Patent Publication Number: US-7907036-B2

Title: Microstripline filter and method for manufacturing the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International Application No. PCT/JP2008/059429, filed May 22, 2008, which claims priority to Japanese Patent Application No. JP2007-183825, filed Jul. 13, 2007, the entire contents of each of these applications being incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a microstripline filter in which striplines are arranged in a dielectric substrate, and a method for manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     In general, microstripline filters in which striplines included in quarter-wavelength resonators are arranged so that open ends thereof are directed to a certain direction and the adjacent resonators are comb-line coupled with one another are used. In such a comb-line microstripline filter, a common electrode may be arranged so as to connect ends of a plurality of resonator lines on short-circuit sides with one another, and the resonators may be inductively coupled with one another (Refer to Patent Documents 1 and 2). 
     A microstripline filter according to Patent Document 1 includes a common electrode perpendicularly extending relative to striplines. First ends of all the striplines are commonly connected to the common electrode. Both ends of the common electrode are connected to a ground electrode in both surfaces which are parallel to the striplines. 
       FIG. 1  is a diagram illustrating an example of a configuration of a microstripline filter according to Patent Document 2. In a microstripline filter  101 , striplines  102 A to  102 C are commonly connected to a common electrode  103  at first ends thereof. Furthermore, the common electrode  103  is connected to a short-circuit electrode  104 . The short-circuit electrode  104  extends in parallel to the striplines  102 A to  102 C and is grounded in a ground electrode  105 .
     [Patent Document 1] Japanese Unexamined Utility Model Application Publication No. 56-105902   [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-270508   

     SUMMARY OF THE INVENTION 
     In known filters, resonant frequencies of resonators and coupling coefficients among the resonators are set by controlling line lengths and line widths of striplines, gaps among adjacent striplines, a line width of a common electrode, and a line width of a short-circuit electrode. However, even if forms of the electrodes are thus controlled, due to restriction of configurations of the electrodes, it is not necessarily the case that desired resonant frequencies and desired coupling coefficients can be realized. Therefore, a desired frequency characteristic is not obtained. 
     Accordingly, the present invention provides a microstripline filter capable of enhancing a degree of freedom of setting of resonant frequencies of resonators and setting of coupling coefficients among the resonators and precisely controlling the setting of the resonant frequencies of the resonators and setting of the coupling coefficients among the resonators. 
     A microstripline filter according to this invention includes a ground electrode, a plurality of main-surface lines, common electrodes, a plurality of short-circuit electrodes, and input/output electrodes. The ground electrode is arranged on a lower surface of a dielectric substrate having a rectangular plate shape. The plurality of main-surface lines are arranged on an upper surface of the dielectric substrate and are included in respective resonators. The common electrodes connect some of the main-surface lines to one another in conduction states. The plurality of short-circuit electrodes connect a group of the main-surface lines which are brought to conduction states by the common electrodes to the ground electrode through an identical side surface of the dielectric substrate. The input-and-output electrodes are connected to corresponding ones of the resonators. 
     With this configuration, characteristics of the resonators including the main-surface lines connected to the common electrodes and degree of coupling can be controlled by controlling electrode patterns of the plurality of short-circuit electrodes connected to a pair of the common electrodes, that is, by controlling line widths of the short-circuit electrodes, positions where the common electrodes and the short-circuit electrodes are connected, or gaps between the adjacent short-circuit electrodes. Accordingly, resonant frequencies of the resonators and coupling coefficients among the adjacent resonators can be set in high degree of freedom. Since the coupling coefficients and the resonant frequencies in a case where shapes of the short-circuit electrodes are changed are less affected when compared with a case where shapes of the common electrodes and the main-surface lines are changed, the resonant frequencies of the resonators and the coupling coefficients among the resonators can be accurately controlled. 
     The short-circuit electrodes are individually arranged on portions of the common electrodes where pairs of the adjacent main-surface lines are connected to each other. 
     With this configuration, in three resonators including respective three main-surface lines adjacent to one another, degrees coupling among the resonators are determined in accordance with arrangement of two short-circuit electrodes. 
     Mass production of the microstripline filter in which the resonant frequencies of the resonator and the coupling coefficients among the resonators are accurately controlled is realized by controlling the plurality of short-circuit electrodes which are connected to one another in conduction states through the common electrodes. 
     According to the present invention, resonant frequencies and coupling coefficients of a plurality of resonators which are connected to one another in a comb-line coupling are determined by electrode patterns of a plurality of short-circuit electrodes connected to identical common electrodes, and the coupling coefficients are accurately set. Since the setting of the short-circuit electrodes is performed separately from setting of plurality of main-surface lines connected to the common electrodes, a frequency characteristic is easily set and the electrode patterns are easily designed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a known microstripline filter. 
         FIGS. 2A and 2B  are perspective views illustrating an example of a configuration of a microstripline filter. 
         FIG. 3  is a graph illustrating an example of a frequency characteristic of the microstripline filter. 
         FIG. 4  is a flowchart illustrating examples of steps of manufacturing the microstripline filter. 
         FIG. 5  is a perspective view illustrating an example of another configuration of the microstripline filter. 
     
    
    
     REFERENCE NUMERALS 
     
         
         
           
               1  microstripline filter 
               2 A to  2 E main surface line 
               3 A,  3 B common electrode 
               4 A to  4 D side-surface short-circuit electrode 
               5  ground electrode 
               6 A,  6 B side-surface extraction electrode 
               7 A,  7 B input/output electrode 
               8 A,  8 B extraction electrode 
               10  dielectric substrate 
               60  glass layer 
               61  coupling electrode 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An example of a configuration of a microstripline filter will be described hereinafter. 
     The microstripline filter described herein corresponding to a bandpass filter. This filter is used in UWB (Ultra Wide Band) communications in a range from 3 GHz to 5 GHz. 
       FIG. 2A  is a perspective view illustrating a dielectric substrate, viewed from an upper surface thereof, included in the microstripline filter, and  FIG. 2B  is a perspective view illustrating the dielectric substrate viewed from a lower surface thereof. 
     A microstripline filter  1  includes a dielectric substrate  10  and a glass layer (not shown). Note that the glass layer is disposed on the upper surface of the dielectric substrate  10  so as to enhance environment resistance of the microstripline filter. 
     The substrate  10  is a sintered ceramic substrate of a small cube shape having a specific inductive capacity of approximately 111, and the substrate  10  is formed of titanium oxide or the like. Composition and a size of the substrate  10  are appropriately determined taking a frequency characteristic, for example, into consideration. 
     On the upper surface of the substrate  10 , an upper-surface electrode pattern including main-surface lines  2 A to  2 E, common electrodes  3 A and  3 B, and extraction electrodes  8 A and  8 B are arranged. The upper-surface electrode pattern is formed of a silver electrode having a thickness of 6 μm or more. The upper-surface electrode pattern is formed by applying a photosensitive silver paste on the substrate  10 , patterning the substrate  10  by a photolithography processing, and performing sintering. 
     The substrate  10  has a side-surface electrode pattern including side-surface short-circuit electrodes  4 A to  4 D on a front surface thereof. Furthermore, the substrate  10  has a side-surface electrode pattern including side-surface extraction electrodes  6 A and  6 B on a rear surface thereof. These side-surface electrode patterns are formed of silver electrodes having thicknesses of 12 μm or more. These side-surface electrode patterns are formed by applying a nonphotosensitive silver paste on the front and rear surfaces of the substrate  10  using a screen mask or a metal mask, and performing sintering. 
     The lower surface of the substrate  10  corresponds to an implementing surface of the microstripline filter  1 . A lower-surface electrode pattern including a ground electrode  5  and input/output electrodes  7 A and  7 B are arranged on the lower surface of the substrate  10 . The input/output electrodes  7 A and  7 B are formed so as to be separated from the ground electrode  5 . The input/output electrodes  7 A and  7 B are connected to high-frequency-signal input/output terminals when the microstripline filter  1  is implemented on an implementing substrate. The ground electrode  5  serves as a ground surface of resonators, and is connected to a ground electrode of the implementing substrate. The lower-surface electrode pattern is formed of a silver electrode having a thickness of approximately 12 μm. The lower-surface electrode pattern is formed by applying a nonphotosensitive silver paste on the lower surface of the substrate  10  using a screen mask or a metal mask, and performing sintering. 
     Note that since the thicknesses of the electrodes of the side-surface electrode patterns are larger than those of the electrodes of the upper-surface electrode pattern, current supplied to portions on a ground terminal side on which current is generally concentrated is dispersed so that a conduction loss is reduced. With this configuration, the microstripline filter attains a small insertion loss. 
     Here, in the upper-surface electrode pattern, the main-surface lines  2 A to  2 E extend from a boundary between the front surface and the upper surface of the substrate  10  toward the rear surface of the substrate  10 , and first ends of the main-surface lines  2 A to  2 E are opened. Furthermore, the main-surface lines  2 A to  2 E face the ground electrode  5  of the lower-surface electrode pattern. Accordingly, the main-surface lines  2 A to  2 E and the ground electrode  5  constitute resonators in five stages which are comb-line coupled with one another. 
     The extraction electrode  8 A is arranged near the rear surface of the substrate  10 . The extraction electrode  8 A has one end which continues to the main-surface line  2 D arranged on the upper surface of the substrate  10 , and the other end which continues to the side-surface extraction electrode  6 A arranged on the rear surface of the substrate  10 . Note that the side-surface extraction electrode  6 A continues to the input/output electrode  7 A arranged on the lower surface of the substrate  10 . Therefore, the extraction electrode  8 A connects the resonator including the main-surface line  2 D to the input/output electrode  7 A through the side-surface extraction electrode  6 A in a tap-coupling manner. 
     The main-surface line  2 D has one end which continues to the extraction electrode  8 A arranged on the upper surface of the substrate  10 , and the other end which is connected to the side-surface short-circuit electrode  4 C arranged on the front surface of the substrate  10 . Note that the side-surface short-circuit electrode  4 C continues to the ground electrode  5  arranged on the lower surface of the substrate  10 . Therefore, the main-surface line  2 D is connected to the ground electrode  5  through the side-surface short-circuit electrode  4 C in a conduction state and constitutes a quarter-wavelength resonator in an input stage (or an output stage). 
     The main-surface line  2 B has one end which is arranged on the upper surface and opened toward the rear surface of the substrate  10 , and the other end which continues to the common electrode  3 A arranged on the front surface side of the upper surface of the substrate  10 . Note that the common electrode  3 A continues to the side-surface short-circuit electrode  4 A arranged on the front surface of the substrate  10 , and the side-surface short-circuit electrode  4 A continues to the ground electrode  5  on the lower surface of the substrate  10 . Therefore, the main-surface line  2 B is connected to the ground electrode  5  through the side-surface short-circuit electrode  4 A in a conduction state, and constitutes a quarter-wavelength resonator in a second stage. 
     The center of the line width of the main-surface line  2 D is shifted from the center of the line width of the side-surface short-circuit electrode  4 C. The center of the line width of the main-surface line  2 B is shifted from the center of the line width of the side-surface short-circuit electrode  4 A. The main-surface line  2 B is arranged close to the main-surface line  2 D whereas the side-surface short-circuit electrode  4 C is arranged far from the side-surface short-circuit electrode  4 A. Therefore, the resonator in the input stage (or the output stage) including the main-surface line  2 D is coupled with the resonator in the second stage including the main-surface line  2 B in a manner of capacity coupling. Due to this capacity coupling, on a lower band side of the frequency characteristic of the microstripline filter  1 , a first low-band attenuation pole falls. 
     The main-surface line  2 A has one end which is arranged on the upper surface and opened toward the rear surface of the substrate  10 , and the other end which continues to the common electrodes  3 A and  3 B arranged on the front surface side of the upper surface of the substrate  10 . Note that the common electrode  3 A continues to the side-surface short-circuit electrode  4 A arranged on the front surface of the substrate  10 , the common electrode  3 B continues to the side-surface short-circuit electrode  4 B arranged on the front surface of the substrate  10 , and the side-surface short-circuit electrodes  4 A and  4 B continue to the ground electrode  5  arranged on the lower surface of the substrate  10 . Therefore, the main-surface line  2 A faces to the ground electrode  5  through the dielectric substrate  10 , is connected to the ground electrode  5  through the side-surface short-circuit electrodes  4 A to  4 B in a conduction state, and constitutes a quarter-wavelength resonator in a third stage. 
     The main-surface lines  2 A and  2 B are connected to each other near a short-circuit end side through the common electrode  3 A, and accordingly, enhanced inductive coupling is attained. Due to the inductive coupling, on a higher band side of the frequency characteristic of the microstripline filter  1 , a first high-band attenuation pole falls. 
     The main-surface line  2 C has one end which is arranged on the upper surface and opened toward the rear surface of the substrate  10 , and the other end which continues to the common electrode  3 B arranged on the front surface side of the upper surface of the substrate  10 . Note that the common electrode  3 B continues to the side-surface short-circuit electrode  4 B arranged on the front surface of the substrate  10 , and the side-surface short-circuit electrode  4 B continues to the ground electrode  5  arranged on the lower surface of the substrate  10 . Therefore, the main-surface line  2 C is connected to the ground electrode  5  through the side-surface short-circuit electrode  4 B in a conduction state, and constitutes a quarter-wavelength resonator in a fourth stage. 
     The main-surface lines  2 A and  2 C are connected to each other near a short-circuit end side through the common electrode  3 B, and accordingly, enhanced inductive coupling is attained. Due to the inductive coupling, on a higher band side of the frequency characteristic of the microstripline filter  1 , a second high-band attenuation pole falls. 
     The main-surface line  2 E has one end which continues to the extraction electrode  8 B arranged on the upper surface of the substrate  10 , and the other end which is connected to the side-surface short-circuit electrode  4 D arranged on the front surface of the substrate  10 . Note that the side-surface short-circuit electrode  4 D continues to the ground electrode  5  arranged on the lower surface of the substrate  10 . Therefore, the main-surface line  2 E is connected to the ground electrode  5  through the side-surface short-circuit electrode  4 D in a conduction state and constitutes a quarter-wavelength resonator in an output stage (or an input stage). 
     The center of the line width of the main-surface line  2 E is shifted from the center of the line width of the side-surface short-circuit electrode  4 D. The center of the line width of the main-surface line  2 C is shifted from the center of the line width of the side-surface short-circuit electrode  4 B. The main-surface line  2 E is arranged close to the main-surface line  2 C whereas the side-surface short-circuit electrode  4 B is arranged far from the side-surface short-circuit electrode  4 D. Therefore, the resonator in the output stage (or the input stage) including the main-surface line  2 E is coupled with the resonator in the fourth stage including the main-surface line  2 C in a manner of capacity coupling. Due to this capacity coupling, on a lower band side of the frequency characteristic of the microstripline filter  1 , a second low-band attenuation pole falls. 
     The extraction electrode  8 B is arranged near the rear surface of the substrate  10 . The extraction electrode  8 B has one end which continues to the main-surface line  2 E arranged on the upper surface of the substrate  10 , and the other end which continues to the side-surface extraction electrode  6 B arranged on the rear surface of the substrate  10 . Note that the side-surface extraction electrode  6 B continues to the input/output electrode  7 B arranged on the lower surface of the substrate  10 . Therefore, the extraction electrode  8 B connects the resonator including the main-surface line  2 E to the input/output electrode  7 B through the side-surface extraction electrode  6 B in a tap-coupling manner. 
     As described above, the microstripline filter  1  constitutes a filter including the resonators in the five stages. The microstripline filter  1  corresponds to a bandpass filter and has two low-pass-band attenuation poles and two high-pass-band attenuation poles. 
       FIG. 3  shows the frequency characteristic of the microstripline filter  1 . Here, an example of the characteristic in which frequencies of the two low-pass-band attenuation poles are matched with each other, and frequencies of the two high-pass-band attenuation poles are matched with each other. A dashed line of  FIG. 3  denotes an S 11  characteristic of the microstripline filter  1 . A solid line of  FIG. 3  denotes an S 21  characteristic of the microstripline filter  1 . 
     When focusing on the S 21  characteristic of the microstripline filter  1 , a pass band having an attenuation amount of −1.5 dB is realized in a range from 3168 MHz to 4752 MHz in the microstripline filter  1 . Furthermore, an attenuation pole is positioned around in a range from approximately 2400 MHz to approximately 2500 MHz which is a lower side of the pass band, and an attenuation amount is approximately −39 dB. Another attenuation pole is positioned around in a range approximately 5150 MHz to approximately MHz which is a higher side of the pass band, and an attenuation amount is −27 dB or less. 
     Since the microstripline filter  1  has the two side-surface short-circuit electrodes  4 A and  4 B for the three main-surface lines  2 A to  2 C, a gap between the side-surface short-circuit electrodes  4 A and  4 B, line widths of the side-surface short-circuit electrodes  4 A and  4 B, a position of the connection between the side-surface short-circuit electrode  4 A and the common electrode  3 A, and a position of the connection between the side-surface short-circuit electrode  4 B and the common electrode  3 B affect resonant frequencies and coupling coefficients between the main-surface lines  2 A to  2 C. 
     Specifically, as the gap between the side-surface short-circuit electrodes  4 A and  4 B becomes larger, the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 B and the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 C become larger. In addition, resonant frequencies of the resonators including the respective main-surface lines  2 A to  2 C become higher. On the other hand, as the gap between the side-surface short-circuit electrodes  4 A and  4 B becomes smaller, the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 B and the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 C become smaller. In addition, the resonant frequencies of the resonators including the respective main-surface lines  2 A to  2 C become lower. 
     Furthermore, as the line widths of the side-surface short-circuit electrodes  4 A and  4 B become larger, the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 B and the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 C become larger. In addition, the resonant frequencies of the resonators including the respective main-surface lines  2 A to  2 C become higher. On the other hand, as the line widths of the side-surface short-circuit electrodes  4 A and  4 B become smaller, the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 B and the coupling coefficient between the resonators including the respective main-surface lines  2 A and  2 C become smaller. In addition, the resonant frequencies of the resonators including the respective main-surface lines  2 A to  2 C become lower. 
     Accordingly, by setting a electrode pattern including the side-surface short-circuit electrodes  4 A and  4 B, the resonant frequencies and the coupling coefficients among the main-surface lines  2 A to  2 C which are connected to one another in a conduction state through the common electrodes  3 A and  3 B can be controlled. In addition, the coupling coefficients and the resonant frequencies are less affected when compared with a case where shapes of the common electrodes and the main-surface lines are changed. Accordingly, it is recognized that the resonant frequencies of the resonators and the coupling coefficients between the resonators can be precisely controlled. 
     A method for manufacturing the microstripline filter  1  will now be described. 
       FIG. 4  is a flowchart illustrating the method of manufacturing the microstripline filter  1 . 
     In steps of manufacturing the microstripline filter  1 , (S 1 ) a dielectric body in which no electrode is formed on surfaces thereof is prepared as a master substrate. 
     (S 2 ) Then, the master substrate is subjected to screen printing using a conductive paste on the lower surface thereof, and further subjected to drying and sintering so that a ground electrode and input/output electrodes are formed. 
     (S 3 ) The master substrate is subjected to printing using a photosensitive conductive paste on the upper surface thereof, subjected to photolithography processing including drying, exposing, and developing, and further subjected to sintering so that a main-surface electrode pattern is formed. 
     (S 4 ) The master substrate is subjected to printing using a glass paste on the upper surface thereof, and subjected to sintering so that a glass layer is formed. 
     (S 5 ) A plurality of dielectric substrates are cut out of the master substrate configured as described above by dicing, for example. After the cutting out, preliminary measurements of electric characteristics are performed on electrode patterns arranged on upper surfaces of some of the dielectric substrates. 
     (S 6 ) One or a small number of dielectric substrates are extracted from the plurality of dielectric substrates which have been cut out, side-surface short-circuit electrodes are formed as a test, and electrode patterns suitable for the side-surface short-circuit electrodes which are optimized so that a desired filter characteristic is obtained are selected. 
     (S 7 ) After the side-surface short-circuit electrodes are formed on the extracted dielectric substrates as the test and the electrode patterns suitable for obtaining the desired filter characteristic are selected, a conductive paste is printed with optimized intervals on side surfaces of the plurality of dielectric substrates having an identical substrate lot, and the plurality of dielectric substrates are subjected to sintering so that the side-surface short-circuit electrodes are formed. 
     With the manufacturing method described above, after the main-surface electrode pattern is formed on the upper surface of the dielectric substrate, a filter characteristic can be controlled through the formation of the side-surface short-circuit electrodes on the side surfaces, and accordingly, a desired filter characteristic is reliably obtained. 
     Note that in the test formation of step S 6 , the following process may be performed: first, electrodes are also formed on gaps among the side-surface short-circuit electrodes and then the filter characteristic is measured; the filter characteristic is measured for different widths of the gaps while the widths of the gaps are gradually increased by cutting, for example; sizes of the gaps in which a desired filter characteristic is obtained are obtained; and in the next step, i.e., a main formation step, the side-surface short-circuit electrodes are formed with the gap having the selected sizes. 
     The electrode pattern arranged on the upper surface of the dielectric substrate  10  may considerably affect a frequency characteristic of the microstripline filter in accordance with degree of accuracy of a shape thereof, and therefore, accuracy of the electrodes are improved by photolithography processing for the formation. 
     Next, an example of another configuration of the microstripline filter will be described.  FIG. 5  is a perspective view illustrating the microstripline filter. A microstripline filter  51  is configured substantially similarly to the microstripline filter  1  described above, but is different from the microstripline filter  1  in that the microstripline filter  51  further includes coupling electrodes  61 A and  61 B on an upper surface of a glass layer  60 . In a description below, the reference numerals that are the same as those of the microstripline filter  1  are used for components substantially the same as those of the microstripline filter  1 , and therefore, detailed descriptions thereof are omitted. 
     The coupling electrode  61 A is arranged so as to face a main-surface line  2 D included in a resonator in an input stage (output stage) and a main-surface line  2 B included in a resonator in a second stage through the glass layer  60 . The coupling electrode  61 A is arranged so as to enhance capacity coupling between the resonator in the input stage (output stage) and the resonator in the second stage. On the other hand, the coupling electrode  61 B is arranged so as to face a main-surface line  2 E included in a resonator in an output stage (input stage) and a main-surface line  2 C included in a resonator in a fourth stage through the glass layer  60 . The coupling electrode  61 B is arranged so as to enhance capacity coupling between the resonator in the output stage (input stage) and the resonator in the fourth stage. 
     The microstripline filter may be configured as described above. 
     Although the microstripline filter  1  has the configuration in which side-surface electrodes other than the side-surface extraction electrodes  6 A and  6 B are not arranged on the rear surface of the dielectric substrate  10 , other side-surface electrodes may be arranged. For example, on the rear surface of the dielectric substrate  10 , side-surface electrodes may be formed congruent to the side-surface short-circuit electrodes  4 A to  4 D. In this case, it is not necessary to separately print the side-surface electrodes on the front surface and the rear surface. Accordingly, the side-surface electrodes can be printed without totally aligning directions of the dielectric substrates. Therefore, the printing step can be simplified. 
     Note that the arrangement positions and shapes of the main-surface lines and the side-surface electrodes are determined in accordance with product specifications, and any positions and shapes may be employed as long as the positions and the shapes are determined in accordance with the product specifications. This invention may be employed in configurations other than those described above, and is applicable to various pattern shapes of a filter element. In addition, another configuration (high-frequency circuit) may be included in the filter element.