Abstract:
A bandpass filter capable of creating a dual mode with a simple configuration and stably adjusting the filter characteristics of the bandpass filter is disclosed. The bandpass filter includes a dielectric base substrate; a disk resonator formed over the dielectric base substrate; and a dielectric block disposed over a part of the dielectric base substrate and in substantially the same plane as the disk resonator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on Japanese Priority Application No. 2007-119710 filed on Apr. 27, 2007, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention generally relates to high-frequency circuit elements used in, for example, the wireless communication field, and more particularly to a structure of a bandpass filter using a resonator for passing only a desired frequency and a manufacturing method of the bandpass filter. 
         [0004]    2. Description of the Related art 
         [0005]    Recently, with prevalence and development of cell phones, fast and high-capacity transmission technologies have become indispensable. To realize such a fast and high-capacity transmission technology, a wide frequency range is required to be secured. Therefore, the frequency range used in wireless communications is being shifted to a higher frequency range. Accordingly, as a filter used in a base station of a mobile communication system, a bandpass filter capable of effectively passing a desired frequency in a high frequency range is necessary. In such circumstances, a superconductor is a promising material for a filter used in a base station for a mobile communication system because the surface resistance of a superconductor is much less than that of a general good conductor even in a high frequency range, thereby a low-loss resonator having a high Q value is expected. 
         [0006]    When a superconductor is used as a transmission filter in a transmission frontend, it is suggested that a circular (disk-shaped) resonator pattern be used instead of a strip-type resonator pattern so as to control the increase of current loss by an input of high RF power. This is because when a circular pattern is used, it is possible to control the concentration of current density that is likely to be generated at an edge and a corner part of a microstrip line. 
         [0007]    When a signal is applied to a disk resonator and a signal corresponding to the resonance frequency is taken, a steeper filter characteristic can be obtained by arranging input and output ports (signal input and output lines) at orthogonal positions with respect to the resonator so as to create a dual mode compared with a case where the input and output ports are arranged at 180 degrees with respect to the resonator. When a notch is formed on a disk resonator it is possible to operate the resonator in a dual mode. However, there is a problem that the concentration of the current into the notch part is increased, thereby lowering the withstand power characteristics of the filter. 
         [0008]    To solve the problem, a method of controlling the concentration of current by forming a circular (arch-shaped) notch on a disk resonator (see, for example, Patent Document 1), and a method of avoiding the current concentration and creating a dual mode by displacing a dielectric unit on a disk resonator where a conductor pattern is formed on the dielectric unit (see, for example, Patent Document 2) are proposed. 
         [0009]    Patent Document 1: Japanese Patent Application Publication No. 2006-101187 
         [0010]    Patent Document 2: Japanese Patent Application Publication No. 2006-115416 
         [0011]    In the method of Patent Document 2, the dielectric unit is preferably required to be on the upper surface of the dielectric unit. Furthermore, there is a problem that if there were even a small gap between the dielectric unit and the disk-shaped resonator pattern the filter characteristics would be changed, thereby complicating the adjustment. 
       SUMMARY 
       [0012]    According to an aspect of the present invention, there is provided a bandpass filter including a dielectric base substrate; a disk resonator formed over the dielectric base substrate; and a dielectric block disposed over a part of the dielectric base substrate and in substantially the same plane as the disk resonator. 
         [0013]    According to another aspect of the present invention, there is provided a method of forming a bandpass filter. The method includes 
         [0014]    (a) forming an disk resonator and input and output signal lines over a dielectric base substrate, the input and the output signal lines extending at substantially 90 degrees from each other with respect to the disk resonator; and 
         [0015]    (b) disposing a dielectric block at a position other than a position that is opposite to the input port and the output port with respect to a center of the disk resonator and that is on an extended line passing though the center of the disk resonator and the input port or on an extended line passing though the center of the disk resonator and the output port, the dielectric block having a size to cover a part of the dielectric base substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Other objects, features, and advantages of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings, in which: 
           [0017]      FIGS. 1A and 1B  are drawings schematically showing a configuration of bandpass filter according to an embodiment of the present invention; 
           [0018]      FIGS. 2A and 2B  are drawings each schematically showing an example where the bandpass filter in  FIG. 1  is practically implemented; 
           [0019]      FIG. 3  is a graph showing a filter characteristic of a bandpass filter according to an embodiment of the present invention including a dielectric block compared with a filter characteristic of a bandpass filter having no dielectric block; 
           [0020]      FIG. 4  is a drawing showing mutual positions of a disk resonator and the dielectric block; 
           [0021]      FIG. 5  is a graph showing relationship between the position of the dielectric block and the filter characteristics; 
           [0022]      FIG. 6  is a graph showing relationships between the film thickness of the dielectric block and the filter characteristics; 
           [0023]      FIG. 7  is a graph showing relationships between the permittivity of the dielectric block and the filter characteristics; 
           [0024]      FIG. 8  is a graph showing relationships between the size of the dielectric block and the filter characteristics; and 
           [0025]      FIG. 9  is a graph showing relationships between the shape of the dielectric block and the filter characteristics. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    In the following, an exemplary embodiment of the present invention is described with reference to the accompanying drawings.  FIGS. 1A and 1B  are a plan view and a side view, respectively, of a bandpass filter  10  according to an embodiment of the present invention. The bandpass filter  10  includes a dielectric base substrate  11 , a disk resonator  12  disposed on the dielectric substrate  11 , an input port  14   a  and an output port  14   b  disposed at 90 degrees relative to each other and with respect to the disk resonator  12 , an input feeder  13   a  and an output feeder  13   b  connected to the input port  14   a  and the output port  14   b , respectively, and a dielectric block  15  disposed only on a part of an upper surface of the dielectric base substrate  11 . The cross-sectional shapes of the input port  14   a  and the output port  14   b  expand like a trumpet approaching the disk resonator  12  and face the disk resonator  12  so as to be electromagnetically connected to the disk resonator  12 . The input feeder  13   a  and the input port  14   a  constitute an input signal line  17   a , and the output feeder  13   b  and the output port  14   b  constitute an output signal line  17   b . The disk resonator  12  and the input and output signal lines  17   a  and  17   b  are made of, for example, a superconducting material, but may be formed of a good conductor material. 
         [0027]    The dielectric base substrate  11  is, for example, an MgO substrate having a ground film  16  formed on the entire rear surface of the MgO substrate. 
         [0028]    The dielectric block  15  disposed on a part of the upper surface of the dielectric base substrate  11  is, for example, an STO (SrTiO 3 ) block. 
         [0029]    To form a bandpass filter as described above, for example, YBCO thin films (the composition formula is YBa 2 Cu 3 O 6+x ) having a film thickness of 500 nm are formed on both sides of the MgO (100) substrate having a thickness of 0.5 mm by, for example, a Pulsed Laser Deposition (PLD) method. One of the formed YBCO thin films is used as the ground film  16 . On the other YBCO thin film, a resist film (not shown) having a prescribed patterns is formed utilizing photolithography technique, and the YBCO film patterns having the shapes of the disk resonator  12  and the input and output signal lines  17   a  and  17   b  are formed by Ar milling (dry etching). Then the resist film is removed using a remover. When a bandpass filter of, for example, 5 GHz band is formed, the diameter of the disk resonator should be 11 mm. The distance between the ends of the input and output ports  14   a  and  14   b  and the disk resonator  12  is, for example, 100 μm. 
         [0030]    On the other hand, an STO (100) substrate having a thickness of 0.5 mm is cut into a 2.1 mm block to form the STO block  15 . The STO block  15  is disposed at 45 degrees rotated from the extended lines of the input and the output feeders  13   a  and  13   b , respectively, and near the circumference of the disk resonator  12 . In the configuration of  FIG. 1 , the STO block  15  is disposed slightly outward from the disk resonator  12 . However, in an example described below, the STO block  15  is displaced so as to overlap with the disk resonator  12  by 0.1 mm. 
         [0031]      FIGS. 2A and 2B  show an example where a bandpass filter of  FIG. 1  is implemented.  FIG. 2A  is a perspective view of the bandpass filter contained in a package.  FIG. 2B  is a drawing schematically showing the package disposed in an adiabatic vacuum container of a cooling system. 
         [0032]    As shown in  FIG. 2A , the bandpass filter  10  is housed in a filter package  40 . Each of connection electrodes  45  connected to the input and the output feeders  13   a  and  13   b  is connected to a center conducting part (not shown) of corresponding coaxial connector  41 . The filter package  40  is, for example, a copper-shielded case with gilded surfaces. In this case, any method of connecting the connection electrode  45  to the corresponding center conducting part of the corresponding coaxial connector  41 , including wirebonding by ultrasonic thermal compression bonding, tape bonding, and soldering may be used. After the connection between the coaxial cables  41  and the corresponding connection electrodes  45  are completed, the filter package  40  is covered with a package cover (not shown) to be hermetically sealed. A signal to be filtered is input into the bandpass filter via a coaxial cable connected to the coaxial connector  41  (see  FIG. 2B ). The filtered output signal is output to the coaxial cable on the output side. 
         [0033]    When the resonator  12  of a bandpass filter is formed of a superconducting material, the bandpass filter in the package is to be housed in a cooling system as shown in  FIG. 2B . More specifically, the package is disposed on a cold plate  51  in an adiabatic vacuum container  50 , and after being evacuated to 10 to 3 Pa, the air is cooled to a prescribed temperature of, for example, 70 K. The air is cooled by using a cooling system expansion section  55  and a cooling system compression section  56  together. 
         [0034]    Each of the coaxial connectors  42  on the package  40  is connected to the corresponding hermetic coaxial connector  58  on the adiabatic vacuum container  50  to input and output signals from and to, respectively, the outside of the adiabatic vacuum container  50 . 
         [0035]      FIG. 3  is a graph showing an electromagnetic simulation result of a bandpass filter configured as described above. The dotted lines in  FIG. 3  show the S 11  and S 21  characteristics when there is no STO block  50 . On the other hand, the full lines in  FIG. 3  show the S 11  and S 12  characteristics when the STO block  15  is disposed so as to partially overlap with the circumference of the disk resonator  12 . 
         [0036]    As shown in  FIG. 3 , without the STO block  15 , there is no connection between the input and the output. However, when the STO block is disposed as described above, a dual mode is created and good characteristics of the bandpass filter are obtained. 
         [0037]    Next, the relationship between the shape and the position of the dielectric block  15  is described.  FIG. 4  shows the positions of the disk resonator  12  and the dielectric block  15 . As shown in  FIG. 4 , each straight center line extending in the longitudinal direction of input and output feeders is extended through the disk resonator  12 . The two extended center lines of the input and output feeders cross at the center of the disk resonator  12 . Next, the other straight line extending though the center of the dielectric block  15  also crosses the other two lines at the center of the disk resonator  12 . In  FIG. 4 , the dielectric block  15  is disposed so that the angle between the straight line passing though the dielectric block  15  and each of the straight lines passing through the input and the output feeders  13   a  and  13   b  is 45 degrees. Then the dielectric block  15  is moved on the straight line passing through the center of the dielectric block  15 , that is, in the radial direction of the disk resonator  12 . As shown in  FIG. 4 , a tangent line passing through the intersection between the circumference of the disk resonator  12  and the straight line extended from the dielectric block  15  is drawn. The distance between the tangent line and an end surface  15   s  of the dielectric block  15  is changed. The end surface  15   s  faces the disk resonator  12 . As shown in  FIG. 4 , it is assumed that when the end surface  15   s  is on the tangent line, the distance is “0”. While the distance is changed, the characteristic at each point is measured. The distance has a positive value when the surface  15   s  is separated from the tangent line. On the other hand, the distance has a negative value when the surface  15   s  passes the tangent line, enters into, and overlaps the disk resonator  12 . 
         [0038]      FIG. 5  is graph showing relationships between the position of the dielectric block  15  and the filter characteristics. In the graph, the end surface  15   s  of the dielectric block  15  is moved from the position 0.74 mm separated outward from the edge part of the disk resonator  12  through the position on the tangent line (distance=0 mm) and inside the disk resonator  12  to gradually increase the overlap distance. According to the results of this movement, when the position of the dielectric block  15  is moved, one of the resonant frequencies can be shifted to a lower frequency range while the other resonant frequency is unchanged. Therefore, a desired dual-mode filter can be obtained by controlling the position of the dielectric block  15  in the design stage. For example, when the overlap distance is −0.1 mm (namely, the dielectric block  15  overlaps 0.1 mm inside the disk resonator  12 ), flat characteristics of approximately −1 dB in the band are obtained. 
         [0039]      FIG. 6  is a graph showing relationships between the film thickness of the dielectric block  15  and the filter characteristics. As shown in  FIG. 6 , the film thickness of the dielectric block  15  is changed from 1 mm to 0.1 mm. Though only a slight change is observed when the film thickness is 0.1 mm, the filter characteristics can only be slightly changed even when the film thickness of the dielectric block  15  is changed. Namely, the thickness of the dielectric block  15  on the dielectric base substrate  11  does not have much influence on the filter characteristics. 
         [0040]      FIG. 7  is a graph showing relationships between the permittivity of the dielectric block  15  and the filter characteristics. During this measurement, the distance of the dielectric block  15  is fixed at −0.1 mm inside the disk resonator  12  (that is, the dielectric block  15  overlaps the disk resonator  12  by 0.1 mm). Then, when the permittivity of the dielectric block  15  is changed from 300 to 10, there are only slight changes on the bandwidth and the center frequency. However, the change has little influence on creating the dual mode. Namely, creating the dual mode has little dependence on the permittivity of the dielectric block  15 , and the dual mode can be created by the dielectric block  15  having a permittivity between 300 and 100. 
         [0041]      FIGS. 8A and 8B  are graphs showing relationships between the size of the dielectric block  15  and the filter characteristics. During the measurement, the thickness of the dielectric block  15  and the distance of the dielectric block  15  are fixed at 0.5 mm and −0.1 mm, respectively. Then, the size of the dielectric block  15  is changed from (1.0 mm)×(1.0 mm) to (3.0 mm)×(3.0 mm), and the obtained filter characteristics are shown in  FIG. 8A .  FIG. 8B  is an enlarged view of an circled area “A” in  FIG. 8A . As shown especially in  FIG. 8B , as the size of the dielectric block  15  becomes larger, a coupling becomes stronger. Namely, the coupling coefficient of a dual mode can be adjusted by changing the size of the dielectric block  15 . 
         [0042]      FIG. 9  is a graph showing relationships between a shape of the dielectric block  15  and the filter characteristics. The dotted line shows filter characteristics of the dielectric block  15  having a circular shape (diameter: 2.1 mm) in plan view. On the other hand, the solid line shows filter characteristics of the dielectric block  15  having a square shape (each side: 2.1 mm) in plan view. As  FIG. 9  shows, as regarding the dielectric block  15 , a square block has better filter characteristics than a circular block. 
         [0043]    From the above results, the thickness and the permittivity of the dielectric block  15  do not have much effect on creating a dual mode (strength of coupling). However, by changing the position, the size, and the shape of the dielectric block  15 , the coupling coefficient of a dual mode can be desirably adjusted. 
         [0044]    Especially, a bandpass filter for the 5 GHz band having a dual mode and good frequency cut-off characteristics can be obtained when the diameter of the disk resonator  12  is 10 mm; the center lines of the input and the output ports  14   a  and  14   b  cross at 90 degrees; and the dielectric block  15  has permittivity between 50 and 300, the film thickness between 0.1 mm and 1 mm, length of each side between 2.0 mm and 2.4 mm, and overlaps the disk resonator  12 . 
         [0045]    In the above embodiment, the dielectric block  15  is disposed so that the center line of the dielectric block passing through the center of the disk resonator  12  has an angle of 45 degrees with respect to each of the center lines of the input and the output feeders  13   a  and  13   b , respectively. However, an embodiment of the present invention is not limited to this case. More specifically, the dielectric block  15  may be disposed at any position other than positions on the center lines of the input and the output feeders  13   a  and  13   b , respectively including the opposite positions of the input and the output ports  14   a  and  14   b  with respect to the disk resonator  12 . 
         [0046]    Further, in the above embodiment, STO (SrTiO 3 ) is used as the material of the dielectric block  15 . However, in an embodiment of the present invention, the dielectric block  15  is not limited to STO. For example, TiO 2 , CaTiO 3 , (Ba, Sr)TiO 3 , (called “BST”), and Bi 1.5 Zn 1 Nb 1.5 O 7  (called “BNZ”) may be preferably used. 
         [0047]    Still further, as the disk resonator  12 , instead of using YBa 2 Cu 3 O 6+x , RBCO (R—Ba—Cu—O: as “R” element, Nd, Gd, Sm, or Ho is used), BSCCO(Bi—Sr—Ca—Cu—O), PBSCCO (Pb—Bi—Sr—Ca—Cu—O), and CBCCO(Cu—Ba p —Ca q —Cu r —O x ) 1.5&lt; p &lt;2.5, 2.5&lt; q &lt;3.5, 3.5&lt; r &lt;4.5) may be used. 
         [0048]    The present invention is not limited to the above exemplary embodiments, and variations and modifications may be made without departing from the scope of the present invention. Further, the present invention should not be interpreted to be limited by the description and accompanying drawings.