Patent Publication Number: US-8536956-B2

Title: Directional coupler

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a directional coupler for communication equipment. 
     2. Description of the Related Art 
     For example, a directional coupler described in Japanese Unexamined Patent Application Publication No. 11-261313 is known. As shown in  FIG. 9 , the directional coupler described in Japanese Unexamined Patent Application Publication No. 11-261313 includes multiple dielectric layers that are arranged on one another. Electrode patterns are provided on the dielectric layers. The directional coupler includes a first main line  33 , a second main line  34 , and a first sub-line  35 , each of which is composed of a strip line. Both the first main line  33  and the second main line  34  are coupled to the first sub-line  35 . The directional coupler is capable of realizing the basic operation in the same manner even if the function of the main lines is exchanged with that of the sub-line because of its structure. The same applies to a technical problem and solutions to the problem described below. 
     However, since the first main line  33  and the second main line  34  are electromagnetically coupled to a common part of the first sub-line  35  in the directional coupler described in Japanese Unexamined Patent Application Publication No. 11-261313, there is a problem in that the isolation between the first main line  33  and the second main line  34  is poor. 
     SUMMARY OF THE INVENTION 
     In order to resolve the above problems, preferred embodiments of the present invention provide a directional coupler having excellent isolation between main lines or between sub-lines. 
     A directional coupler according to a preferred embodiment of the present invention includes a main line including a first terminal and a second terminal; a first sub-line that is electromagnetically coupled to the main line and that includes a third terminal and a fourth terminal; a second sub-line that is electromagnetically coupled to the main line and that includes a fifth terminal and a sixth terminal; and a capacitive element connected between the fourth terminal and the fifth terminal. The fourth terminal and the fifth terminal are each terminated with a load. 
     With the above configuration, it is possible to improve the isolation characteristics between the first and second sub-lines in the directional coupler. 
     A directional coupler according to a preferred embodiment of the present invention preferably includes a multilayer body including a plurality of insulating layers that are stacked on each other. The main line, the sub-lines, and the capacitive element are preferably defined by conductive layers provided in the multilayer body. 
     With the above configuration, it is possible to improve the isolation characteristics between the first and second sub-lines to reduce the size of the directional coupler. 
     In the directional coupler according to a preferred embodiment of the present invention, a first main surface of the directional coupler is preferably used as a mounting surface, and the capacitive element is preferably provided between the main line and the sub-lines and the first main surface in the multilayer body. 
     With the above configuration, it is possible to reduce various electromagnetic effects of the mounting surface on the directional coupler that is mounted on the mounting surface. 
     In a circuit apparatus according to another preferred embodiment of the present invention, the directional coupler according to one of the above-described preferred embodiments of the present invention is preferably mounted on a board having a shielding effect. 
     With the above configuration, it is not necessary to provide a ground layer in the directional coupler which reduces the size of the directional coupler. 
     According to various preferred embodiments of the present invention, it is possible to improve the isolation characteristics between the first and second sub-lines in the directional coupler. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a directional coupler according to a preferred embodiment of the present invention. 
         FIGS. 2A and 2B  include an external perspective view and a top view of the directional coupler according to a preferred embodiment of the present invention. 
         FIG. 3  is an exploded perspective view of a multilayer body in the directional coupler according to a preferred embodiment of the present invention. 
         FIGS. 4A and 4B  include characteristic diagrams of the directional coupler according to a preferred embodiment of the present invention. 
         FIGS. 5A and 5B  include characteristic diagrams of a directional coupler of a modification of a preferred embodiment of the present invention. 
         FIGS. 6A and 6B  include characteristic diagrams of the directional coupler according to a preferred embodiment of the present invention. 
         FIGS. 7A and 7B  include characteristic diagrams of the directional coupler of a modification of a preferred embodiment of the present invention. 
         FIG. 8  is a diagram for describing how the directional coupler according to a preferred embodiment of the present invention is mounted on a mounting surface. 
         FIG. 9  is a diagram for describing a layering structure of a directional coupler in the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will herein be described in detail with reference to the attached drawings. 
       FIG. 1  is a circuit diagram of a directional coupler  10  according to a preferred embodiment of the present invention. 
       FIGS. 2A and 2B  include external views of the directional coupler  10 .  FIG. 3  is an exploded perspective view of the directional coupler  10 . 
     An exemplary circuit configuration of the directional coupler  10  will now be described. The directional coupler  10  includes external electrodes (terminals)  1  to  6 , a main line M, sub-lines S 1  and S 2 , termination resistors R 1  and R 2 , and a capacitive element C 1 . The main line M is connected between the external electrodes  1  and  2 . The sub-line S 1  is connected between the external electrodes  3  and  4  and is electromagnetically coupled to the main line M. The sub-line S 2  is connected between the external electrodes  5  and  6  and is electromagnetically coupled to the main line M. One end of the termination resistor R 1  is connected to the external electrode  4  and the other end thereof is grounded. One end of the termination resistor R 2  is connected to the external electrode  5  and the other end thereof is grounded. The capacitive element C 1  is connected between the external electrodes  4  and  5 . 
     Signals transmitted on the main line M of the directional coupler  10  include forward-direction signals that enter the directional coupler  10  from the external electrode  1  and exit from the external electrode  2  and backward-direction signals generated by the forward-direction signals that are reflected from a downstream circuit, enter the directional coupler  10  from the external electrode  2 , and exit from the external electrode  1 . For the forward-direction signals, the external electrode  1  functions as an input port and the external electrode  2  functions as an output port. For the backward-direction signals, the external electrode  2  functions as the input port and the external electrode  1  functions as the output port. The external electrode  3  functions as a coupling port for the forward-direction signals and the external electrode  6  functions as the coupling port for the backward-direction signals. The external electrodes  4  and  5  are each preferably used as a 50 Ω-termination port, for example. 
     In the directional coupler  10  having the above configuration, the electromagnetic coupling between the main line M and the sub-line S 1  causes a signal having power that is proportional to the power of the forward-direction signals to be output from the external electrode  3 . The electromagnetic coupling between the main line M and the sub-line S 2  causes a signal having power that is proportional to the power of the backward-direction signals to be output from the external electrode  6 . A predetermined frequency of these signals preferably is, for example, a frequency between 824 MHz and 915 MHz (Global System for Mobile Communications (GSM)800/900) or a frequency between 1,710 MHz and 1,910 MHz (GSM1800/1900). Signals output from the external electrodes  3  and  6  of the directional coupler are preferably input into an automatic gain control apparatus (not shown). 
     A coupling characteristic, an isolation characteristic, and a directional characteristic are preferably used as the main characteristics representing the performance of the directional coupler. The coupling characteristic indicates the relationship between the ratio in power between a signal input into the input port and a signal output from the coupling port, that is, an amount-of-attenuation S (3, 1) and the frequency. The isolation characteristic indicates the relationship between the ratio in power between a signal input from the output port and a signal output from the coupling port (that is, the amount-of-attenuation S (3, 2)) and the frequency. The directional characteristic indicates the relationship between the ratio between the coupling characteristic and the isolation characteristic (that is, the amount-of-attenuation S (3, 2)/(3, 1)) and the frequency. 
     A specific configuration of the directional coupler  10  will now be described.  FIG. 2A  is an external perspective view of the directional coupler  10 .  FIG. 2B  is a top view of the directional coupler  10 .  FIG. 3  is an exploded perspective view of a multilayer body  11  of the directional coupler  10  according to a preferred embodiment of the present invention. The layering direction is defined as the z-axis direction, the long-side direction of the directional coupler  10  in a plan view from the z-axis direction is defined as the x-axis direction, and the short-side direction of the directional coupler  10  in a plan view from the z-axis direction is defined as the y-axis direction in the following description. The x axis, the y axis, and the z axis are orthogonal to each other. 
     The multilayer body  11  includes external electrodes  14   a  to  14   f  (collectively referred to as an external electrode  14 ), the main line M, the sub-lines S 1  and S 2 , and the capacitive element C 1 , as shown in  FIGS. 2A and 2B  and  FIG. 3 . 
     The multilayer body  11  preferably has a rectangular or substantially rectangular parallelepiped shape, for example, as shown in  FIGS. 2A and 2B . The multilayer body  11  preferably includes insulating layers  12   a  to  12   g  (collectively referred to as an insulating layer  12 ) that are arranged from the positive direction to the negative direction of the z-axis direction in this order, as shown in  FIG. 3 . A mounting surface  15  of the directional coupler  10  is at the rear surface side of the layering surface of the insulating layer  12   g , which is the lowermost layer. The insulating layer  12  preferably is made of dielectric ceramics and has a rectangular or substantially rectangular shape. 
     The external electrodes  14   a ,  14   e , and  14   b  are provided on a side at the negative direction side of the y-axis direction of the multilayer body  11  so as to be arranged from the positive direction side to the negative direction side of the x-axis direction in this order and is arranged so as to extend over all the layers in the z-axis direction. The external electrodes  14   c ,  14   f , and  14   d  are provided on a side at the positive direction side of the y-axis direction of the multilayer body  11  so as to be arranged from the positive direction side to the negative direction side of the x-axis direction in this order and is arranged so as to extend over all the layers in the z-axis direction. 
     The main line M preferably includes a line portion  21 , as shown in  FIG. 3 . The line portion  21  preferably is a linear conductive layer provided on the insulating layer  12   e  and is connected to the external electrodes  14   a  and  14   b.    
     The sub-line S 1  preferably includes line portions  22   a ,  22   b , and  22   c  and via-hole conductors b 1  to b 2 , as shown in  FIG. 3 . The sub-line S 1  has a helical shape in which the sub-line S 1  circles counterclockwise from the positive side to the negative side of the z-axis direction. In the sub-line S 1 , an upper-side end in the counterclockwise circle is called an upper end and a lower-side end in the counterclockwise circle is called a lower end. 
     The line portion  22   a  is a linear conductive layer provided on the insulating layer  12   b . The upper end of the line portion  22   a  is connected to the external electrode  14   d . The line portion  22   b  is a linear conductive layer provided on the insulating layer  12   c . The line portion  22   c  is a linear conductive layer provided on the insulating layer  12   d . The lower end of the line portion  22   c  is connected to the external electrode  14   e . The via-hole conductor b 1  penetrates through the insulating layer  12   b  in the z-axis direction and connects the line portion  22   a  to the line portion  22   b . The via-hole conductor b 2  penetrates through the insulating layer  12   c  in the z-axis direction and connects the line portion  22   b  to the line portion  22   c.    
     The sub-line S 1  is connected between the external electrodes  14   d  and  14   e  in the above manner. In a plan view from the z-axis direction, an area m 11  of the main line M opposes areas s 11 , s 12 , and s 13  of the sub-line S 1  so as to be parallel or substantially parallel to the areas s 11 , s 12 , and s 13 . The main line M is electromagnetically coupled to the sub-line S 1  with these areas. 
     The sub-line S 2  preferably includes line portions  23   a ,  23   b , and  23   c  and via-hole conductor b 3  to b 4 , as shown in  FIG. 3 . The sub-line S 2  has a helical shape in which the sub-line S 2  circles clockwise from the positive side to the negative side of the z-axis direction. In the sub-line S 2 , an upper-side end in the clockwise circle is called an upper end and a lower-side end in the clockwise circle is called a lower end. 
     The line portion  23   a  preferably is a linear conductive layer provided on the insulating layer  12   b . The upper end of the line portion  23   a  is connected to the external electrode  14   c . The line portion  23   b  preferably is a linear conductive layer provided on the insulating layer  12   c . The line portion  23   c  preferably is a linear conductive layer provided on the insulating layer  12   d . The lower end of the line portion  23   c  is connected to the external electrode  14   f . The via-hole conductor b 3  penetrates through the insulating layer  12   b  in the z-axis direction and connects the line portion  23   a  to the line portion  23   b . The via-hole conductor b 4  penetrates through the insulating layer  12   c  in the z-axis direction and connects the line portion  23   b  to the line portion  23   c.    
     The sub-line S 2  is connected between the external electrodes  14 c and  14   f  in the above manner. In a plan view from the z-axis direction, an area m 21  of the main line M opposes areas s 21 , s 22 , and s 23  of the sub-line S 2  so as to be parallel or substantially parallel to the areas s 21 , s 22 , and s 23 . The main line M is electromagnetically coupled to the sub-line S 2  with these areas. 
     The capacitive element C 1  preferably includes planar conductive layers  24   a  and  24   b . The planar conductive layer  24   a  is provided on the insulating layer  12   f  and is connected to the external electrode  14   f . The planar conductive layer  24   b  is provided on the insulating layer  12   g  and is connected to the external electrode  14   e . The planar conductive layers  24   a  and  24   b  each preferably have a rectangular or substantially rectangular shape and are overlaid with each other in a plan view from the z-axis direction. Accordingly, a capacitance occurs between the planar conductive layers  24   a  and  24   b . The capacitive element C 1  is connected between the external electrode  14   f  and the external electrode  14   e.    
     It is possible to improve the isolation characteristic and the directional characteristic with the directional coupler  10  having the above configuration. 
       FIG. 4A  is a graph indicating a coupling characteristic E and an isolation characteristic F of a forward-direction signal of the directional coupler  10  in  FIG. 1  and  FIG. 4B  is a graph indicating a directional characteristic G thereof.  FIG. 5A  is a graph indicating the coupling characteristic E and the isolation characteristic F of a forward-direction signal in a configuration in the related art according to a modification and  FIG. 5B  is a graph indicating the directional characteristic G thereof.  FIG. 6A  is a graph indicating the coupling characteristic E and the isolation characteristic F of a backward-direction signal of the directional coupler  10  in  FIG. 1  and  FIG. 6B  is a graph indicating the directional characteristic G thereof.  FIG. 7A  is a graph indicating the coupling characteristic E and the isolation characteristic F of a backward-direction signal in the configuration in the related art and  FIG. 7B  is a graph indicating the directional characteristic G thereof. Marker frequencies m 1 , m 5 , and m 9  in the respective graphs indicate lower cut-off frequencies of GSM800/900, marker frequencies m 2 , m 6 , and m 10  in the respective graphs indicate upper cut-off frequencies of GSM800/900, marker frequencies m 3 , m 7 , and m 11  in the respective graphs indicate lower cut-off frequencies of GSM1800/1900, and maker frequencies m 4 , m 8 , and m 12  in the respective graphs indicate upper cut-off frequencies of GSM1800/1900. 
     In the directional coupler having the configuration in the related art, that is, in the circuit configuration before the capacitive element C 1  is added in  FIG. 1 , the isolation characteristic F and the directional characteristic G are increased with the increasing frequency, as shown in  FIGS. 5A and 5B . In contrast, in the directional coupler  10  in  FIG. 1 , the inductance of the sub-lines and the capacitance of the capacitive element cause series resonance and poles to appear around 1.5 GHz in the isolation characteristic F and the directional characteristic G. In addition, the frequencies at the poles are capable of being adjusted by adjusting the capacitance value of the capacitive element.  FIGS. 4A and 4B  include the graphs when the capacitance value is adjusted so that the most preferable isolation characteristic is acquired in a predetermined frequency domain.  FIGS. 4A and 4B  and  FIGS. 5A and 5B  show that the addition of the capacitive element C 1  increases the amount of attenuation, in addition to the isolation characteristic and the directional characteristic. 
     Since the lines are designed so as to be symmetrical to each other in terms of their lengths with respect to the input-output directions and the symmetry is maintained even with the capacitive element C 1  added in the directional coupler  10 , the above advantages for the forward-direction signals are also achieved for the backward-direction signals, as shown in  FIG. 6  and  FIG. 7 . 
     Furthermore, since the directional coupler  10  has symmetry, it is possible to receive the forward-direction signals and the backward-direction signals at the same sensitivity. Accordingly, it is possible to apply integrated circuits (ICs) having the same specifications to both of the sub-lines S 1  and S 2 . 
     The directional coupler  10  is fixed to a mounting board  13  shown in  FIG. 8  with solder  16  with the mounting surface  15  opposing the mounting board  13 . Various electrode patterns are provided on or in the mounting board  13  although not shown in  FIG. 8 . Various electromagnetic waves are emitted from the electrode patterns. 
     In the directional coupler  10 , the layers on which the sub-lines S 1  and S 2  are provided, the layer on which the main line M is provided, the layers on which the capacitive element C 1  is provided, and the mounting surface are arranged in this order from the positive direction side to the negative direction side of the z-axis direction. Accordingly, the capacitive element C 1  is positioned between the main line M and the sub-lines S 1  and S 2 , which are signal lines of the directional coupler  10 , and the mounting board. As a result, the signal lines of the directional coupler  10  are kept away from the mounting board by the distance corresponding to the capacitive element C 1  so as to reduce electromagnetic effects of the various electrode patterns on or in the mounting board on the directional coupler  10 . 
     Although the external electrodes  4  and  5  preferably have a 50 Ω termination impedance at the termination resistors R 1  and R 2 , respectively, the termination impedance may be shifted from 50 Ω. 
     The directional coupler  10  preferably has no shield conductive layer having ground voltage in the multilayer body. Accordingly, in a circuit apparatus (not shown) including the directional coupler, shielding is performed at the side of electronic components or the board so that no electromagnetic mutual interference occurs between the directional coupler and the electronic components or the electrode patterns on or in the mounting board. As a result, it is possible to reduce the space where the shield conductive layer or a shield terminal is provided and the material and the manufacturing cost of the shield conductive layer and the shield terminal in the directional coupler  10 . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.