Abstract:
A first passive component includes one unbalanced line having one unbalance input terminal, one balanced line installed opposite to the unbalanced line and having two balanced output terminals (first balanced output terminal and second balanced output terminal), and a capacitor formed between the balanced line and a fixed potential (e.g. the ground potential). Furthermore, the relation d 1 &gt;d 2  is satisfied, where d 1  is the physical length of the unbalanced line and d 2  is the physical length of the balanced line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a passive component, and more particularly to a passive component suitable for use in an unbalanced to balanced converting circuit for converting an unbalanced input signal into a balanced output signal or for use in a composite circuit which includes a filter having at least one resonator and an unbalanced to balanced converting circuit. 
     2. Description of Related Art 
     Generally, there is known a balun transformer (unbalanced to balanced converter) as a circuit component for converting an unbalanced input signal into a balanced output signal or converting a balanced input signal into an unbalanced output signal. 
     Recently, semiconductor components such as integrated circuits (ICs) or the like have been highly integrated, while semiconductor components have quickly become smaller in their size. Accordingly, balun transformers have also become smaller in size. 
     One conventional balun transformer has a ½-wavelength unbalanced transmission line and a pair of ¼-wavelength balanced transmission lines (see Japanese Laid-Open Patent Publication No. 2002-299127). 
     The unbalanced transmission line has an end serving as an unbalanced input terminal of the balun transformer and the other end serving as an open end. Each of the balanced transmission lines has an end serving as a balanced output terminal of the balun transformer and the other end connected to ground. 
     Conventional passive components incorporating such a balun transformer are disclosed in Japanese Laid-Open Patent Publication No. 2004-056745 and Japanese Laid-Open Patent Publication No. 2003-087008. 
     The passive component disclosed in Japanese Laid-Open Patent Publication No. 2004-056745 is a high-frequency component incorporating a balun and a filter. The disclosed passive component includes a balun for converting a balanced line signal into an unbalanced line signal and vice versa, and a filter electrically connected to the balun, for passing or attenuating a certain frequency component. The disclosed passive component has an electrode layer which provides electrode patterns of the balun and the filter, and a dielectric layer. The electrode layer and the dielectric layer are stacked into an integral assembly. 
     The passive component disclosed in Japanese Laid-Open Patent Publication No. 2003-087008 includes, within a dielectric substrate, a filter section having input resonant electrodes and output resonant electrodes of two ¼-wavelength resonators, a converter having a plurality of striplines, and a connector which connects the filter section and the converter to each other. 
     A passive component disclosed in Japanese Laid-Open Patent Publication No. 2005-080248 is a stacked bandpass filter which is capable of outputting a balanced signal and which is small in size and can easily be adjusted. The disclosed passive component includes an unbalanced input terminal, a balanced output terminal, and a bandpass filter section connected between the unbalanced input terminal and the balanced output terminal. The bandpass filter section has a multilayer substrate on which a plurality of resonators comprising respective TEM lines are integrated. The bandpass filter section has resonators including an input resonator and a balanced-output ½-wavelength resonator which comprises a double-open-ended ½-wavelength resonator. The unbalanced input terminal is connected to the input resonator through a capacitor, and the balanced output terminal is connected to the balanced-output ½-wavelength resonator through a capacitor. 
     Although the passive components disclosed in Japanese Laid-Open Patent Publication No. 2004-056745 and Japanese Laid-Open Patent Publication No. 2003-087008 have the filter and the balun integrally combined with each other by the multilayer substrate or the dielectric substrate, since filter and the balun are separate circuits, the number of parts is so large that a circuit including the bandpass filter and the balun causes a large loss and has a large size. 
     In the stacked bandpass filter disclosed in Japanese Laid-Open Patent Publication No. 2005-080248, on the other hand, the two balanced output terminals are connected to the balanced-output ½-wavelength resonator which comprises a double-open-ended ½-wavelength resonator. Therefore, the stacked bandpass filter is capable of outputting balanced signals from the two balanced output terminals without using a balun. 
     SUMMARY OF THE INVENTION 
     However, the stacked bandpass filter disclosed in Japanese Laid-Open Patent Publication No. 2005-080248 is problematic in that it suffers a low degree of design freedom. 
     Specifically, FIGS. 1, 5, 8, 11, 15 of Japanese Laid-Open Patent Publication No. 2005-080248 disclose that an output resonator and a resonator adjacent thereto comprise λ/2 resonators, respectively, and these resonators have the same physical length. FIG. 13 thereof discloses that an output resonator comprises a λ/2 resonator, a resonator adjacent thereto comprises a λ/4 resonator, and the physical length of the output resonator is twice the physical length of the adjacent resonator. FIGS. 21 and 24 through 30 thereof disclose that an output resonator comprises two λ/4 resonators and a resonator adjacent thereto comprises a λ/2 resonator. 
     As can be understood from the disclosed arrangements, according to the example in which the output resonator has an electrical length λ/2, if the resonator adjacent thereto has an electrical length λ/2, then their physical lengths have to be the same as each other, or if the resonator adjacent thereto has an electrical length λ/4, then the physical length of the output resonator has to be twice the physical length of the adjacent resonator. Therefore, the degree of design freedom is limited. Accordingly, the disclosed stacked bandpass filter may not be able to meet various requirements. Even if the disclosed stacked bandpass filter can meet various requirements, it suffers problems in that it causes a large loss and has a large size. 
     Furthermore, as disclosed in Japanese Laid-Open Patent Publication No. 2002-299127, Japanese Laid-Open Patent Publication No. 2004-056745, and Japanese Laid-Open Patent Publication No. 2003-087008, the electromagnetic coupling between the unbalanced transmission line and the pair of balanced transmission lines tends to suffer characteristic degradations because no electromagnetic coupling is provided between the balanced transmission lines. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide a passive component which allows the physical length of a balanced line to be smaller than the physical length of an unbalanced line even if the electrical length of each of the unbalanced line and the balanced line is λ/2, or which allows the physical lengths of an unbalanced line and a balanced line to be the same as each other even if the electrical lengths of the unbalanced line and the balanced line are different from each other, so that an unbalanced to balanced converter can be designed with an increased degree of freedom. 
     Another object of the present invention is to provide a passive component which allows one of resonant electrodes to double as an unbalanced line of an unbalanced to balanced converter, which allows the physical length of a balanced line to be smaller than the physical length of an unbalanced line even if the electrical length of each of the unbalanced line and the balanced line is λ/2, or which allows the physical lengths of an unbalanced line and a balanced line to be the same as each other even if the electrical lengths of the unbalanced line and the balanced line are different from each other, so that the passive component which has a filter section and unbalanced to balanced converter in integral combination can effectively be designed with an increased degree of freedom, effectively has a reduced size, and effectively reduces the loss which it causes. 
     A passive component according to a first invention includes an unbalanced line, a balanced line disposed in confronting relation to the unbalanced line, and a capacitor occurring between the balanced line and a fixed potential. 
     The passive component may be designed in various modes. For example, even if each of the unbalanced line and the balanced line has an electrical length λ/2, the physical length of the balanced line can be made smaller than the physical length of the unbalanced line. Alternatively, even if the unbalanced line and the balanced line have different electrical lengths, the physical lengths of the unbalanced line and the balanced line can be made equal to each other. Therefore, an unbalanced to balanced converter can be designed with an increased degree of freedom. Unlike Japanese Laid-Open Patent Publication No. 2002-299127, Japanese Laid-Open Patent Publication No. 2004-056745, and Japanese Laid-Open Patent Publication No. 2003-087008, since one balanced line is disposed in confronting relation to one unbalanced line, the balanced line includes nothing that cannot be electromagnetically coupled. Consequently, the passive component is free of characteristics degradations. 
     In the first invention, the unbalanced line has a physical length d 1  and the balanced line has a physical length d 2 , and the physical lengths (d 1 , d 2 ) may be related to each other as follows:
 
d1&gt;d2
 
     Alternatively, the unbalanced line may have an electrical length λ/4 and the balanced line may have an electrical length λ/2. 
     In the first invention, the passive component may further include a dielectric substrate with an upper shield electrode and/or a lower shield electrode disposed thereon, a first stripline electrode disposed in the dielectric substrate and serving as the unbalanced line, a second stripline electrode disposed in the dielectric substrate and serving as the balanced line, and a capacitor-forming electrode disposed in the dielectric substrate and forming the capacitor between itself and the second stripline electrode, wherein the capacitor-forming electrode may be connected to the second stripline electrode at a position adjusted for the phase difference and balanced characteristics of balanced output. 
     In the first invention, the first stripline electrode and the second stripline electrode may be disposed on different surfaces, respectively, or may be disposed on one surface. 
     In the first invention, the capacitor-forming electrode may be disposed between the second stripline electrode and the upper shield electrode or the lower shield electrode in confronting relation to the second stripline electrode and the upper shield electrode or the lower shield electrode, and the fixed potential may comprise a ground potential. 
     In the first invention, the capacitor-forming electrode may be disposed between the second stripline electrode and the upper shield electrode or the lower shield electrode in confronting relation to the second stripline electrode and the upper shield electrode or the lower shield electrode, and may be clamped to a DC potential different from a ground potential, and the fixed potential may comprise the DC potential. 
     A passive component according to a second invention includes a filter section having at least one resonator, and an unbalanced to balanced converter for converting an unbalanced output signal from at least the filter section into a balanced output signal, wherein the unbalanced to balanced converter includes a resonator of the filter section, a balanced line disposed in confronting relation to the resonator, and a capacitor occurring between the balanced line and a fixed potential. 
     With the above arrangement, one resonance electrode can also function as an unbalanced line of the unbalanced to balanced converter. The passive component may be designed in various modes. For example, even if each of an unbalanced line and the balanced line has an electrical length λ/2, the physical length of the balanced line can be made smaller than the physical length of the unbalanced line. Alternatively, even if the unbalanced line and the balanced line have different electrical lengths, the physical lengths of the unbalanced line and the balanced line can be made equal to each other. Therefore, the passive component which includes the filter section and the unbalanced to balanced converter can be designed with an increased degree of freedom, can be reduced in size, and can reduce the loss which it causes. 
     In the second invention, the resonator includes an unbalanced line having a physical length d 1  and the balanced line has a physical length d 2 , and the physical lengths (d 1 , d 2 ) may be related to each other as follows:
 
d1&gt;d2
 
     Alternatively, the resonator may include an unbalanced line having an electrical length λ/4 and the balanced line may have an electrical length λ/2. 
     In the second invention, the passive component may further includes a dielectric substrate with an upper shield electrode and/or a lower shield electrode disposed thereon, a resonance electrode of the resonator which is disposed in the dielectric substrate, a stripline electrode disposed in the dielectric substrate and serving as the balanced line, and a capacitor-forming electrode disposed in the dielectric substrate and forming the capacitor between itself and the stripline electrode, wherein the capacitor-forming electrode may be connected to the stripline electrode at a position adjusted for the phase difference and balanced characteristics of balanced output. 
     In the second invention, the resonance electrode and the stripline electrode may be disposed on different surfaces, respectively, or may be disposed on one surface. 
     In the second invention, the capacitor-forming electrode may be disposed between the stripline electrode and the upper shield electrode or the lower shield electrode in confronting relation to the stripline electrode and the upper shield electrode or the lower shield electrode, and the fixed potential may comprise a ground potential. 
     In the second invention, the capacitor-forming electrode may be disposed between the stripline electrode and the upper shield electrode or the lower shield electrode in confronting relation to the stripline electrode and the upper shield electrode or the lower shield electrode, and may be clamped to a DC potential different from a ground potential, and the fixed potential may comprise the DC potential. 
     As described above, the passive component according to the present invention may be designed in various modes. For example, even if each of the unbalanced line and the balanced line has an electrical length λ/2, the physical length of the balanced line can be made smaller than the physical length of the unbalanced line. Alternatively, even if the unbalanced line and the balanced line have different electrical lengths, the physical lengths of the unbalanced line and the balanced line can be made equal to each other. Therefore, the unbalanced to balanced converter can be designed with an increased degree of freedom. 
     With the passive component according to the present invention, furthermore, one resonance electrode can also function as the unbalanced line of the unbalanced to balanced converter. The passive component may be designed in various modes. For example, even if each of an unbalanced line and the balanced line has an electrical length λ/2, the physical length of the balanced line can be made smaller than the physical length of the unbalanced line. Alternatively, even if the unbalanced line and the balanced line have different electrical lengths, the physical lengths of the unbalanced line and the balanced line can be made equal to each other. Therefore, the passive component which includes the filter section and the unbalanced to balanced converter can be designed with an increased degree of freedom, can be reduced in size, and can reduce the loss which it causes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a first passive component; 
         FIG. 2A  is a view showing a passive component according to a comparative example; 
         FIG. 2B  is view showing a passive component according to a comparative example with a shortened balanced line; 
         FIG. 3  is a characteristic diagram showing changes in the phase difference depending on the frequency of the inventive and comparative examples; 
         FIG. 4  is a view showing a second passive component; 
         FIG. 5  is a view showing a third passive component; 
         FIG. 6  is a view showing a fourth passive component; 
         FIG. 7  is a view showing a fifth passive component; 
         FIG. 8  is a view showing a sixth passive component; 
         FIG. 9  is a perspective view showing the appearance of a first balun; 
         FIG. 10  is an exploded perspective view showing the structure of the first balun; 
         FIG. 11  is a perspective view showing the appearance of a second balun; 
         FIG. 12  is an exploded perspective view showing the structure of the second balun; 
         FIG. 13  is an exploded perspective view showing the structure of a third balun; 
         FIG. 14  is an exploded perspective view showing the structure of a fourth balun; 
         FIG. 15  is a perspective view showing the appearance of a first filter (and a second filter); 
         FIG. 16  is an exploded perspective view showing the structure of the first filter; 
         FIG. 17  is an exploded perspective view showing the structure of the second filter; 
         FIG. 18  is an exploded perspective view showing the structure of a third filter; and 
         FIG. 19  is an exploded perspective view showing the structure of the fourth filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Passive components according to exemplary embodiments of the present invention will be described below with reference to  FIGS. 1 through 19 . 
     As shown in  FIG. 1 , a passive component according to a first exemplary embodiment (hereinafter referred to as “first passive component  10 A”) includes an unbalanced line  14  having an unbalanced input terminal  12 , a balanced line  18  disposed in confronting relation to the unbalanced line  14  and having two balanced output terminals (a first balanced output terminal  16   a  and a second balanced output terminal  16   b ), and a capacitor  20  occurring between the balanced line  18  and a fixed potential (e.g., a ground potential). 
     In the first passive component  10 A, if the unbalanced line  14  has a physical length d 1  and the balanced line  18  has a physical length d 2 , then the physical lengths d 1 , d 2  are related to each other as follows:
 
d1&gt;d2
 
In the example shown in  FIG. 1 , the fixed potential is represented by the ground potential. However, the fixed potential may be any desired DC potential.
 
     Operation of the first passive component  10 A will be described with reference to  FIGS. 2A through 3  in comparison with a passive component  300  according to a comparative example. 
     The passive component  300  according to the comparative example includes an unbalanced line  304  having an unbalanced input terminal  302  and a balanced line  308  disposed in confronting relation to the unbalanced line  304  and having two balanced output terminals  306   a ,  306   b . If the unbalanced line  304  has a physical length d 1  and the balanced line  308  has a physical length d 2 , then the physical lengths d 1 , d 2  are related to each other as follows:
 
d1=d2
 
Each of the unbalanced line  304  and the balanced line  308  has an electrical length λ/2. The passive component  300  is free of the capacitor  20  of the first passive component  10 A.
 
     Specifically, according to the comparative example, the unbalanced line  304  has an electric field distribution K such that there is no electric field at the longitudinal center of the unbalanced line  304  and there are maximum electric fields at the both ends of the unbalanced line  304 . In order for the balanced line  308  to have the same electric field distribution K, the physical length of the balanced line  308  is made equal to the physical length of the unbalanced line  304 . With this arrangement, the two balanced output terminals  306   a ,  306   b  output signals whose phase difference is of 180 degrees. 
     If the physical length d 2  of the balanced line  308  is made shorter than the physical length d 1  of the unbalanced line  304 , as shown in  FIG. 2B , for example, by shortening the both ends of the balanced line  308  by a phase of α degrees, then the electric field distribution K of the balanced line  308  is such that there is no electric field at the longitudinal center of the balanced line  308  and there are no maximum electric fields at the both ends of the balanced line  308 . Accordingly, the phase difference between the signals output from the two balanced output terminals  306   a ,  306   b  is not of 180 degrees, but of (180−2×α) degrees deviating from the prescribed value of 180 degrees. 
     According to the comparative example, in order to provide the phase difference of 180 degrees between the signals output from the two balanced output terminals  306   a ,  306   b , it is essential to make the physical length d 2  of the balanced line  308  equal to the physical length d 1  of the unbalanced line  304 . Therefore, it will be seen that the passive component  300  according to the comparative example has almost no design freedom. 
     With the first passive component  10 A, on the other hand, by appropriately setting the value of the capacitor  20  occurring between the balanced line  18  and the fixed potential, even if the physical length d 2  of the balanced line  308  is smaller than the physical length d 1  of the unbalanced line  304 , the balanced line  18  has an electric field distribution K such that there is no electric field at the longitudinal center of the balanced line  18  and there are maximum electric fields at the both ends of the balanced line  18 . Specifically, by appropriately setting the value of the capacitor  20 , the resonant frequency of the balanced line  18  is changed and hence the phase is changed, making it possible to provide a phase difference of 180 degrees between signals output from the first balanced output terminal  16   a  and the second balanced output terminal  16   b.    
     An experimental example will be illustrated below. The experimental example was conducted to observe changes in the phase difference depending on the frequency if the physical length d 2  of the balanced line is made smaller in the passive component  300  according to the comparative example and the passive component (the first passive component  10 A) according to the exemplary embodiment. The results of the experimental example are shown in  FIG. 3 . 
     In  FIG. 3 , the curve A represents the characteristics of the passive component  300  according to the comparative example shown in  FIG. 2A , indicating the prescribed phase difference of 180 degrees at a central frequency fa. The curve B represents the characteristics of the passive component  300  according to the comparative example shown in  FIG. 2B , indicating a phase difference of 170 degrees, smaller than the prescribed phase difference of 180 degrees, at the central frequency fa. The phase difference of 170 degrees results from the smaller physical length d 2  of the balanced line  308 . 
     With the passive component (the first passive component  10 A) according to the exemplary embodiment, as indicated by the curve C in  FIG. 3 , it is possible to adjust the phase difference at the central frequency fa to the prescribed 180 degrees by appropriately varying the value of the capacitor  20  even if the physical length d 2  of the balanced line  18  is smaller. 
     With the first passive component  10 A, therefore, even if the physical length d 2  of the balanced line  18  is smaller than the physical length d 1  of the unbalanced line  14 , the balanced characteristics of the balanced output signals can easily be controlled by appropriately setting the value of the capacitor  20 . As a consequence, it is possible to increase the degree of design freedom of the first passive component  10 A. 
     As shown in  FIG. 4 , a passive component according to a second exemplary embodiment (hereinafter referred to as “second passive component  10 B”) is substantially identical in structure to the first passive component  10 A, but is different therefrom in that the unbalanced line  14  has an electrical length λ/4 and the physical length d 2  of the balanced line  18  is substantially the same as the physical length d 1  of the unbalanced line  14 . 
     With the second passive component  10 B, the unbalanced line  14  has an electric field distribution K such that there is no electric field at a short-circuited end of the unbalanced line  14  and there is a maximum electric field at an open end of the unbalanced line  14 . With the second passive component  10 B, by appropriately setting the value of the capacitor  20  occurring between the balanced line  18  and the fixed potential, even if the physical length d 2  of the balanced line  18  is substantially the same as the physical length d 1  of the unbalanced line  14 , the balanced line  18  has an electric field distribution K such that there is no electric field at the longitudinal center of the balanced line  18  and there are maximum electric fields at the both ends of the balanced line  18 . Specifically, it is possible to provide a phase difference of 180 degrees between signals output from the first balanced output terminal  16   a  and the second balanced output terminal  16   b.    
     Furthermore, even if the physical lengths of the unbalanced line  14  and the balanced line  18  are the same as each other in design, they actually tend to be different from each other due to manufacturing variations, thus degrading balanced characteristics of the balanced output. In other words, the phase difference may not be of 180 degrees. Even in such a case, the manufacturing variations may be absorbed by appropriately setting the value of the capacitor, thereby increasing the yield (increasing the productivity) of second passive components  10 B. This leads to a reduction in the cost of the second passive component  10 B. 
     A passive component according to a third exemplary embodiment (hereinafter referred to as “third passive component  10 C”) will be described below with reference to  FIG. 5 . 
     As shown in  FIG. 5 , the third passive component  10 C is substantially identical in structure to the first passive component  10 A, but is different therefrom in that it includes a filter section  22  and a balun  24 . 
     The filter section  22  includes a resonator  26  having an unbalanced input terminal  12 . The resonator  26  comprises an unbalanced line  14  having an electrical length λ/2. 
     The balun  24  includes the unbalanced line  14  of the resonator  26  of the filter section  22 , a balanced line  18  disposed in confronting relation to the unbalanced line  14 , and a capacitor  20  occurring between the balanced line  18  and the fixed potential. 
     In the third passive component  10 C, if the unbalanced line  14  has a physical length d 1  and the balanced line  18  has a physical length d 2 , then the physical lengths d 1 , d 2  are related to each other as follows:
 
d1&gt;d2
 
     With the third passive component  10 C, it is possible to provide a phase difference of 180 degrees between signals output from the first balanced output terminal  16   a  and the second balanced output terminal  16   b . Furthermore, since the resonator  26  of the filter section  22  can also function as the unbalanced line  14  of the balun  24 , the third passive component  10 C is reduced in size. 
     The third passive component  10 C makes it possible to effectively increase the degree of design freedom, reduce the size, and reduce the loss, of a passive component which includes the filter section  22  and the balun  24  in integral combination. 
     A passive component according to a fourth exemplary embodiment (hereinafter referred to as “fourth passive component  10 D”) will be described below with reference to  FIG. 6 . 
     As shown in  FIG. 6 , the fourth passive component  10 D is substantially identical in structure to the third passive component  10 C, but is different therefrom in that the filter section  22  has an input resonator  26 A and an output resonator  26 B. Each of the input resonator  26 A and the output resonator  26 B comprises an unbalanced line  14  having an electric length λ/2. 
     The balun  24  includes the unbalanced line  14  of the output resonator  26 B of the filter section  22 , a balanced line  18  disposed in confronting relation to the unbalanced line  14 , and a capacitor  20  occurring between the balanced line  18  and the fixed potential. In the fourth passive component  10 D, if the unbalanced line  14  of the output resonator  26 B has a physical length d 1  and the balanced line  18  has a physical length d 2 , then the physical lengths d 1 , d 2  are related to each other as follows:
 
d1&gt;d2
 
     With the fourth passive component  10 D, it is possible to provide a phase difference of 180 degrees between signals output from the first balanced output terminal  16   a  and the second balanced output terminal  16   b . Furthermore, since the output resonator  26 B of the filter section  22  can also function as the unbalanced line  14  of the balun  24 , the fourth passive component  10 D is effectively reduced in size and effectively reduces the loss which it causes. 
     A passive component according to a fifth exemplary embodiment (hereinafter referred to as “fifth passive component  10 E”) will be described below with reference to  FIG. 7 . 
     As shown in  FIG. 7 , the fifth passive component  10 E is substantially identical in structure to the third passive component  10 C, but is different therefrom in that the resonator  26  of the filter section  22  comprises an unbalanced line  14  having an electric length λ/4 and the physical length d 2  of the balanced line  18  is substantially the same as the physical length d 1  of the unbalanced line  14 . 
     With the fifth passive component  10 E, as with the second passive component  10 B, it is possible to provide a phase difference of 180 degrees between signals output from the first balanced output terminal  16   a  and the second balanced output terminal  16   b . Furthermore, by appropriately setting the value of the capacitor  20 , the yield (increasing the productivity) of fifth passive components  10 E can be increased, and the cost of the fifth passive component  10 E can be reduced. 
     A passive component according to a sixth exemplary embodiment (hereinafter referred to as “sixth passive component  10 F”) will be described below with reference to  FIG. 8 . 
     As shown in  FIG. 8 , the sixth passive component  10 F is substantially identical in structure to the fifth passive component  10 E, but is different therefrom in that the filter section  22  has an input resonator  26 A and an output resonator  26 B. Each of the input resonator  26 A and the output resonator  26 B comprises an unbalanced line  14  having an electric length λ/4. 
     The balun  24  includes the unbalanced line  14  of the output resonator  26 B of the filter section  22 , a balanced line  18  disposed in confronting relation to the unbalanced line  14 , and a capacitor  20  occurring between the balanced line  18  and the fixed potential. 
     With the sixth passive component  10 F, it is possible to provide a phase difference of 180 degrees between signals output from the first balanced output terminal  16   a  and the second balanced output terminal  16   b . Furthermore, since the output resonator  26 B of the filter section  22  can also function as the unbalanced line  14  of the balun  24 , the sixth passive component  1 OF is effectively reduced in size and effectively reduces the loss which it causes. 
     In the fourth passive component and the sixth passive component, the resonators of the filter section include the input resonator and the output resonator. However, the resonators of the filter section may further include one or more resonators between the input resonator and the output resonator. 
     Specific examples (embodiments) of the above various exemplary embodiments will be described below with reference to  FIGS. 9 through 19 . 
     [Embodiment 1] 
     A balun according to Embodiment 1 (hereinafter referred to as “first balun  100 A”) represents a first specific example of the second passive component  10 B. As shown in  FIG. 9 , the first balun  100 A has a dielectric substrate  102  comprising a plurality of dielectric layers stacked and sintered together. The dielectric substrate  102  has outer surfaces including a first side surface  102   a  with an unbalanced input terminal  12  disposed thereon, a second side surface  102   b  (a side surface facing the first side surface  102   a ) with two balanced output terminals (a first balanced output terminal  16   a  and a second balanced output terminal  16   b ) disposed thereon, and a third side surface  102   c  and a fourth side surface  102   d  with shield terminals  104  disposed respectively thereon. 
     As shown in  FIG. 10 , the dielectric substrate  102  comprises first through fifth dielectric layers S 1  through S 5  which are stacked successively from above. Each of the first through fifth dielectric layers S 1 -S 5  comprises a single layer or a plurality of layers. 
     The first balun  100 A includes an upper shield electrode  106   a  disposed on an upper end of the dielectric substrate  102  and a lower shield electrode  106   b  disposed on a lower end of the dielectric substrate  102 . Specifically, the upper shield electrode  106   a  is disposed on a principal surface of the first dielectric layer S 1 , and the lower shield electrode  106   b  is disposed on a principal surface of the fifth dielectric layer S 5 . The upper shield electrode  106   a  and the lower shield electrode  106   b  are connected to the shield terminals  104 . 
     The first balun  100 A also includes a first stripline electrode  108  disposed on a principal surface of the second dielectric layer S 2  and serving as the unbalanced line  14 . The first stripline electrode  108  includes, at a position near an end thereof (open end), a lead electrode  110  connected to the unbalanced input terminal  12 , and has the other end (short-circuited end) connected to one of the shield terminals  104 . 
     The first balun  100 A also includes a second stripline electrode  112  disposed on a principal surface of the third dielectric layer S 3  at a position facing the first stripline electrode  108  and serving as the balanced line  18 . The second stripline electrode  112  includes, at a position near an end thereof (open end), a first lead electrode  114   a  connected to the first balanced output terminal  16   a , and also includes, at a position near the other end thereof, a second lead electrode  114   b  connected to the second balanced output terminal  16   b.    
     The first balun  100 A also includes a capacitor-forming electrode  116  disposed on a principal surface of the fourth dielectric layer S 4  and forming a capacitor  20  between the second stripline electrode  112  and the lower shield electrode  106   b . The capacitor-forming electrode  116  is disposed in confronting relation to the second stripline electrode  112  and the lower shield electrode  106   b , and is connected to a longitudinally central portion of the second stripline electrode  112  through a via hole  118  that is defined in the third dielectric layer S 3 . 
     If the value of the capacitor  20  between the second stripline electrode  112  and the lower shield electrode  106   b  is to be changed, then the dielectric constant or thickness of the fourth dielectric layer S 4  may be changed or the area of the capacitor-forming electrode  116  may be changed. 
     If the phase difference and balanced characteristics between balanced output signals from the first balanced output terminal  16   a  and the second balanced output terminal  16   b  are to be changed, then the position of the via hole  118  defined in the third dielectric layer S 3  may be changed. 
     [Embodiment 2] 
     A balun according to Embodiment 2 (hereinafter referred to as “second balun  100 B”) represents a second specific example of the second passive component  10 B. The second balun  100 B is of substantially the same structure as the first balun  100 A, but is different therefrom as follows: 
     As shown in  FIG. 11 , the second balun  100 B includes an unbalanced input terminal  12  and a DC voltage input terminal (DC input terminal  120 ) which are disposed on the first side surface  102   a  among the outer surfaces of the dielectric substrate  102 . 
     As shown in  FIG. 12 , the capacitor-forming electrode  116  is connected to the DC input terminal  120  through a lead electrode  122 , and also functions as an electrode (DC electrode  124 ) to which a DC voltage is applied. Therefore, the first balanced output terminal  16   a  and the second balanced output terminal  16   b  output balanced output signals including the DC voltage applied to the DC electrode  124  as a bias voltage. 
     [Embodiment 3] 
     A balun according to Embodiment 3 (hereinafter referred to as “third balun  100 C”) represents a third specific example of the second passive component. The third balun  100 C is of substantially the same structure as the first balun  100 A, but is different therefrom as follows: 
     As shown in  FIG. 13 , the dielectric substrate  102  comprises first through sixth dielectric layers S 1  through S 6  which are stacked successively from above. 
     The second stripline electrode  112  is disposed on the principal surface of the third dielectric layer S 3 . A first matching circuit element  126 A and a second matching circuit element  126 B for matching the output impedance with the input impedance of an external circuit are disposed on the principal surface of the fourth dielectric layer S 4 . 
     The first matching circuit element  126 A includes a first inductance electrode  128   a  having a spiral shape and a first lead electrode  114   a  which connects the first inductance electrode  128   a  to the first balanced output terminal  16   a . The first inductance electrode  128   a  is connected to the second stripline electrode  112  through a first via hole  130   a  defined in the third dielectric layer S 3 . 
     Similarly, the second matching circuit element  126 B includes a second inductance electrode  128   b  having a spiral shape and a second lead electrode  114   b  which connects the second inductance electrode  128   b  to the second balanced output terminal  16   b . The second inductance electrode  128   b  is connected to the second stripline electrode  112  through a second via hole  130   b  defined in the third dielectric layer S 3 . 
     The capacitor-forming electrode  116  is disposed on the principal surface of the fifth dielectric layer S 5 . The lower shield electrode  106   b  is disposed on a principal surface of the sixth dielectric layer S 6 . 
     If the phase difference and balanced characteristics between balanced output signals from the first balanced output terminal  16   a  and the second balanced output terminal  16   b  are to be changed, then the position of the via hole  118  which is defined in the third dielectric layer S 3  and the fourth dielectric layer S 4  may be changed. 
     [Embodiment 4] 
     A balun according to Embodiment 4 (hereinafter referred to as “fourth balun  100 D”) represents a fourth specific example of the second passive component  10 B. The fourth balun  100 D is of substantially the same structure as the first balun  100 A, but is different therefrom as follows: 
     As shown in  FIG. 14 , the dielectric substrate  102  comprises first through fourth dielectric layers S 1  through S 4  which are stacked successively from above. 
     The first stripline electrode  108  and the second stripline electrode  112  are disposed on the principal surface of the second dielectric layer S 2 . The capacitor-forming electrode  116  is disposed on the principal surface of the fourth dielectric layer S 4 . 
     The fourth balun  100 D is of a structure that is advantageous to make itself low in profile though the coupling between the striplines is somewhat weak, because the first stripline electrode  108  and the second stripline electrode  112  are disposed on the same surface (the principal surface of the second dielectric layer S 2 ). 
     [Embodiment 5] 
     A filter according to Embodiment 5 (hereinafter referred to as “first filter  200 A”) represents a first specific example of the sixth passive component  10 F. As shown in  FIG. 15 , the dielectric substrate  102  has outer surfaces including a first side surface  102   a  with an unbalanced input terminal  12 , a first NC terminal  132   a , and a second NC terminal  132   b  disposed thereon, a second side surface  102   b  (a side surface facing the first side surface  102   a ) with two balanced output terminals (a first balanced output terminal  16   a  and a second balanced output terminal  16   b ) and a third NC terminal  132   c  disposed thereon, and a third side surface  102   c  and a fourth side surface  102   d  with shield terminals  104  disposed respectively thereon. 
     As shown in  FIG. 16 , the dielectric substrate  102  comprises first through sixth dielectric layers S 1  through S 6  which are stacked successively from above. 
     The upper shield electrode  106   a  is disposed on a principal surface of the first dielectric layer S 1 , and the lower shield electrode  106   b  is disposed on a principal surface of the sixth dielectric layer S 6 . The upper shield electrode  106   a  and the lower shield electrode  106   b  are connected to the shield terminals  104 . 
     An input resonance electrode  134   a  of the input resonator  26 A of the filter section  22  and an output resonance electrode  134   b  of the output resonator  26 B of the filter section  22  are disposed on the principal surface of the third dielectric layer S 3 . The input resonance electrode  134   a  includes, at a position near an end thereof (open end), a lead electrode  110  connected to the unbalanced input terminal  12 , and has the other end (short-circuited end) connected to one of the shield terminals  104 . The output resonance electrode  134   b  has the other end (short-circuited end) connected to the same shield terminal  104 . 
     A first inner-layer shield electrode  136   a  is disposed on the principal surface of the second dielectric layer S 2  at a position facing the open end of the input resonance electrode  134   a . A second inner-layer shield electrode  136   b  is disposed on the principal surface of the second dielectric layer S 2  at a position facing the open end of the output resonance electrode  134   b . A coupling adjusting electrode  138  for adjusting the coupling between the input resonator  26 A and the output resonator  26 B is disposed on the principal surface of the second dielectric layer S 2 . 
     A stripline electrode  140  of the balanced line  18  of the balun  24  is disposed on the principal surface of the fourth dielectric layer S 4  at a position facing the output resonance electrode  134   b . The stripline electrode  140  includes, at a position near an end thereof (open end), a first lead electrode  114   a  connected to the first balanced output terminal  16   a , and also includes, at a position near the other end thereof, a second lead electrode  114   b  connected to the second balanced output terminal  16   b.    
     The first filter  200 A also includes a capacitor-forming electrode  116  disposed on the principal surface of the fifth dielectric layer S 5  and forming a capacitor  20  between the stripline electrode  140  and the lower shield electrode  106   b . The capacitor-forming electrode  116  is disposed in confronting relation to the stripline electrode  140  and the lower shield electrode  106   b , and is connected to a longitudinally central portion of the stripline electrode  140  through a via hole  118  that is defined in the fourth dielectric layer S 4 . 
     If the value of the capacitor  20  between the stripline electrode  140  and the lower shield electrode  106   b  is to be changed, then the dielectric constant or thickness of the fifth dielectric layer S 5  may be changed or the area of the capacitor-forming electrode  116  may be changed. 
     If the phase difference and balanced characteristics between balanced output signals from the first balanced output terminal  16   a  and the second balanced output terminal  16   b  are to be changed, then the position of the via hole  118  defined in the fourth dielectric layer S 4  may be changed. 
     [Embodiment 6] 
     A filter according to Embodiment 6 (hereinafter referred to as “second filter  200 B”) represents a second specific example of the sixth passive component  10 F. The second filter  200 B is of substantially the same structure as the first filter  200 A, but is different therefrom as follows: 
     As shown in  FIG. 15 , a DC voltage input terminal (DC input terminal  120 ) is disposed, in place of the first NC terminal  132   a , on the first side surface  102   a  among the outer surfaces of the dielectric substrate  102 . 
     As shown in  FIG. 17 , the capacitor-forming electrode  116  is connected to the DC input terminal  120  through a lead electrode  122 , and also functions as an electrode (DC electrode  124 ) to which a DC voltage is applied. Therefore, the first balanced output terminal  16   a  and the second balanced output terminal  16   b  output balanced output signals including the DC voltage applied to the DC electrode  124  as a bias voltage. 
     [Embodiment 7] 
     A filter according to Embodiment 7 (hereinafter referred to as “third filter  200 C”) represents a third specific example of the sixth passive component  10 F. The third filter  200 C is of substantially the same structure as the first filter  200 A, but is different therefrom as follows: 
     As shown in  FIG. 18 , the dielectric substrate  102  comprises first through seventh dielectric layers S 1  through S 7  which are stacked successively from above. 
     The stripline electrode  140  is disposed on the principal surface of the fourth dielectric layer S 4 . The first matching circuit element  126 A and the second matching circuit element  126 B for matching the output impedance with the input impedance of an external circuit are disposed on the principal surface of the fifth dielectric layer S 5 . The first matching circuit element  126 A and the second matching circuit element  126 B will not be described in detail below as their structures have already been described above. 
     The capacitor-forming electrode  116  is disposed on the principal surface of the sixth dielectric layer S 6 . The lower shield electrode  106   b  is disposed on a principal surface of the seventh dielectric layer S 7 . 
     If the phase difference and balanced characteristics between balanced output signals from the first balanced output terminal  16   a  and the second balanced output terminal  16   b  are to be changed, then the position of the via hole  118  which is defined in the fourth dielectric layer S 4  and the fifth dielectric layer S 5  may be changed. 
     [Embodiment 8] 
     A filter according to Embodiment 8 (hereinafter referred to as “fourth filter  200 D”) represents a specific example of the fifth passive component  10 E. The fourth filter  200 D is of substantially the same structure as the first filter  200 A, but is different therefrom as follows: 
     As shown in  FIG. 19 , the dielectric substrate  102  comprises first through fifth dielectric layers S 1  through S 5  which are stacked successively from above. 
     A resonance electrode  134  of the resonator  26  of the filter section  22  and the stripline electrode  140  of the balanced line  18  of the balun  24  are disposed on the principal surface of the third dielectric layer S 3 . An inner-layer shield electrode  136  is disposed on the principal surface of the second dielectric layer S 2  at a position facing the open end of the resonance electrode  134 . The capacitor-forming electrode  116  is disposed on the principal surface of the fourth dielectric layer S 4 , and the lower shield electrode  106   b  is disposed on the principal surface of the fifth dielectric layer S 5 . 
     The fourth filter  200 D is of a structure that is advantageous to make itself low in profile though the coupling between the resonance electrode  134  and the stripline electrode  140  is somewhat weak, because the resonance electrode  134  and the stripline electrode  140  are disposed on the same surface (the principal surface of the third dielectric layer S 3 ). 
     The passive components according to the present invention are not limited to the above exemplary embodiments. Rather, the passive components may incorporate various structural details without departing from the scope of the present invention.