Patent Publication Number: US-6222431-B1

Title: Balanced dielectric filter

Description:
FIELD OF THE INVENTION 
     The present invention relates to a balanced dielectric filter mainly used for high-frequency circuits, such as those used for radio communication apparatuses. 
     PRIOR ART 
     In accordance with the recent progress of mobile communications including the cellular phone system, filters have been required to be more compact in size and higher in performance, and dielectric filters suited for these requirements have been used widely. Such dielectric filters are used in a microwave band ranging from a few hundred megahertzs to about five gigaherzs, which are mounted on circuit boards in communications equipment, in particular, in the cellular phones. For this purpose, ceramic-multilayered filters have been used in larger quantity to be suited to be made especially smaller and thinner. 
     FIG. 9B shows the structure of a conventional unbalanced dielectric filter in which five ceramic dielectric layers  61  to  65  are laminated into a multilayer structure. Between the dielectric layers  63  and  64 , a resonator comprising a pair of strip lines  66 ,  66  as a resonator is formed on a plane in the structure, and the strip lines have lengths of a quarter of a resonant wavelength with short-circuited ends. 
     In this example, an input capacitance electrode  68  is coupled to one end of one of the strip lines  66 , and an output capacitance electrode  69  is coupled to one end of the other strip lines  66 . The two strip lines as resonating elements are disposed in parallel and coupled electrostatically through an interstage-coupling capacitance electrode  70 . 
     The resonator of the strip lines  66  and  66  is interposed between two shield electrodes  71 ,  72  through dielectric layers, thereby forming a tri-plate structure. The strip lines  66  and  66  in a pair are grounded through a loading capacitance electrode  67 . Furthermore, an input terminal  73  ( 11 ) and an output terminal  74  ( 51 ) are connected to the one and the other of the strip lines  66 , respectively, through the input capacitance electrode  68  and the output capacitance electrode  69 , respectively. Moreover, grounding terminals  75 ,  76  ( 4 ) are connected to the shield electrodes  71 ,  72  and the above loading-capacitance electrode  67  so that they are grounded. 
     FIG. 9A is a view showing the connections of the terminals to the conventional dielectric filter. A high-frequency input signal is applied between the input terminal  11  and the grounding terminal  4 , and then an output signal is delivered between the output terminal  51  and the grounding terminal  4 . 
     In the dielectric filter as described above, the two strip lines  66  and  66  of the resonator are first coupled electromagnetically to each other to form a comb-line type filter. The loading-capacitance electrode  67  is used to connect a capacitance in parallel with the strip lines, thereby lowering the resonant frequency for the strip line having the same length. 
     In this filter, the input and output stages of the filter are capacitance coupling, and parallel-plate capacitors are formed at the portions of the input/output capacitance electrode  68 ,  69  on the dielectric layer opposed to the strip-line resonator  66 . The interstage-coupling capacitance electrode  70  can attain the interstage coupling between the strip line resonators by combining electromagnetic field coupling with electric field coupling, then, generating an attenuation pole in transfer characteristics (see Japanese Patent Publication JP-A 5-95202, for example). 
     An unbalanced filter using stepped impedance resonators integrally formed in a dielectric ceramic-multilayered structure is disclosed in Japanese Patent Publication JP-A 7-312503. In this filter, a pair of strip lines are stepped impedance resonators, each comprising a first line portion, one end of which is grounded, and a second line portion, one end of which is open, and which has a characteristic impedance lower than that of the first line portion. The coupling factor between the first line portions and the coupling factor between the second line portions are changed to control the transfer characteristics of the filter circuit. 
     Furthermore, as still another conventional example, an attempt to balance the output and/or input terminal arrangement of the filter has already been proposed in PCT international publication WO92/02969, as shown in FIG.  10 . In this example, the filter comprises two split ring resonators  80  and  81  of a microstrip-line type, wherein one of the resonators is connected to an unbalanced input terminal  82  through an input coupling capacitance  85 , and the other resonator is connected to balanced output terminals  83 ,  84 . The split ends of the rings are connected to each other by loading capacitances  86  and  87 , respectively. In this arrangement, the split-ring resonators  80 ,  81  are coupled electromagnetically to form a filter. 
     In the conventional strip-line type filters described above, since the input terminals are unbalanced, a balance-unbalance transformer (BALUN) is required to connect the unbalanced terminals to a high-frequency balanced amplifier or semiconductor integrated circuit. Furthermore, in the unbalanced circuit, current flows in the grounding circuit thereof, thereby causing a problem of having low resistance against electromagnetic interference and being easily susceptible to noise or the like. 
     Furthermore, the PCT international publication WO92/02969 discloses another embodiment of a filter using a pair of the ring strip resonators with balanced input and output terminals. This balanced filter must occupy a large area on the dielectric substrate to locate the twin resonators on the same surface. Therefore, a new type of balanced filters with more compact sizes is eagerly desired and it is required to arrange the resonators compactly for fabricating a balanced filter. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a dielectric filter having balanced input and output terminals, being highly resistant against electromagnetic interference and capable of being designed easily. 
     Another object of the present invention is to provide a balanced dielectric filter having excellent characteristics for connection to balanced circuits or balanced integrated circuits. 
     Yet another object of the present invention is to provide a balanced dielectric filter being mountable efficiently on circuit boards. 
     The balanced dielectric filter in accordance with the present invention comprises two resonators, each comprising plural TEM mode resonating elements which are disposed in parallel and mutually coupled electromagnetically, two input terminals each of which is connected to the corresponding resonator, and two output terminals each of which is connected to the corresponding resonator, functioning as balanced input and output terminals, wherein the two resonators are disposed in a dielectric to face each other and to have mirror symmetry with each other. 
     In the balanced dielectric filter in accordance with the present invention, both the input and output terminals are of a balanced type, and the two resonators are disposed so as to be mirror images of each other. When high-frequency signals opposite in phase are applied to the two input terminals of the two resonators, an electric wall having the zero potential is formed in the mirror-symmetry plane between the two resonators. Therefore, both the input and output sides of the filter are balanced excellently. External electromagnetic interference is cancelled, and not produced on the output side. As a result, it is possible to configure a filter highly resistant against external electromagnetic interference. 
     In the present invention, the TEM mode resonating element comprises a strip line formed of a thin conductor embedded in a dielectric. 
     A hollow resonator included in a dielectric may be used as another type of TEM mode resonating element. In this case, holes, acting as resonant cavities, arranged in a dielectric block may be used to constitute a dielectric block type balanced filter. 
     The strip-line resonating elements may act as a quarter-wavelength resonator, the ends of the strip lines thereof formed of a conductor being grounded. More particularly, the filter of the present invention comprises a pair of resonators having plural strip lines as a resonating element disposed in parallel and mutually coupled electromagnetically. The resonators of the plural strip lines are disposed so as to be mirror images of each other in a dielectric and each acts as a quarter-wavelength resonator. Both ends on the input sides of the strip lines in the two resonators are connected to both of the input terminals, respectively, and a pair of output terminals are connected to both ends on the output sides of the resonator. 
     The strip lines of each of the two resonators, being symmetrical to each other, may be connected to each other at the ends of the strip lines so as to form a half-wavelength resonator. In other words, the present invention includes a balanced dielectric filter comprising a strip line producing half-wavelength resonance, being bent to be mirror-image symmetrical. 
     More specifically, the balanced dielectric filter comprises plural strip-line resonating elements disposed in parallel and mutually coupled electromagnetically, a pair of input terminals connected to both end sides of the resonating elements on the input side, respectively, and a pair of output terminals connected to both end sides of the resonating elements on the output side, wherein the resonating elements are connected to form a bent shape so as to be mirror images of each other and located in a dielectric. 
     Particularly, in this balanced dielectric filter, both strip lines, having been bent, are disposed so as to be face-to-face with relation to mirror images of each other. Therefore, an electric wall is formed on the mirror symmetry plane in the dielectric filter, and both the input and output sides of the filter can remain almost completely balanced. 
     In the above-mentioned strip-line resonator, the electric wall is formed on the mirror-symmetry plane between the bent strip lines. However, the electric wall may be an imaginary wall without any conductor. Preferably, an intermediate shield of a metallic conductor should be located on the electric wall and grounded, whereby the filter circuit can be securely balanced. 
     In order to form the filter, two strip-line resonators are independently disposed so as to be symmetrical to each other in a dielectric ceramic-multilayered structure. In other words, each resonator is formed on the top and bottom sides of a ceramic layer so that two set of quarter-wavelength strip-line conductors face each other. In each of the these two resonators, the strip-line conductors adjacent to each other are located in parallel with each other with a small clearance therebetween to be coupled electro-magnetically. 
     Furthermore, the input/output capacitance electrodes of the resonators are located separately on other dielectric ceramic layers adjacent to the resonators, and electrically coupled to the ends of the strip lines, respectively. As described above, two resonators, input/output capacitance electrodes, and coupling capacitance electrodes between the resonators when required are integrated into a ceramic-multilayered structure, whereby a compact filter structure can be attained. 
     The present invention can attain a balanced filter being compact in size and excellent in performance by electromagnetically coupling quarter-wavelength resonating elements formed of strip lines in a dielectric ceramic-multilayered structure as described above. Furthermore, the present invention can attain a compact balanced dielectric filter formed of two half-wavelength resonators, each obtained by connecting the ends of the two strip lines of the quarter-wavelength resonating elements in a folded-back form. Moreover, an imaginary electric wall is attained on the basis of electric symmetry between the resonators by providing a multilayered structure having resonators among dielectric ceramic layers. As a result, the filter can be made compact in size, excellent in performance and balanced on both the input and output sides. 
     The balanced dielectric filter with accordance to the present invention can preferably be used as a band pass filter in a frequency range of a few hundred megahertzs to about five gigahertzs in a microwave band which is mounted on circuit boards in communications equipment, in particular, in cellular phones. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described below in detail referring to the accompanying drawings, in which; 
     FIG. 1A shows a block diagram of the connections of input/output terminals of a balanced dielectric filter; and 
     FIGS. 1B and 1C are equivalent circuits of balanced dielectric filters in accordance with the present invention; 
     FIG. 2 is an exploded view showing a balanced dielectric filter having balanced input/output terminals in accordance with an embodiment of the present invention; 
     FIG. 3 is an exploded view of a multilayered filter in an embodiment showing an arrangement of strip lines and electrodes on sheets in a dielectric filter in accordance with the present invention; 
     FIG. 4 is a vertical sectional view showing the balanced dielectric filter having a ceramic-multilayered structure in accordance with the embodiment of the present invention; 
     FIG. 5 is a view similar to FIG. 2 showing a balanced dielectric filter having balanced input/output terminals in accordance with another embodiment of the present invention; 
     FIG. 6 is a view similar to FIG. 2 showing the structure of a balanced dielectric filter in accordance with yet another embodiment of the present invention; 
     FIG. 7 is a view similar to FIG. 3 showing the structure of the balanced dielectric filter in accordance with the embodiment of the present invention; 
     FIG. 8 is a view similar to FIG. 2 showing a balanced dielectric filter in accordance with still yet another embodiment of the present invention; 
     FIG. 9A is a view showing the connections of the terminals of a conventional dielectric filter; and 
     FIG. 9B is an exploded view showing the structure of the conventional dielectric filter; and, 
     FIG. 10 is a circuit diagram of the conventional dielectric filter. 
    
    
     DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION 
     FIG. 1A shows a block diagram of a balanced dielectric filter to illustrate the operation of each embodiment of the present invention. Referring to this figure, a pair of balanced input terminals  11  and  12  and a pair of balanced output terminals  51  and  52  are connected to a balanced dielectric filter  1  with grounding terminals  4  and  4  further connected to the input side and the output side, respectively, of the filter. 
     An input signal is applied across the two terminals  11 ,  12  used as balanced input terminals  10  in opposite phase, the grounding terminal  4  on the input side is essentially at the zero potential at all times, and no grounding current flows to the grounding terminal  4 . In the same way, an output signal is produced across a pair of balanced output terminals  50  ( 51  and  52 ) in opposite phase, the grounding terminal  4  on the output side is essentially at the zero potential at all times, and no grounding current flows to the grounding terminal  4 . 
     In order to attain this kind of filter balanced on both the input and output sides, FIGS. 1B and 1C are schematic views showing the balanced dielectric filter  1  of the present invention. 
     Referring to FIG. 1B, the filter is provided with balanced input/output terminals  10  and four quarter-wavelength resonating elements constituting two resonators. The balanced dielectric filter in accordance with the present embodiment has pairs of strip lines ( 210   a  and  210   b ) and ( 220   a  and  220   b ) of the two resonators, and every two of these strip lines are located in parallel and coupled to each other electromagnetically. On one end sides of these strip lines, the input terminals  11  and  12  and the output terminals  51 ,  52  are connected in parallel through a coupling capacitor C 1 , and the other end sides of the strip lines are grounded, thereby to form the quarter-wavelength resonating elements. 
     One end of a first resonating element  210   a  is coupled to the positive terminal  11  of the balanced input terminals  10 , and one end of a second resonating element  220   a  is coupled to the negative terminal  12 . One end of a third resonating element  210   b  is coupled to the positive terminal  51  of the balanced input terminals  50 , and one end of a fourth resonating element  220   b  is coupled to the negative terminal  52 . The other ends of all the resonating elements are grounded electrically. 
     Furthermore, the strip lines of the resonating elements  210   a ,  210   b  used in a pair are disposed to face the resonating elements  220   a  and  220   b  used as another pair with a clearance therebetween so as to be mirror images of each other. With this configuration, input signals to the pair of the input terminals  11  and  12  pass through input capacitors C 11  and C 12 , respectively, and applied to the two pairs of the resonating elements  210   a ,  210   b  and  220   a ,  220   b  in opposite phase. However, in the resonators, each of the pairs of the strip lines is coupled electro-magnetically, independently of other to filtrate each signal in a constant frequency range. The resonators are connected to the output terminals through the output capacitors C 51  and C 52 , and filtrated signals being in opposite phase are output from the resonators to the output terminals. 
     The resonating elements  210   a  and  220   a  face each other, and the resonating elements  210   b  and  220   b  also face each other. Input signals applied to the resonators are opposite in phase (difference of 180° in phase). Therefore, the strip lines of the resonators  210  and  220  have the same potential, with opposite signs with respect to a mirror-symmetry plane between the resonators, at any positions corresponding to each other, whereby both the resonators are balanced completely. 
     The above-mentioned symmetry plane has the zero potential at all times, which is considered to be an imaginary electric wall  9 . In particular, it is desired that an intermediate shield electrode  271  is located at the position of the electric wall  9  between the resonators  210  and  220 , and is grounded. 
     Because the resonators are electrically symmetrical with respect to the intermediate shield electrode  271 , even if external magnetic or electric field are applied to the resonators  210  and  220 , the field is cancelled no to appear on both the input and output sides of the filter, thereby obtaining the filter made completely balanced. 
     FIG. 1C shows another embodiment of a balanced filter. In resonators  3   a ,  3   b , half-wavelength strip lines  3   a  and  3   b  used in a pair are disposed in parallel, coupled electromagnetically to each other, and are folded so that the halves  310   a  and  320   a  of the strip line  3   a  are disposed to be mirror images of each other. A pair of input terminals are connected to both ends of one of the strip lines, through input capacitors C 11  and C 12 . Also, a pair of output terminals are connected to both ends of the other strip line, through output capacitors C 51  and C 52 . In this embodiment, the midpoint  39  of the strip line is not grounded. However, a standing wave of current appears at the zero potential (voltage node). The symmetry plane between the halves of the folded strip and the above-mentioned midpoint of the strip line has the zero potential at all times, this plane being an imaginary electric wall  9 . In this case, it is also desired that a shield electrode  271  is located at the position of the electric wall  9  between the resonators  310   a  and  320   a , and between the resonators  310   b  and  320   b.    
     The space between the resonators  210 ,  220  and the peripheral portions thereof are supported and integrated in a multilayered condition by using ceramic sheets having excellent high-frequency characteristics as a dielectric as described below. 
     Furthermore, an interstage-coupling capacitance C 1  is coupled across the strip lines disposed in a pair and in parallel to form an attenuation pole adjacent to a pass band. In addition, the ends on the input/output sides of the strip lines are grounded through loading capacitors C 2 , whereby the lengths of the strip lines can be made shorter than the length of resonance. 
     An outstanding feature of this arrangement of the filter is that no grounding current flows at the balanced input/output terminals. In an extreme condition, that is, without grounding terminals, this arrangement acts normally as a balanced four-terminal filter circuit. 
     In this respect, this balanced filter significantly differs from an unbalanced filter that requires ideal grounding to attain a normal filter characteristic, i.e., a high attenuation level. In the case of the unbalanced filter, ideal grounding is hardly attained in an actual high-frequency circuit, whereby the characteristics of the dielectric filter are deteriorated. 
     In the case of the balanced dielectric filter of a balanced input/output type, excellent filter characteristics can be obtained at all times regardless of grounding condition. 
     In the descriptions of embodiments described below, as disclosed in Japanese Patent Publication JP-A 7-312503, two or more strip lines, each comprising a wide strip portion and a narrow strip portion being integrated in series, are arranged, the wide strip portions are coupled electromagnetically to each other, and the narrow strip portions are also coupled electromagnetically to each other to form a quarter-wavelength resonator and a half-wavelength resonator. 
     Embodiment 1 
     FIG. 2 is an exploded view showing the configuration of a balanced dielectric filter having balanced input/output terminals in accordance with a first embodiment of the present invention. This filter comprises seven layers  201  to  207  of laminated dielectric ceramic sheets. Two strip-line resonators  210  and  220 , provided separately in the vertical direction, are located on the ceramic sheets  204  and  205 , respectively. In these strip-line resonators  210  and  220 , provided separately in the vertical direction, the strip lines thereof face each other so as to be mirror images of each other. 
     Furthermore, on the ceramic sheet  203  above the resonator  210 , an input capacitance electrode  231 , an output capacitance electrode  232 , an interstage-coupling capacitance electrode  233  and a loading-capacitance electrode  230  are formed, and these capacitance electrodes are coupled with the strip-line resonator  210 . 
     In the similar way, on the ceramic sheet  206  below the lower resonator  220 , an input capacitance electrode  241 , an output capacitance electrode  242 , an interstage-coupling capacitance  243  and a loading capacitance electrode  240  are formed, and these capacitance electrodes are coupled with the strip-line resonator  220 . 
     In this example, each of the resonators  210  and  220  comprises a pair of strip-line resonating elements. In a pair of strip-line resonating elements  210   a ,  210   b  and another pair of strip-line resonating elements  220   a ,  220   b , two parallel narrow line portions  211  are connected to two parallel wide line portions  212 , and two parallel narrow line portions  221  are connected to two parallel wide line portions  222 . The input capacitance electrode  241  and the output capacitance electrode  242  are electrostatically coupled to the wide line portions. The ends of the two parallel narrow portions of the resonator  210  are connected to a common grounding terminal  213 , and the ends of the two parallel narrow portions of the resonator  220  are connected to a common grounding terminal  223 . The two parallel narrow line portions  211 ,  221  are coupled electromagnetically to each other, and the two parallel wide line portions  212 ,  222  are also coupled electromagnetically to each other. At the same time, the wide line portions  212  are coupled electrostatically through the interstage-coupling capacitance  233 , and the wide line portions  222  are also coupled electrostatically through the interstage-coupling capacitance  243 . 
     The electrodes and the strip lines mentioned above are held between shield electrodes  250  and  251  in the vertical direction. Input terminals  261  ( 11 ) and  262  ( 12 ) are formed in a pair on one side of the multilayered filter and connected to the input capacitance electrodes, and output terminals  263  ( 51 ) and  264  ( 52 ) are formed in a pair on the other side of the multilayered filter and connected to the output capacitance electrodes. Furthermore, in the case of this embodiment, input/output grounding electrodes  265  and  266  ( 4 ) are provided so as to be connected to the shield electrodes  250  and  251 , an intermediate electrode  271  and the grounding electrodes  213  and  223 . 
     In the two resonators of the present invention, that is, the resonator  210  (resonating elements  210   a  and  210   b ) and the resonator  220  (resonating elements  220   a  and  220   b ), the two strip lines, each comprising the wide line portion  212  and the narrow line portion  211  coupled to each other, are provided in parallel. Furthermore, the two strip lines, each comprising the wide line portion  222  and the narrow line portion  221  coupled to each other, are provided in parallel. The resonating elements used in a pair for one of the two resonators are located separately from the resonating elements used in a pair for the other resonator in the vertical direction so as to face each other and to be mirror images of each other. With this configuration, an input signal supplied to the input terminal  261  passes through the input capacitance electrode  231  and is applied to the resonator  210 , and an input signal supplied to the input terminal  262  passes through the input capacitance electrode  241  and is applied to the resonator  220 . The resonating elements of the resonator  210  and those of the resonator  220  are independently coupled electromagnetically, and filtration is carried out in a constant frequency range. Each resonator is connected to the output terminal through the output capacitance electrode corresponding thereto, and filtration signals being 180 degrees out of phase with each other are output to the output terminals of the resonators. 
     The resonators  210 ,  220  are disposed to face each other, and input signals applied to the resonators are opposite in phase (180 degrees out of phase). Therefore, the potential on the strip line of one of the resonators is the same as that on the strip line of the other resonator, but opposite in sign with respect to the symmetry plane between the resonators  210 ,  220  at any positions on the strip lines corresponding to each other, whereby both the resonators are balanced completely. The symmetry plane  9  has the zero potential at all times, and this surface becomes an imaginary electric wall. In the present embodiment, an intermediate shield electrode described below is not formed, whereby high-frequency current does not flow through such an intermediate shield electrode. As a result, the present embodiment is advantageous in that filter-passing loss is low, and filter production is attained simply. 
     A filter is produced as described below. Silver paste is applied into thick films printing onto green sheets of ceramic material such as, for example, Bi—Ca—Nb—O based ceramic in order to form patterns of resonators and electrodes, and the plural green sheets are laminated and then fired to an integrated ceramic filter. 
     FIG. 3 is an exploded view showing an arrangement of strip lines and electrodes located on every sheet in a ceramic-multilayered filter having balanced input/output terminals in accordance with the present embodiment. Input electrodes  261 ,  262  are formed in a pair on one side of the filter, and these are used as input terminals  11 ,  12 , respectively. In addition, output electrodes  263 ,  264  are formed in a pair on the opposite side thereof, and these are used as output terminals  51 ,  52 , respectively. Furthermore, a grounding electrode  265  is formed so as to be exposed on another side of the filter, and a grounding electrode  266  is also formed so as to be exposed on the opposite side thereof, and these grounding electrodes are used as a grounding terminal  4 . 
     FIG. 4 is a sectional view showing the filter of a ceramic-multilayered structure in accordance with the present embodiment. Referring to this figure, electrodes are disposed symmetrical with respect to a mirror-image symmetry plane  40  in the vertical direction, and the imaginary electric wall  9  is formed on the symmetry plane. 
     Embodiment 2 
     In the case of a second embodiment of the present invention, an intermediate shield electrode is formed at the position of the electric wall  9  in addition to the electrodes located in the balanced dielectric filter having the balanced input/output terminals in accordance with the first embodiment. 
     Referring to FIG. 5, in a balanced dielectric filter in accordance with the second embodiment, an intermediate shield electrode  271  is located between the two strip-line resonators  210 ,  220  in accordance with the above-mentioned embodiment. The intermediate shield electrode  271  is positioned nearly close to the symmetry plane  40  between the two strip-line resonators  210 ,  220  located in the vertical direction and grounded. 
     This filter of the present embodiment is the same as the filter of the first embodiment except that the intermediate shield electrode  271  is provided so as to act as the electric wall  9  between the resonators  210 ,  220 . This filter is electrically symmetrical with respect to the electric wall  9  in the vertical direction as described in the explanation of the first embodiment. Since this filter is electrically symmetrical with respect to the shield electrode  271 , even if external magnetic and electric fields are exerted on the filter, such fields are not produced at the input and output of the filter. 
     Embodiment 3 
     FIG. 6 is a view showing another structure of a balanced dielectric filter having balanced input/output terminals in accordance with the present embodiment. This dielectric filter comprises multilayers of dielectric ceramic sheets  201  to  207 . Two resonators, that is, a resonator  310  (resonating elements  310   a ,  310   b ) and a resonator  320  (resonating elements  320   a ,  320   b ), are formed as strip lines having a nearly quarter-wavelength. 
     The resonators are detailed as follows. The wide line portions  212  comprising two parallel strip lines are coupled electromagnetically to the wide line portions  222  comprising two parallel strip lines. The narrow line portions  211  comprising two parallel strip lines connected to the wide line portions  212  are coupled electromagnetically to the narrow line portions  221  comprising two parallel strip lines connected to the wide line portions  222 . The ends of the two narrow line portions  211  are connected to connection electrodes  313  and  314 , respectively, and the ends of the two narrow line portions  221  are connected to connection electrodes  323  and  324 , respectively. Furthermore, the connection electrode  313  is connected to the connection electrode  323  through a short-circuit electrode  366 , and the connection electrode  314  is connected to the connection electrode  324  through a short-circuit electrode  367 , thereby forming a pair of half-wavelength strip lines. 
     On the input sides of the wide line portions  212 ,  222 , input capacitance electrodes  231 ,  241  are located respectively through a dielectric ceramic layer. On the output sides of the wide line portions  212 ,  222 , output capacitance electrodes  232 ,  242  are located respectively through the dielectric ceramic layer. Furthermore, above the pair of the wide line portions  212 , a loading-capacitance electrode  230  and an interstage-coupling capacitance electrode  233  are located on a dielectric ceramic layer  203 . Below the pair of the wide line portions  222 , a loading capacitance electrode  240  and an interstage-coupling capacitance electrode  243  are located on a dielectric ceramic layer  206 . With this configuration, capacitances are formed between these capacitance electrodes and the wide line portions. Moreover, a shield electrode  250  is formed on a dielectric ceramic layer  202  above the capacitance electrodes  230 ,  233 , and a shield electrode  251  is formed on a dielectric ceramic layer  207  below the capacitance electrodes  240 ,  243 . 
     The input capacitance electrodes  231 ,  241  are connected to input terminals  261 ,  262 , respectively. The output capacitance electrodes  232 ,  242  are connected to output terminals  263 ,  264 , respectively. These input/output capacitance electrodes are formed on two sides of the ceramic-multilayered filter. On the other two sides of the multilayered filter, grounding terminals  265 ,  368 ,  369  are formed and connected to the shield electrodes  250 ,  251 . A terminal electrode  366  is a connection end electrode used to connect the connection electrodes  313  and  323 , and a terminal electrode  367  is a connection end electrode used to connect the connection electrodes  314  and  324 . 
     A dielectric filter is produced as described below. A dielectric ceramic green sheet of Bi—Ca—Nb—O ceramic material for example is formed. Silver paste is applied by thick film printing onto each sheet to form electrode and strip line patterns having predetermined shapes. The green Sheets are laminated and then fired, which is integrated into a ceramic-multilayered filter. 
     FIG. 7 is an exploded view of a ceramic-multilayered filter according to this embodiment showing an arrangement of strip lines and electrodes on every sheet in the dielectric filter. In the filter  1  having the shape of a rectangular parallelepiped, the input terminals  11 ,  12  used in a pair and formed of the electrodes  261 ,  262 , respectively, are attached to the end surface of one side of the filter. The output terminals  51 ,  52  used in a pair and formed of the electrodes  263 ,  264 , respectively, are attached to the end surface of the opposite side. The grounding terminals  4  comprising the grounding electrodes  265 ,  368  and  369  are formed on the end surfaces of the remaining sides. In the case when this filter  1  is used, the electrodes  261 ,  262  and the electrodes  263 ,  264  thereof are secured by soldering to corresponding electrodes located at predetermined positions on a circuit board. 
     Furthermore, on the end surface of one side of the dielectric filter  1 , the short-circuit electrode  366  is formed to short between the end of one of the two narrow line portions  211  and the end of one of the two narrow line portions  221 , and the short-circuit electrode  367  is formed to short between the end of the other narrow line portion  211  and the end of the other narrow line portion  221 . 
     This embodiment is significantly different from the first embodiment in that, instead of grounding the end of each resonating element, the end of a quarter-wavelength resonating element is connected to the end of the other quarter-wavelength resonating element disposed therebelow to form a nearly half-wavelength resonator. As a result, the resonating elements are floated from the grounding potential. 
     By forming the half-wavelength resonator by electrically connecting the resonating elements as described above, it is possible to attain a filter being excellent in filter characteristics and compact in size. In addition, even if variations are present between the upper and lower resonating elements, balanced conditions can be attained automatically, since there is no point to be forcibly grounded on the resonating elements. The filter can thus have excellent characteristics at all times. 
     Since this filter is not provided with the intermediate shield electrode  371  (see FIG.  8 ), the filter can be produced simply. Furthermore, since no high-frequency current flows to the intermediate shield electrode  371 , a loss due to such current can be eliminated, and the passing characteristics of the filter can be improved. 
     As described above, the present embodiment provides outstanding effects of simplifying filter production, eliminating filter loss, and improving filter characteristics. 
     Embodiment 4 
     FIG. 8 is a view showing a structure of a balanced dielectric filter having balanced input/output terminals in accordance with a fourth embodiment of the present invention. The balanced dielectric filter of the present embodiment is the same as the balanced dielectric filter of the third embodiment except that an intermediate shield electrode  371  used to act as an electric wall is provided between the resonators  310  and  320  of the dielectric filter of the third embodiment. 
     The intermediate shield electrode  371  is located between upper and lower resonating elements  310   a  and  320   a  and between upper and lower resonating elements  310   b  and  320   b , and is connected to a grounding electrode  266  so as to be grounded, thereby acting as an electric wall  9 . Since this dielectric filter is symmetrical electrically in the vertical direction with respect to the intermediate shield electrode  371 , the filter functions as a completely balanced dielectric filter as viewed from the input/output terminals thereof. 
     As described above, the balanced dielectric filter of the present invention has two resonators, each comprising plural TEM mode resonating elements, disposed in parallel and mutually coupled electromagnetically. The two resonators are connected so as to be balanced and parallel between a pair of input terminals and a pair of output terminals, and the resonators are disposed in parallel and symmetrical so as to be mirror images of each other. As a result, both the input and output sides of the filter are balanced completely, and the filter is completely shielded against external electromagnetic interference, thereby preventing adverse effects due to the interference from being produced on both the input and output sides. 
     The filter comprises a pair of input terminals, resonators, each comprising plural TEM mode resonating elements, disposed in parallel and mutually coupled electromagnetically and both the input side ends of the resonators being connected to the input terminals, respectively, and a pair of output terminals connected to both the output side ends of the resonators, respectively. Each resonator is in a bent form so that the resonating elements thereof are mirror images of each other. Therefore, just as described above, the input and output sides of the filter are balanced completely, and the filter is completely shielded against external electromagnetic interference, thereby preventing adverse effects due to the interference from being produced on both the input and output sides. 
     Since the TEM mode resonating elements are located on dielectrics to form a strip-line resonator, it is possible to obtain a filter being very compact in size and excellent in mass production capability. 
     In addition, the above-mentioned resonator is formed as a quarter-wavelength resonator by grounding the ends of the strip lines thereof. Or the above-mentioned resonator is formed as a half-wavelength resonator by connecting the ends of the strip lines being symmetrical to each other. In both types of these resonators, filters nearly as large as a quarter-wavelength filter can be obtained. 
     Since the two strip-line resonators are embedded in dielectric layers, the resonators can be held at predetermined positions. In addition, by using ceramics with a high dielectric constant, the strip-line resonators can be shortened, whereby dielectric filters can be made compact. 
     Furthermore, between the resonators being mirror images of each other, the electric wall is formed near a plane of symmetry, and in particular, a grounded conductor is located between the resonators. As a result, the balanced dielectric filter is balanced completely, whereby the filter can function as a filter highly resistant against electromagnetic interference. 
     Furthermore, the balanced dielectric filer according to the present invention may be used as an unbalanced input-balanced output filter. In this case, one of the pair of the input terminals may be connected with another unbalanced circuit, and the other input terminal is unconnected free, or, preferably, grounded to a grounded terminal. Signals from the circuit are applied between the input terminal and the grounded terminal. Alternatively, as a balanced-unbalanced filter the balanced dielectric filter may be used for connecting the unbalanced output terminals to another unbalanced circuit. The filters to be used for such unbalanced-balanced filtration have the same configuration of the resonators as shown in FIGS. 2,  5 ,  6  and  8 , the difference being only in a method of wiring to outer circuits, which are balanced or unbalanced.