Patent Publication Number: US-6903633-B2

Title: Electronic tunable filters with dielectric varactors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of patent Ser. No. 09/734,969, entitled, “ELECTRONIC TUNABLE FILTERS WITH DIELECTRIC VARACTORS”, filed Dec. 12, 2000, by Yongfei Zhu et al., that issued as U.S. Pat. No. 6,686,817 on Feb. 3, 2004. 
    
    
     BACKGROUND OF INVENTION 
     The present invention generally relates to electronic filters and, more particularly, to such filters that include tunable dielectric capacitors (dielectric varactors). 
     One of most dramatic developing areas in communications over the past decade has been mobile and portable communications. This has led to continual reductions in the size of the terminal equipment such as the handset phone. Size reduction of the electronic circuits is progressing with the development of recent semiconductor technologies. However, microwave filters occupy a large volume in communications circuits, especially in multi-band applications. Multi-band applications typically use fixed filters to cover different frequency bands, with switches to select among the filters. Therefore, compact, high performance tunable filters are extremely desirable for these applications, to reduce the number of filters and simplify the control circuits. 
     Electrically tunable filters are suitable for mobile and portable communication applications, compared to other tunable filters such as mechanically and magnetically tunable filters. Both mechanically and magnetically tunable filters are relatively large in size and heavy in weight. Electronically tunable filters have the important advantages of small size, lightweight, low power consumption, simple control circuits, and fast tuning capability. Electronically tunable filters can be divided into two types: one is tuned by tunable dielectric capacitors (dielectric varactors), and the other is tuned by semiconductor diode varactors. The dielectric varactor is a voltage tunable capacitor in which the dielectric constant of a dielectric material in the capacitor can be changed by a voltage applied thereto. Compared to semiconductor diode varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP 3 , and faster tuning speed. Third intermodulation distortion happens when two close frequency signals (f 1  and f 2 ) are input into a filter. The two signals generate two related signals at frequencies of 2f 2 −f 1  (say f 3 ), and 2f 1 −f 2  (say f 4 ), in addition to the two main signals f 1  and f 2 . F 3  and f 4  should be as low as possible compared to f 1  and f 2 . The relationship between f 1 , f 2 , f 3  and f 4  is characterized by IP 3 . The higher the IP 3  value is, the lower the third intermodulation. Considering the additional attributes of low power consumption, low cost, variable structures, and compatibility to integrated circuit processing, dielectric varactors are suitable for tunable filters in mobile and portable communication applications. 
     Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents. 
     Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled “Ceramic Ferroelectric Material”; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO—MgO”; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO—ZrO 2 ”; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO-Magnesium Based Compound”; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled “Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled “Electronically Graded Multilayer Ferroelectric Composites”; and U.S. Pat. No. 5,635,433 to Sengupta, entitled “Ceramic Ferroelectric Composite Material—BSTO—ZnO”. These patents are hereby incorporated by reference. A copending, commonly assigned U.S. patent application Ser. No. 09/594,837, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTO—MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage. 
     Commonly used compact fixed filters in mobile and portable communications are ceramic filters, combline filters, and LC-lumped filters. This invention provides tunable filters, utilizing advanced dielectric varactors. 
     SUMMARY OF THE INVENTION 
     Radio frequency electronic filters constructed in accordance with this invention include an input, an output, and first and second resonators coupled to the input and the output, with the first resonator including a first tunable dielectric varactor and the second resonator including a second tunable dielectric varactor. The resonators can take the form of a lumped element resonator, a ceramic resonator, or a microstrip resonator. Additional tunable dielectric varactors can be connected between the input and the first resonator and between the second resonator and the output. Tunable dielectric varactors can also be connected between the first and second resonators. Further embodiments include additional resonators and additional tunable dielectric varactors. 
     The compact tunable filters of this invention are suitable for mobile and portable communication applications such as handset phones. The high Q dielectric varactors used in the preferred embodiments of the invention utilize low loss tunable thin film dielectric materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a lumped element LC tunable filter constructed in accordance with one embodiment of the invention; 
         FIG. 2  is a schematic diagram of a DC bias circuit for varactors used in the filters of this invention; 
         FIG. 3  is a schematic diagram of another lumped element LC tunable filter constructed in accordance with the invention; 
         FIG. 4  is a schematic diagram of another lumped element LC tunable filter constructed in accordance with the invention; 
         FIG. 5  is a plan view of a varactor that can be used in filters constructed in accordance with the present invention; 
         FIG. 6  is a sectional view of the varactor of  FIG. 5  taken along line  6 — 6 ; 
         FIG. 7  is a plan view of another varactor that can be used in filters constructed in accordance with the present invention; 
         FIG. 8  is a sectional view of the varactor of  FIG. 7  taken along line  8 — 8 ; 
         FIG. 9  is a plan view of another varactor that can be used in filters constructed in accordance with the present invention; 
         FIG. 10  is a sectional view of the varactor of  FIG. 9  taken along line  10 — 10 ; 
         FIG. 11  is a plan view of another varactor that can be used in filters constructed in accordance with the present invention; 
         FIG. 12  is a sectional view of the varactor of  FIG. 11  taken along line  12 — 12 ; 
         FIG. 13  is a plan view of another varactor that can be used in filters constructed in accordance with the present invention; 
         FIG. 14  is a sectional view of the varactor of  FIG. 13  taken along line  14 — 14 ; 
         FIG. 15  is a plan view of another varactor that can be used in filters constructed in accordance with the present invention; 
         FIG. 16  is a sectional view of the varactor of  FIG. 15  taken along line  16 — 16 ; 
         FIG. 17  is an isometric view of a prior art ceramic filter that can be modified to include tunable varactors in accordance with the present invention; 
         FIG. 18  is a longitudinal vertical cross sectional view of the filter of  FIG. 17 ; 
         FIG. 19  is a top plan view of ceramic filter with a schematically illustrated varactor constructed in accordance with the present invention; 
         FIG. 20  is a schematic diagram of the filter of  FIG. 19 ; 
         FIG. 21  is a top plan view of another ceramic filter with a schematically illustrated varactor constructed in accordance with the present invention; 
         FIG. 22  is a top plan view of another ceramic filter with a schematically illustrated varactor constructed in accordance with the present invention; 
         FIG. 23  is a schematic representation of a combline filter constructed in accordance with the present invention; 
         FIGS. 24 ,  25 ,  26  and  27  are schematic representations of additional combline filters constructed in accordance with the present invention; and 
         FIGS. 28 and 29  are schematic diagrams of other lumped element LC tunable filters constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings,  FIG. 1  is a schematic diagram of a three pole lumped element LC tunable filter  10  constructed in accordance with one embodiment of the invention. The filter includes an input  12  and an output  14 . A plurality of resonant circuits  16 ,  18  and  20  are electrically coupled to the input and the output. Resonant circuit  16  includes inductor L 1  and capacitor C 1 . Resonant circuit  18  includes inductor L 2  and capacitor C 2 . Resonant circuit  20  includes inductor L 3  and capacitor C 3 . Capacitor C 4  couples resonant circuit  16  to the input  12 . Capacitor C 5  couples resonant circuit  16  to resonant circuit  18 . Capacitor C 6  couples resonant circuit  18  to resonant circuit  20 . Capacitor C 7  couples resonant circuit  20  to the output  14 . Capacitors C 1 , C 2  and C 3  are tunable dielectric varactors. C 4  and C 7  are port coupling capacitors used to provide a specific port impedance, typically 50 ohms or 75 ohms. More or fewer resonators can be used in the filter to obtain specific filter rejection. Each of the tunable varactors is connected to a voltage bias circuit not shown in  FIG. 1 , but shown in  FIG. 2  as bias circuit  22 . 
       FIG. 2  shows a voltage source  24  connected to varactor C 1  through an inductor  26 . A blocking capacitor  28  is electrically connected in series with the varactor. By varying the voltage supplied by source  24 , the capacitance of the varactor changes. This enables tuning of the filter. The DC blocking capacitor is used to prevent the DC bias voltage from entering into the other parts of the filter. Inductor  26  works as an RF choke to prevent RF signal leaking into the bias circuit. 
       FIG. 3  is a schematic diagram of another lumped element LC tunable filter  30  constructed in accordance with the invention. Filter  30  is similar to filter  10  of  FIG. 1 , except that capacitors fixed C 4  and C 7  in  FIG. 1  have been replaced by varactors C 8  and C 9  in FIG.  3 . 
       FIG. 4  is a schematic diagram of another lumped element LC tunable filter  32  constructed in accordance with the invention. Filter  32  is similar to filter  30  of  FIG. 3 , except that capacitors fixed C 5  and C 6  in  FIG. 3  have been replaced by varactors C 10  and C 11  in FIG.  4 . 
     The lumped element tunable filters of  FIGS. 1-4  are particularly applicable for use in mobile and portable communications. Lumped element tunable filters have the advantages of small size, simple structure, and low cost. In order to tune the filters, the fixed resonating capacitors in a conventional LC lumped element filter are replaced by dielectric varactors. The tuning range of the filter is determined by the tuning range of the varactors. In order to control the frequency response (such as bandwidth and return loss) in the tuning range, the coupling between resonators and resonator-ports may be tunable. To do so, varactors may replace the fixed port coupling capacitors, as shown in  FIGS. 3 and 4 .  FIG. 4  shows a fully controlled filter for controlling center frequency, bandwidth, and return loss in the tuning range. Since each capacitance in the filter is tunable, the lumped element tunable filter of  FIG. 4  has the highest tuning range compared to other tunable filters for a certain varactor tuning range. However, LC lumped element filters suffer from high insertion losses, and frequency limitations caused by lumped element behaviors vs. frequency. 
     In the preferred embodiments of the invention, each of the filters includes varactors comprising a substrate, a first conductor positioned on a surface of the substrate, a second conductor positioned on the surface of the substrate and forming a gap between the first and second conductors, a tunable dielectric material positioned on the surface of the substrate and within the gap, the tunable dielectric material having a top surface, with at least a portion of said top surface being positioned above the gap opposite the surface of the substrate, and a first portion of the second conductor extending along at least a portion of the top surface of the tunable dielectric material. The second conductor can overlap or not overlap a portion of the first conductor. 
       FIGS. 5 and 6  are top plan and cross-sectional views of a varactor  60  that can be used in filters constructed in accordance with the present invention. The varactor includes a substrate  62  and a first electrode  64  positioned on first portion  66  of a surface  68  of the substrate. A second electrode  70  is positioned on second portion  72  of the surface  68  of the substrate and separated from the first electrode to form a gap  74  therebetween. A tunable dielectric material  76  is positioned on the surface  68  of the substrate and in the gap between the first and second electrodes. A section  78  of the tunable dielectric material  76  extends along a surface  80  of the first electrode  64  opposite the substrate. The second electrode  70  includes a projection  82  that is positioned on a top surface  84  of the tunable dielectric layer opposite the substrate. In this embodiment of the invention projection  82  has a rectangular shape and extends along the top surface  84  such that it vertically overlaps a portion  86  of the first electrode. The second electrode can be referred to as a “T-type” electrode. A DC bias voltage, as illustrated by voltage source  88 , is applied to the electrodes  64  and  70  to control the dielectric constant of the tunable dielectric material lying between the electrodes  64  and  70 . An input  90  is provided for receiving an electrical signal and an output  92  is provided for delivering the signal. 
     The tunable dielectric layer  76  can be a thin or thick film. The capacitance of the varactor of  FIGS. 5 and 6  can be expressed as: 
       C   =       ɛ   o     ⁢     ɛ   r     ⁢     A   t           
 
where C is capacitance of the capacitor; ∈ o  is permittivity of free-space; ∈ r  is dielectric constant (permittivity) of the tunable film; A is area of the electrode  64  that is overlapped by electrode  70 ; and t is thickness of the tunable film in the overlapped section. An example of these parameters for a 1 pF capacitor is: ∈ r =200; A=170 μm 2 ; and t=0.3 μm. The horizontal distance (HD) along the surface of the substrate between the first and second electrodes is much greater than the thickness (t) of the dielectric film. Typically, thickness of tunable film is &lt;1 micrometer for thin films, and &lt;5 micrometers for thick film, and the horizontal distance is greater than 50 micrometers. Theoretically, if the horizontal distance is close to t, the capacitor will still work, but its capacitance would be slightly greater than that calculated from the above equation. However, from a processing technical view, it is difficult and not necessary to make the horizontal distance close to t. Therefore, the horizontal distance mainly depends on the processing used to fabricate the device, and is typically about &gt;50 micrometers. In practice, we choose HD&gt;10t.
 
     The substrate layer  62  may be comprised of MgO, alumina (Al 2 O 3), LaAlO   3 , sapphire, quartz, silicon, gallium arsenide, and other materials that are compatible with the various tunable films and the electrodes, as well as the processing used to produce the tunable films and the electrodes. 
     The bottom electrode  64  can be deposited on the surface of the substrate by electron-beam, sputtering, electroplating or other metal film deposition techniques. The bottom electrode partially covers the substrate surface, which is typically done by etching processing. The thickness of the bottom electrode in one preferred embodiment is about 2 μm. The bottom electrode should be compatible with the substrate and the tunable films, and should be able to withstand the film processing temperature. The bottom electrode may typically be comprised of platinum, platinum-rhodium, ruthenium oxide or other materials that are compatible with the substrate and tunable films, as well as with the film processing. Another film may be required between the substrate and bottom electrode as an adhesion layer, or buffer layer for some cases, for example platinum on silicon can use a layer of silicon oxide, titanium or titanium oxide as a buffer layer. 
     The thin or thick film of tunable dielectric material  76  is then deposited on the bottom electrode and the rest of the substrate surface by techniques such as metal-organic solution deposition (MOSD or simply MOD), metal-organic chemical vapor deposition (MOCVD), pulse laser deposition (PLD), sputtering, screen printing and so on. The thickness of the thin or thick film that lies above the bottom electrode is preferably in range of 0.2 μm to 4 μm. It is well known that the performance of a varactor depends on the quality of the tunable dielectric film. Therefore low loss and high tunability films should be selected to achieve high Q and high tuning of the varactor. In the varactors used in the preferred embodiment of the invention, these tunable dielectric films have dielectric constants of 2 to 1000, and tuning of greater than 20% with a loss tangent less than 0.005 at around 2 GHz. To achieve low capacitance, low dielectric constant (k) films should be selected. However, high k films usually shows high tunability. The typical k range is about 100 to 500. 
     In the preferred embodiment the tunable dielectric layer is preferably comprised of Barium-Strontium Titanate, Ba x Sr 1-x TiO 3  (BSTO), where x can range from zero to one, or BSTO-composite ceramics. Examples of such BSTO composites include, but are not limited to: BSTO—MgO, BSTO—MgAl 2 O 4 , BSTO—CaTiO 3 , BSTO—MgTiO 3 , BSTO—MgSrZrTiO 6 , and combinations thereof. Other tunable dielectric materials may be used partially or entirely in place of barium strontium titanate. An example is Ba x Ca 1-x TiO 3 , where x ranges from 0.2 to 0.8, and preferably from 0.4 to 0.6. Additional alternative tunable ferroelectrics include Pb x Zr 1-x TiO 3  (PZT) where x ranges from 0.05 to 0.4, lead lanthanum zirconium titanate (PLZT), lead titanate (PbTiO 3 ), barium calcium zirconium titanate (BaCaZrTiO 3 ), sodium nitrate (NaNO 3 ), KNbO 3 , LiNbO 3 , LiTaO 3 , PbNb 2 O 6 , PbTa 2 O 6 , KSr(NbO 3 ), and NaBa 2 (NbO 3 ) 5  and KH 2 PO 4 . 
     The second electrode  70  is formed by a conducting material deposited on the surface of the substrate and at least partially overlapping the tunable film, by using similar processing as set forth above for the bottom electrode. Metal etching processing can be used to achieve specific top electrode patterns. The etching processing may be dry or wet etching. The top electrode materials can be gold, silver, copper, platinum, ruthenium oxide or other conducting materials that are compatible with the tunable films. Similar to the bottom electrode, a buffer layer for the top electrode could be necessary, depending on electrode-tunable film system. Finally, a part of the tunable film should be etched away to expose the bottom electrode. 
     For a certain thickness and dielectric constant of the tunable dielectric film, the pattern and arrangement of the top electrode are key parameters in determining the capacitance of the varactor. In order to achieve low capacitance, the top electrode may have a small overlap (as shown in  FIGS. 5 and 6 ) or no overlap with the bottom electrode.  FIGS. 7 and 8  are top plan and cross-sectional views of a varactor  94 , that can be used in filters of the invention, having a T-type top electrode with no overlap electrode area. The structural elements of the varactor of  FIGS. 7 and 8  are similar to the varactor of  FIGS. 5 and 6 , except that the rectangular projection  96  on electrode  98  is smaller and does not overlap electrode  64 . Varactors with no electrode overlap area may need more tuning voltage than those in which the electrodes overlap. 
       FIGS. 9 and 10  are top plan and cross-sectional views of a varactor  100 , that can be used in filters of the invention, having a top electrode  102  with a trapezoid-type projection  106  and an overlapped electrode area  104 . The structural elements of the varactor of  FIGS. 9 and 10  are similar to the varactor of  FIGS. 5 and 6 , except that the projection  106  on electrode  102  has a trapezoidal shape. Since the projection on the T-type electrode of the varactor of  FIGS. 5 and 6  is relatively narrow, the trapezoid-type top electrode of the varactor of  FIGS. 9 and 10  is less likely to break, compared to the T-type pattern varactor.  FIGS. 11 and 12  are top plan and cross-sectional views of a varactor  108  having a trapezoid-type electrode  110  having a smaller projection  112  with no overlap area of electrodes to obtain lower capacitance. 
       FIGS. 13 and 14  are top plan and cross-sectional views of a varactor  114 , that can be used in filters of the invention, having triangle-type projection  116  on the top electrode  118  that overlaps a portion of the bottom electrode at region  120 . Using a triangle projection on the top electrode may make it easier to reduce the overlap area of electrodes.  FIGS. 15 and 16  are top plan and cross-sectional views of a varactor  122  having triangle-type projection  124  on the top electrode  126  that does not overlap the bottom electrode. 
     The invention uses voltage tunable thick film and thin film varactors that can be used in room temperature. Vertical structure dielectric varactors with specific electrode patterns and arrangements as described above are used to achieve low capacitance in the present invention. Variable overlap and no overlap structures of the bottom and top electrodes are designed to limit effective area of the vertical capacitor. Low loss and high tunability thin and thick films are used to improve performance of the varactors. Combined with the low loss and high tunability materials, the varactors have low capacitance, higher Q, high tuning, and low bias voltage. 
       FIG. 17  is an isometric view of a prior art ceramic filter  130  that can be modified to include tunable varactors in accordance with the present invention.  FIG. 18  is a longitudinal vertical cross sectional view of the filter of FIG.  17 . Filter  130  includes an input  132  and an output  134 , each coupled to a block  136  of ceramic material. The ceramic block includes a plurality of openings  138 ,  140 ,  142 ,  144 ,  146  and  148 , extending from its top surface to its bottom surface with each hole lined by a metal tube  150 ,  152 ,  154 ,  156 ,  158  and  160 . The dielectric block is covered with a conductive material  162  with the exception of portions near one end of each hole and near the first and second electrodes. Slots  164 ,  166 ,  168 ,  170  and  172  are cut into the sides of the conductive material and the ceramic block. Tabs  174  and  176  are used to connect the ceramic block to the input and output connectors. 
     To make the conventional filter tunable, a dielectric varactor is shunted on the top surface of each of the resonators, as shown in FIG.  19 . The detailed bias circuit for each dielectric varactor is similar to that for LC lumped element tunable filter as shown in FIG.  2 .  FIG. 19  is a top plan view of ceramic filter  178  with a schematically illustrated varactor constructed in accordance with the present invention. The filter  178  includes a metallic housing  180  that holds a ceramic block  182 . Holes  184 ,  186  and  188  are positioned in the ceramic block  182 . Metallic tubes  190 ,  192  and  194  line the holes. Dielectric varactors  196 ,  198  and  200  couple tubes  190 ,  192  and  194  respectively, to the housing. Projections  202 ,  204 ,  206  and  208  extend from the housing into the ceramic block. Tabs  210  and  212  are used to connect the input and output of the filter to an external circuit. 
       FIG. 20  is a schematic diagram of the filter of FIG.  19 . The filter is shown to include three resonant circuits  214 ,  216  and  218 . The resonant circuits are coupled by inductors L 4  and L 5 . Dielectric varactors C 12 , C 13  and C 14  are electrically connected in parallel with resonant circuits  214 ,  216  and  218 , respectively. Capacitor C 15  couples the input  220  to the first resonant circuit  214 . Capacitor C 16  couples the output  222  to the third resonant circuit  218 . Since the capacitance contributed by the dielectric varactors is a part of the capacitance in each resonator, tuning of varactor can tune the resonating frequency. 
     In order to more accurately control filter performance in tuning range, dielectric varactors may be added to the port couplings as well as resonator couplings to tune the couplings.  FIG. 21  is a top plan view of another ceramic filter  224  with schematically illustrated varactors constructed in accordance with the present invention. The filter of  FIG. 21  is similar to that of  FIG. 19 , with the addition of dielectric varactors  226  and  228 . Dielectric varactor  226  couples tube  190  to the input tab  210  and dielectric varactor  228  couples tube  194  to the output tab  212 . 
       FIG. 22  is a top plan view of another ceramic filter  230  with schematically illustrated varactors constructed in accordance with the present invention. The filter of  FIG. 22  is similar to that of  FIG. 21 , with the addition of dielectric varactors  232  and  234 . Dielectric varactor  232  couples tube  190  to the tube  192  and dielectric varactor  228  couples tube  192  to tube  194 . 
     This tunable ceramic tunable filter should have low insertion loss, compact size, and low cost. It should be noted that the ceramic filters of this invention are not limited to those shown in  FIGS. 19 ,  21  and  22 . Any fixed ceramic filters can be modified into tunable filters, as long as the dielectric varactors can be shunted between the resonating hole and its ground plane. 
       FIG. 23  is a schematic representation of a microstrip combline filter  236  constructed in accordance with the present invention. Filter  236  includes an input  238  and an output  240 . A plurality of resonators are formed by microstrips  242 ,  244 ,  246  and  248 . Each resonator is comprised of a microstrip line, a capacitor, and two short-circuited ends. Dielectric varactors  250 ,  252 ,  254  and  256  connect the microstrips to ground. The bias circuit for each varactor is not shown for clarity, but would be similar to that for LC lumped element tunable filter as shown in FIG.  2 . 
       FIGS. 24 ,  25 ,  26  and  27  are schematic representations of additional combline filters constructed in accordance with the present invention.  FIG. 24  is a top plan view of another ceramic filter  260  with schematically illustrated varactors constructed in accordance with the present invention. The filter of  FIG. 24  is similar to that of  FIG. 23 , with the addition of dielectric varactors  262  and  264 . Dielectric varactor  262  couples microstrip  242  to the input  238  and dielectric varactor  264  couples microstrip  248  to the output  240 . 
       FIG. 25  is a top plan view of another ceramic filter  266  with schematically illustrated varactors constructed in accordance with the present invention. The filter of  FIG. 25  is similar to that of  FIG. 24 , with the addition of dielectric varactors  268 ,  270  and  272 . Dielectric varactor  268  couples microstrip  242  to microstrip  244 , dielectric varactor  270  couples microstrip  244  to microstrip  242  and dielectric varactor  272  couples microstrip  246  to microstrip  242 . 
       FIG. 26  is a top plan view of another ceramic filter  274  with schematically illustrated varactors constructed in accordance with the present invention. Filer  274  is similar to that shown in  FIG. 23 , except for the use of transformer coupled input  276  and output  278 . 
       FIG. 27  is a top plan view of another ceramic filter  280  with schematically illustrated varactors constructed in accordance with the present invention. Filer  280  is similar to that shown in  FIG. 24 , except for the connection points for dielectric varactors  282  and  284 . 
     The port couplings can be tunable, as shown in  FIG. 24 , as well as resonator coupling (FIG.  25 ), to improve filter performance in tuning range. It should be also noted that the invention is not limited to tapped combline filters as shown in  FIG. 23 , but encompasses transformer, capacitive loaded, and others combline filters, shown in  FIGS. 24 ,  25 ,  26  and  27 . 
     It is an object of the present invention to provide relatively compact, high performance tunable filters for mobile and portable communication as well as other applications. Tunable filters with ceramic filters, combline filters, and LC-lumped element filters are disclosed as examples of the dielectric varactor applications. The dielectric varactors may be located in resonators and/or in couplings in the filters to make filter tunable and to optimize performance of the filter during tuning processing. 
     It should be noted that the lumped element filters are not limited to those discussed above. Some examples of other filter structures are illustrated in  FIGS. 28 and 29 . In the filter of  FIG. 28 , resonators  286 , and  290  are coupled to input  292  and output  294 . Resonator  286  includes the parallel connection of varactor  296  and inductor  298 . Resonator  288  includes the parallel connection of varactor  300  and inductor  302 . Resonator  290  includes the parallel connection of varactor  304  and inductor  306 . Resonators  286  and  288  are coupled to each other by a series circuit including inductor  308  and capacitor  310 . Resonators  288  and  290  are coupled to each other by a series circuit including inductor  312  and capacitor  314 . 
     The filter of  FIG. 29  is similar to that of  FIG. 28  except that the resonators  286  and  288  are coupled by a parallel connection of inductor  316  and capacitor  318 , and resonators  288  and  290  are coupled by a parallel connection of inductor  320  and capacitor  322 . In addition, resonator  286  is coupled to the input be capacitor  324  and resonator  290  is coupled to the output by capacitor  326 . In  FIGS. 28 and 29 , some or all of the capacitors can be replaced with dielectric varactors in accordance with the invention. 
     RF microwave filters typically include multiple resonators with specific resonating frequencies. These adjacent resonators are coupled to each other by reactive coupling. In addition, the RF signal input and output are coupled to the first and last resonator with a specific port impedance. The resonator is electrically equivalent to an LC circuit. Either a change of capacitance or a change in inductance of the resonator can shift the resonating frequency. 
     Accordingly, the present invention, by utilizing the unique application of high Q tunable dielectric varactor capacitors, provides high performance electronically tunable filters. Several tunable filter structures have been described as illustrative embodiments of the present invention. However, it will be apparent to those skilled in the art that these examples can be modified without departing from the scope of the invention, which is defined by the following claims.