Abstract:
A tuning circuit for use in an electrical circuit network is disclosed. The tuning circuit includes a bridge circuit. The bridge circuit comprises a first adjustable capacitance grouping, a second adjustable capacitance grouping and a third adjustable capacitance grouping. Each adjustable capacitance grouping includes at least one tunable capacitor and a bias port. In one embodiment, the tunable capacitor is comprised of a tunable thin-film barium strontium titanate (“BST”) capacitor. A first lead and a second lead couple to, or extend from, the bridge circuit. The leads are configured to couple with a coupling element and a shunt element of the electrical circuit network so that when the leads are grounded, one adjustable capacitance group is grounded and the remaining adjustable capacitance group couple with a bias voltage to appropriately tune the electrical circuit to achieve desired electrical characteristics.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims a priority benefit under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 60/464,675, filed Apr. 21, 2003, the contents of which are herein incorporated by reference.  
         [0002]    This application is related to U.S. patent application Ser. No. 10/144,185, “Voltage-Variable Capacitor with Increased Current Conducting Perimeter,” by Robert A. York, filed May 10, 2002, now U.S. Pat. No. 6,683,341, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 60/337,364, “Ferroelectric Varactor Design,” by Robert A. York, filed Dec. 5, 2001; and is related to U.S. patent application Ser. No. 10/211,765, “Circuit Configuration for DC-Biased Capacitors,” by Robert A. York, filed Aug. 1, 2002, now U.S. Pat. No. 6,674,321, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 60/335,191, “Thin-Film Ferroelectric Tuning Circuit,” by Robert A. York, filed Oct. 31, 2001. The appropriate subject matter of all of the foregoing is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    This invention generally relates to tuning an electrical circuit network, and more particularly, to a tuning circuit for the electrical circuit network.  
           [0005]    2. Description of the Related Art  
           [0006]    Circuit technology for use as a filter or other tuning mechanism for an electrical circuit is known. A basic filter for an electrical circuit is a well-known resistive-capacitive (“RC”) circuit, inductive-capacitive (“LC”) circuit or resistive-inductive-capacitive (“RLC”) circuit.  
           [0007]    Electrical filters and matching networks are frequently constructed as a “ladder” network. The ladder network consists of a number of shunt circuit elements that are electrically interconnected using series coupling elements. The shunt elements may be collections of reactive lumped elements such as resistors, capacitors, and/or inductors, or they may be distributed components such as small sections of transmission lines. The coupling elements may similarly be comprised of reactive lumped elements or transmission-line sections. The shunt elements and coupling elements may also be resonant or non-resonant, depending on the specific filter implementation and the desired electrical characteristics. For example, many bandpass filter designs use shunted short-circuited transmission-line resonators that are capacitively coupled together.  
           [0008]    A problem with a conventional ladder network is that it is often sensitive to slight variations in any of its components values. For example, small deviations in any element value as a result of mechanical tolerances in the manufacturing process can degrade the electrical characteristics of the entire network. Another problem with a conventional ladder network is that it is often inflexible. Specifically, tuning the network is cumbersome and time-consuming because any changes to any element may result in changing the values of other elements to achieve the desired electrical characteristics.  
           [0009]    Conventional solutions for tuning circuits may include use of a voltage-variable capacitor (“varactor”) in the ladder network. A varactor adds flexibility for tuning a circuit because the capacitance can be tuned by changing a bias voltage across the capacitor. However, conventional varactors made from gallium arsenide (“GaAs”) have relatively higher radio frequency (“RF”) losses, poor power handling, poor intermodulation distortion (“IMD”), and are expensive to produce.  
           [0010]    Therefore, there is a need for a tunable circuit that allows for adjusting circuit characteristics on an ad hoc basis and that allows for adjusting electrical characteristics so that a single network can be used for multiple functions. Moreover, from both manufacturing and operational standpoints, there is a need for a filter structure with adjustable component values that also has strong power handling capabilities and is relatively inexpensive to produce.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides a tuning circuit for adjusting electrical characteristics in an electrical circuit network having at least one coupling element and at least one shunt element.  
           [0012]    In one embodiment, a tuning circuit includes a bridge circuit configured to couple an electrical circuit, for example, a ladder network having a coupling element coupled with a shunt element. The bridge circuit includes a first adjustable capacitance grouping, a second adjustable capacitance grouping and a third adjustable capacitance grouping. Each adjustable capacitance grouping includes at least one tunable capacitor and a bias port. Connected with, or extended from, the bridge circuit are a first lead (or terminal or node) and a second lead. The leads are configured to couple the bridge circuit with an electrical circuit.  
           [0013]    When one of the adjustable capacitance groups is at a direct current (“DC”) ground potential, e.g., when a shunt element it couples with provides a DC path to ground, the other two adjustable capacitance groups may be isolated. This beneficially allows for application of a direct current (“DC”) control voltage to the bias port of the respective other adjustable capacitance groups. In turn, this allows for appropriately adjusting the tuning capacitors in these adjustable capacitance groups to achieve the desired electrical characteristics.  
           [0014]    In an alternative embodiment, a tunable capacitive bridge is configured to couple a ladder network. The ladder network includes a coupling element having a first side and a second side. The first side of the coupling element couples with a first shunt element and the second side of the coupling element couples with a second shunt element.  
           [0015]    The tunable capacitive bridge that couples to this ladder network includes a first tunable capacitor, a second tunable capacitor, and a third tunable capacitor. The first tunable capacitor couples in parallel with the coupling element. In addition, a first end of the first tunable capacitor couples the first shunt element and a second end of the first tunable capacitor couples the second shunt element. The second tunable capacitor and the third tunable capacitor couple in parallel with the first tunable capacitor. A first end of the second tunable capacitor couples the first end of the first tunable capacitor and the first shunt element. A first end of the third tunable capacitor couples with the second end of the first tunable capacitor and the second shunt element.  
           [0016]    In one embodiment, one or more of the tunable capacitors is a tunable thin-film barium strontium titanate (“BST”) capacitor. The tunable thin-film BST capacitor may be a passive integrated circuit device that is capable of handling large voltage swings. Hence, the tunable thin-film BST capacitor configured within a tunable capacitive bridge in accordance with the present invention beneficially provides a mechanism for easily obtaining desired circuit characteristics. For example, tuning of electrical circuits such as ladder networks is accomplished by appropriately adjusting either the tunable capacitor in parallel with the coupling element or the tunable capacitors in parallel with the shunt elements. Thus, the electrical circuit is tuned without having to conduct undue experimentation involving identification and replacement of specific shunt elements to obtain desired characteristics for the electrical circuit.  
           [0017]    The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawing, in which:  
         [0019]    FIG. (“FIG.”)  1  is a circuit diagram of a conventional ladder network electrical circuit.  
         [0020]    [0020]FIG. 2 is a circuit diagram of a tunable capacitive bridge coupled with a ladder network electrical circuit in accordance with one embodiment of the present invention.  
         [0021]    [0021]FIG. 3 is a circuit diagram of a tuning circuit coupled with a ladder network electrical circuit in accordance with one embodiment of the present invention.  
         [0022]    [0022]FIG. 4 is a circuit diagram of a tuning circuit comprised of tunable capacitors in accordance with one embodiment of the present invention.  
         [0023]    [0023]FIG. 5 is a circuit diagram of a tuning circuit comprised of bulk capacitors and tuning capacitors in accordance with one embodiment of the present invention.  
         [0024]    [0024]FIG. 6 is an illustration of a tuning circuit configured as an integrated passive device in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    FIG. (“FIG.”)  1  is a circuit diagram of a conventional ladder network electrical circuit  100 . The ladder network includes a node for an input (e.g., Vi), an output (e.g., Vo or ground), one or more coupling elements  110  (e.g.,  110   a,    110   a,    110   b, . . .    110   m,    110   n ), and one or more shunt elements  120  (e.g.,  120   a,    120   b . . .    120   n ). Generally, the conventional ladder network includes the shunt circuit elements  120  configured to electrically interconnect through the series coupling elements  110 .  
         [0026]    As an example, in the conventional ladder network  100  illustrated in FIG. 1, the input, e.g., Vi, couples with an input coupling element  110   m.  The input coupling element  110   m  couples with a first shunt element  120   a  and a first coupling element  110   a.  The first coupling element  110   a  couples with the first shunt element  120   a,  a second shunt element  120   b,  and a second coupling element  110   b.  The second coupling element  110   b  couples with the second shunt element  120   b,  a third shunt element, e.g.,  120   n,  and an output coupling element  110   n.  The output coupling element  110   n  couples with the third shunt element, e.g.,  120   n,  and the output, e.g., Vo or ground. Each of the shunt elements  120  couples with ground.  
         [0027]    It is noted that each coupling element  110  may be reactive lumped elements, for example, resistors, capacitors, and/or inductors. They may also be distributed components, for example, small sections of transmission lines. Similarly, the shunt elements  120  may also be reactive lumped elements or small section transmission lines. Each coupling element  110  and shunt element  120  may be either resonant or non-resonant. Use of resonant or non-resonant elements depends on a particular electrical circuit, e.g., filter implementation and desired electrical characteristics sought. For example, a many bandpass filter designs use shunted short-circuited transmission line resonators that capacitively couple together.  
         [0028]    As previously mentioned, electrical circuits constructed like the conventional ladder network  100  are sensitive to slight variations in element values. Small deviations in the element values, for example, as a result of mechanical tolerances in a manufacturing process, can degrade the electrical characteristics of the circuit. Hence, it may be advantageous to adjust the electrical characteristics so that a single network can be used for multiple functions, for example, tuning after manufacturing or during operation.  
         [0029]    [0029]FIG. 2 is a circuit diagram of a tunable capacitive bridge  210  coupled with an electrical circuit, e.g., a ladder network, in accordance with one embodiment of the present invention. In the illustrated embodiment, the tunable capacitive bridge  210  couples with a portion of the conventional ladder network  100  previously described. Specifically, the first end (e.g., terminal, node, or lead) of the tunable capacitive bridge (or “pi network”)  210  couples with a first end of the first coupling element  110   a  and the first shunt element  120   a,  which also couples with ground. A second end (e.g., a terminal, node or lead) of the tunable capacitive bridge  210  couples with a second end of the first coupling element  110   a  and the second shunt element  120   b,  which also couples with ground.  
         [0030]    The tunable capacitive bridge  210  includes a first tunable variable capacitor (C 1 )  215 , a second tunable variable capacitor (C 2 )  220 , and a third tunable variable capacitor (C 3 )  225 . Each tunable variable capacitor has a capacitance that can be tuned with a direct current (“DC”) bias that is applied across each tunable variable capacitor individually. It is noted that a biasing circuit (not shown in FIG. 2) for use with the tunable capacitive bridge  210  is a conventional biasing circuit.  
         [0031]    As an example, the first tunable variable capacitor C 1  provides capacitance in parallel with the first coupling element  110   a.  By voltage-tuning capacitor C 1 , the total impedance of the coupling element  110   a  is proportionally changed. The second tunable capacitor C 2  and the third tunable capacitor C 3  provide an additional capacitance in parallel with the first shunt filter element  120   a  and the second shunt filter element  120   b.  By voltage-tuning the tunable variable capacitors C 2  and C 3 , the total impedance of the shunt elements  120   a,    120   b  is proportionally changed.  
         [0032]    It is noted that the tunable capacitive bridge  210  of the present invention can be configured for tuning using the formulaic principles. Further, it is also noted that each tunable variable capacitor may be a varactor having an associated conventional biasing circuit (not shown). A varactor is a voltage-variable capacitor. This means that the capacitance of the varactor varies according to a DC bias voltage that is applied across that varactor.  
         [0033]    Referring to FIG. 3, illustrated is a circuit diagram of a first embodiment of a tuning circuit  310  coupled with an electrical circuit  305  in accordance with one embodiment of the present invention. The electrical circuit  305  couples with the first embodiment of the tuning circuit  310  at a node  1  and a node  2 . The electrical circuit  305  includes a coupling element, e.g., the first coupling element  110   a  previously described with respect to FIG. 1, coupled on each end (or side) with a shunt element, e.g., the first shunt element  120   a  and the second shunt element  120   b  previously described with respect to FIG. 1. In this exemplary embodiment, the first and the second shunt elements  120   a,    120   b  couple in parallel with each other and the coupling element  110 . The coupling element  110  also couples the first embodiment of the tuning circuit  310  in parallel.  
         [0034]    The first embodiment of the tuning circuit  310  in FIG. 3 includes three reactance groups. Each reactance group includes reactance elements that may include a capacitor, an inductor and/or a resistor. FIG. 3 illustrates each reactance group as having two reactance elements, as is described below. Preferably, at least one reactance element is a variable reactance element. Moreover, although there are two reactance elements in each reactance group, the number of reactance elements can be varied to achieve desired electrical characteristics for a particular electrical circuit.  
         [0035]    Turning now to each reactance group, the first reactance group includes elements between nodes (terminals or ports)  1  and  2 . Specifically, this includes a first reactance element  315   a  and a second reactance element  315   b.  A first end of the first reactance element  315   a  couples with node  1  and a second end of the first reactance element  315   a  couples a first biasing circuit. A first end of the second reactance element  315   b  couples with node  2  and a second end of the second reactance element  315   b  couples with the first biasing circuit. The first biasing circuit includes a bias resistor, R b5 , and node  5 . A biasing voltage, V b5 , can be applied to node  5  when nodes  1  and  2  provide a DC path to ground.  
         [0036]    The second reactance group includes elements between nodes  1  and  3 . Specifically, this includes a third reactance element  320   a  and a fourth reactance element  320   b.  A first end of the third reactance element  320   a  couples with node  1  and a second end of the third reactance element  320   a  couples a second biasing circuit. A first end of the fourth reactance element  320   b  couples with node  3  and a second end of the fourth reactance element  320   b  couples with the second biasing circuit. The second biasing circuit includes a bias resistor, R b4 , and node  4 . A biasing voltage, V b4 , can be applied to node  4  when nodes  1  and  3  provide a DC path to ground.  
         [0037]    The third reactance group includes elements between nodes  2  and  3 . Specifically, this includes a fifth reactance element  325   a  and a sixth reactance element  325   b.  A first end of the fifth reactance element  325   a  couples with node  2  and a second end of the fifth reactance element  325   a  couples a third biasing circuit. A first end of the sixth reactance element  325   b  couples with node  3  and a second end of the sixth reactance element  325   b  couples with the third biasing circuit. The third biasing circuit includes a bias resistor, R b6 , and node  6 . A biasing voltage, V b6 , can be applied to node  6  when nodes  2  and  3  provide a DC path to ground.  
         [0038]    An advantage of the first embodiment of the tuning circuit  310  is that it allows for generation of higher power radio frequency (“RF”) fields by keeping particular biasing voltages low and providing a DC path to ground for the appropriate associated nodes. Further, the first embodiment of the tuning circuit  310  also provides circuit tuning functionality for use in small form factor integrated passive devices as will be further described below.  
         [0039]    [0039]FIG. 4 is a circuit diagram of a second embodiment of a tuning circuit  410  comprised of a plurality of tunable (or variable) capacitors in accordance with one embodiment of the present invention. The second embodiment of the tuning circuit  410  includes three tunable capacitance groups distributed between nodes  1  and  2 ,  1  and  3 , and  2  and  3 . The first tunable capacitance group, e.g., between nodes  1  and  2 , includes a first tunable capacitor  415   a,  a second tunable capacitor  415   b,  and a biasing circuit of node  5 . The biasing circuit of node  5  includes a biasing resistor, R b5 , and a node for application of a biasing voltage, V b5 . The first tunable capacitor  415   a  couples with node  1  and the biasing circuit of node  5 . The second tunable capacitor  415   b  couples with node  2  and the biasing circuit of node  5 .  
         [0040]    Likewise, the second tunable capacitance group, e.g., between nodes  1  and  3 , includes a third tunable capacitor  420   a,  a fourth tunable capacitor  420   b,  and a biasing circuit of node  4 . The biasing circuit of node  4  includes a biasing resistor, R b4 , and a node for application of a biasing voltage, V b4 . The third tunable capacitor  420   a  couples with node  1  and the biasing circuit of node  4 . The second tunable capacitor  420   b  couples with node  3  and the biasing circuit of node  4 .  
         [0041]    Similarly, the third tunable capacitance group, e.g., between nodes  2  and  3 , includes a fifth tunable capacitor  425   a,  a sixth tunable capacitor  425   b,  and a biasing circuit of node  6 . The biasing circuit of node  6  includes a biasing resistor, R b6 , and a node to application of a biasing voltage, V b6 . The fifth tunable capacitor  425   a  couples with node  2  and the biasing circuit of node  6 . The second tunable capacitor  425   b  couples with node  3  and the biasing circuit of node  6 .  
         [0042]    Generally, the second embodiment of the tuning circuit  410  couples with an electrical circuit (e.g.,  305 ) at, for example, nodes  1  and  2 . In one operational embodiment of the third embodiment of the tuning circuit  410 , nodes  1  and  2  are at a DC ground potential when the shunt elements, e.g.,  120   a,    120   b,  provide a DC path to ground. In this embodiment, DC control voltages applied to nodes  4 ,  5 , and  6  can be isolated from an electrical circuit, such as an RF circuit. Examples of RF circuits with which the second embodiment of the tuning circuit  410  may couple with are disclosed in U.S. patent application Ser. No. 10/211,765, titled “Circuit Configuration for DC-Biased Capacitors” by Robert A. York and assigned to the same assignee as the present application, the relevant portions of which are herein incorporated by reference.  
         [0043]    In addition, it is noted that in one embodiment, the second embodiment of the tuning circuit  410 , the tunable capacitors of each tunable capacitance group have a one to one relationship. For example, the first tunable capacitor  415   a  and the second tunable capacitor  415   b  of the first capacitance group, the third tunable capacitor  420   a  and the fourth tunable capacitor  420   b  of the second capacitance group, and the fifth tunable capacitor  425   a  and the sixth tunable capacitor  425   b  of the third capacitance group each may have a value of 2×C x  (or 2C x ). This would allow for the value of the capacitance for a particular capacitance group to be 2C x +2C x =C x . It is noted that the value C x  would be the same within each tunable capacitance group, but may differ between tunable capacitance groups. However, it is also noted that other capacitance values may apply within each tunable capacitor, as well as between tunable capacitance groups, to achieve the desired electrical characteristics for a particular circuit.  
         [0044]    The second embodiment of the tuning circuit  410  is an attractive approach because it conveniently isolates the DC control voltages applied at nodes  4 ,  5 , and  6  through respective bias resistors, R b4 , R b5 , and R b6 , from electrical circuits, e.g., an RF circuit. Moreover, the series combination of tunable capacitors allows for greater power handling. Power handling with regard to tunable capacitors is further described-in U.S. patent application Ser. No. 10/211,765, titled “Circuit Configuration for DC-Biased Capacitors” by Robert A. York and assigned to the same assignee as the present application, the relevant portions of which are herein incorporated by reference.  
         [0045]    [0045]FIG. 5 is a circuit diagram of a third embodiment of a tuning circuit  510  that includes bulk capacitors and tuning capacitors in accordance with one embodiment of the present invention. As with the second embodiment of the tuning circuit  410 , the third embodiment of the tuning circuit  510  includes three tunable capacitance groups distributed between nodes  1  and  2 ,  1  and  3 , and  2  and  3 .  
         [0046]    Each tunable capacitance group in the third embodiment of the tuning circuit  510  includes at least one tunable capacitor and a biasing circuit, similar to each capacitance group in the second embodiment of the tuning circuit  410 . However, in the third embodiment of the tuning circuit  510 , a fixed value capacitor (e.g., a bulk capacitor) replaces at least one tunable capacitor in each capacitance group. For example, fixed value capacitors, e.g.,  515   a,    520   a,    525   a,  may be used in place of the first  415   a,  third  420   a,  and fifth  425   a  tunable capacitors described with regard to the second embodiment of the tuning circuit  410 .  
         [0047]    It is noted that in one embodiment, each fixed value capacitor  515   a,    520   a,    525   a  may be the same value, e.g., C x =C b . Alternatively, at least two of the fixed value capacitors may also differ in value in order to achieve desired electrical characteristics. The fixed value capacitors may be gallium arsenide capacitors that provide an alternative current (“AC”) short condition when the value of the capacitor, C b  is large. In addition, reducing the value of the capacitor, C b , provides a greater Q factor. It is also noted that the third embodiment of the tuning circuit  510  allows for a circuit having greater tunability along with a higher Q factor.  
         [0048]    Turning now to FIG. 6, it provides an illustration of the second embodiment of the tuning circuit  410  as it is configured as an integrated passive device  610  in accordance with one embodiment of the present invention. It is noted that the principles illustrated and described with regard to the second embodiment of the tuning circuit  410  in FIG. 6 also apply to the third embodiment of the tuning circuit  510 , with the one tunable capacitor in each capacitance group being replaced with a fixed value capacitor as previously described. An advantage of the tuning circuits (including bridge circuits) as the integrated passive device  610  is that they allow for implementation in a small form factor. Hence, it offers benefits for small form factor devices and applications, for example, an RF circuit in a handheld communication device.  
         [0049]    Further, in one embodiment of the present invention the integrated passive device  610  may be configured so that the capacitors may be barium strontium titanate (“BST”) capacitors. Moreover the BST capacitor may be thin-film BST capacitors. In some applications, the BST capacitor has a fixed capacitance. One advantage of using BST as the dielectric material is that BST thin-film materials have a high intrinsic capacitance density. As a result, compared with other technologies, BST capacitors of a given capacitance can be realized with small active regions.  
         [0050]    In other applications, the capacitance of the BST capacitor can be voltage-variable (e.g., a BST tunable (or variable) capacitor or BST varactor). The BST dielectric region can be voltage-variable because the BST dielectric material has a field-dependent electrical permittivity. Electrical permittivity, or tunability, is an electric field dependent property. Therefore, using BST thin films in a parallel plate capacitor configuration produces tunabilities with low control voltages (e.g., &lt;20 volts) by maximizing field concentrations in the field.  
         [0051]    Thus, changing the voltage applied across the BST dielectric region can change the capacitance of the tunable capacitor. For example, changing the voltage applied across the two electrodes, e.g., nodes  1  and  2 , changes the electric field within the BST dielectric region of the tunable capacitor, e.g.,  415   b.  This, in turn, changes the dielectric constant of the BST material, thus changing the capacitance of the tunable capacitor, e.g.,  415   b.  The BST thin film dielectric region preferably exhibits a field-dependent permittivity in a (non-hysteretic) paraelectric state over a useful temperature range (e.g., −30C to +90C).  
         [0052]    Examples of suitable BST thin film materials for dielectric region include barium titanate, strontium titanate, and composites of the two. For convenience, the term “BST” refers to all of these materials even though, strictly speaking, barium titanate does not contain strontium and strontium titanate does not contain barium. The BST materials can also include small concentrations of one or more dopants to modify certain properties.  
         [0053]    To reduce costs, inexpensive insulating substrates are usually preferred, including but not limited to high-resistivity silicon (HR Si), crystalline sapphire (Al 2 O 3 ), aluminum nitride (AlN), quartz and glass. These substrates are polished for low surface roughness for compatibility with growth of smooth ferroelectric films with high breakdown fields. This approach results in low-cost, small size, reliable components which are suitable for mass production and for integration with other circuit elements.  
         [0054]    BST thin-film capacitors (including varactors) can be used in a variety of applications such as radio-frequency (RF) or wireless electronics, voltage-controlled oscillators, impedance matching networks, tunable filters, and numerous other applications. As one example application, BST varactors can be used in RC tuning circuits for RF applications, such as mobile phones. While specific numbers will vary by application, 2:1 capacitance variations and capacitances in the range of 0.01 pF to 10 nF are not unheard of. Similarly, DC control voltages may be in the range of −100 to +100 volts, depending on the BST film thickness and the specific application. The tunable capacitors (or varactors) preferably are operated at voltages that are less than half their intrinsic breakdown voltage.  
         [0055]    A thin-film capacitor is attractive because it can be easily integrated alongside other active and passive electrical components on many different host substrates, including semiconductors (such as silicon, gallium arsenide, silicon carbide, gallium nitride, etc.) and insulators (such as glass, quartz, sapphire, etc.). However, processing steps for BST thin films can require conditions that limit the choice of materials for the electrodes and other structures.  
         [0056]    As an example, the growth of BST thin films can require high temperature processing that limits electrode materials (at least for the materials that are present when the high temperature processing occurs) to those that have high melting points and that also do not oxidize easily. Examples of such materials include platinum and other refractory metals such as palladium and tungsten, but generally exclude commonly used conductors such as gold, copper and aluminum. Other suitable materials can include noble metals and conductive oxides. Unfortunately, materials such as platinum typically have higher resistivity and can also be quite expensive.  
         [0057]    In some cases, the electrode can be shaped to reduce resistance. For example, an electrode that is narrow in the active region can open up into a wide area that has significantly less resistance compared to the narrow portion. Alternately, the electrodes can contact other layers that have better conductivity, for example thick gold layers that are formed after the BST processing steps are completed. Generally speaking, the fabrication process for BST capacitors, including even the order of processing steps, can affect the overall design, cost and performance of the capacitor.  
         [0058]    Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs for a tuning circuit, including a tuning circuit that includes a tunable thin-film BST capacitor, in accordance with the disclosed principles of the present invention. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.