Abstract:
An impedance matching circuit includes a conductor line having an input port and an output port, a ground conductor, a tunable dielectric material positioned between a first section of the conductor line and the ground conductor, a non-tunable dielectric material positioned between a second section of the conductor line and the ground conductor, and means for applying a DC voltage between the conductor line and the ground conductor. The impedance matching circuit may alternatively include a first planar ground conductor, a second planar ground conductor, a strip conductor having an input port and an output port, and positioned between the first and second planar ground conductors to define first and second gaps, the first gap being positioned between the strip conductor and the first planar ground conductor and the second gap being positioned between the strip conductor and the second planar ground conductor. A non-tunable dielectric material supports the first and second planar ground conductors and the strip conductor in the same plane. A connection is provided for applying a DC voltage between the strip conductor and the first and second planar ground conductors. A plurality of tunable dielectric layer sections are positioned between the strip conductor and the first and second planar ground conductors so as to bridge the gaps between the said first and second planar ground conductors and the strip conductor at a plurality of locations, leaving non-bridged sections in between, defining a plurality of alternating bridged and non-bridged co-planar waveguide sections.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/219,500, filed Jul. 20, 2000. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of tunable microwave devices. More specifically, the invention relates to impedance matching circuits that utilize a bias voltage to alter the permittivity of a tunable dielectric material. 
     BACKGROUND OF THE INVENTION 
     Microwave devices typically include a plurality of components that may have different characteristic impedances. In order to propagate the microwave signal through the device with minimal loss, the impedances of the various components are matched to the characteristic impedance of the input and output signal. By transitioning the impedances so that an input transmission line is matched, most of the available power from the input is delivered to the device. Historically, impedance matching techniques have treated the matching of components with constant characteristic impedances to a constant characteristic impedance of the input line, e.g. to 50 Ω. Multi-stage matching circuits have been utilized to obtain minimal reflection loss over a specified frequency range of operation of a device. Numerous techniques, such as the use of radial stubs, quarter wave transformers, and multistage matching circuits with specific distributions, such as Binomial or Tchebychef, etc., have been developed in order to achieve maximum power transfer from the input to the device. 
     However, the characteristic impedance of the tunable components in tunable microwave devices is not a constant value. The characteristic impedance of the tunable component varies over the operating range of the device from a minimum to a maximum impedance value. In tunable dielectric devices, a bias voltage applied to tunable dielectric material provides the ability to alter the dielectric constant. The change in the dielectric constant provides a variation in the electrical path length of a microwave signal. As the electrical properties of the tunable dielectric material are varied, the characteristic impedance is also affected. 
     In practice, a single characteristic impedance within the tunable components minimum/maximum impedance range is selected. This single impedance value is matched using one of the state of the art impedance matching techniques. However, as the tunable microwave device is operated, the impedance of the tunable component varies from the matched impedance and a degradation in the impedance match occurs. 
     Prior tunable dielectric microwave transmission lines have utilized tuning stubs and quarter wave matching transformers to transition the impedance between the input and output. The technique is best for matching a fixed impedance mismatch. U.S. Pat. No. 5,479,139 by Koscica et al. discloses quarter wavelength transformers using non-tunable dielectric material for the purpose of impedance matching to a ferroelectric phase shifter device. Similar impedance matching configurations using non-tunable dielectric substrate of background interest are shown in U.S. Pat. No. 5,561,407, U.S. Pat. No. 5,334,958, and U.S. Pat. No. 5,212,463. The disadvantage of the above technique is that the impedance match is optimal at one selective tuning point of the device and degrades as the device is tuned through its range. Hence, the reflection loss due to impedance match increases when the device is tuned away from the matched point. 
     Another impedance matching approach for tunable devices is presented in U.S. Pat. No. 5,307,033 granted to Koscica et al. That patent discloses the use of spacing of a half wavelength between elements or matching networks for the purpose of impedance matching. 
     Still another approach utilizes quarter wavelength transformers on tunable dielectric material as disclosed in U.S. Pat. No. 5,032,805, granted to Elmer et al. Other impedance matching configurations are shown in U.S. Pat. Nos. 6,029,075; 5,679,624; 5,496,795; and 5,451,567. Since it is also desirable to reduce the insertion loss of the matching network, a disadvantage of the above approach is that the quarter wavelength transformer on tunable dielectric material increases the insertion loss. 
     The disclosures of all of the above-mentioned patents are expressly incorporated by reference. 
     It would be desirable to minimize the impedance mismatch in tunable microwave device applications. There is a need for a technique for improving impedance matching for tunable microwave components that achieves minimal reflection and insertion losses throughout the range of operation of tunable devices. 
     SUMMARY OF THE INVENTION 
     This invention provides an impedance matching circuit comprising a conductive line having an input port and an output port, a ground conductor, a tunable dielectric material positioned between a first section of the conductive line and the ground conductor, a non-tunable dielectric material positioned between a second section of the conductor line and the ground conductor, and means for applying a DC voltage between the conductive line and the ground conductor. 
     The invention further encompasses an impedance matching circuit comprising a first ground conductor, a second ground conductor, a strip conductor having an input port and an output port. The strip conductor is positioned between the first and second ground conductors and to define first and second gaps, the first gap being positioned between the strip conductor and the first ground conductor and the second gap being positioned between the strip conductor and the second ground conductor. A non-tunable dielectric material supports the first and second ground conductors and the strip conductor in a plane. A connection point is provided for applying a DC voltage between the strip conductor and the first and second ground conductors. A plurality of tunable dielectric layer sections are positioned between the strip conductor and the first and second ground conductors so as to bridge the gaps between the first and second ground conductors and the strip conductor at a plurality of locations, leaving non-bridged sections in between, defining a plurality of alternating bridged and non-bridged co-planar waveguide sections. 
     The matching circuits form tunable impedance transformers that are able to match a constant microwave source impedance connected at the input port to a varying load impedance connected at the output port, thereby reducing signal reflections between the microwave source and a variable load impedance. 
     This invention provides an impedance matching circuit capable of matching a range of impedance values to a tunable microwave device in order to reduce reflections from impedance mismatch during tuning of the microwave device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a first embodiment of the auto adjusting matching network of this invention in the form of the microstrip; 
     FIG. 2 is a cross-sectional view of the embodiment of FIG. 1 taken along line  2 — 2 , showing a microstrip line geometry; 
     FIG. 3 is a cross-sectional view of the embodiment of FIG. 1 taken along line  3 — 3 , showing a microstrip line geometry; 
     FIG. 4 is a plan view of a second embodiment of the auto adjusting matching network of this invention in the form of the stripline; 
     FIG. 5 is a cross-sectional view of the embodiment of FIG. 4 taken along line  5 — 5 , showing a stripline geometry; 
     FIG. 6 is a cross-sectional view of the embodiment of FIG. 4 taken along line  6 — 6 , showing a stripline geometry; 
     FIG. 7 is a cross-sectional view of a third embodiment for the auto adjusting matching network of this invention based on a coaxial geometry; 
     FIG. 8 is a cross-sectional view of the embodiment of FIG. 7 taken along line  8 — 8 , showing the coaxial transmission line geometry; 
     FIG. 9 is a plan view of another embodiment for the auto adjusting matching network of this invention including multiple partial stages on tunable material; 
     FIG. 10 is a plan view of another embodiment for the auto adjusting matching network of this invention based on a slotline or finline geometry, and including multiple partial stages; 
     FIG. 11 is a cross-sectional view of the embodiment of FIG. 10 taken along line  11 — 11 , showing a slotline geometry; 
     FIG. 12 is a plan view of another embodiment for the auto adjusting matching network of this invention based on a co-planar waveguide geometry, and including multiple partial stages; 
     FIG. 13 is a cross-sectional view of the embodiment of FIG. 12 taken along line  13 — 13 , showing a coplanar waveguide geometry; and 
     FIG. 14 is a block diagram showing a matching network of this invention coupled to a tunable dielectric device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments described herein are each designed for use within a certain arbitrary frequency range. For this reason all references to a “wavelength” will refer to the center frequency of the design. 
     Referring to the drawings, FIG. 1 is a plan view of a first embodiment of an auto adjusting matching network of this invention in the form of the microstrip circuit  10 . FIG. 2 is a cross-sectional view of FIG. 1 taken along line  2 — 2 , showing the microstrip line geometry. FIG. 3 is a cross-sectional view of FIG. 1 taken along line  3 — 3 . 
     The device has two ports  12  and  14  for input and output of a guided electromagnetic wave. It includes a multi-stage microstrip line  16 , having sections  18  and  20  of various widths and lengths, deposited on a non-tunable dielectric substrate  22 , which in turn is supported by ground plane  24 ; and a microstrip line section  26  deposited on a voltage tunable dielectric substrate  28  which in turn is supported by ground plane  30 . A biasing electrode  32  in the form of a high impedance microstrip line is connected to microstrip section  20 . 
     The biasing electrode  32  serves as a means for connecting an external variable DC bias voltage supply  34  to the auto-adjusting impedance matching circuit. The connection of the biasing electrode  32  to the circuit is not limited to microstrip section  20 , but may be made to any other part of the circuit that is electrically connected to microstrip line section  26 . Ground planes  24  and  30  are electrically connected to each other. While ground planes  24  and  30  are shown as separate elements, it should be understood that they may alternatively be constructed as a single ground plane. 
     The microstrip line section  26 , which comprises a conducting strip, is directly supported by a dielectric layer  28 , which is the voltage tunable layer. A ground plane  30  supports the dielectric layer  28 . The microstrip line  26  is less than a quarter wavelength long and forms an approximately quarter wavelength long transformer when joined to section  20  of the matching network on the non-tunable dielectric substrate  22 . 
     The non-tunable stages  36  and  38  of the matching network  10  form a multistage matching circuit  40  directly supported by the non-tunable dielectric layer  22 . The multi-stage matching circuit  40  can be any number of stages of varying widths and lengths, not limited to quarter wavelength sections. If the non-tunable and tunable substrates  22  and  28  respectively are not of the same height, the last stage  38  of the matching network, which abuts the tunable dielectric  28 , would be electrically connected to microstrip line section  26  via a step  42 . 
     FIG. 4 is a plan view of a second embodiment of the auto adjusting matching network  44  of the invention in the form of a stripline. FIG. 5 is a cross-sectional view of FIG. 4 taken along line  5 — 5 , showing a stripline geometry. FIG. 6 is a cross-sectional view of FIG. 4 taken along line  6 — 6 . 
     The device  44  has two ports  46  and  48  for input and output of the guided electromagnetic wave. It comprises a stripline  50  having sections  52  and  54  of various widths and lengths embedded in a non-tunable dielectric substrate  56  supported by top and bottom ground planes  58  and  60 , an additional section  62  of stripline  50  embedded in a tunable dielectric substrate  64  supported by top and bottom ground planes  66  and  68 , and a biasing electrode  70  in the form of a high impedance stripline is connected to stripline section  54 . 
     The connection of biasing electrode  70  to the circuit is not limited to stripline section  54 , but may be made to any other part of the circuit that is electrically connected to stripline section  62 . The biasing electrode  70  serves as means for connecting the auto adjusting impedance matching circuit to an external adjustable DC voltage bias source  72 . Ground planes  66  and  68  may or may not be the same ground planes as for the tunable microwave device to which the matching circuit is connected. Ground planes  66  and  68  are electrically connected to the ground planes of the tunable microwave device. Ground planes  66  and  68  are electrically connected to ground planes  58  and  60 . 
     The stripline section  62 , which is a conducting strip, is directly embedded in the tunable dielectric layer  64 , which is the voltage tunable layer. Ground planes  66  on the top and  68  on the bottom support the dielectric layer  64 . The stripline  62  is less than a quarter wavelength long and forms an approximate quarter wavelength long transformer when joined to section  54  of the matching network in the non-tunable dielectric substrate  56 . 
     The non-tunable stages  76  and  78  of the matching network  44  form a multistage matching circuit  80  directly supported by the non-tunable dielectric layer  56 . The multi-stage matching circuit  80  can be any number of stages of varying widths and lengths, not limited to and including quarter wavelength sections. The last stage  78  of the matching network, which abuts the tunable dielectric  64 , is electrically connected to microstrip line section  62 . 
     FIG. 7 is a longitudinal cross-sectional view of a third embodiment of the invention for the auto adjusting matching network based on a coaxial geometry. FIG. 8 is a cross-sectional view of FIG. 7 taken along line  8 — 8 , showing the coaxial transmission line geometry. 
     The device  82  of FIGS. 7 and 8 has two ports  84  and  86  for input and output of the guided electromagnetic wave. It comprises a center conductor  88  having sections  90  and  92  of various diameters and lengths surrounded by a non-tunable dielectric substrate  94 , which in turn is surrounded by ground conductor  96 . An additional center conductor section  98  is surrounded by a tunable dielectric substrate  100 , which in turn is surrounded by ground conductor  102 . A thin biasing electrode  104  enters the co-axial structure through a small hole  106  and is connected to the central conductor  88 . 
     The connection of biasing electrode  104  to the circuit is not limited to the center conductor section  92 , but may be made to any other part of the circuit that is electrically connected to center conductor section  98 . The biasing electrode  104  serves as a means for connecting an external adjustable DC voltage bias source  74  to the auto-adjusting impedance matching circuit. Ground conductor  102  may or may not be the same ground conductor as for a tunable microwave device to which the matching circuit would be connected. Ground conductor  102  is electrically connected to the ground conductor of the tunable microwave device. Ground conductors  96  and  102  are electrically connected to each other. 
     The center conductor section  98  is surrounded by a dielectric layer  100 , which is the voltage tunable layer. The voltage tunable dielectric layer  100  is enclosed by a ground conductor  102 . The center conductor section  98  is less than a quarter wavelength long and forms a composite impedance transformer approximately a quarter wavelength long when joined to section  92  of the matching network in the non-tunable dielectric  94 . 
     The matching network  109  is a multi-stage matching circuit surrounded by a dielectric layer  94 , which is a non-tunable dielectric. The dielectric layer  94  is enclosed by a ground conductor  96 . The multi-stage matching circuit can be any number of stages of varying widths and lengths, not limited to and including quarter wavelength sections. The last stage  108  of the matching circuit, which abuts the tunable dielectric  100 , is electrically connected to the center conductor  98 . 
     An extension to the first embodiment is shown in FIG. 9, as a matching circuit having multiple tunable stages  112 ,  114 ,  116 , and multiple non-tunable stages  154 ,  156 ,  114   a  and  116   a . The device has two ports  118  and  120  for input and output of a guided electromagnetic wave. It includes a matching microstrip line section  122 , deposited on a non-tunable dielectric substrate  124 ; multiple pairs of microstrip sections  126 ,  128  and  130 ,  132  and  134 ,  136  deposited on pairs of non-tunable and tunable dielectric substrates  124 ,  138  and  140 ,  142  and  144 ,  146  respectively; a biasing electrode  148  in the form of a high impedance microstrip line connected to microstrip section  126 ; and a ground plane (not shown). The dielectric substrates are supported by the ground plane, which may include different electrically connected sections to adapt to the different thicknesses of the substrates  124 ,  140   144  and  138 ,  142 ,  146 . 
     The connection of biasing electrode  148  to the circuit is not limited to microstrip section  126 , but may be made to any other part of the circuit that is electrically connected to microstrip line sections  128 ,  132  and  136 . The biasing electrode  148  connects the auto-adjusting impedance matching circuit to an adjustable DC voltage bias source  152 . The ground plane may or may not be the same ground plane as for the tunable microwave device to which the matching circuit is connected. 
     Each of the microstrip line sections  128 ,  132  and  136  is less than a quarter wavelength long and forms an approximately quarter wavelength long impedance transformer when joined to microstrip sections  126 ,  130  and  134  respectively. 
     The non-tunable stages of the matching network  154 ,  156  form a multi-stage matching circuit  158  directly supported by the non-tunable dielectric layer  124 . The multistage matching circuit can be any number of stages of varying widths and lengths, not limited to quarter wavelength sections. Microstrip section  126  of the last stage  156  of the non-tuning part of the matching network, which abuts the tunable stage  112 , is electrically connected to microstrip line section  128 . The latter abuts non-tunable tunable stage  114   a  and is electrically connected to microstrip line section  130 . The latter abuts tunable stage  114  and is electrically connected to microstrip line section  132 . The latter abuts non-tunable tunable stage  134  and is electrically connected to microstrip line section  134 . The latter abuts tunable stage  116  and is electrically connected to microstrip line section  136 . 
     The multiple stage pairs in FIG. 9 ensure impedance matching over a wider frequency and impedance range than the more simple geometry of FIG.  1 . It should be understood that a similar extension to multiple stage pairs can be made for the second (stripline) and third (co-axial) embodiments as well. 
     FIG. 10 is a plan view of another embodiment for the auto adjusting matching network  160  based on a slotline or finline geometry, and including multiple partial stages  162 ,  164 , and  166 . FIG. 11 is a cross-sectional view of FIG. 10 taken along line  11 — 11 , showing a slotline geometry. 
     The device  160  has two ports  168  and  170  for input and output of the guided electromagnetic wave. It includes two conducting coplanar conductors  172  and  174 , supported by non-tunable dielectric layer  176 , and separated by a gap  178  to form a slotline (or finline if integrated into a waveguide) geometry. For comparison with the first, second and third embodiments, one of these coplanar conductors  174  can be considered to be the ground conductor. The slot  178  may be of uniform width, or it can be of non-uniform width as shown in FIG.  10 . At multiple locations (three shown in FIG. 10)  162 ,  164  and  166 , the slot is bridged by a tunable dielectric layer  180 ,  182  and  184 , which can be deposited on the supporting dielectric layer  176  using thick or thin film technology prior to depositing the metal layers  172  and  174  on the supporting dielectric layer  176 . Between locations  162 ,  164  and  166 , there remain sections  186  and  188  as well as  190   a  and  190   b , which are not bridged with a tunable material layer. Planar conductor  172  is connected to an adjustable DC voltage bias source  192 , and planar conductor  174  is connected to DC ground. The coplanar conductors may or may not be the same coplanar conductors as for the tunable microwave device to which the matching circuit is connected. 
     Each of the tunable slotline sections  162 ,  164  and  166  is less than a quarter wavelength long, but together with the non-tunable intermediate sections  186 ,  188 ,  190   a  and  190   b  which are also typically shorter than a quarter wavelength long, these cascaded slotline sections form a cascaded network that may be a multiple of quarter wavelengths long. The network can be made longer by simply adding more pairs of tunable and non-tunable slotline sections. By careful choice of the relative lengths of each tunable and non-tunable slotline section, the cascaded network forms a tunable impedance matching network over a wide frequency band. 
     The slotline sections  186 ,  188   190   a  and  190   b  may also be bridged by a tunable layer, similar to the tunable sections  162 ,  164  and  166 , but which may be less tunable. Reduced tunability in regions  186 ,  188   190   a  and  190   b  can be achieved by using a material that is less tunable and/or by using wider slot gaps to reduce the bias field strength in these regions. Instead of using different types of materials in the strongly tunable and lesser tunable slot regions, a tunable material can be deposited which may have varying tunability along the slot length. 
     FIG. 12 is a plan view of another embodiment for the auto adjusting matching network  194  based on a co-planar waveguide geometry, and including multiple partial stages. FIG. 13 is a cross-sectional view taken along line  13 — 13  in FIG. 12, showing a coplanar waveguide geometry. 
     The device  194  has two ports  196  and  198  for input and output of the guided electromagnetic wave. It includes two coplanar conducting ground conductors  200 ,  202  and a central strip conductor  204 , supported by non-tunable dielectric layer  206 , and separated by gaps  208  and  210  to form a co-planar waveguide geometry. The slots  208  and  210  may be of uniform width, or they can be of non-uniform width as shown in FIG.  12 . At multiple locations  212 ,  214  and  216 , the slots  208  and  210  are bridged by a tunable dielectric layers  218   220  and  222 , which can be deposited on the supporting dielectric layer  206  using thick or thin film technology prior to depositing the metal layers  200  and  202  and  204  on the supporting dielectric layer  206 . Between locations  212 ,  214  and  216 , there remain sections  224 ,  226  as well as  228   a  and  228   b,  which are not bridged with a tunable material layer. Strip conductor  204  is connected to an adjustable DC voltage bias source  230 , and planar ground conductors  200  and  202  are connected to DC ground. The coplanar conductors  200  and  202  and strip  204  may or may not be the same coplanar conductors as for the tunable microwave device to which the matching circuit is connected. 
     Each of the tunable co-planar waveguide sections  212 ,  214  and  216  is less than a quarter wavelength long, but together with the non-tunable intermediate sections  224 ,  226 ,  228   a  and  228   b , which are also typically shorter than a quarter wavelength long, these cascaded co-planar waveguide sections form a cascaded network that may be a multiple of quarter wavelengths long. The network can be made longer by simply adding more pairs of tunable and non-tunable co-planar waveguide sections. By careful choice of the relative lengths of each tunable and non-tunable section, the cascaded network forms a tunable impedance matching network over a wide frequency band. 
     The slots in co-planar waveguide sections  224 ,  226 ,  228   a  and  228   b  may also be bridged by a tunable layer, similar to the tunable sections  212 ,  214  and  216 , but in that case the layer may be less tunable. Reduced tunability in regions  224 ,  226 ,  228   a  and  228   b  can be achieved by using a material that is less tunable and/or by using wider slot gaps to reduce the bias field strength in these regions. Instead of using different types of materials in the strongly tunable and lesser tunable slot regions, a tunable material can be deposited which may have varying tunability along the co-planar waveguide length. 
     FIG. 14 is a block diagram showing a matching network  10  constructed in accordance with this invention coupled to a tunable microwave device  232 . The tunable microwave device  232  could be one of many devices which have varying input/output characteristic impedances such as tunable phase shifters, delay lines, filters, etc. In the arrangement shown in FIG. 14, the adjustable external DC voltage source is used to supply bias voltage to the matching network  10  and the tunable microwave device  232  in tandem. As the voltage supplied by the external DC voltage source changes, the characteristic input/output impedance of the tunable dielectric device will also change. At the same time the impedance characteristics of the matching network will change to maximize power transfer from/to the microwave source/load  234  to/from the tunable microwave device  232 . Alternatively, the tunable microwave device  232  and the matching network  10  can be controlled by two different external DC voltage sources. 
     The first preferred embodiment of the auto adjusting matching network uses a microstrip geometry. The second preferred embodiment of the auto-adjusting matching circuit has a stripline geometry, the third has a coaxial geometry, the fourth has a slotline or finline geometry and the fifth has a co-planar waveguide geometry. 
     In some embodiments, this invention provides a multi-stage impedance circuit functionally interposed between a conductor line and an entry point of a tunable microwave device, wherein the multi-stage impedance matching circuit reduces the signal reflection of a microwave signal propagating through the tunable impedance transformer into the microwave device, by matching the wave impedance of a microwave signal at the entry point, to the microwave source impedance. 
     This invention provides electrically controlled auto-adjusting matching networks that contribute to the tunable applications of microwave devices, while improving upon the range of operation of such devices. It overcomes the problem of matching to a microwave transmission line with a varying characteristic impedance. It is well suitable for tunable phase shifters, delay lines, and impedance matching for power amplifiers used as general-purpose microwave components in a variety of applications such as handset power amplifiers, radar, microwave instrumentation and measurement systems and radio frequency phased array antennas. The devices are applicable over a wide frequency range, from 500 MHz to 40 GHz. 
     The invention provides an impedance matching circuit having minimal reflection loss and reduced insertion loss over the tuning range of the device. 
     The auto-adjusting matching circuits of this invention may have a dual function. The main objective of the auto-adjusting matching circuit is to operate as an impedance matching network. Additionally, the auto-adjusting matching circuit has the ability to contribute to the tunable range of the microwave device to which it is coupled. Hence, the auto-adjusting matching circuit may incorporate tunable applications in its design as well. For example, the length of a tunable phase shifter may be decreased since the matching network provides a small amount of tunable phase shift through its operating range. Thus, both objectives also lead to a decrease in the insertion loss. 
     The present invention is advantageous because it has wide application to tunable microwave transmission line applications that make use of a static electric field to produce the desired tuning effect. This invention is also applicable to tunable microwave device applications that operate over a frequency band or at a single frequency. 
     The auto-adjusting matching circuit according to the present invention may or may not contribute to the design criteria of the tunable application and may use a common DC Voltage bias or a different DC Voltage bias. The invention minimizes reflection loss and increases the useable bandwidth of the microwave application. 
     The auto-adjusting matching circuit is a multi-stage impedance matching circuit that includes both non-tunable and tunable dielectric material. For example in one preferred embodiment, the impedance matching transformer stage supported by the tunable dielectric material is less than a quarter wavelength long and is connected to the adjacent transformer supported by the non-tunable dielectric to form a composite quarter wavelength impedance transformer. Individual sections of such a composite quarter wave transformer can be referred to as “partial stages”. The matching transformers are tuned in tandem with the microwave device, in order to obtain low insertion loss as well as reducing the reflections from impedance mismatch. Thus, minimal insertion loss in the matching network is achieved. Additionally, the reflections from impedance mismatch due to the tuning of the microwave device are also minimized. 
     The auto-adjusting matching circuit is a two-port device, which in its simplest form includes a conducting matching network supported by a low-loss, conventional non-tunable dielectric substrate which in turn is a supported by a first ground conductor; at least one conducting partial-stage supported by a low-loss voltage-tunable dielectric layer, which in turn is supported by a second ground conductor; and a biasing electrode for connection to an external variable DC voltage source, preferably by way of a microwave choke. 
     The partial-stage on the tunable dielectric layer and the adjoining partial stage on the non-tunable dielectric layer together can form an approximate quarter wavelength long impedance transformer. The port leading to the partial stage supported by the tunable dielectric can be connected at the input or output of a tunable microwave device. The other port of the auto-adjusting matching circuit, which is connected to the matching section supported by the non-tunable dielectric substrate, forms a microwave signal input/output port, which has a substantially constant characteristic impedance. 
     The low-loss voltage-tunable dielectric layer of the partial-stage of the matching circuit may be comprised of the same tunable dielectric material as the tunable microwave device to which it is connected, or it may be comprised of a different tunable dielectric material. The auto-adjusting matching circuit may be biased with the same bias voltage as the tunable microwave device to which it is connected, or it may have a separate bias voltage applied. If more than one tunable material is used, i.e. one tunable dielectric material for the microwave tunable device and another for the partial-stage of the matching circuit, each may have its own separate bias voltage source or use a common (shared) bias voltage source. 
     As is well known, the bandwidth of the impedance matching network may be improved by additional matching stages. The additional matching stages may each be comprised of a partial stage supported by a tunable dielectric substrate in series with a partial stage supported by a non-tunable dielectric substrate. The tunable dielectric substrate sections for the additional matching stages may be comprised of the same tunable dielectric material as the first tunable partial-stage, or it may be comprised of a different tunable dielectric material. 
     The microwave matching section, which is supported by the tunable dielectric substrate for the dual purpose of reducing signal reflections and providing good transmission to and from the microwave transmission line application as well as contributing to the tunability of the application, may be a partial stage less than a quarter wave length long, or may include more than one matching section and is not limited to the use of one tunable dielectric substrate. 
     The biasing electrode may be connected to the auto-adjusting matching circuit by way of a microwave choke such as a high impedance transmission line, or by a highly inductive wire attached directly to the auto-adjusting matching circuit at any point that is ultimately electrically connected to the partial stage supported by the tunable dielectric. 
     The first and second ground conductors are electrically connected and if both are of a planar construction, they should preferably form one continuous ground plane. The non-tunable matching stages are electrically connected to the tunable partial-stage. 
     The objective of a matching network is to ensure that a guided electromagnetic wave entering one port (as such defined as the input port) will enter the microwave device and leave it at the other port (output), with minimum residual reflections at each port. The ground plane is kept at zero voltage, while a voltage bias is applied to the electrodes. The voltage bias causes a DC electric field across the voltage tunable dielectric, which affects the dielectric permittivity of the medium. Since the characteristic impedance of the microstrip is inversely proportional to the square root of the effective dielectric permittivity of the medium around the strip, the biasing voltage can be used to control the characteristic impedance of the auto-adjusting matching network. In this way, the characteristic impedance of the invention can be controlled by the voltage bias. The advantages of this invention are low insertion loss and improved bandwidth operation for tunable devices. 
     Tunable dielectric materials have been described in several patents. Barium strontium titanate (BaTiO 3 —SrTiO 3 ), also referred to as BSTO, is used for its high dielectric constant (200-6,000) and large change in dielectric constant with applied voltage (25-75 percent with a field of 2 Volts/micron). Tunable dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,427,988 by Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material-BSTO—MgO”; U.S. Pat. No. 5,635,434 by Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material-BSTO-Magnesium Based Compound”; U.S. Pat. No. 5,830,591 by Sengupta, et al. entitled “Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No. 5,846,893 by Sengupta, et al. entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S. Pat. No. 5,766,697 by Sengupta, et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No. 5,693,429 by Sengupta, et al. entitled “Electronically Graded Multilayer Ferroelectric Composites”; U.S. Pat. No. 5,635,433 by Sengupta entitled “Ceramic Ferroelectric Composite Material BSTO—ZnO”; U.S. Pat. No. 6,074,971 by Chiu et al. entitled “Ceramic Ferroelectric Composite Materials with Enhanced Electronic Properties BSTO—Mg Based Compound-Rare Earth Oxide”. These patents are incorporated herein by reference. 
     The electronically tunable materials that can be used in the present invention include at least one electronically tunable dielectric phase, such as barium strontium titanate, in combination with at least two additional metal oxide phases. Barium strontium titanate of the formula Ba x Sr 1-x TiO 3  is a preferred electronically tunable dielectric material due to its favorable tuning characteristics, low Curie temperatures and low microwave loss properties. In the formula Ba x Sr 1-x TiO 3 , x can be any value from 0 to 1, preferably from about 0.15 to about 0.6. More preferably, x is from 0.3 to 0.6. 
     Other electronically 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 is in a range from about 0.2 to about 0.8, preferably from about 0.4 to about 0.6. Additional electronically tunable ferroelectrics include Pb x Zr 1-x TiO 3  (PZT) where x ranges from about 0.05 to about 0.4, lead lanthanum zirconium titanate (PLZT), PbTiO 3 , BaCaZrTiO 3 , NaNO 3 , KNbO 3 , LiNbO 3 , LiTaO 3 , PbNb 2 O 6 , PbTa 2 O 6 , KSr(NbO 3 ) and NaBa 2 (NbO 3 )5KH 2 PO 4 . 
     In addition, the following U.S. Patent Applications, assigned to the assignee of this application, disclose additional examples of tunable dielectric materials: U.S. application Ser. No. 09/594,837 filed Jun. 15, 2000, entitled “Electronically Tunable Ceramic Materials Including Tunable Dielectric and Metal Silicate Phases”; U.S. application Ser. No. 09/768,690 filed Jan. 24, 2001, entitled “Electronically Tunable, Low-Loss Ceramic Materials Including a Tunable Dielectric Phase and Multiple Metal Oxide Phases”; U.S. application Ser. No. 09/882,605 filed Jun. 15, 2001, entitled “Electronically Tunable Dielectric Composite Thick Films And Methods Of Making Same”; and U.S. Provisional Application Ser. No. 60/295,046 filed Jun. 1, 2001 entitled “Tunable Dielectric Compositions Including Low Loss Glass Frits”. These patent applications are incorporated herein by reference. 
     The tunable dielectric materials can also be combined with one or more non-tunable dielectric materials. The non-tunable phase(s) may include MgO, MgAl 2 O 4 , MgTiO 3 , Mg 2 SiO 4 , CaSiO 3 , MgSrZrTiO 6 , CaTiO 3 , Al 2 O 3 , SiO 2  and/or other metal silicates such as BaSiO 3  and SrSiO 3 . The non-tunable dielectric phases may be any combination of the above, e.g., MgO combined with MgTiO 3 , MgO combined with MgSrZrTiO 6 , MgO combined with Mg 2 SiO 4 , MgO combined with Mg 2 SiO 4 , Mg 2 SiO 4  combined with CaTiO 3  and the like. 
     Additional minor additives in amounts of from about 0.1 to about 5 weight percent can be added to the composites to additionally improve the electronic properties of the films. These minor additives include oxides such as zirconnates, tannates, rare earths, niobates and tantalates. For example, the minor additives may include CaZrO 3 , BaZrO 3 , SrZrO 3 , BaSnO 3 , CaSnO 3 , MgSnO 3 , Bi 2 O 3 /2SnO 2 , Nd 2 O 3 , Pr 7 O 11 , Yb 2 O 3 , Ho 2 O 3 , La 2 O 3 , MgNb 2 O 6 , SrNb 2 O 6 , BaNb 2 O 6 , MgTa 2 O 6 , BaTa 2 O 6  and Ta 2 O 3 . 
     Thick films of tunable dielectric composites can comprise Ba 1-x Sr x TiO 3 , where x is from 0.3 to 0.7 in combination with at least one non-tunable dielectric phase selected from MgO, MgTiO 3 , MgZrO 3 , MgSrZrTiO 6 , Mg 2 SiO 4 , CaSiO 3 , MgAl 2 O 4 , CaTiO 3 , Al 2 O 3 , SiO 2 , BaSiO 3  and SrSiO 3 . These compositions can be BSTO and one of these components or two or more of these components in quantities from 0.25 weight percent to 80 weight percent with BSTO weight ratios of 99.75 weight percent to 20 weight percent. 
     The electronically tunable materials can also include at least one metal silicate phase. The metal silicates may include metals from Group 2A of the Periodic Table, i.e., Be, Mg, Ca, Sr, Ba and Ra, preferably Mg, Ca, Sr and Ba. Preferred metal silicates include Mg 2 SiO 4 , CaSiO 3 , BaSiO 3  and SrSiO 3 . In addition to Group 2A metals, the present metal silicates may include metals from Group 1A, i.e., Li, Na, K, Rb, Cs and Fr, preferably Li, Na and K. For example, such metal silicates may include sodium silicates such as Na 2 SiO 3  and NaSiO 3 -5H 2 O, and lithium-containing silicates such as LiAlSiO 4 , Li 2 SiO 3  and Li 4 SiO 4 . Metals from Groups 3A, 4A and some transition metals of the Periodic Table may also be suitable constituents of the metal silicate phase. Additional metal silicates may include Al 2 Si 2 O 7 , ZrSiO 4 , KalSi 3 O 8 , NaAlSi 3 O 8 , CaAl 2 Si 2 O 8 , CaMgSi 2 O  6 , BaTiSi 3 O 9  and Zn 2 SiO 4 . Tunable dielectric materials identified as Parascan™ materials, are available from Paratek Microwave, Inc. The above tunable materials can be tuned at room temperature by controlling an electric field that is applied across the materials. 
     In addition to the electronically tunable dielectric phase, the electronically tunable materials can include at least two additional metal oxide phases. The additional metal oxides may include metals from Group 2A of the Periodic Table, i.e., Mg, Ca, Sr, Ba, Be and Ra, preferably Mg, Ca, Sr and Ba. The additional metal oxides may also include metals from Group 1A, i.e., Li, Na, K, Rb, Cs and Fr, preferably Li, Na and K. Metals from other Groups of the Periodic Table may also be suitable constituents of the metal oxide phases. For example, refractory metals such as Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta and W may be used. Furthermore, metals such as Al, Si, Sn, Pb and Bi may be used. In addition, the metal oxide phases may comprise rare earth metals such as Sc, Y, La, Ce, Pr, Nd and the like. 
     The additional metal oxides may include, for example, zirconnates, silicates, titanates, aluminates, stannates, niobates, tantalates and rare earth oxides. Preferred additional metal oxides include Mg 2 SiO 4 , MgO, CaTiO 3 , MgZrSrTiO 6 , MgTiO 3 , MgAl 2 O 4 , WO 3 , SnTiO 4 , ZrTiO 4 , CaSiO 3 , CaSnO 3 , CaWO 4 , CaZrO 3 , MgTa 2 O 6 , MgZrO 3 , MnO 2 , PbO, Bi 2 O 3  and La 2 O 3 . Particularly preferred additional metal oxides include Mg 2 SiO 4 , MgO, CaTiO 3 , MgZrSrTiO 6 , MgTiO 3 , MgAl 2 O 4 , MgTa 2 O 6  and MgZrO 3 . 
     The additional metal oxide phases may alternatively include at least two Mg-containing compounds. In addition to the multiple Mg-containing compounds, the material may optionally include Mg-free compounds, for example, oxides of metals selected from Si, Ca, Zr, Ti, Al and/or rare earths. In another embodiment, the additional metal oxide phases may include a single Mg-containing compound and at least one Mg-free compound, for example, oxides of metals selected from Si, Ca, Zr, Ti, Al and/or rare earths. 
     This invention provides minimal loss auto-adjusting matching circuits for application to microwave transmission line devices that utilize a bias voltage for tuning. Each embodiment of the auto-adjusting matching circuit is comprised of a microwave transmission line configuration, a tunable dielectric material, means for connecting to a bias voltage, and a non-tunable low-loss dielectric material. In operation, the auto-adjusting matching circuit is placed adjacent to the tunable microwave device in order to reduce the reflections from impedance mismatch. 
     The invention contemplates various dielectric materials, tunable dielectric materials, tunable liquid crystals, bias line geometries, matching stages, impedances of microstrip lines, and operating frequencies of the auto-adjusting matching circuit. It should be understood that the foregoing disclosure relates to only typical embodiments of the invention and that numerous modifications or alternatives may be made therein by those skilled in the art without departing from the scope of the invention as set forth in the appended claims.