Patent Publication Number: US-6987426-B2

Title: Nonreciprocal circuit element with input and output characteristic impedances matched

Description:
This application claims the benefit of priority to Japanese Patent Application No. 2003-111913, herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a nonreciprocal circuit element, particularly to a nonreciprocal circuit element capable of matching the input and output characteristic impedances. 
   2. Description of the Related Art 
   A lumped-constant nonreciprocal circuit element (isolator) is a high-frequency component for allowing a signal to pass in the transmission direction without loss while blocking a signal traveling in the reverse direction. It is typically used in a transmission circuit of a mobile communication apparatus such as a mobile phone. A known example of such an isolator is described in Japanese Unexamined Patent Application Publication No. 2000-151217. 
   The isolator described in the Japanese Unexamined Patent Application Publication No. 2000-151217 includes three pairs of central conductors, the three pairs crossing one another at an angle of about 120° relative to one another and being insulated from one another. In this isolator, the two conductors of each pair are not parallel to each other. With this structure, the isolator exhibits wideband electrical characteristics and isolation characteristics in a desired frequency band. 
   In general, in order to reduce the insertion loss of an isolator, the characteristic impedances of at least two central conductors connected to the input and output terminals of the isolator are preferably matched. 
   In the isolator described in the Japanese Unexamined Patent Application Publication No. 2000-151217, however, one of the two central conductors connected to the input and output terminals is disposed off the ferrite at their intersection. This means that one of the two central conductors is farther away from the shield plate (common electrode) than the other, the shield plate being disposed on a surface of the ferrite remote from the surface where the central conductors are disposed. Due to this difference between the two central conductors in distance to the ferrite, the characteristic impedances of the central conductors become mismatched, thus the insertion loss increases, and accordingly the transmission efficiency of a signal decreases. 
   One possible approach for matching the characteristic impedances of two central conductors is to make the width of one central conductor shorter than that of the other. Unfortunately, reducing the width of a central conductor makes the conductor mechanically weak. This is disadvantageous in the production of central conductors. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a nonreciprocal circuit element that is made superior in transmission efficiency by suppressing insertion loss without reducing the width of central conductors. 
   According to an aspect of the present invention, a nonreciprocal circuit element includes an input terminal, an output terminal, a magnetic plate, and a common electrode disposed on a first surface of the magnetic plate. The nonreciprocal circuit element further includes a first central conductor, a second central conductor, and a third central conductor, each including a pair of divisions. The three central conductors extend from the circumference of the common electrode in three different directions and are bent along the circumference of the magnetic plate towards a second surface of the magnetic plate so as to cross one another on the second surface of the magnetic plate at a predetermined angle relative to one another. The first and second central conductors are connected to the input and output terminals. In this nonreciprocal circuit element, the relationship θ 1 &gt;θ 2  is satisfied, where θ 1  is the angle between the pair of divisions of the first central conductor and θ 2  is the angle between the pair of divisions of the second central conductor, when the first central conductor is farther away from the magnetic plate than the second central conductor. 
   In the present invention, an angle between a pair of divisions is defined as an angle between two imaginary center lines crossing each other, the two imaginary center lines corresponding to the pair of divisions, respectively. 
   An imaginary center line of a division is defined as a line connecting the centers in the width direction at both extremities of the division so as to extend along the longitudinal direction of the division. 
   An extremity of a division is defined as a longitudinal end of the segment of the division, i.e., the segment overlapping the second surface of the magnetic plate. 
   According to the nonreciprocal circuit element of the present invention, the characteristic impedances of the first and second central conductors connected to the input and output terminals can be matched by satisfying the relationship θ 1 &gt;θ 2 , where θ 1  and θ 2  are as defined above. The insertion loss of the nonreciprocal circuit element can be reduced by matching the above-described characteristic impedances, and thereby the signal transmission efficiency can be improved. 
   The characteristic impedance of a central conductor increases as the angle between its divisions becomes larger. On the other hand, the characteristic impedance of a central conductor decreases as the distance between the central conductor and the opposing common electrode increases, the distance being defined by the thickness of the magnet plate. 
   In the present invention, the first central conductor which has a longer distance from the magnetic plate than the second central conductor is compensated for a decrease in characteristic impedance by making the angle between the divisions of the first central conductor larger than the angle between the divisions of the second central conductor. As a result of this compensation, the characteristic impedances of the first and second central conductors that are connected to the input and output terminals can be matched. 
   Furthermore, the characteristic impedances of the first and second central conductors can be matched only by adjusting θ 1  and θ 2 . This eliminates the need to reduce the width of divisions of the central conductors. This advantageously retains the mechanical strength of the divisions, and therefore the nonreciprocal circuit element can easily be produced. 
   In the nonreciprocal circuit element according to the present invention, the angle θ 2  is preferably 0°. This means that the divisions of the second central conductor are parallel to each other. 
   In order to match the characteristic impedances of the first and second central conductors, it is sufficient to adjust the angle between the divisions of the first central conductor if the divisions of the second central conductor are set parallel to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic plan view showing the main section of an isolator as an example of a nonreciprocal circuit element according to a first embodiment of the present invention; 
       FIG. 2  is a schematic perspective view showing the main section of an isolator as an example of a nonreciprocal circuit element according to a first embodiment of the present invention; 
       FIG. 3  is an exploded perspective view showing an isolator as an example of a nonreciprocal circuit element according to a first embodiment of the present invention; 
       FIG. 4  is an example of a circuit of a mobile phone including an isolator according to a first embodiment; 
       FIG. 5  is a schematic plan view showing the main section of an isolator as an example of a nonreciprocal circuit element according to a second embodiment of the present invention; 
       FIG. 6  is a schematic plan view showing the main section of an isolator as an example of a nonreciprocal circuit element according to a third embodiment of the present invention; 
       FIG. 7  is a schematic plan view showing the main section of an isolator as an example of a nonreciprocal circuit element according to a fourth embodiment of the present invention; 
       FIG. 8  is a schematic plan view showing the main section of an isolator as an example of a nonreciprocal circuit element according to a fifth embodiment of the present invention; 
       FIG. 9  is a Smith chart for isolators according to EXAMPLE 1 and COMPARATIVE EXAMPLE 1; 
       FIG. 10  is a graph showing a relationship between frequency and isolation of isolators according to EXAMPLE 1 and COMPARATIVE EXAMPLE 1; and 
       FIG. 11  is a graph showing a relationship between insertion loss and frequency of isolators according to EXAMPLE 1 and COMPARATIVE EXAMPLE 1. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   A first embodiment according to the present invention will now be described with reference to the attached drawings.  FIG. 1  is a schematic plan view showing the main section of an isolator as an example of a nonreciprocal circuit element according to the present invention.  FIG. 2  is a perspective view of the main section of the isolator.  FIG. 3  is an exploded perspective view of the isolator. 
   Referring to  FIGS. 1 and 2 , an isolator  1  according to this embodiment includes a magnetic assembly  10  and a permanent magnet  16  as major components. The magnetic assembly  10  includes a flat magnetic plate  15  made of ferrite; a common electrode  14  in the form of a metal plate provided on a bottom surface (a first surface)  15   b  of the magnetic plate  15 ; and first, second, and third central conductors  11 ,  12 , and  13 . Each of the three central conductors  11 ,  12 , and  13  extends radially in a different direction from the common electrode  14  and is bent along the magnetic plate  15  towards a top surface (a second surface)  15   a  of the magnetic plate  15 . 
   On the top surface  15   a,  the three central conductors  11 ,  12 , and  13  cross one another at a predetermined angle relative to one another, one overlapping another. Although not shown in the figures, the central conductors  11 ,  12 , and  13  are insulated from one another by an insulating sheet on the top surface  15   a  of the magnetic plate  15 . 
   The positional relationship among the central conductors  11 ,  12 , and  13  is described with reference to  FIG. 1 . The second central conductor  12  is disposed closest to the magnetic plate  15 , the first central conductor  11  is disposed on the second central conductor  12 , and the third central conductor  13  is disposed on the first central conductor  11 . In other words, the first central conductor  11  is farther away from the magnetic plate  15  than the second central conductor  12 . If this positional relationship between the first central conductor  11  and the second central conductor  12  is satisfied, the third central conductor  13  may be disposed on the first central conductor  11 , as shown in  FIGS. 1 and 2 , or may be disposed closest to the magnetic plate  15 . 
   Referring to  FIGS. 1 and 2 , the ends of the central conductors  11 ,  12 , and  13  are provided with ports P 1 , P 2 , and P 3 , respectively, such that the ports P 1 , P 2 , and P 3  protrude from the magnetic plate  15 . Matching capacitors C 1  and C 2  are connected to the ports P 1  and P 2 , respectively. A capacitor C 3  and a terminating resistor (resistor element) R are connected to the port P 3 . The magnetic assembly  10  including these electrical components and the permanent magnet  16  are disposed in a magnetic yoke functioning as a magnetic circuit. In this manner, the isolator  1  is constructed where the permanent magnet  16  applies a DC magnetic field to the magnetic assembly  10 . 
   In the isolator  1 , the port P 1  and the port P 2  are connected to an input terminal and an output terminal, respectively, of the isolator  1 . Thus, the first central conductor  11  and the second central conductor  12  are connected to the input and output terminals, respectively. 
   As shown in  FIGS. 1 and 2 , the central conductors  11  to  13  are integrally connected to one another at the common electrode  14  functioning as a grounding portion and extend from the common electrode  14  in three different directions. The central conductors  11  to  13  are accurately disposed at a predetermined angle relative to the magnetic plate  15 , and are bent towards the top surface  15   a  of the magnetic plate  15  so as to face the common electrode  14  disposed on the remote bottom surface  15   b  of the highly dielectric magnetic plate  15 . In this state, the central conductors  11  to  13  function as microstrip lines. 
   Referring to  FIGS. 1 and 2 , the first central conductor  11 , the second central conductor  12 , and the third central conductor  13  are provided with a slit  11   a,  a slit  12   a,  and a slit  13   a,  respectively. Each of the three central conductors  11  to  13  includes two conductor divisions generated by the corresponding slit. More specifically, the first central conductor  11  includes a division  11   b  and a division  11   c,  the second central conductor  12  includes a division  12   b  and a division  12   c,  and the third central conductor  13  includes a division  13   b  and a division  13   c.  The divisions  11   b,    11   c,    12   b,    12   c,    13   b,  and  13   c  are substantially linear conductors extending, with a constant width maintained, along the longitudinal direction of the respective central conductors  11 ,  12 , and  13 . 
   As shown in  FIG. 1 , the divisions  11   b  and  11   c  of the first central conductor  11  extend such that the slit  11   a  between the divisions  11   b  and  11   c  becomes narrower from the common electrode  14  towards the port P 1 . In other words, an imaginary center line L 11b , which is a longitudinal center line of the division  11   b,  and an imaginary center line L 11c , which is a longitudinal center line of the division  11   c,  are not parallel to each other. Hence, the imaginary center lines L 11b  and L 11c  cross each other at an angle θ 1 . In the present invention, θ 1  is defined as an angle between the divisions  11   b  and  11   c.    
   The imaginary center line L 11b  is defined as a line connecting the centers in the width direction at both extremities of the division  11   b  so as to extend along the longitudinal direction of the division  11   b.  The imaginary center line L 11c  is defined in the same manner in relation to the division  11   c.  From a different viewpoint, the imaginary center lines L 11b  and L 11c  divide the divisions  11   b  and  11   c,  respectively, into two equal subdivisions, because segments of the divisions  11   b  and  11   c  according to this embodiment, i.e., the segments overlapping the top surface  15   a  of the magnetic plate  15 , are substantially linear conductors extending, with a constant width maintained, along the longitudinal direction of the respective central conductors  11  and  12 . 
   Similarly, the divisions  12   b  and  12   c  extend such that the slit  12   a  between the divisions  12   b  and  12   c  becomes narrower from the common electrode  14  towards the port P 2 . In other words, an imaginary center line L 12b , which is a longitudinal center line of the division  12   b,  and an imaginary center line L 12c , which is a longitudinal center line of the division  12   c,  are not parallel to each other. Hence, the imaginary center lines L 12b  and L 12c  cross each other at an angle θ 2 . In the present invention, θ 2  is defined as an angle between the divisions  12   b  and  12   c.  Consequently, similarly with the divisions  11   b  and  11   c,  the imaginary center lines L 12b  and L 12c  divide the divisions  12   b  and  12   c,  respectively, into two equal subdivisions. 
   On the other hand, the divisions  13   b  and  13   c  of the third central conductor  13  extend parallel to each other. 
   According to this embodiment, θ 2  for the second central conductor  12  and θ 1  for the first central conductor  11 , which overlaps the second central conductor  12  and is farther away from the magnetic plate  15  than the second central conductor  12 , are determined so as to satisfy the relationship θ 1 &gt;θ 2 . 
   The angle θ 1  preferably ranges from 2° to 10°, and more preferably from 4° to 6°. The angle θ 2  preferably ranges from 0° to 4°, and more preferably from 0° to 2°. 
   In general, the characteristic impedance of a central conductor decreases as the distance between the central conductor and an opposing common electrode (e.g., common electrode  14 ) increases, the distance being defined by the thickness of a magnet plate (e.g., magnetic plate  15 ). In this embodiment, the first central conductor  11  has a longer distance from the magnetic plate  15  than the second central conductor  12 . So far as the characteristic impedance affected by the above-described distance is concerned, therefore, the first central conductor  11  has a smaller measurement than the second central conductor  12 . 
   On the other hand, the characteristic impedance of a central conductor increases as the angle between its divisions (e.g., divisions  11   b  and  11   c ) becomes larger. In this embodiment, it follows from the relationship θ 1 &gt;θ 2  that, for the characteristic impedance affected by the above-described angle, the first central conductor  11  has a larger measurement than the second central conductor  12 . 
   Consequently, in this embodiment, the first central conductor  11 , which has a longer distance from the magnetic plate  15  than the second central conductor  12 , is compensated for a decrease in characteristic impedance by making θ 1  larger than θ 2 , where θ 1  is the angle between the divisions  11   b  and  11   c  as defined above, and θ 2  is the angle between the divisions  12   b  and  12   c  as defined above. As a result of this compensation, the characteristic impedances of the central conductors  11  and  12  that are connected to the input and output terminals can be matched. To make the characteristic impedances match each other, θ 1  and θ 2  are adjusted. 
   Although the divisions  13   b  and  13   c  of the third central conductor  13  are parallel to each other in this embodiment, the divisions  13   b  and  13   c  may be formed such that the slit  13   a  between the division  13   b  and  13   c  becomes narrower from the common electrode  14  towards the port P 3 , as with the central conductors  11  and  12 , or may be formed such that the slit  13   a  becomes wider from the common electrode  14  to a halfway point and then narrower from the halfway point towards the port P 3 . Furthermore, the slit  13   a  may extend straight to a halfway point and then becomes narrower from the halfway point towards the port P 3 . 
   Regarding the respective capacitances Cap 1  and Cap 2  of the matching capacitors C 1  and C 2  connected to the central conductors  11  and  12 , the capacitance Cap 1  may be larger than or equal to the capacitance Cap 2 . The capacitance Cap 3  of the capacitor C 3  connected to the third central conductor  13  may be equal to either the capacitance Cap 1  or the capacitance Cap 2  or may be different from the capacitances Cap 1  and Cap 2 . 
   If the capacitance Cap 1  is larger than the capacitance Cap 2 , the center frequency for the reflection coefficient in the first central conductor  11  can be made to match that in the second central conductor  12 . This advantageously reduces insertion loss, and thereby increases the transmission efficiency of a signal. 
   Referring to  FIG. 3 , the isolator  1  includes a closed magnetic circuit (magnetic yoke) composed of a top yoke component  21  and a bottom yoke component  22 . A resin casing  23  is disposed between the top yoke component  21  and the bottom yoke component  22 . The resin casing  23  contains the rectangular permanent magnet  16 , a spacer  17 , the magnetic assembly  10 , capacitor plates  24 ,  25 , and  26  (C 1 , C 2 , and C 3 ), and a terminating resister  27  (R). The magnetic assembly  10  includes the magnetic plate  15  and the first, second, and third central conductors  11 ,  12 , and  13  wound around the magnetic plate  15 . The capacitor plate  24  is disposed on the first central conductor  11 , the capacitor plate  25  is disposed on the second central conductor  12 , and the capacitor  26  and the terminating resister  27  are disposed on the third central conductor  13 . 
   The plate capacitors  24 ,  25 , and  26  include the capacitors C 1 , C 2 , and C 3 , respectively. The terminating resister  27  includes the terminating resistor element R. 
     FIG. 4  is an example of a circuit of a mobile phone including the isolator  1  according to this embodiment. In this circuit, a duplexer  141  is connected to an aerial  140 ; an intermediate frequency (IF) circuit  144  is connected to an output of the duplexer  141  via a low-noise amplifier  142 , an inter-stage filter  148 , and a mixer  143 ; an IF circuit  147  is connected to an input of the duplexer  141  via the isolator  1 , a power amplifier  145 , and a mixer  146 ; and a local oscillator  150  is connected to the mixers  143  and  146  via a distributing transformer  149 . 
   The duplexer  141  includes, for example, two ladder SAW filters  138 . The input terminal of each of the ladder SAW filters  138  is connected to the aerial  140 , the output terminal of one ladder SAW filter  138  is connected to the low-noise amplifier  142 , and the output terminal of the other ladder SAW filter  138  is connected to the isolator  1 . 
   The isolator  1  described above, which is used in a circuit of a mobile phone, allows signals from the isolator  1  to the duplexer  141  to pass at low insertion loss, but causes high insertion loss with signals from the duplexer  141  to the isolator  1  to block such signals in that direction. Thus, the isolator  1  prevents undesired signals such as noise in the duplexer  141  from entering the power amplifier  145  in the reverse direction. 
   Second Embodiment 
   A second embodiment of the present invention will now be described with reference to the drawings.  FIG. 5  is a schematic plan view of the main section of an isolator according to this embodiment. In this embodiment, the angle θ 2  between the two divisions of a second central conductor is 0°. The reference numerals and symbols in  FIG. 5  refer to the same components as those with the same reference numerals and symbols in  FIG. 1 , and repeated descriptions of the same components are omitted or provided only briefly. 
   Referring to  FIG. 5 , a magnetic assembly  30  of an isolator according to this embodiment includes a magnetic plate  15 ; a common electrode (not shown) disposed on the bottom surface of the magnetic plate  15 ; and first, second, and third central conductors  31 ,  32 , and  13  protruding in three directions from the common electrode and being wrapped towards a top surface  15   a  of the magnetic plate  15 . 
   The positional relationship among the three central conductors at their intersection is as with the first embodiment. That is, the first central conductor  31  is farther away from the magnetic plate  15  than the second central conductor  32 . 
   As shown in  FIG. 5 , the first central conductor  31 , the second central conductor  32 , and the third central conductor  13  are provided with a slit  31   a,  a slit  32   a,  and a slit  13   a,  respectively. Each of the three central conductors  31 ,  32 , and  13  includes two conductor divisions generated by the corresponding slit. More specifically, the first central conductor  31  includes two divisions  31   b  and  31   c,  the second central conductor  32  includes two divisions  32   b  and  32   c,  and the third central conductor  13  includes two divisions  13   b  and  13   c.  The divisions  31   b,    31   c,    32   b,    32   c,    13   b,  and  13   c  are substantially linear conductors extending, with a constant width maintained, along the longitudinal direction of the respective central conductors  31 ,  32 , and  13 . 
   As shown in  FIG. 5 , the divisions  31   b  and  31   c  of the first central conductor  31  extend such that the slit  31   a  between the divisions  31   b  and  31   c  becomes narrower from the common electrode towards the port P 1 . In other words, an imaginary center line L 31b , which is a longitudinal center line of the division  31   b,  and an imaginary center line L 31c , which is a longitudinal center line of the division  31   c,  are not parallel to each other. Hence, the imaginary center lines L 31b  and L 31c  cross each other at an angle θ 1 . 
   In contrast, the divisions  32   b  and  32   c  extend such that the width of the slit  32   a  between the divisions  32   b  and  32   c  is constant from the common electrode towards the port P 2 . In other words, an imaginary center line L 32b , which is a longitudinal center line of the division  32   b,  and an imaginary center line L 32c , which is a longitudinal center line of the division  32   c,  are parallel to each other. Hence, the imaginary center lines L 32b  and L 32c  do not cross each other, that is, θ 2  is 0° in this embodiment of the present invention. 
   As a result, in this embodiment, the relationship between θ 1  for the first central conductor  31  and θ 2  for the second central conductor  32  is represented by θ 1 &gt;θ 2 =0°. 
   Here, the angle θ 1  preferably ranges from 2° to 10°, and more preferably from 4° to 6°. 
   In the isolator with the structure described above, as with the first embodiment, the characteristic impedances of the first and second central conductors  31  and  32  connected to the input and output terminals can be matched. 
   In this embodiment, since the divisions  32   b  and  32   c  of the second central conductor  32  are parallel to each other, it is sufficient to adjust only θ 1 , i.e., the angle between the divisions  31   b  and  31   c  of the first central conductor  31 , for characteristic impedance adjustment. 
   Third Embodiment 
   A third embodiment of the present invention will now be described with reference to the drawings.  FIG. 6  is a schematic plan view of the main section of an isolator according to this embodiment. In this embodiment, the two divisions of a first central conductor are parallel to each other from the common electrode to a halfway point and extend so as to converge from the halfway point towards the port, and the angle θ 2  between the two divisions of a second central conductor is 0°. The reference numerals and symbols in  FIG. 6  refer to the same components as those with the same reference numerals and symbols in  FIG. 1 , and repeated descriptions of the same components are omitted or provided only briefly. 
   Referring to  FIG. 6 , a magnetic assembly  40  of an isolator according to this embodiment includes a magnetic plate  15 ; a common electrode (not shown) disposed on the bottom surface of the magnetic plate  15 ; and first, second, and third central conductors  41 ,  42 , and  13  protruding in three directions from the common electrode and being wrapped towards a top surface  15   a  of the magnetic plate  15 . 
   The positional relationship among the three central conductors at their intersection is as with the first embodiment. That is, the first central conductor  41  is farther away from the magnetic plate  15  than the second central conductor  42 . 
   As shown in  FIG. 6 , the first central conductor  41 , the second central conductor  42 , and the third central conductor  13  are provided with a slit  41   a,  a slit  42   a,  and a slit  13   a,  respectively. Each of the three central conductors  41 ,  42 , and  13  includes two conductor divisions generated by the corresponding slit. More specifically, the first central conductor  41  includes two divisions  41   b  and  41   c,  the second central conductor  42  includes two divisions  42   b  and  42   c,  and the third central conductor  13  includes two divisions  13   b  and  13   c.  The divisions  41   b,    41   c,    42   b,    42   c,    13   b,  and  13   c  are substantially linear conductors extending, with a constant width maintained, along the longitudinal direction of the respective central conductors  41 ,  42 , and  13 . 
   As shown in  FIG. 6 , the divisions  41   b  and  41   c  of the first central conductor  41  on the top surface  15   a  of the magnetic plate  15  extend in parallel to each other from the common electrode to a halfway point and, from the halfway point, the divisions  41   b  and  41   c  extend such that the slit  41   a  between the divisions  41   b  and  41   c  becomes narrower towards the port P 1 . In other words, an imaginary center line L 41b  for the division  41   b  and an imaginary center line L 41c  for the division  41   c  are not parallel to each other. Hence, the imaginary center lines L 41b  and L 41c  cross each other at an angle θ 1 . 
   The imaginary center line L 41b  is defined as a line connecting the centers in the width direction at both extremities of the division  41   b  so as to extend along the longitudinal direction of the division  41   b.  The imaginary center line L 41c  is defined in the same manner in relation to the division  41   c.  Here, an extremity of a division of a central conductor is defined as a longitudinal end of the segment of the division, i.e., the segment overlapping the top surface  15   a  of the magnetic plate  15 . In short, the imaginary center lines L 41b  and L 41c  are as shown in  FIG. 6 , where the divisions  41   b  and  41   c  according to this embodiment are substantially linear conductors with a constant width along the longitudinal direction, and extend in parallel to each other up to a halfway point and, from the halfway point extend so as to converge towards the port  1 . 
   As a result, the imaginary center line L 41b  is defined as a line connecting points  41   b   1  and  41   b   2 , as shown in  FIG. 6 , where the points  41   b   1  and  41   b   2  are respectively the centers in the width direction at both longitudinal extremities of the division  41   b.  The imaginary center line L 41c  is defined as a line connecting points  41   c   1  and  41   c   2  in the same manner in relation to the division  41   c.    
   In contrast, the divisions  42   b  and  42   c  extend such that the width of the slit  42   a  between the divisions  42   b  and  42   c  is constant from the common electrode towards the port P 2 . In other words, an imaginary center line L 42b , which is a longitudinal center line of the division  42   b,  and an imaginary center line L 42c , which is a longitudinal center line of the division  42   c,  are parallel to each other. Hence, the imaginary center lines L 42b  and L 42c  do not cross each other, that is, θ 2  is 0° in this embodiment of the present invention. 
   As a result, in this embodiment, the relationship between θ 1  for the first central conductor  41  and θ 2  for the second central conductor  42  is represented by θ 1 &gt;θ 2 =0°. 
   Here, the angle θ 1  preferably ranges from 2° to 10°, and more preferably from 4° to 6°. 
   In the isolator with the structure described above, as with the first embodiment, the characteristic impedances of the first and second central conductors  41  and  42  connected to the input and output terminals can be matched. 
   In this embodiment, since the divisions  42   b  and  42   c  of the second central conductor  42  are parallel to each other, it is sufficient to adjust only θ 1 , i.e., the angle between the divisions  41   b  and  41   c  of the first central conductor  41 , for characteristic impedance adjustment. 
   Fourth Embodiment 
   A fourth embodiment of the present invention will now be described with reference to the drawings.  FIG. 7  is a schematic plan view of the main section of an isolator according to this embodiment. In this embodiment, the two divisions of a first central conductor extend so as to diverge from the common electrode to a halfway point and so as to converge from the halfway point towards the port, and the angle θ 2  between the two divisions of a second central conductor is 0°. The reference numerals and symbols in  FIG. 7  refer to the same components as those with the same reference numerals and symbols in  FIG. 1 , and repeated descriptions of the same components are omitted or provided only briefly. 
   Referring to  FIG. 7 , a magnetic assembly  50  of an isolator according to this embodiment includes a magnetic plate  15 ; a common electrode (not shown) disposed on the bottom surface of the magnetic plate  15 ; and first, second, and third central conductors  51 ,  52 , and  13  protruding in three directions from the common electrode and being wrapped towards a top surface  15   a  of the magnetic plate  15 . 
   The positional relationship among the three central conductors at their intersection is as with the first embodiment. That is, the first central conductor  51  is farther away from the magnetic plate  15  than the second central conductor  52 . 
   As shown in  FIG. 7 , the first central conductor  51 , the second central conductor  52 , and the third central conductor  13  are provided with a slit  51   a,  a slit  52   a,  and a slit  13   a,  respectively. Each of the three central conductors  51 ,  52 , and  13  includes two conductor divisions generated by the corresponding slit. More specifically, the first central conductor  51  includes two divisions  51   b  and  51   c,  the second central conductor  52  includes two divisions  52   b  and  52   c,  and the third central conductor  13  includes two divisions  13   b  and  13   c.  The divisions  51   b,    51   c,    52   b,    52   c,    13   b,  and  13   c  are substantially linear conductors extending, with a constant width maintained, along the longitudinal direction of the respective central conductors  51 ,  52 , and  13 . 
   As shown in  FIG. 7 , the divisions  51   b  and  51   c  of the first central conductor  51  on the top surface  15   a  of the magnetic plate  15  extend such that the slit  51   a  between the divisions  51   b  and  51   c  becomes wider from the common electrode to a halfway point and, from the halfway point, the slit  51   a  becomes narrower towards the port P 1 . In other words, an imaginary center line L 51b  for the division  51   b  and an imaginary center line L 51c  for the division  51   c  are not parallel to each other. Hence, the imaginary center lines L 51b  and L 51c  cross each other at an angle θ 1 . 
   The imaginary center line L 51b  is defined as a line connecting the centers in the width direction at both extremities of the division  51   b  so as to extend along the longitudinal direction of the division  51   b.  The imaginary center line L 51c  is defined in the same manner in relation to the division  51   c.  Here, an extremity of a division of a central conductor is defined as a longitudinal end of the segment of the division, i.e., the segment overlapping the top surface  15   a  of the magnetic plate  15 . In short, the imaginary center lines L 51b  and L 51c  are as shown in  FIG. 7 , where the divisions  51   b  and  51   c  according to this embodiment are substantially linear conductors with a constant width along the longitudinal direction, and extend so as to diverge up to a halfway point and, from the halfway point extend so as to converge towards the port  1 . 
   As a result, the imaginary center line L 51b  is defined as a line connecting points  51   b   1  and  51   b   2 , as shown in  FIG. 7 , where the points  51   b   1  and  51   b   2  are respectively the centers in the width direction at both longitudinal extremities of the division  51   b.  The imaginary center line L 51c  is defined as a line connecting points  51   c   1  and  51   c   2  in the same manner in relation to the division  51   c.    
   In contrast, the divisions  52   b  and  52   c  extend such that the width of the slit  52   a  between the divisions  52   b  and  52   c  is constant from the common electrode towards the port P 2 . In other words, an imaginary center line L 52b , which is a longitudinal center line of the division  52   b,  and an imaginary center line L 52c , which is a longitudinal center line of the division  52   c,  are parallel to each other. Hence, the imaginary center lines L 52b  and L 52c  do not cross each other, that is, θ 2  is 0° in this embodiment of the present invention. 
   As a result, in this embodiment, the relationship between θ 1  for the first central conductor  51  and θ 2  for the second central conductor  52  is represented by θ 1 &gt;θ 2 =0°. 
   Here, the angle θ 1  preferably ranges from 2° to 10°, and more preferably from 4° to 6°. 
   The isolator with the structure described above can offer the similar advantages to those of the isolators according to the second and third embodiments. 
   Fifth Embodiment 
   A fifth embodiment of the present invention will now be described with reference to the drawings.  FIG. 8  is a schematic plan view of the main section of an isolator according to this embodiment. In this embodiment, the two divisions of a first central conductor are shaped like arcs and extend so as to converge towards the port, and the angle θ 2  between the two divisions of a second central conductor is 0°. The reference numerals and symbols in  FIG. 8  refer to the same components as those with the same reference numerals and symbols in  FIG. 1 , and repeated descriptions of the same components are omitted or provided only briefly. 
   Referring to  FIG. 8 , a magnetic assembly  60  of an isolator according to this embodiment includes a magnetic plate  15 ; a common electrode (not shown) disposed on the bottom surface of the magnetic plate  15 ; and first, second, and third central conductors  61 ,  62 , and  13  protruding in three directions from the common electrode and being wrapped towards a top surface  15   a  of the magnetic plate  15 . 
   The positional relationship among the three central conductors at their intersection is as with the first embodiment. That is, the first central conductor  61  is farther away from the magnetic plate  15  than the second central conductor  62 . 
   As shown in  FIG. 8 , the first central conductor  61 , the second central conductor  62 , and the third central conductor  13  are provided with a slit  61   a,  a slit  62   a,  and a slit  13   a,  respectively. Each of the three central conductors  61 ,  62 , and  13  includes two conductor divisions generated by the corresponding slit. More specifically, the first central conductor  61  includes two divisions  61   b  and  61   c,  the second central conductor  62  includes two divisions  62   b  and  62   c,  and the third central conductor  13  includes two divisions  13   b  and  13   c.  The divisions  61   b,    61   c,    62   b,    62   c,    13   b,  and  13   c  are substantially linear or curved conductors extending, with a constant width maintained, along the longitudinal direction of the respective central conductors  61 ,  62 , and  13 . 
   As shown in  FIG. 8 , the segments of the divisions  61   b  and  61   c  of the first central conductor  61  on the top surface  15   a  of the magnetic plate  15  are shaped like arcs in plan view, and extend such that the slit  61   a  between the divisions  61   b  and  61   c  becomes narrower towards the port P 1 . In other words, an imaginary center line L 61b  for the division  61   b  and an imaginary center line L 61c  for the division  61   c  are not parallel to each other. Hence, the imaginary center lines L 61b  and L 61c  cross each other at an angle θ 1 . 
   The imaginary center line L 61b  is defined as a line connecting the centers in the width direction at both extremities of the division  61   b  so as to extend along the longitudinal direction of the division  61   b.  The imaginary center line L 61c  is defined in the same manner in relation to the division  61   c.  Here, an extremity of a division of a central conductor is defined as a longitudinal end of the segment of the division, i.e., the segment overlapping the top surface  15   a  of the magnetic plate  15 . In short, the imaginary center lines L 61b  and L 61c  are as shown in  FIG. 8 , where the divisions  61   b  and  61   c  according to this embodiment are substantially arc conductors in plan view with a constant width along the longitudinal direction, and extend so as to converge towards the port  1 . 
   As a result, the imaginary center line L 61b  is defined as a line connecting points  61   b   1  and  61   b   2 , as shown in  FIG. 8 , where the points  61   b   1  and  61   b   2  are respectively the centers in the width direction at both longitudinal extremities of the division  61   b.  The imaginary center line L 61c  is defined as a line connecting points  61   c   1  and  61   c   2  in the same manner in relation to the division  61   c.    
   In contrast, the divisions  62   b  and  62   c  extend such that the width of the slit  62   a  between the divisions  62   b  and  62   c  is constant from the common electrode towards the port P 2 . In other words, an imaginary center line L 62b , which is a longitudinal center line of the division  62   b,  and an imaginary center line L 62c , which is a longitudinal center line of the division  62   c,  are parallel to each other. Hence, the imaginary center lines L 62b  and L 62c  do not cross each other, that is, θ 2  is 0° in this embodiment of the present invention. 
   As a result, in this embodiment, the relationship between θ 1  for the first central conductor  61  and θ 2  for the second central conductor  62  is represented by θ 1 &gt;θ 2 =0°. 
   Here, the angle θ 1  preferably ranges from 2° to 10°, and more preferably from 4° to 6°. 
   The isolator with the structure described above can offer the similar advantages to those of the isolators according to the second, third, and fourth embodiments. 
   EXAMPLES 
   Isolator According to EXAMPLE 1 
   The characteristic impedance, isolation value, and insertion loss of an isolator with the same structure as the isolator according to the second embodiment in  FIG. 5  were measured. 
   The isolator included a magnetic plate in the form of a substantially hexagonal plate made of yttrium iron garnet ferrite (YIG ferrite) 1.8 mm in long side, 1.5 mm in short side, and 0.35 mm in thickness. A first, second, and third central conductors were copper foils 1.6 mm in length, 0.15 mm in effective width, and 0.04 mm in thickness. The widths of the divisions of each central conductor were 0.15 mm, and the widths of the slits of the central conductors ranged from about 0.2 mm to 0.25 mm. These three central conductors extended in three directions from a substantially hexagonal common electrode. 
   Angle θ 1  between the divisions of the first central conductor was 7°, and angle θ 2  between the divisions of the second central conductor was 0°. 
   The common electrode was disposed on the bottom surface of the magnetic plate and the first, second, and third central conductors were folded towards the top surface of the magnetic plate to produce a magnetic assembly as shown in  FIG. 5 . 
   Next, a capacitor C 1  was mounted on a port P 1 , which was at the end of the first central conductor, a capacitor C 2  was mounted on a port P 2 , which was at the end of the second central conductor, and capacitor C 3  was mounted on a port P 3 , which was at the end of the third central conductor. Furthermore, a terminating resistor R was mounted on the capacitor C 3 . Then, the magnetic assembly with a permanent magnet attached on the magnetic plate was placed in a closed magnetic circuit composed of a top yoke component and a bottom yoke component to produce the isolator used in EXAMPLE 1. 
   In this isolator, the capacitance of the capacitor C 1  was 5.1 pF, the capacitance of the capacitor C 2  was 5.1 pF, the capacitance of the capacitor C 3  was 12.0 pF, and the resistance of the terminating resistor R was 120 Ω. The isolator was designed to have a characteristic impedance of 50 Ω and a center frequency of 1.88 GHz for isolation value. 
   Isolator According to COMPARATIVE EXAMPLE 1 
   An isolator same as the isolator according to EXAMPLE 1 was produced, with the exception of the angle θ 1  between the divisions of the first central conductor being 0°. The isolator for COMPARATIVE EXAMPLE 1 was also designed to have a characteristic impedance of 50 Ω and a center frequency of 1.88 GHz for isolation value. 
   The characteristics impedance, isolation value, and insertion loss of each of the isolators for EXAMPLE 1 and COMPARATIVE EXAMPLE 1 were measured.  FIGS. 9 to 11  show the results. 
     FIG. 9  is a Smith chart showing a relationship between the reflection coefficient and the characteristic impedance of each of the isolator according to EXAMPLE 1 and the isolator according to COMPARATIVE EXAMPLE 1. 
   In  FIG. 9 , compared with the isolator according to COMPARATIVE EXAMPLE 1, the curve of the isolator according to EXAMPLE 1 was closer to 50 Ω at the circled portions. This means that the isolator according to EXAMPLE 1 exhibited a characteristic impedance more faithfully representing the design value. This is because the divisions of the first central conductor of the isolator according to EXAMPLE 1 were made so as to converge. 
     FIG. 10  shows the frequency characteristics of isolation. Table 1 shows the isolation values at frequencies of 1.85 GHz and 1.91 GHz. As shown in  FIG. 10  and Table 1, the isolator according to EXAMPLE 1 and the isolator according to COMPARATIVE EXAMPLE 1 exhibited almost the same isolation characteristics at the center frequency and its surroundings (1.85 to 1.91 GHz). This means that the isolation characteristics of the isolator according to EXAMPLE 1, where the divisions of the first central conductor were made to converge, were not degraded. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Frequency (GHz) 
               Isolation Value (dB) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               EXAMPLE 1 
               1.85 
               −20.44 
             
             
                 
               EXAMPLE 1 
               1.91 
               −21.02 
             
             
                 
               COMPARATIVE 
               1.85 
               −21.87 
             
             
                 
               EXAMPLE 1 
             
             
                 
               COMPARATIVE 
               1.91 
               −20.82 
             
             
                 
               EXAMPLE 1 
             
             
                 
                 
             
          
         
       
     
   
     FIG. 11  shows the frequency characteristics of insertion loss. The isolator according to EXAMPLE 1 exhibited superior frequency characteristics because it had less insertion loss than the isolator according to COMPARATIVE EXAMPLE 1 at the center frequency and its surroundings (1.85 to 1.91 GHz). 
   From the results of  FIGS. 10 and 11 , it follows that the isolator according to EXAMPLE 1 reduces insertion loss without degrading the isolation characteristics.