Abstract:
A compact dielectric resonator of high Qu, in which an electrode formed of an oxide superconducting material is provided on a surface of the dielectric so as to serve as an electrode. A dielectric filter, dielectric duplexer and a communications device, in which the compact resonator is incorporated, are also provided. The dielectric which constitutes the dielectric resonator of the present invention is preferably a Ba(Mg, Ma)0 3 -based dielectric (wherein Ma is at least one pentavalent elemental metal but cannot be Ta alone), and the oxide superconducting electrode is formed of an oxide superconducting material selected from among a RE—M—Cu—O-based oxide superconducting material (wherein RE is a rare earth element and M is an alkaline earth metal element), a Bi—Sr—Ca—Cu—O-based oxide superconducting material (which encompasses those in which Bi is partially substituted by Pb), and a Tl—Ba—Ca—Cu—O-based oxide superconducting material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a compact dielectric resonator of a very high value of Q, to a dielectric filter making use of the resonator, to a dielectric duplexer and to a communications device. 
     2. Background Art 
     Recently, dielectric resonators utilizing a dielectric as a material for constructing the resonator have been widely used so as to miniaturize the resonant system of an electric circuit which handles high-frequency waves such as microwaves. Such dielectric resonators utilize the phenomenon that the wavelength of an electromagnetic wave in a dielectric is 1/(∈r) ½ (wherein ∈r represents relative dielectric constant) that measured in free space. Dielectric resonators are used in a variety of resonant modes, including the TE, TM and TEM modes. In order to prevent electromagnetic energy from being scattered and lost, dielectric resonators are usually housed in a metallic casing, or alternatively, metal electrodes are formed on the dielectric surface. 
     In resonant systems of the above-mentioned types, Qu (i.e., Q under no-load) varies not only depending on Qd (=1/tan δ, Q of the dielectric per se) but also on Qc (i.e., Q attributed to a conductor loss which is caused by the current that flows in the surface of metal). Qu is expressed by the following equation: 1/Qu=(1/Qd)+(1/Qc). Therefore, in order to realize a resonant system of a high Qu, it is essential that a dielectric material of high Qd be used, and in addition, it is essential that electrodes of high Qc—in other words, electrodes of small conductor loss—be used. 
     Japanese Patent Application Laid-Open (kokai) No. 1-154603 discloses a method for achieving a high Qu (Q under no-load) by forming RE—M—Cu—O-based superconducting electrodes on a dielectric ceramic of any of a variety of types, including MgTiO 3 —(Ca, Me)TiO 3 -based dielectric ceramic, Ba(Zr, Zn, Ta)O 3 -based dielectric ceramic, (Zr, Sn)TiO 4  and BaO—PbO—Nd 2 O 3 —TiO 2 -based dielectric ceramic. Also, Japanese Patent Application Laid-Open (kokai) No. 9-298404 discloses a method which utilizes Ba(Mg, Ta)O 3  as a dielectric material. 
     FIGS. 1 and 2 are graphs showing temperature-dependent characteristics of tan δ(=1/Qd) at 10 GHz for a variety of dielectric materials. As shown in FIGS. 1 and 2, MgTiO 3 —(Ca, Me)TiO 3 -based material, Ba(Zr, Zn, Ni, Ta)O 3 -based material, BaO—PbO—Nd 2 O 3 —TiO 2 -based material, and Ba(Mg, Ta)O 3 -based material exhibit disadvantageously poor low-temperature characteristics because in each case, tan δ does not decrease at a constant rate across an entire range of low temperatures. 
     In a (Zr, Sn)TiO 4 -based dielectric material, tan δ decreases at a constant rate throughout the low temperature range. However, this material has a disadvantage in that a violent interface reaction occurs between the resultant dielectric and superconducting electrodes. Particularly when a thick film is formed through screen printing, interfacial reaction between a dielectric and oxide superconducting material raises a critical issue; violent interfacial reaction degrades the superconducting material and therefore no superconducting characteristic can be obtained. Therefore, in order to pursue practical use of various products derived from superconducting materials, there exists a strong need for a new substrate material that does not cause interfacial reaction. MgO is a candidate dielectric material that does not cause interfacial reaction between the dielectric and oxide superconducting material, and thus is suitable for use with high-frequency waves. However, MgO has an ∈r (relative dielectric constant) of 9-10, which is low as compared to that of the above-mentioned dielectric (∈r=20-30), making MgO disadvantageous in terms of miniaturizing the resonant system. 
     SUMMARY OF THE INVENTION 
     Accordingly, a primary object of the present invention is to provide a compact dielectric resonator of high Qu, in which an electrode formed of oxide superconducting material is provided on a surface of the dielectric. 
     Another object of the present invention is to provide a dielectric filter making use of such a compact resonator. 
     A further object of the present invention is to provide a dielectric duplexer making use of the compact resonator. 
     A still further object of the present invention is to provide a communications device making use of the compact resonator. 
     In a first aspect of the present invention, there is provided a dielectric resonator comprising a dielectric and an oxide superconducting electrode provided on a surface of the dielectric, wherein the dielectric is a Ba(Mg, Ma)O 3 -based dielectric (wherein Ma is at least one pentavalent elemental metal but cannot be Ta alone), and the oxide superconducting electrode is formed of an oxide superconducting material selected from among a RE—M—Cu—O-based oxide superconducting material (wherein RE is a rare earth element and M is an alkaline earth metal element), a Bi—Sr—Ca—Cu—O-based oxide superconducting material (which encompasses those in which Bi is partially substituted by Pb), and a Tl—Ba—Ca—Cu—O-based oxide superconducting material. 
     Preferably, Ma is at least one element selected from among Ta, Sb and Nb (except the case where Ta is used alone). 
     In a second aspect of the present invention, there is provided a dielectric resonator comprising a dielectric and an oxide superconducting electrode provided on a surface of the dielectric, wherein the dielectric is a Ba(Mb, Mg, Ta)O 3 -based dielectric (wherein Mb is a tetravalent or pentavalent elemental metal), and the oxide superconducting electrode is formed of an oxide superconducting material selected from among a RE—M—Cu—O-based oxide superconducting material (wherein RE is a rare earth element and M is an alkaline earth metal element), a Bi—Sr—Ca—Cu—O-based oxide superconducting material (which encompasses those in which Bi is partially substituted by Pb), and a Tl—Ba—Ca—Cu—O-based oxide superconducting material. 
     Preferably, Mb is at least one element selected from among Sn, Zr, Sb and Nb. 
     Preferably, the Ba(Mb, Mg, Ta)O 3 -based dielectric is a Ba(Sn, Mg, Ta)O 3 -based dielectric. Preferably, the composition of the Ba (Sn, Mg, Ta)O 3 -based dielectric is Ba(Sn x , Mg y , Ta z )O 7/2−x/2−3y/2  (wherein x+y+z=1, 0.04≦x≦0.26, 0.23≦y≦0.31, and 0.51≦z≦0.65). 
     In a dielectric resonator according to the second aspect of the present invention, the Ba(Mb, Mg, Ta)O 3 -based dielectric may be a Ba(Mg, Sb, Ta)O 3 -based dielectric. In this case, the composition of the Ba(Mg, Sb, Ta)O 3 -based dielectric is Ba x Mg y (Sb v , Ta l−v ) z O w  (wherein x+y+z=1, w is an arbitrary number, x, y, and z fall within the tetrahedron defined by connecting points A, B, C, and D shown in Table 1, and 0.00≦v≦0.300). 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 x 
                 y 
                 z 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A 
                 0.495 
                 0.175 
                 0.330 
               
               
                   
                 B 
                 0.495 
                 0.170 
                 0.335 
               
               
                   
                 C 
                 0.490 
                 0.170 
                 0.340 
               
               
                   
                 D 
                 0.490 
                 0.180 
                 0.330 
               
               
                   
                   
               
             
          
         
       
     
     In the first and second aspects of the present invention, the RE—M—Cu—O-based oxide superconducting material may be YBa 2 Cu 3 O 7-x , the Bi—Sr—Ca—Cu—O-based oxide superconducting material may be (Bi,Pb) 2 Sr 2 Ca 2 CU 3 O x  or Bi 2 , Sr 2 CaCu 2 O x , and the Tl—Ba—Ca—Cu—O-based oxide superconducting material may be Tl 2 Ba 2 Ca 2 Cu 3 O x . 
     In a third aspect of the present invention, there is provided a dielectric filter comprising a dielectric resonator according to any of the above aspects of the present invention, and an external connecting means. 
     In a fourth aspect of the present invention, there is provided a dielectric duplexer comprising at least two dielectric filters, input-output connection means for each of the dielectric filters, and antenna connecting means which is connected to the dielectric filter, wherein at least one of the dielectric filters is a dielectric filter of the present invention. 
     In a fifth aspect of the present invention, there is provided a communications device comprising a dielectric duplexer as described above, a transmitting circuit which is connected to at least one input-output connection means of the dielectric duplexer, a receiving circuit which is connected to at least one input-output connection means other than that to be connected to the transmitting circuit, and an antenna which is connected to the antenna connecting means of the dielectric duplexer. 
     Examples of the RE element that serves as a constituent of the RE—M—Cu—O-based oxide superconducting material include Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. M (i.e., an alkaline earth metal element) is preferably Ba or Sr among others. 
     Since the surface resistance (Rs) of an oxide superconducting material is lower than that of metal at a temperature lower than a critical temperature (Tc), smaller conductor loss occurs in electrodes, to thereby greatly improve Qc. Also, the dielectric used in the present invention exhibits an excellent tan δ characteristic at a low temperature, and does not cause interfacial reaction with an oxide superconducting material. Therefore, the dielectric of the present invention is suitable for forming an oxide superconducting electrode on the surface thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: 
     FIG. 1 is a graph showing the temperature versus tan δ(at 10 GHz) curves of different dielectrics; 
     FIG. 2 is another graph showing the temperature versus tan δ(at 10 GHz) curves of a variety of dielectrics; 
     FIG. 3 is an explanatory sketch showing an example dielectric resonator according to the present invention; 
     FIG. 4 is a graph showing the low-temperature Qu (Q under no load) characteristics of TE 011 -mode dielectric resonators; 
     FIG. 5 is an explanatory sketch showing another example dielectric resonator according to the present invention; 
     FIG. 6 is a graph showing the low-temperature Qu (Q under no load) characteristics of TE 010 -mode dielectric resonators; and 
     FIG. 7 is a block diagram showing an example communications device according to the present invention; 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 3 is an explanatory sketch of an example TE 011 -mode dielectric resonator of the present invention. 
     The resonant system of the dielectric resonator  10  uses a both-terminal-short-circuit-type dielectric resonator method (Hakki &amp; Colemann method), which is a method generally employed for evaluation of microwave-band dielectric characteristics of a dielectric material and for measuring surface resistance of a superconductor. The Hakki &amp; Colemann method generally employs a structure in which a dielectric is sandwiched between two metal plates; however, the dielectric resonator  10  shown in FIG. 3 has a structure in which one of the metal plates is substituted by a superconducting electrode formed on the surface of the dielectric. That is, the dielectric resonator  10  shown in FIG. 3 includes a dielectric substrate  12 , and a film-shaped superconducting electrode  14  is formed on the surface of the dielectric substrate  12 . A copper plate  16  is disposed to face the superconducting electrode  14 . A dielectric  18  is sandwiched between the superconducting electrode  14  and the copper plate  16 . Further, two excitation cables  20  and  22  are disposed on opposite sides of the dielectric  18  and between the superconducting electrode  14  and the copper plate  16 , such that the cables  20  and  22  face each other. 
     In the dielectric resonator of FIG. 3, a Ba(Sn, Mg, Ta)O 3 -based dielectric (size: φ8.5 mm×t3.8 mm) is used as a dielectric  18 . The composition is Ba (Sn x Mg y Ta z )O 7/2−x/2−3y/2  (in which x+y+z=1, 0.04≦x≦0.26, 0.23≦y≦0.31, 0.51≦z≦0.65). The dielectric substrate  12  on which the superconducting electrode  14  is formed was also fabricated from Ba(Sn, Mg, Ta)O 3 . 
     In this dielectric resonator, Bi—Pb—Sr—Ca—Cu—O film or Y—Ba—Cu—O film is used as the superconducting electrode  14 . More specifically, for example, (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x  or YBa 2 Cu 3 O 7−x  is used. The superconducting electrode  14  using one of these materials can be formed, for example, in the following manner. 
     A Bi—Pb—Sr—Ca—Cu—O film can be formed by use of the following method. A powder of the composition Bi—Pb—Sr—Ca—Cu—O (2223 phase) and an organic vehicle are mixed, subjected to adjustment of the viscosity thereof, and screen-printed on the dielectric substrate  12 . The resultant film is dried at 100° C. to 150° C., and the dried film is fired at 840° C. to 860° C. for 100 to 200 hours in air. 
     A Y—Ba—Cu—O film can be formed by use of the following method. A powder of the composition. Y—Ba—Cu—O and an organic vehicle are mixed, subjected to adjustment of the viscosity thereof, and screen-printed on the dielectric ceramic. The resultant film is fired at 860° C. to 880° C. for 5 to 10 hours in an oxygen atmosphere. 
     A dielectric resonator  10  having the Bi—Pb—Sr—Ca—Cu—O film serving as the superconducting electrode  14  and a dielectric resonator  10  having the Y—Ba—Cu—O film were formed, and low-temperature Qu was measured. The results are plotted by use of open circles and open triangles in FIG.  4 . BPSCCO appearing in FIG. 4 represents Bi—Pb—Sr—Ca—Cu—O, and YBCO therein represents Y—Ba—Cu—O. 
     Further, as a first comparative example, there was fabricated a dielectric resonator having the same structure as the dielectric resonator  10  shown in FIG. 3 except that a copper plate was provided in place of the superconducting electrode  14 . In other words, the dielectric resonator of the first comparative example has the same structure as the dielectric resonator  10  shown in FIG. 3 except that the dielectric  18  is sandwiched between two copper plates. Low-temperature Qu of the dielectric resonator of the first comparative example was measured, and the results are plotted by use of filled rhombuses in FIG.  4 . 
     As is apparent from FIG. 4, the dielectric resonators  10  can achieve Qu higher than that of the dielectric resonator in the first comparative example in which the dielectric is sandwiched between two copper plates. Namely, the superconducting electrode  14  formed on the dielectric substrate  12  does not undergo interfacial reaction with the dielectric but exhibits superconducting characteristics. 
     FIG. 5 is an explanatory sketch of an example TM 010 -mode dielectric resonator of the present invention. The dielectric resonator  30  shown in FIG. 5 includes a dielectric substrate  32 . Film-shaped superconducting electrodes  34  and  36  are formed on the top and bottom surfaces of the dielectric substrate  32 , respectively. The dielectric substrate  32  is fixed within a metal casing  40  through the mediation of a Teflon sheet  38 . An excitation cable  42  is disposed at one end of the metal casing  40 , and an excitation cable  44  is disposed at the other end. 
     The dielectric substrate  32  of this resonator  30  was also fabricated from Ba(Sn, Mg, Ta)O 3 -based dielectric as in the dielectric resonator  10 . The superconducting electrodes  34  and  36  were fabricated from Bi—Pb—Sr—Ca—Cu—O film by use of the above-mentioned method. Low-temperature Qu was measured, and the results are plotted by use of open circles in FIG.  6 . BPSCCO appearing in FIG. 6 represents Bi—Pb—Sr—Ca—Cu—O. 
     Further, as a second comparative example there was fabricated a dielectric resonator having the same structure as the dielectric resonator  30  shown in FIG. 5, except that a copper thin film was formed on the dielectric substrate  32  instead of the superconducting electrodes  34  and  36 . In other words, the dielectric resonator of the second comparative example has the same structure as the dielectric resonator  30  shown in FIG. 5 except that the dielectric  32  is sandwiched between two copper thin films. The low-temperature Qu of the dielectric resonator of the second comparative example was measured, and the results are plotted by use of black rhombuses in FIG.  6 . 
     As is apparent from FIG. 6, the dielectric resonators  30  can achieve a Qu higher than that of the dielectric resonator of the second comparative example. Namely, the superconducting electrodes  34  and  36  formed on the top and bottom surfaces of the dielectric substrate  32  do not undergo an interfacial reaction with the dielectric but exhibit superconducting characteristics. 
     The case in which Ba(Sn, Mg, Ta)O 3 -based dielectric was used as a dielectric has been described with reference to embodiment examples and the related data shown in FIGS. 3 through 6; however, when other dielectrics described hereinabove are used, the same effect can be produced. Further, the oxide superconducting material is not limited only to the materials used in the embodiments as described with reference to FIGS. 3 and 5; when other oxide superconducting materials hereinabove are used, the same effect can be produced. 
     A TE 011 -mode dielectric resonator and a TE 010 -mode dielectric resonator have been described with reference to FIGS. 3 through 6; however, the present invention is not limited to only these types of resonators. The invention can be also applied to other types of dielectric resonators, for example, other TE-mode, TM-mode, TEM-mode dielectric resonators or resonators in which strip lines are fabricated on the dielectric substrate thereof. 
     FIG. 7 is a block diagram of an example communications device using the dielectric resonator of the present invention. The communications device  50  includes a dielectric duplexer  52 , a transmitting circuit  54 , a receiving circuit  56 , and an antenna  58 . The transmitting circuit  54  is connected to an input means  60  of the dielectric duplexer  52 , and the receiving circuit  56  is connected to an output means  62  of the dielectric duplexer  52 . The antenna  58  is connected to an antenna connecting means  64  of the dielectric duplexer  52 . The dielectric duplexer  52  includes two dielectric filters  66  and  68 . The dielectric filters  66  and  68  each include the dielectric resonator of the present invention and external connecting means connected to the resonator. In this example communications device, the filters are formed by connecting external connecting means  70  to the excitation cables of the dielectric resonators  10  ( 30 ); one dielectric filter  66  is connected between the input means  60  and the antenna connecting means  64 , and the other dielectric filter  68  is connected between the antenna connecting means  64 . and the output means  62 . 
     As described above, in the dielectric resonator according to the present invention, no interfacial reaction occurs between the dielectric and the superconducting material to thereby provide an excellent superconducting characteristic, achieving a higher Qu than the case in which metal electrodes are used. Therefore, when such a dielectric resonator of the present invention is incorporated into a dielectric filter, dielectric duplexer or a communications device, excellent working characteristics can be obtained.