Patent Publication Number: US-7595703-B2

Title: Balun and electronic device using this

Description:
This application is a U.S. National Phase Application of PCT International Application PCT/JP2007/051054. 
     TECHNICAL FIELD 
     The present invention relates to a balun for converting an unbalanced signal to a balanced signal and to electronic apparatuses such as portable terminals using the balun. 
     BACKGROUND ART 
     Description of a conventional balun will be given in the following referring to  FIG. 4 ,  FIG. 5 , and  FIG. 6 . 
       FIG. 4  is a circuit diagram of a conventional balun. In  FIG. 4 , conventional balun  1  comprises input terminal  2 , coupled line  3  connected to input terminal  2 , and first output terminal  4  and second output terminal  5  connected to coupled line  3 . 
     Additionally, first capacitor  6  is connected between input terminal  2  and coupled line  3  in shunt with ground. Also, the lines from coupled lines  3  to first output terminal  4  and from coupled line  3  to second output terminal  5  are RF-coupled through second capacitor  7 . 
     Also, coupled line  3  includes first unbalanced line  8  connected to input terminal  2 , and second unbalanced line  9  connected between first unbalanced line  8  and ground. Furthermore, coupled line  3  includes first balanced line  10  connected between ground and first output terminal  4  and electromagnetically coupled to first unbalanced line  8 , and second balanced line  11  connected between ground and second output terminal  5  and electromagnetically coupled to second unbalanced line  9 . 
       FIG. 5  is a sectional view of a conventional balun. In  FIG. 5 , conventional balun  1  includes first dielectric layer  12  and second dielectric layer  13  on top of first dielectric layer  12 . First unbalanced line  8  and second unbalanced line  9  are disposed on the same plane on top of second dielectric layer  13 . Furthermore, first balanced line  10  and second balanced line  11  are disposed on the same plane on top of first dielectric layer  12  under first unbalanced line  8  and second unbalanced line  9 , respectively, through second dielectric layer  13 . 
       FIG. 6  is a perspective view of a coupled line of a conventional balun. As can be seen in  FIG. 6 , first unbalanced line  8  and second unbalanced line  9  are connected by folding back. Size reduction of balun  1  is achieved with this design. 
     As a related art document relating to the invention of this application, Patent Document 1 is known, for example: 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-190413. 
     However, as first unbalanced line  8  and second unbalanced line  9  are connected by folding back, first unbalanced line  8  and second unbalanced line  9  come close to each other. As a result, the current of a radio frequency signal propagating in first unbalanced line  8  and the current of a radio frequency signal propagating in second unbalanced line  9  cancel each other thus resulting in attenuation of balanced signals obtained from first output terminal  4  and second output terminal  5  of balun  1  when compared with an unbalanced signal inputted to input terminal  2  of conventional balun  1 . 
     SUMMARY OF THE INVENTION 
     The present invention provides a balun in which, when a first unbalanced line and a second unbalanced line are connected by folding back, attenuation of a balanced signal output from the output terminal is suppressed compared with an unbalanced signal inputted to the input terminal. 
     The balun in accordance with the present invention includes an input terminal, a coupled line connected to the input terminal, and a first output terminal and a second output terminal connected to the coupled line. And the coupled line includes a first unbalanced line connected to the input terminal, a second unbalanced line connected to the first unbalanced line by folding back with its free end connected to ground, a first balanced line connected between ground and the first output terminal, and a second balanced line connected between ground and the second output terminal. Also, the first balanced line and the second balanced line are disposed substantially on the same plane. The first unbalanced line is disposed above the first balanced line at a predetermined distance from the first balanced line. The second unbalanced line is disposed below the second balanced line at the predetermined distance from the second balanced line. 
     With this configuration, the distance between the first unbalanced line and the second unbalanced line is increased thereby suppressing a current flowing in the first unbalanced line and a current flowing in the second unbalanced line from greatly cancelling with each other. As a result, when the first unbalanced line and the second unbalanced line are connected by folding back, attenuation of a balanced signal outputted from the output terminal can be suppressed compared with the unbalanced signal inputted to the input terminal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a circuit diagram of a balun in a first exemplary embodiment of the present invention. 
         FIG. 1B  is a block diagram of an electronic apparatus that uses a balun in the first exemplary embodiment of the present invention. 
         FIG. 2  is a sectional view of a balun in the first exemplary embodiment of the present invention. 
         FIG. 3  is a perspective view of a coupled line of a balun in the first exemplary embodiment of the present invention. 
         FIG. 4  is a circuit diagram of a conventional balun. 
         FIG. 5  is a sectional view of a conventional balun. 
         FIG. 6  is a perspective view of a coupled line of a conventional balun. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           14  Balun 
           15  Input terminal 
           16  Coupled line 
           17  First output terminal 
           18  Second output terminal 
           21  First unbalanced line 
           22  Second unbalanced line 
           23  First balanced line 
           24  Second balanced line 
           100  Electronic apparatus 
           101  BPF (bandpass filter) 
           102  RF circuit 
           103  Demodulator 
           104  Display 
           105  Antenna 
       
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Exemplary Embodiment 
     Referring to  FIG. 1A ,  FIG. 1B ,  FIG. 2 , and  FIG. 3 , description of the first exemplary embodiment will be given below. 
       FIG. 1A  is a circuit diagram of a balun in the first exemplary embodiment of the present invention. In  FIG. 1A , balun  14  includes input terminal  15 , coupled line  16  connected to input terminal  15 , and first output terminal  17  and second output terminal  18  connected to an output side of coupled line  16 . First capacitor  19  is connected between input terminal  15  and coupled line  16  in shunt with ground. Also, the lines between coupled line  16  and first output terminal  17  and between coupled line  16  and second output terminal  18  are RF (radio frequency)-coupled through second capacitor  20 . 
     Also, coupled line  16  includes first unbalanced line  21  connected to input terminal  15  and second unbalanced line  22  connected between first unbalanced line  21  and ground. Furthermore, coupled line  16  includes first balanced line  23  connected between ground and first output terminal  17  and electromagnetically coupled to first unbalanced line  21 , and second balanced line  24  connected between ground and second output terminal  18  and electromagnetically coupled to second unbalanced line  22 . 
     Each of first unbalanced line  21 , second unbalanced line  22 , first balanced line  23 , and second balanced line  24  is of a length equal to odd number multiples of nearly quarter-wavelength of the respective input/output signals. Generally, the length is nearly quarter-wavelength of the input/output signals for the purpose of miniaturization. 
     In the above configuration, balun  14  converts an unbalanced signal inputted at input terminal  15  into a balanced signal which is reversed in phase and equal in amplitude, and outputs the balanced signal from first output terminal  17  and second output terminal  18 . 
       FIG. 1B  is a block diagram of an electronic apparatus that uses a balun in accordance with the first exemplary embodiment of the present invention. Electronic apparatus  100  using balun  14  is a television receiver. The television receiver includes RF circuit  102  connected to output sides of first output terminal  17  and second output terminal  18 , demodulator  103  connected to an output side of RF circuit  102 , and display  104  connected to an output side of demodulator  103 . An output signal from antenna  105  is inputted to input terminal  15  through BPF  101 . 
     RF circuit  102  is implemented as an integrated circuit for the sake of miniaturization. Also, display  104  is configured with components including CRT, LCD, PDP. 
       FIG. 2  is a sectional view of a balun in the first exemplary embodiment of the present invention. In  FIG. 2 , balun  14  includes first dielectric layer  25 , second dielectric layer  26  on top of first dielectric layer  25 , and third dielectric layer  27  on top of second dielectric layer  26 . First balanced line  23  and second balanced line  24  are disposed on the same plane on second dielectric layer  26 . Accordingly, the distance between first balanced line  23  and a shield pattern (not shown) and the distance between second balanced line  24  and the shield pattern, for example, are made nearly equal, and uniformity of the amplitude of a signal to be outputted from first balanced line  23  and the amplitude of a signal to be outputted from second balanced line  24  can be enhanced. 
     Furthermore, first unbalanced line  21  is disposed on third dielectric layer  27  and above first balanced line  23  at a predetermined distance d from first balanced line  23 . Also, second unbalanced line  22  is disposed on first dielectric layer  25  and below second balanced line  24  at the predetermined distance d from second balanced line  24 . Though attenuation of a balanced signal can be suppressed by choosing a larger value of d, it is not appropriate for miniaturization. With the balun in accordance with the present application, the d-value chosen is in the range 0.1 mm to 0.2 mm for optimization of attenuation suppression of the balanced signal and miniaturization. 
       FIG. 3  is a perspective view of a coupled line of a balun in the first exemplary embodiment of the present invention. As shown in  FIG. 3 , first unbalanced line  21  and second unbalanced line  22  are connected by folding back. Further miniaturization of balun  14  is intended with this. 
     By adopting the above configuration, the distance between first unbalanced line  21  and second unbalanced line  22  is increased thus greatly suppressing mutual cancellation of the current flowing in first unbalanced line  21  and the current flowing in second unbalanced line  22 . As a result, when first unbalanced line  21  and second unbalanced line  22  are connected by folding back, the attenuation of balanced signals outputted from output terminals  17 ,  18  can be suppressed compared with an unbalanced signal inputted to input terminal  15 . 
     Here, it is preferable that the widths of first unbalanced line  21  and second unbalanced line  22  be greater than the widths of first balanced line  23  and second balanced line  24 . With this, the input impedance of balun  14  can be made smaller than the output impedance of balun  14 . As a result, impedance matching between balun  14  and an RF circuit connected to the output side of balun  14  can be made easier. 
     INDUSTRIAL APPLICABILITY 
     When a first unbalanced line and a second unbalanced line are connected by folding back, the balun in accordance with the present invention can suppress attenuation of a balanced signal to be outputted from the output terminal compared with an unbalanced signal inputted at the input terminal, thus lending itself to electronic apparatuses such as portable terminals and broadcast receivers.