferrite  12 , the area can be utilized more efficiently and size and weight can be further reduced. 
     Since the present embodiment uses the single plate capacitors C 1 -C 3 , manufacture is easy and mass-production is possible, as described above. Therefore, product cost can be reduced. Furthermore, processing and assembling are easier than when round holes and capacitor electrodes are formed on a thin flat board as in the conventional case. As a result, damage such as breakage can be avoided and reliability of product quality can be improved. 
     Furthermore, it is possible to prevent deterioration of attenuation characteristics of the isolator outside the band by reducing electrostatic coupling between the single plate capacitors C 1 -C 3 . That is, as shown in FIG. 10, when capacitor electrodes are formed on a conventional dielectric substrate, attenuation characteristics are liable to deteriorate at the second and third harmonies (broken line in FIG.  10 ). By contrast, in the present embodiment, it can be seen that attenuation characteristics outside the band are better (solid line in FIG.  10 ). This has the advantageous effect of attenuating unnecessary waves outside the waveband, thereby improving the electrical characteristics of the mobile communications device. 
     According to the present invention, since the ferrite and the permanent magnet are both square, there is the advantage that an optimum magnetic field can be applied to the ferrite, improving the electrical properties.