Abstract:
A microstripline filter capable of strengthening capacitive coupling between resonators and suppressing fineness of electrodes is provided. The microstripline filter includes principal-surface lines, a ground electrode, and input/output electrodes. A first principal-surface line is capacitively coupled to a second principal-surface line. The second principal-surface line is inductively coupled to a third principal-surface line. The third principal-surface line is capacitively coupled to a fourth principal-surface line. The first through fourth principal-surface lines include open-end-side electrodes, short-circuit-end-side electrodes, and end-opened electrodes. A first pair of the end-opened electrodes are adjacent to each other, whereas a first pair of the short-circuit-end-side electrodes are separate from each other. A second pair of end-opened electrodes are adjacent to each other, whereas a second pair of short-circuit-end-side electrodes are separate from each other.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of International Application No. PCT/JP2008/059428, filed May 22, 2008, which claims priority to Japanese Patent Application No. JP2007-185703, filed Jul. 17, 2007, the entire contents of each of these applications being incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a microstripline filter including striplines provided on a dielectric substrate. 
       BACKGROUND OF THE INVENTION 
       [0003]    There have been widespread combline-type microstripline filters in which open ends of striplines constituting quarter-wavelength resonators are unidirectionally placed such that adjacent resonators couple to each other. In the combline-type microstripline filter, each resonant line may have a step structure in which the line width of an open-end-side electrode is different from the line width of a short-circuit-end-side electrode. Also, in the microstripline filter having the step structure, a depressed portion may be provided in an open-end-side electrode so that a short-circuit-end-side electrode extends from the bottom of the depressed portion of the open-end-side electrode to a ground electrode (e.g., see Patent Document 1). 
         [0004]    Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-111602 
       SUMMARY OF THE INVENTION 
       [0005]    In radio LAN communication requiring a wideband frequency characteristic in a very high frequency region, a deep attenuation pole may be necessary on a low-frequency side of a frequency band of a microstripline filter. An interdigital-type microstripline filter has been used to realize a wideband frequency characteristic. The interdigital-type microstripline filter has a configuration in which open ends of striplines are alternately oriented and has a feature of very strong coupling in a pair of resonators. 
         [0006]    On the other hand, in a conventional combline-type microstripline filter, the coupling in a pair of resonators is not as strong as that of the interdigital-type microstripline filter and thus it is difficult to realize a wideband filter. However, in the combline-type microstripline filter, a plurality of attenuation poles can be set with a high degree of freedom, and using this advantage in radio LAN communication has been demanded. 
         [0007]    When a target frequency region is very high as in the microstripline filter for radio LAN communication, a fine electrode pattern is necessary. However, the degree of fineness of the electrode pattern is limited due to constraints in a manufacturing process. For this reason, it is difficult to make a complicated electrode pattern in the microstripline filter for radio LAN communication. For example, a complicated electrode pattern having a depressed portion provided in an open-end-side electrode of a stripline of a step structure and a short-circuit-end-side electrode extending from the bottom of the depressed portion of the open-end-side electrode to a ground electrode cannot be adopted in the microstripline filter for radio LAN communication. 
         [0008]    Accordingly, an object of the present invention is to provide a combline-type microstripline filter having a wide passband and having a deep attenuation pole on a low-frequency side of a frequency band, while reducing the degree of fineness of an electrode pattern. 
       Means for Solving the Problems 
       [0009]    A microstripline filter according to the present invention includes a ground electrode, a plurality of resonant lines, and input/output electrodes. The ground electrode is provided on a bottom surface of a rectangular flat dielectric substrate. The plurality of resonant lines constitute a plurality of resonators together with the ground electrode and the dielectric substrate. The input/output electrodes couple to the resonator constituted by any of the plurality of resonant lines. 
         [0010]    Any of the resonant lines form a pair of resonators that capacitively couple to each other together with another of the resonant lines. The resonant line includes an open-end-side electrode, an end-opened electrode, and a short-circuit-end-side electrode. The open-end-side electrode is an electrode including an open end of the resonant line. The open-end-side electrode is parallel to a side edge of the other resonant line in the pair. The end-opened electrode is an electrode that extends from an end on a short-circuit-end side of the open-end-side electrode with a line width different from a line width of the open-end-side electrode and that is parallel to the side edge of the other resonant line in the pair together with the open-end-side electrode. The end-opened electrode has an open end. The short-circuit-end-side electrode is an electrode that extends from the end on the short-circuit-end side of the open-end-side electrode to the ground electrode with a line width different from the line width of the open-end-side electrode. A center of the line width of the short-circuit-end-side electrode is displaced in a direction separating from the end-opened electrode. 
         [0011]    With this configuration, the open-end-side electrode and the end-opened electrode are parallel to the other resonant line in the pair. Accordingly, a very strong mutual capacitance occurs on the open-end side of the resonant line. Also, the other resonant line in the pair is separated from the short-circuit-end-side electrode, and the end-opened electrode exists therebetween. Accordingly, the mutual capacitance on the short-circuit-end side of the resonant line is significantly small compared to the mutual capacitance on the open-end side. Therefore, the pair of resonators constituted by the resonant lines capacitively couple to each other very strongly compared to the conventional microstripline filter in which combline coupling is performed. Also, an attenuation pole deeply falls on the low-frequency side of the frequency band of the microstripline filter. 
         [0012]    Since the center position in the width direction of the short-circuit-end-side electrode is displaced, a large line width of the end-opened electrode can be easily ensured, whereby the degree of fineness of electrode forms can be reduced. Also, the gap between the resonant lines forming a pair can be increased in accordance with the mutual capacitance added by the end-opened electrode, whereby the degree of fineness in an electrode absent portion between the lines can be reduced. 
         [0013]    In the microstripline filter according to the present invention, any of the resonant lines includes an open-end-side electrode, an end-opened electrode, and a short-circuit-end-side electrode. This resonant line forms a pair of resonators that capacitively couple to each other together with the resonant line adjacent on the side of the end-opened electrode. Also, this resonant line forms a pair of resonators that inductively couple to each other together with the resonant line adjacent on the side of the short-circuit-end-side electrode. 
         [0014]    With this configuration, this resonant line is capable of capacitively coupling to one resonant line adjacent on the side of the end-opened electrode and inductively coupling to the other resonant line adjacent on the side of the short-circuit-end-side electrode. Accordingly, desired attenuation poles can be caused on the high-frequency side and the low-frequency side of the frequency band of the microstripline filter. Since the center position in the width direction of the short-circuit-end-side electrode is displaced, a line width of the end-opened electrode can be easily ensured. Also, the gap between the resonant lines forming a pair of resonators can be increased in accordance with the mutual capacitance added by the end-opened electrode. 
         [0015]    A plurality of end capacitance electrodes to give a stray capacitance to the respective resonant lines constituting the pair of resonators that inductively couple to each other may be provided. 
         [0016]    An end capacitance is given to the resonant lines constituting the pair of resonators that inductively couple to each other by the end-opened electrodes. The end capacitance functions as a stray capacitance on the resonant lines, and the coupling between the resonators constituted by those resonant lines is biased to inductive coupling. 
         [0017]    The respective resonant lines constituting the pair of resonators that capacitively couple to each other may include the open-end-side electrode, the end-opened electrode, and the short-circuit-end-side electrode. In that case, the respective end-opened electrodes face each other preferably. Accordingly, the mutual capacitance increases, so that the coupling between the resonators constituted by the resonant lines is biased to capacitive coupling. 
         [0018]    The short-circuit-end-side electrode may extend from a top surface to a side surface of the dielectric substrate. With this configuration, side-surface electrodes function as part of a transmission line, so that the filter can be miniaturized while maintaining the same resonant frequency. 
         [0019]    A sum value of the line width of the end-opened electrode and a dimension of a gap between the end-opened electrode and the short-circuit-end-side electrode on one side of the short-circuit-end-side electrode may be larger than 0.5 times of a value calculated by subtracting the line width of the short-circuit-end-side electrode from the line width of the open-end-side electrode. For example, in the microstripline filter described in the above-mentioned Patent Document 1, the width of one side of a step portion is 0.5 times of a value calculated by subtracting the line width of the short-circuit-end-side electrode from the line width of the open-end-side electrode. Compared to this conventional case, the sum value of the line width of the end-opened electrode and the dimension of the gap between the end-opened electrode and the short-circuit-end-side electrode can be larger in the present configuration. Accordingly, the degree of fineness of the electrode pattern can be reduced in the present configuration. 
         [0020]    The sum value of the line width of the end-opened electrode and the dimension of the gap between the end-opened electrode and the short-circuit-end-side electrode on one side of the short-circuit-end-side electrode may be larger than 0.5 times of the line width of the open-end-side electrode. Accordingly, the degree of fineness of the electrode pattern can be significantly reduced. 
         [0021]    According to the present invention, in a combline-type microstripline filter, a wideband frequency characteristic can be realized and an attenuation pole can be arbitrarily set. Also, an attenuation pole on a low-frequency side of a frequency band can fall more deeply compared to a conventional microstripline filter that performs combline coupling. Furthermore, the degree of fineness of an electrode pattern can be reduced and a good product ratio in a manufacturing process can be increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  includes perspective views illustrating an example of a microstripline filter. 
           [0023]      FIG. 2  is a developed view of the microstripline filter. 
           [0024]      FIG. 3  is a graph showing a frequency characteristic of the microstripline filter based on simulation. 
       
    
    
     REFERENCE NUMERALS 
       [0000]    
       
           1  dielectric substrate 
           2  glass layer 
           6  principal-surface line 
           7  end capacitance electrode 
           8  lead electrode 
           11  ground electrode 
           12  input/output electrode 
           9 ,  10 ,  13 , and  14  side-surface electrode 
           61  open-end-side electrode 
           62  short-circuit-end-side electrode 
           63  end-opened electrode 
           100  microstripline filter 
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Hereinafter, an example of the configuration of a microstripline filter is described. 
         [0038]    The microstripline filter described here is a bandpass filter. This filter is used in radio LAN facilities in 5 GHz band. 
         [0039]      FIG. 1(A)  is a perspective view of the microstripline filter.  FIG. 1(B)  is a transparent perspective view of the microstripline filter. 
         [0040]    The microstripline filter  100  includes a dielectric substrate  1  and a glass layer  2 . The substrate  1  is a compact rectangular parallelepiped ceramic-sintered substrate that is composed of titanium oxide or the like and that has a relative permittivity of about 111. The composition and dimensions of the substrate  1  may be appropriately set in view of a frequency characteristic and so on. 
         [0041]    An electrode pattern is formed on a top surface of the substrate  1 . On the top surface side of the substrate  1 , the glass layer  2  having a thickness of 15 μm or more is laminated as a protective layer for mechanical protection of the top-surface electrode pattern and for electrical insulation. The glass layer  2  is composed of an insulating material, such as crystalline SiO 2  or borosilicate glass. The glass layer  2  is formed by printing and sintering a glass paste. A photosensitive glass paste may be used as a glass paste. Also, the glass layer  2  may be formed by laminating a translucent glass paste and a light-shielding glass paste. With the use of the glass layer  2 , the top-surface electrode pattern can be mechanically protected and the resistance to weather can be enhanced. 
         [0042]    Alternatively, an electrode can be formed on the top surface side of the glass layer  2 . In that case, the glass layer  2  can prevent the top-surface electrode of the glass layer  2  from being short circuited to the top-surface electrode pattern on the dielectric substrate  1 . The pattern and dimensions of the glass layer  2  may be appropriately set in view of the degree of adhesion between the dielectric substrate  1  and the glass layer  2 , the resistance to environment, a frequency characteristic, and so on. 
         [0043]      FIGS. 2(A)-2(F)  are a developed view of the dielectric substrate  1 .  FIG. 2(A)  is a front view,  FIG. 2(B)  is a top view,  FIG. 2(C)  is a back view,  FIG. 2(D)  is a bottom view,  FIG. 2(E)  is a left side view, and  FIG. 2(F)  is a right side view. 
         [0044]    On the top surface of the dielectric substrate  1  illustrated in  FIG. 2(B) , a top-surface electrode pattern including principal-surface lines  6 A to  6 D, end capacitance electrodes  7 A to  7 D, and lead electrodes  8 A and  8 B is formed. The top-surface electrode pattern is formed of silver electrodes having a thickness of about 6 μm or more. The top-surface electrode pattern is formed by applying a photosensitive silver paste on a mother substrate, forming a pattern by a photolithography process, and performing sintering. 
         [0045]    On the front surface of the dielectric substrate  1  illustrated in  FIG. 2(A) , a side-surface electrode pattern including side-surface electrodes  10 A to  10 D is formed. On the back surface of the dielectric substrate  1  illustrated in  FIG. 2(C) , a side-surface electrode pattern including side-surface electrodes  9 A to  9 D is formed. Those side-surface electrode patterns are formed of silver electrodes having a thickness of about 12 μm or more. The side-surface electrode patterns on the front and back surfaces have similar forms. This is because a printing process is almost the same in the side-surface electrode patterns on the front and back surfaces. By adopting such forms, the necessity of arranging the orientations of the front, back, top, and bottom surfaces of the side-surface electrode patterns can be eliminated in the printing process. Those side-surface electrode patterns are formed by applying a non-photosensitive silver paste on the front and back surfaces of the dielectric substrate  1  by using a screen mask or a metal mask and performing sintering. 
         [0046]    The bottom surface of the dielectric substrate  1  illustrated in  FIG. 2(D)  is a mount surface of the microstripline filter, and a bottom-surface electrode pattern including a ground electrode  11  and input/output electrodes  12 A and  12 B is formed thereon. The input/output electrodes  12 A and  12 B are separated from the ground electrode  11 . The input/output electrodes  12 A and  12 B are connected to high-frequency signal input/output terminals when the microstripline filter  100  is mounted on a mount substrate. The ground electrode  11  is a ground surface of resonators and is connected to a ground electrode of the mount substrate. The bottom-surface electrode pattern is formed of silver electrodes having a thickness of about 12 μm. The bottom-surface electrode pattern is formed by applying a non-photosensitive silver paste on the bottom surface of the dielectric substrate  1  by using a screen mask or a metal mask and performing sintering. 
         [0047]    On the left surface of the dielectric substrate  1  illustrated in  FIG. 2(E) , a side-surface electrode pattern including a side-surface electrode  13  is formed. Also, on the right surface of the dielectric substrate  1  illustrated in  FIG. 2(F) , a side-surface electrode pattern including a side-surface electrode  14  is formed. Those side-surface electrode patterns are formed of silver electrodes having a thickness of about 12 μm or more. The side-surface electrode patterns on the left and right surfaces have similar forms. This is because a printing process is almost the same in the side-surface electrode patterns on the left and right surfaces. By adopting such forms, the necessity of arranging the orientations of the left, right, top, and bottom surfaces of the side-surface electrode patterns can be eliminated in the printing process. Those side-surface electrode patterns are formed by applying a non-photosensitive silver paste on the left and right surfaces of the dielectric substrate  1  by using a screen mask or a metal mask and performing sintering. 
         [0048]    In the top-surface electrode pattern illustrated in  FIG. 2(B) , the principal-surface lines  6 A to  6 D extend from the boundary of the back surface and the top surface of the dielectric substrate  1  toward the front surface of the dielectric substrate  1 . Accordingly, those lines constitute four stages of quarter-wavelength resonators that couple to each other in a combline manner. 
         [0049]    The lead electrode  8 A has a form bending from the left surface side to the back surface side of the dielectric substrate  1 . The lead electrode  8 A continues to the principal-surface line  6 A on the top surface side of the dielectric substrate  1 . The lead electrode  8 A continues to the side-surface electrode  13  at the boundary of the left surface and the top surface of the dielectric substrate  1 . The side-surface electrode  13  continues to the lead electrode  8 A on the top surface side on the left surface of the dielectric substrate  1  and continues to the input/output electrode  12 A on the bottom surface side. Accordingly, the lead electrode  8 A allows tap-coupling between the input/output electrode  12 A and the resonator constituted by the principal-surface line  6 A. 
         [0050]    The principal-surface line  6 A includes an open-end-side electrode  61 A, a short-circuit-end-side electrode  62 A, and an end-opened electrode  63 A. The open-end-side electrode  61 A is a rectangular electrode that is open on the front surface side of the dielectric substrate  1 , that continues to the short-circuit-end-side electrode  62 A at the corner on the back surface side and the left surface side, and that continues to the end-opened electrode  63 A at the corner on the back surface side and the right surface side. The end-opened electrode  63 A extends from the edge on the back surface side of the open-end-side electrode  61 A toward the back surface side of the dielectric substrate  1 , and the end on the back surface side of the end-opened electrode  63 A is open. The short-circuit-end-side electrode  62 A continues to the open-end-side electrode  61 A on the front surface side, the vicinity of the center connects to the lead electrode  8 A, and continues to the side-surface electrode  9 A at the boundary of the back surface and the top surface of the dielectric substrate  1 . The line width of the short-circuit-end-side electrode  62 A is smaller than that of the open-end-side electrode  61 A, whereby the principal-surface line  6 A has a step structure. The side-surface electrode  9 A continues to the short-circuit-end-side electrode  62 A on the top surface side and continues to the ground electrode  11  at the boundary of the_back surface and the bottom surface of the dielectric substrate  1  on the bottom surface side. Accordingly, the principal-surface line  6 A faces the ground electrode  11  via the dielectric substrate  1  and is brought into conduction with the ground electrode  11  via the side-surface electrode  9 A. Accordingly, the principal-surface line  6 A constitutes a quarter-wavelength resonator in an input stage (or output stage). 
         [0051]    The principal-surface line  6 B includes an open-end-side electrode  61 B, a short-circuit-end-side electrode  62 B, and an end-opened electrode  63 B. The open-end-side electrode  61 B is a rectangular electrode that is open on the front surface side of the dielectric substrate  1 , that continues to the short-circuit-end-side electrode  62 B on the right surface side of the edge on the back surface side, and that continues to the end-opened electrode  63 B on the left surface side of the edge on the back surface side. The end-opened electrode  63 B extends from the edge on the back surface side of the open-end-side electrode  61 B toward the back surface side of the dielectric substrate  1 , and the end on the back surface side of the end-opened electrode  63 B is open. The short-circuit-end-side electrode  62 B continues to the open-end-side electrode  61 B on the front surface side, and continues to the side-surface electrode  9 B at the boundary of the back surface and the top surface of the dielectric substrate  1 . The line width of the short-circuit-end-side electrode  62 B is smaller than that of the open-end-side electrode  61 B, whereby the principal-surface line  6 B has a step structure. The side-surface electrode  9 B continues to the short-circuit-end-side electrode  62 B on the top surface side and continues to the ground electrode  11  at the boundary of the back surface and the bottom surface of the dielectric substrate  1 . Accordingly, the principal-surface line  6 B faces the ground electrode  11  via the dielectric substrate  1  and is brought into conduction with the ground electrode  11  via the side-surface electrode  9 B. Accordingly, the principal-surface line  6 B constitutes a quarter-wavelength resonator in a second stage. 
         [0052]    The open-end-side electrode  61 A and the end-opened electrode  63 A of the principal-surface line  6 A and the open-end-side electrode  61 B and the end-opened electrode  63 B of the principal-surface line  6 B are parallel to each other and face each other with a predetermined gap of an electrode absent portion therebetween. Accordingly, a large mutual capacitance is given to the open-end side between the resonator constituted by the principal-surface line  6 A and the resonator constituted by the principal-surface line  6 B. The end-opened electrode  63 A and the end-opened electrode  63 B exist between the short-circuit-end-side electrode  62 A of the principal-surface line  6 A and the short-circuit-end-side electrode  62 B of the principal-surface line  6 B. Accordingly, a mutual capacitance is hardly given to the short-circuit-end side between the resonator constituted by the principal-surface line  6 A and the resonator constituted by the principal-surface line  6 B. Therefore, these resonators capacitively couple to each other. This capacitive coupling causes an attenuation pole to fall on the low frequency side of the frequency band of the microstripline filter  100 . 
         [0053]    The principal-surface line  6 C includes an open-end-side electrode  61 C, a short-circuit-end-side electrode  62 C, and an end-opened electrode  63 C. The principal-surface line  6 C has a form similar to that of the principal-surface line  6 B, but the orientation on the right surface side and the left surface side is inverted. The short-circuit-end-side electrode  62 C continues to the side-surface electrode  9 C at the boundary of the back surface and the top surface of the dielectric substrate  1 . The side-surface electrode  9 C continues to the short-circuit-end-side electrode  62 C on the top surface side and continues to the ground electrode  11  at the boundary of the back surface and the bottom surface of the dielectric substrate  1  on the bottom surface side. The principal-surface line  6 C constitutes a quarter-wavelength resonator in a third stage. 
         [0054]    The open-end-side electrode  61 B and the short-circuit-end-side electrode  62 B of the principal-surface line  6 B and the open-end-side electrode  61 C and the short-circuit-end-side electrode  62 C of the principal-surface line  6 C are parallel to each other and face each other with a predetermined gap of an electrode absent portion therebetween. Accordingly, a mutual capacitance is evenly given from the open-end side to the short-circuit-end side between the resonator constituted by the principal-surface line  6 B and the resonator constituted by the principal-surface line  6 C. Additionally, the ends on the front surface side of the open-end-side electrodes  61 B and  61 C face end capacitance electrodes  7 B and  7 C described below. The end capacitance electrodes  7 B and  7 C continue to the ground electrode  11  via side-surface electrodes  10 B and  10 C. Therefore, an end capacitance is added to the open-end-side electrodes  61 B and  61 C. The end capacitance functions as a stray capacitance in the resonators, and the resonators inductively couple to each other. This inductive coupling causes an attenuation pole to fall on the high frequency side of the frequency band of the microstripline filter  100 . 
         [0055]    The principal-surface line  6 D includes an open-end-side electrode  61 D, a short-circuit-end-side electrode  62 D, and an end-opened electrode  63 D. The principal-surface line  6 D has a form similar to that of the principal-surface line  6 A, but the orientation on the right surface side and the left surface side is inverted. The short-circuit-end-side electrode  62 D continues to the lead electrode  8 B at the vicinity of the center and continues to the side-surface electrode  9 D at the boundary of the back surface and the top surface of the dielectric substrate  1 . The side-surface electrode  9 D continues to the short-circuit-end-side electrode  62 D on the top surface side and continues to the ground electrode  11  at the boundary of the back surface and the bottom surface of the dielectric substrate  1  on the bottom surface side. Therefore, the principal-surface line  6 D constitutes a quarter-wavelength resonator in an output stage (or input stage). The principal-surface line  6 D and the above-described principal-surface line  6 C constitute a pair of resonators that capacitively couple to each other. This capacitive coupling causes a second attenuation pole to fall on the low frequency side of the frequency band of the microstripline filter  100 . 
         [0056]    The lead electrode  8 B has a form similar to that of the lead electrode  8 A, but the orientation on the right surface side and the left surface side is inverted. The lead electrode  8 B continues to the principal-surface line  6 D on the back surface side of the dielectric substrate  1 . The lead electrode  8 B continues to the side-surface electrode  14  at the boundary of the right surface and the top surface of the dielectric substrate  1 . The side-surface electrode  14  continues to the lead electrode  8 B on the top surface side of the right surface of the dielectric substrate  1  and continues to the input/output electrode  12 B on the bottom surface side. Accordingly, the lead electrode  8 B allows tap-coupling between the input/output electrode  12 B and the resonator constituted by the principal-surface line  6 D. 
         [0057]    The end capacitance electrodes  7 A to  7 D have forms similar to each other, continue to the side-surface electrodes  10 A to  10 D at the boundary of the front surface and the top surface of the dielectric substrate  1 , and their ends on the back surface side are open. The end capacitance electrodes  7 A to  7 D are separated from the open ends of the principal-surface lines  6 A to  6 D by a predetermined gap. Accordingly, the end capacitance electrodes  7 A to  7 D give an end capacitance to the principal-surface lines  6 A to  6 D. The value of the end capacitance depends on the dimension of the gap and the facing length between the end capacitance electrodes  7 A to  7 D and the principal-surface lines  6 A to  6 D. The frequency characteristic can be adjusted by adjusting the gap and the facing length. 
         [0058]    The entire end portions of the end capacitance electrodes  7 B and  7 C face the principal-surface lines  6 B and  6 C. Therefore, the end capacitance given to the principal-surface lines  6 B and  6 C by the end capacitance electrodes  7 B and  7 C is very large, so that the coupling between the principal-surface lines  6 B and  6 C is biased to inductive coupling. 
         [0059]    On the other hand, the centers in the horizontal direction in the figure of the end capacitance electrodes  7 A and  7 D are significantly displaced from the centers in the horizontal direction in the figure of the principal-surface lines  6 A and  6 D, and only parts of the end portions of the end capacitance electrodes  7 A and  7 D face the principal-surface lines  6 A and  6 D. Therefore, the end capacitance given to the principal-surface lines  6 A and  6 D by the end capacitance electrodes  7 A and  7 D is very small, and the mutual capacitance on the open end side between the principal-surface lines  6 A and  6 B and the mutual capacitance on the open end side between the principal-surface lines  6 C and  6 D keep the coupling between the principal-surface lines  6 A and  6 B and the coupling between the principal-surface lines  6 C and  6 D biased to capacitive coupling. 
         [0060]    The end capacitance electrodes  7 A and  7 D and the side-surface electrodes  10 A and  10 D need not always be provided. However, it is preferable to provide the end capacitance electrodes  7 A and  7 D in the case where the side-surface electrodes  10 A and  10 D are provided to form similar side-surface electrode patterns. For example, in the case where only the side-surface electrodes  10 A and  10 D are provided with the end capacitance electrodes  7 A and  7 D not being provided, an end capacitance is given to the principal-surface lines  6 A and  6 D by the side-surface electrodes  10 A and  10 D. The end capacitance easily varies depending on a cut error and so on of the dielectric substrate, which leads to a risk of affecting stability of the frequency characteristic of the microstripline filter  100 . On the other hand, in the case where the end capacitance electrodes  7 A and  7 D are provided, the end capacitance given to the principal-surface lines  6 A and  6 D by the end capacitance electrodes  7 A and  7 D is stable even if the dielectric substrate has a cut error, which contributes to the stability of the frequency characteristic of the microstripline filter  100 . 
         [0061]    With the above-described configuration, the microstripline filter  100  constitutes a filter including four stages of resonators. Specifically, the input/output electrode  12 A achieves tap-coupling to the resonator constituted by the principal-surface line  6 A. The resonator constituted by the principal-surface line  6 A capacitively couples to the resonator constituted by the principal-surface line  6 B. The resonator constituted by the principal-surface line  6 B inductively couples to the resonator constituted by the principal-surface line  6 C. The resonator constituted by the principal-surface line  6 C capacitively couples to the resonator constituted by the principal-surface line  6 D. The input/output electrode  12 B achieves tap-coupling to the resonator constituted by the principal-surface line  6 D. 
         [0062]    When capacitive coupling between adjacent resonators is discussed, it is necessary to discuss a difference in resonant frequency between an odd mode where an electric wall exists at the center between the resonators and an even mode where a magnetic wall exists at the center between the resonators. When the principal-surface lines  6 A to  6 D have a step structure, the resonant frequency in the even mode is lower than the resonant frequency in the odd mode in each principal-surface line. Accordingly, the resonant frequency in the even mode is higher than the resonant frequency in the odd mode, so that a stronger capacitive coupling can be obtained. 
         [0063]    Furthermore, the capacitive coupling among the resonators is strengthened also by the end-opened electrodes  63 A to  63 D of the respective principal-surface lines  6 A to  6 D. Providing the end-opened electrodes  63 A and  63 B causes the resonator length in the odd mode to be very long and the resonant frequency in the odd mode to be significantly low. On the other hand, the resonator length in the even mode slightly becomes long, but the degree of extension is small, and the resonant frequency in the even mode becomes low only slightly. Therefore, the resonant frequency in the even mode is higher than the resonant frequency in the odd mode, and thus stronger capacitive coupling can be obtained. 
         [0064]    Also, since the short-circuit-end-side electrodes  62 A to  62 D and the end-opened electrodes  63 A to  63 D continue to the ends on the back surface side of the open-end-side electrodes  61 A to  61 D in the principal-surface lines  6 A to  6 D, respectively, each line width can be a little smaller than half of the line width of the open-end-side electrodes  61 A to  61 D. Accordingly, the degree of fineness of the top-surface electrode pattern can be reduced. 
         [0065]    For example, in the above-described microstripline filter according to Patent Document 1, the width of one side of the step portion is half of the difference between the line width on the open-end side and the line with on the short-circuit-end side. 
         [0066]    However, in this configuration, the width of one side of the step portion: the sum value of the dimension of the gap between the short-circuit-end-side electrode  62 A and the end-opened electrode  63 A and the line width of the end-opened electrode  63 A, can be more than half of the value calculated by subtracting the line width of the short-circuit-end-side electrode  62 A from the line width of the open-end-side electrode  61 A. Furthermore, it is even possible that the sum value is larger than 0.5 times the line width of the open-end-side electrode  61 A. Accordingly, the degree of fineness of the top-surface electrode pattern can be reduced. 
         [0067]    The top-surface electrode pattern provided on the top surface of the dielectric substrate  1  has a large influence on the frequency characteristic of the microstripline filter depending on its form and precision, and is thus formed in a photolithography process by improving the electrode precision as much as possible. 
         [0068]    Furthermore, since the electrode thickness of the side-surface electrode pattern is larger than the electrode thickness of the top-surface electrode pattern, the current in a portion on the ground-end side where a current typically concentrates is dispersed so as to reduce conductor loss. With this configuration, the microstripline filter serves as a device with a small insertion loss. 
         [0069]    Next, an example of a frequency characteristic of the microstripline filter determined by simulation is described. 
         [0070]      FIG. 3(A)  is a diagram illustrating the dimensions of respective parts of a microstripline filter  101  used in the simulation. In the figure, the parts same as those in the above-described microstripline filter  100  are denoted by the same reference numerals. The unit of the dimensions shown in the figure is μm (micrometer).  FIG. 3(B)  is a graph showing the frequency characteristic of the microstripline filter  101  obtained through the simulation. The broken line shown in the graph indicates characteristic S 11  of the microstripline filter  101 . The solid line shown in the graph indicates characteristic S 21  of the microstripline filter  101 . 
         [0071]    When attention is focused on characteristic S 21  of the microstripline filter  101 , the microstripline filter  101  realizes a passband with an insertion loss of about −2 dB from about 5100 MHz to about 5900 MHz. Also, two attenuation poles exist at the vicinity of about 4200 MHz to about 4800 MHz on the low-frequency side in the passband, and the attenuation amount in the region of 4600 GHz or less is about −40 dB or less. 
         [0072]    In this configuration, the end-opened electrodes  63 A and  63 B strengthen the capacitive coupling in the pair of resonators of the principal-surface lines  6 A and  6 B, and the end-opened electrodes  63 C and  63 D strengthen the capacitive coupling in the pair of resonators of the principal-surface lines  6 C and  6 D. Accordingly, the two attenuation poles on the low-frequency side of the passband deeply fall with an attenuation amount of about −40 dB or less. Furthermore, the inductive coupling in the pair of resonators of the principal-surface lines  6 B and  6 C causes the high-frequency side of the passband to fall relatively abruptly. 
         [0073]    The position and form of the principal-surface lines and the side-surface electrodes in the above-described configuration example are based on product specifications, and any position and form may be adopted based on product specifications. The present invention can be applied to a configuration other than the above-described configuration, and can be adopted to various pattern forms of filter devices. Furthermore, another configuration (high-frequency circuit) may be provided in this filter device.