Patent Publication Number: US-7587229-B2

Title: Superconducting tunable filter having a patch resonator pattern tuned by a variable dielectric constant top plate

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to a superconducting microwave device, and more particularly, to a superconducting tunable filter applied to an ultralow temperature RF front-end of a transmitter of a base station in mobile communications systems. 
   2. Description of the Related Art 
   Along with rapid development and spread of mobile phones in recent years, high-speed and high-volume data transmission technologies have becomes indispensable. Because of extremely small surface resistances, as compared with typical good conductors, superconductors have great potential for application to RF filters used in base stations of mobile communications systems, and application to low-loss and high-Q resonators are especially expected. 
   As illustrated in  FIG. 1 , an RF signal received at an antenna (ANT)  151  is processed through a bandpass filter (BPF)  152 R, a low-noise amplifier (LNA)  153 , down converter (D/C)  154 , and a demodulator (DEMOD)  155 , which components constitute a receiver RF front-end, and then processed at a baseband processing unit  156 . 
   At the transmitter RF front-end, a baseband-processed transmission signal is subjected to processes successively through a modulator (MOD)  157 , an up converter (U/C)  158 , a high power amplifier (HPA)  159 , and a band-pass filter (BPF)  152 T, and then transmitted as an RF signal from the antenna  151 . 
   When a superconducting filter is applied to the receiving-end bandpass filter  152 R, transmission loss is small and a steep frequency cut-off characteristic can be expected. When it is applied to the transmission-end bandpass filter  152 T, an effect of removing distortion generated due to the high power amplifier  159  can be expected. However, the transmitter RF front-end needs a high power system to transmit radio frequency signals, and therefore, today&#39;s issue is balancing the compact structure and satisfactory power quality. 
   In application to the mobile communications field, frequency tunability is in strong demand. In order to achieve a tunable superconducting filter, it is proposed to arrange a plate having a surface covered with a conductive film above a pattern of a superconducting resonator such that the conducting surface faces the superconducting resonator. A piezoelectric device is inserted between the superconducting resonator and the conducting face of the plate to adjust the distance between the two to vary the resonant frequency. See, for example, WO 01/041251. 
   Because this method uses a mechanical mechanism of an actuator, there are problems of high susceptibility to shaking or vibration and slow response speed. 
   Another known technique for achieving frequency tunability is to make use of a dielectric having a highly bias-dependent permittivity. It is proposed to form a dielectric film with a bias-dependent permittivity over the pattern of a resonator filter, and to apply an electric voltage to the dielectric film to vary the dielectric constant. See, for example, JP 9-307307A. In this method, an electric voltage is applied in the lateral direction, and the rate of change is small. In addition, since the power durability of this filter device is insufficient, it is only applicable to the receiving front-end. 
   Still another known method is to place a superconducting dielectric resonator of a parallel plate type on a microstrip line and tune the frequency characteristic of the resonator by making use of the bias-voltage dependency of the permittivity of the dielectric plate. See, for example, WO 97/23012. 
     FIG. 2A  and  FIG. 2B  illustrate the structure of a tunable superconducting filter disclosed in WO 97/23012. In  FIG. 2A , the resonator  111  has a dielectric plate  112  with a nonlinear permittivity, and high temperature superconductor (HTS) plates  113   a  and  113   b  are arranged on both sides of the dielectric plate  112 . The HTS plates  113   a  and  113   b  are covered with conducting films  114 .  FIG. 2B  depicts the resonator  111  ( FIG. 2A  disposed on a microstrip line  115 . One of the superconducting plates ( 113   b  in this example) is electrically connected to the center strip  118  of the microstrip line  115  via the conducting film  114 , as illustrated in  FIG. 2B . A signal propagates through the resonator  111  from a particular side (designated as INPUT) to the side (designated as OUTPUT) opposing to the particular side. A DC bias voltage is applied between the upper superconducting plate  113   a  of the resonator  111  and the microstrip line  115  using a voltage supply  119 . By changing the DC bias voltage, the dielectric constant of the nonlinear dielectric plate  112  is changed to vary the resonant frequency of the resonator  111 . 
   This structure, however, is inferior in power durability, and therefore, it is applicable only to a filter of a receiving-end. Poor power durability in the conventional superconducting filter is attributed to concentration of electric current at the corners or edges of the superconducting resonator patterns. 
   It may be effective to form a resonator in a patch pattern or a plane FIGURE pattern, including a disk pattern, an oval pattern, an elliptic pattern, and a polygonal pattern, with less sharp corners or edges. Such shapes are effective in reducing local concentration of electric current on the superconducting resonator, and a large power response required for a transmission filter can be achieved. Such a patch shaped (plane FIGURE shaped) superconducting filter may be further developed by arranging a conductive pattern with a certain shape above the superconducting resonator via a dielectric between them to cause coupling corresponding to a desired bandwidth. By generating two orthogonal resonating modes (dual mode) in a round or polygonal resonator, the power characteristic and the frequency characteristic can be improved through reduction of concentration of electric current, and the device can be made compact because of the dual-mode structure. 
   However, the above-described dual-mode resonator does not have frequency tunability, and it cannot deal with correction of deviation in characteristic features due to variation in manufacturing nor with positive adjustment of characteristic features. 
   SUMMARY OF THE INVENTION 
   The present invention was conceived in view of the above-described problems, and the embodiments provide a simple and novel structure of a transmission filter, in which the center carrier frequency and the bandwidth of a superconducting resonator can be adjusted simultaneously or independently of each other, while maintaining satisfactory power characteristics. 
   To realize such a filter, a dielectric with an electric-field dependent permittivity is provided over a superconducting resonator pattern of a superconducting filter. The resonator pattern is shaped in a patch pattern, including a disk pattern, an elliptic pattern, and a polygonal pattern. A conducting pattern is formed on the overlaid dielectric to produce dual-mode resonance. A bias voltage is applied between the superconducting resonator pattern and the conducting pattern to vary the permittivity or the dielectric constant of the overlaid dielectric so as to tune the filter characteristics. 
   To be more precise, in one aspect of the invention, a superconducting filter comprises:
     (a) a dielectric base plate;   (b) a patch-shaped resonator pattern formed of a superconducting material on the dielectric base plate;   (c) a top dielectric provided over the resonator pattern and having a non-linear electric-field dependency of permittivity;   (d) a conducting pattern formed on the top dielectric to produce coupling corresponding to a prescribed bandwidth; and   (e) a bias voltage supply configured to apply a bias voltage to the top dielectric.   

   With this structure, the dielectric constant of a dual-mode filter can be controlled by applying a bias voltage to the top dielectric, and consequently, the center carrier frequency and/or the bandwidth of the filter can be tuned. 
   In a preferred example, the bias voltage supply includes bias application wiring connected to the conducting pattern and the superconducting resonator pattern, and the bias application wiring has an inductance component for removing high-frequency components. The bias application wiring is patterned into, for example, a hairpin pattern. 
   It is preferred that the top dielectric be made of a perovskite oxide or a pyrochlore oxide. 
   In another aspect of the invention, a superconducting filter comprises:
     (a) a dielectric base plate;   (b) a patch-shaped resonator pattern formed of a superconducting material on the dielectric base plate;   (c) a dielectric top plate locally positioned on a part of the superconducting resonator pattern and formed of a material with an electric-field dependent permittivity;   (d) a conducting film formed on a surface of the dielectric top plate; and   (e) a bias voltage supply configured to apply a bias voltage between the conducting film and the resonator pattern.   

   In a preferred example, the superconducting filter further includes an input feeder for supplying a signal to the resonator pattern, and an output feeder for outputting the signal from the resonator. The dielectric top plate is positioned on a line extending so as to be symmetric to the input feeder and the output feeder with respect to the center of the resonator pattern. 
   In another example, the bias voltage supply includes first bias application wiring formed on the dielectric base plate and electrically connected to the resonator pattern, and second bias application wiring formed on the dielectric base plate and electrically connected to the conducting pattern formed on the dielectric top plate. 
   In a preferred structure, the first and second bias application wirings include repeat patterns serving as inductance components. 
   With the above-described structures, the center carrier frequency and the bandwidth can be tuned precisely in a dual-mode superconducting filter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates a general structure of the RF front-end of a conventional mobile communications base station; 
       FIG. 2A  and  FIG. 2B  are schematic diagram of conventional superconducting tunable filters; 
       FIG. 3A  is a top view of a superconducting filter according to the first embodiment of the invention, and  FIG. 3B  is a schematic diagram of the cross-sectional view of the superconducting filter; 
       FIG. 4  is a plan view of the superconducting filter of  FIG. 3  accommodated in a metal package; 
       FIG. 5  is a cross-sectional view of the superconducting filter of  FIG. 3  accommodated in the metal package; 
       FIG. 6A  through  FIG. 6F  are graphs of filter characteristics under different bias voltages applied between the superconducting resonator pattern and the dual-mode generating conducting pattern; 
       FIG. 7A  and  FIG. 7B  are graphs showing voltage dependency of the dielectric constants of different materials; 
       FIG. 8  is a plan view of a superconducting filter according to the second embodiment of the invention; 
       FIG. 9A  is a perspective view of the superconducting filter shown in  FIG. 8 , and  FIG. 9B  is a schematic cross-sectional diagram of the superconducting filter; 
       FIG. 10A  through  FIG. 10E  illustrate a fabrication process of the superconducting filter according to the second embodiment of the invention; and 
       FIG. 11  is a graph showing the bandpass characteristic of the superconducting filter with and without application of a bias voltage according to the second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the invention are now described in conjunction with the attached drawings. 
     FIG. 3A  and  FIG. 3B  illustrate a preferred structure of a superconducting filter  10  ( FIG. 3A ) according to the first embodiment of the invention.  FIG. 3A  is a top view of the filter, and  FIG. 3B  schematically illustrates in a cross-sectional view the tunable structure of the superconducting filter  10 . 
   The superconducting filter  10  includes a dielectric base plate  11  made of, for example, MgO single-crystal substrate, a disk-shaped superconducting resonator pattern  12  formed on the top face of the dielectric base plate  11 , and signal input/output feeders  13  extending to and from the vicinity of the superconducting resonator pattern  12 . The superconducting filter  10  also includes a top dielectric  14  provided over the superconducting resonator pattern  12 , a round or elliptic conducting pattern  15  formed on the top face of the top dielectric  14 , and wirings  16  for applying a bias voltage to the top dielectric  14 . The conducting pattern  15  formed on the top dielectric  14  generates coupling corresponding to a desired bandwidth and allows the superconducting resonator  12  to operate in dual modes. 
   The dielectric base plate  11  is made of a material showing a low transmission loss at radio frequencies. Examples of such a material include sapphire, LaAlO3 (referred to as “LAO”), and TiO3, in addition to MgO. In general, transmission loss is low in a single-crystal dielectric, as compared with a poly crystalline dielectric, and therefore, a single-crystal dielectric is suitably used for the dielectric base plate  11  of a superconducting filter. 
   An arbitrary superconductor can be used as the superconducting material of the resonator pattern  12 . For example, a metal such as Nb, a nitride such as NbN, or an oxide such as YBCO (Y—Ba—Cu composite oxide), can be suitably used. From the viewpoint of easiness of handling, it is desired to use a high temperature oxide superconductor (such as YBCO) having a high critical temperature. Preferred examples of oxide superconductor include RBCO (R—Ba—Cu—O) in which Nd, Sm, Gd, Dy, or Ho can be used, in place of yttrium (Y), as the element R. Other oxide superconductors, such as BSCCO (Bi—Sr—Ca—Cu—O), PBSCCO (Pb—Bi—Sr—Ca—Cu—O), CBCCO (Cu—Ba p —Ca q —Cu r — x , 1.5&lt;p&lt;2.5, 2.5&lt;q&lt;3.5, 3.5&lt;r&lt;4.5) can also be applicable to the superconducting resonator. 
   It is desired to shape the superconducting resonator pattern  12  into a round shape from the viewpoint of reducing concentration of electric current and improving the power durability. Different plane FIGURE patterns, such as ellipses, polygons, and annulii, can also be used because these shapes are advantageous in power durability. The term “patch pattern” or “plane FIGURE pattern” used in the specification and claims represents a two-dimensionally spread pattern including a disk pattern, an oval pattern, an elliptic pattern, a polygonal pattern, and an annular pattern, which is distinguished from a linear pattern (a strip or line pattern). 
   The top dielectric  14  is made of a material with highly electric-field-dependent permittivity (i.e., with non-linear electric-field dependency of the dielectric constant) and with low transmission loss at radio frequencies. For example, perovskite oxide, such as SrTiO3 or (Ba, Sr)TiO3, and pyrochlore oxide, such as BZN (Bi—Zn—Nb composite oxide), are suitably used. The top dielectric  14  may be a poly-crystalline or single-crystal dielectric plate placed over superconducting resonator pattern  12 , or alternatively, it may be a poly-crystalline or single-crystal dielectric layer grown over the superconducting resonator pattern  12 . 
   One of the input/output feeders  13  is used to supply a signal to the superconducting resonator  12 , and the other is used to output the signal from the superconducting resonator  12 . Although not shown in  FIG. 3B , a ground film (i.e., a ground electrode) is formed on the back bottom face of the dielectric base plate  11  using a superconducting material. 
   In order to supply a bias voltage to the top dielectric  14  with electric-field dependent permittivity, a bias application wiring  16  is connected to each of the dual-mode producing conducting pattern  15  formed on the top dielectric  14  and the superconducting resonator pattern  12 . Under the application of a bias voltage, the permittivity or the dielectric constant of the top dielectric  14  is changed to tune the filter characteristics. In this regard, the top dielectric  14  may be called a “permittivity variable dielectric  14 ”. 
   In the example shown in  FIG. 3A , the bias application wiring  16  includes a pattern involving an inductance component so as to block a radio frequency component from entering the DC power source. This arrangement can prevent high frequency loss from occurring in the bias apply wiring  16 . The bias application wiring  16  may be patterned into a hairpin pattern with repeated U, or a zigzag pattern with repeated V. 
     FIG. 4  is a schematic top view of the superconducting filter  10  shown in  FIGS. 3A and 3B  accommodated in a metal package  21  for application to a transmission filter of a base station of a mobile communications system.  FIG. 5  is a cross-sectional view of the packaged filter. In actual use, the superconducting filter  10  packaged in the metal package  21  is placed in a cold chamber furnished with a freezer and a vacuum heat insulator. As shown in  FIGS. 4 and 5 , the superconducting filter  10  includes the dielectric base plate  11  and the top dielectric  14 . The feeders  13  for inputting and outputting signals to and from the superconducting resonator  12  are connected to the input connector and output connectors  22 . The bias application wirings  16  are connected to bias application connectors  24  provided on the metal package  21 . A bias voltage is applied between the dual-mode producing conducting pattern  15  and the superconducting resonator pattern  12  from the external DC power source via the bias application wirings  16 . 
     FIG. 6A  through  FIG. 6F  are simulation graphs showing the tunability of the superconducting filter of the first embodiment. By applying a bias voltage to the superconducting filter  10  shown in  FIGS. 3A and 3B , the dielectric constant ∈ of the top dielectric  14  is changed as follows:  100  in  FIG. 6A ,  180  in  FIG. 6B ,  230  in  FIG. 6C. 250  in  FIG. 6D ,  290  in  FIG. 6E and 620  in  FIG. 6F . In the graphs, the dashed lines indicate input reflection characteristics (S 11 ), and the solid lines indicate transmission characteristics (S 21 ). 
   It is understood from the graphs that along with the changes in dielectric constant from ∈=100, ∈=250, to ∈=620, the center carrier frequency changes from 4.16 GHz, 3.92 GHz, to 3.57 GHz, respectively. In addition, along with the change in dielectric constant, bandwidth also changes. The simulation result at ∈=620 is obtained with no bias voltage application. By increasing the bias voltage form zero level, the dielectric constant decreases. 
   The rate of change in center carrier frequency and bandwidth differs among dielectric materials. 
     FIG. 7A  is a graph of dielectric constant of BST thin film as a function of applied bias voltage, and 
     FIG. 7B  is a graph of dielectric constant of BZN plate as a function of applied bias voltage. It is preferred to use a BST thin film as the top dielectric  14  if efficient change in center carrier frequency and bandwidth is required. On the other hand, if the major purpose is to carry out fine tuning of the filter, a BZN plate is suitably used for the top dielectric  14 . 
   In addition, even if a same dielectric material is used, the rate of change of the permittivity under the application of bias voltage varies depending on the fabrication process. For example, the dielectric constant of a BST thin film becomes 600 or higher without application of a bias voltage depending on the film formation process. 
   Samples (test devices) of superconducting filter  10  were actually fabricated to measure the center carrier frequency under the application of bias voltage to evaluate the bias voltage dependency of the filter characteristics. 
   EXAMPLE 1 
   A 20×20×0.5 [mm] MgO single-crystal plate is used as the dielectric base plate  11  of the superconducting filter  10 . A disk-shaped superconducting resonator pattern  12 , a bias application hairpin wiring  16  extending from the resonator pattern  12 , and input/output feeders  13  are formed on the MgO dielectric base plate  11  by epitaxial growth of a YBCO thin film and a patterning process. The diameter and the thickness of the superconducting resonator pattern  12  are 128 mm and 0.5 μm, respectively. A ground electrode (ground film) is also formed on the back face of the MgO dielectric base plate  11  by epitaxial growth of a YBCO thin film. 
   A BZN plate is used as the top dielectric  14 , and is placed on the YBCO patterned face of the MgO dielectric base plate  11 . On the top face of the BZN plate are formed in advance a dual-mode producing conducting pattern  15  with a diameter of 38 mm and a bias application hairpin wiring  16 . 
   This superconducting filter  10  has a center carrier frequency at 3.95 GHz without application of a bias voltage. Upon application of 60 V bias, the center carrier frequency shifts to 4.05 GHz. The center carrier frequency changes by 0.1 GHz. 
   EXAMPLE 2 
   As in the first example, a disk-shaped superconducting resonator pattern  12 , a bias application hairpin wiring  16  extending from the resonator pattern  12 , and input/output feeders  13  are formed on a 20×20×0.5 [mm] MgO single-crystal plate  11 . The diameter and the thickness of the disk resonator  12  are 128 mm and 0.5 μm, respectively. 
   A (Ba, Sr)TiO3 thin film is formed by epitaxial growth over the dielectric base plate  11 . A YBCO thin film is formed over the (Ba, Sr)TiO3 film by epitaxial growth, and patterned into a dual-mode producing conducting pattern  15  with a diameter of 38 mm and a hairpin wiring  16  for bias application. 
   This superconducting filter  10  has a center carrier frequency at 3.90 GHz without application of a bias voltage. Upon application of 30 V bias, the center carrier frequency shifts to 4.10 GHz. A 0.2 GHz change is achieved. 
   In this manner, in the first embodiment, the permittivity of the top dielectric  14  with a dual-mode producing conducting pattern  15  formed on the top face is changed. Consequently, the center carrier frequency and the bandwidth of a dual-mode resonator can be tuned precisely. 
   Next, the second embodiment of the invention is described below. In the previous embodiment, the top dielectric  14  is provided over the entire surface of the superconducting resonator pattern  12 , and a bias voltage is applied between the dual-mode producing conducting pattern  15  formed on the top dielectric  14  and the superconducting resonator pattern  12  to efficiently change the center carrier frequency. In the second embodiment as depicted in  FIGS. 8 ,  9 A and  9 B, a permittivity variable top dielectric  40  is locally placed on a part of the superconducting resonator pattern  12 , and a bias voltage is applied between a conductor  35  ( FIGS. 9A and 9B ) formed over the permittivity variable top dielectric  40  and the superconducting resonator pattern  12 . This arrangement is more effective in tuning the passband width of the filter. 
     FIG. 8  is a top view of a superconducting filter  30  according to the second embodiment of the invention, and  FIG. 9A  and  FIG. 9B  are a perspective view and a cross-sectional view, respectively, of the filter  30  in  FIG. 8 . In the example shown in  FIG. 8 , a permittivity variable top dielectric  40  with dimensions of 3 mm×2 mm is arranged over a superconducting disk resonator  12  with a diameter φ of 11 mm, the superconducting disk resonator  12  being disposed on a base dielectric plate  11 . The permittivity variable top dielectric  40  is positioned so as to be point-symmetric with the input/output feeders  13  with respect to the center point of the disk resonator  12 . As in the first embodiment, input and output feeders  13  extend to the vicinity of the superconducting resonator pattern  12  at an angle of 90 degrees between them. Accordingly, the permittivity variable top dielectric  40  is positioned in the middle (on the center line C) between the two extended lines of the input and output feeders  13 . 
   In the example shown in  FIGS. 8 ,  9 A and  9 B, the permittivity variable top dielectric  40  is positioned so as to be aligned with the edge of the superconducting resonator pattern  12 ; however, it may be moved along the center line C extending between the orthogonally arranged feeders  13  toward the center of the superconducting resonator  12 . By placing the permittivity variable top dielectric  40  along the center line C on the resonator pattern  12  (so as to be point-symmetric to the input and output feeders  13  with respect to the center point of the disk resonator  12 ), the electric current density reducing effect can be maintained satisfactory. It is desired not to place the permittivity variable top dielectric  40  near the input and output feeders  13  nor on the extended lines of these feeders  13 . 
   As illustrated in  FIG. 9A  and  FIG. 9B , the permittivity variable top dielectric  40  includes a dielectric plate  34  made of, for example, SrTiO3 (referred to as “STO”), and a conducting film  35  formed over the top face of the dielectric plate  34 . Although in this example the conducting film  35  is made of a superconducting material, it may be formed of gold (Au). 
   The permittivity variable top dielectric  40  serves as a varactor (variable-capacitance device). By changing the permittivity of the dielectric plate  34  under the application of a bias voltage, the center carrier frequency and/or the passband width of a signal passing through the superconducting filter  30  (which may be used as a bandpass filter, for example) can be controlled. 
   Bias application wirings  16   a  and  16   b  are formed on the dielectric base plate  11 . ( FIG. 9B ). The bias application wiring  16   a  extends to the disk-shaped superconducting resonator pattern  12 , and a bias application wiring  16   b  is connected to the conductor  35  formed on the dielectric plate  34  by wire bonding  42 . ( FIG. 9A ). As in the first embodiment, the bias application wirings  16   a  and  16   b  have hairpin patterns serving as inductance components for blocking the radio frequency component. The patterns of the bias application wirings  16   a  and  16   b  are not limited to the hairpin patterns, but may have square pulse patterns or zigzag patterns consisting of repeated V. 
   The conducting film  35  of the permittivity variable top dielectric  40  may be formed as a laminate of a superconducting film and a gold (Au) film. In this case, the conducting film  35  serves as a dual-mode producing conductor, and simultaneously, serves as an electrode pad for the wire bonding. The bias application wirings  16   a  and  16   b  are connected to an external DC power source. 
   As illustrated in  FIG. 9B , a ground film  19  is formed of a superconducting material (e.g., YBCO) on the back face of the dielectric base plate (e.g., MgO plate)  11 . The superconducting resonator pattern  12  is formed on the top face (opposite to the ground face) of the dielectric base plate  11 . The permittivity variable top dielectric  40  is placed on a part of the superconducting resonator pattern  12 . By applying a bias voltage between the YBCO film (or the Au film if laminated)  35  of the permittivity variable top dielectric  40  and the superconducting resonator pattern  12  from the DC power source, the dielectric constant of the STO plate  34  can be changed. 
   In general, as the dielectric constant is greater, the dielectric loss caused under the application of a DC bias increases. Accordingly, instead of placing the permittivity variable top dielectric over the entire surface of the superconducting resonator pattern  12 , it is placed only locally on the resonator pattern  12  to reduce the dielectric loss, while maintaining the dual-mode operability and realizing bandwidth tunability. 
     FIG. 10A  through  FIG. 10E  illustrate a fabrication process of the superconducting tunable filter  30  according to the second embodiment of the invention. As illustrated in  FIG. 10A , a superconducting film (e.g. a YBCO film)  41  with a thickness of 500 nm (see  FIG. 9B ) is formed over a 0.5 mm (see  FIG. 9B ) thick dielectric base plate  11  by a PLD method. As in the first embodiment, the dielectric base plate  11  is made of a dielectric material characterized by low transmission loss at radio frequencies, and examples of such a material include MgO, sapphire, and LAO. For the superconducting film  41 , RBCO (R—Ba—Cu—O), BSCCO (Bi—Sr—Ca—Cu—O), PBSCCO (Pb—Bi—Sr—Ca—Cu—O), 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) can be used, in addition to YBCO. 
   Then, as illustrated in  FIG. 10B , the superconducting film on the top face of the dielectric base plate  11  (MgO plate in this case) is patterned into a disk-shaped resonator pattern  12 , bias application wiring  16   a  and  16   b  (see  FIG. 8 ), and input/output feeders  13  (see  FIGS. 8 and 9A ), while maintaining the superconducting film formed on the back face of the dielectric base plate  11 . In the patterning process, a resist mask (not shown) is formed over the superconducting film  41  by a conventional lithography technique to perform dry etching, such as argon (Ar) milling. The pattern of the bias application wirings  16   a  and  16   b  has a narrow width and enough length so as to secure sufficient inductance. The superconducting film  41  on the back face is used as the ground film  19 . 
   Meanwhile, in  FIG. 10C , a YBCO film with a thickness of 500 nm (see  FIG. 9B ) and a gold (Au) film with a thickness of 500 nm are successively formed over the top face of a 0.5 mm (see  FIG. 9B ) thick STO ( 100 ) substrate  34  by an PLD method to provide a conducting film  35 . The STO substrate  34  with the conducting film  35  is cut into a 3 mm×2 mm piece using an ultrasonic processing machine, as illustrated in  FIG. 10D . This dice is used as the permittivity variable top dielectric  40 . For the dielectric substrate  34 , (Ba, Sr)TiO 3 , Bi 1.5 Zn 1.0 Nb 1.5 O 7 , CaTiO 3  can be used other than STO. 
   Finally, as illustrated in  FIG. 10E , the permittivity variable top dielectric  40  including the STO substrate  34  and the conducting film  35  is placed on the superconducting resonator pattern  12  at a prescribed position such that the uncovered back face (opposite to the face covered with the conducting film  35 ) is in contact with the resonator pattern  12 . The conducting film  35  is electrically connected to the bias application wiring  16   b  (see  FIG. 8 ) formed on the dielectric base plate  11  (MgO plate in this case) by Au wire bonding. The ground film  19  on the dielectric base plate  11  is used as ground. 
     FIG. 11  is a simulation graph showing the bandpass filter characteristics of the superconducting filter, that is, transmission characteristics (S 21 ) as a function of frequency (GHz), according to the second embodiment of the invention. The dashed line indicates the transmission characteristic without application of a DC bias, and the solid line indicates the transmission characteristic under the application of a DC bias. Without the application of a DC current, the dielectric constant of the dielectric plate  34  of the top dielectric  40  is 300 (∈=300). Upon application of a DC bias, the dielectric constant changes to 200 (∈=200). As a result, the bandwidth can be changed efficiently with little change in the center carrier frequency. In addition, the structure of the second embodiment is advantageous from the viewpoint of the fast response in permittivity change because the permittivity variable top dielectric  40  is locally arranged on the resonator pattern  12  at a prescribed position. 
   This patent application is based upon and claims the benefit of the earlier filing dates of Japanese Patent Application Nos. 2006-095250 and 2006-346212 filed Mar. 30, 2006 and Dec. 22, 2006, respectively, the entire contents of which are incorporated herein by reference.