Abstract:
An embodiment of the present invention provides an apparatus capable of interference cancellation, comprising a first antenna capable of receiving an electrical signal, said electrical signal comprising a main signal component and an interfering signal component; a second antenna capable of receiving said electrical signal and passing it to a tunable delay line, said tunable delay line capable of varying the time delay of said electrical signal; and a combiner capable of receiving the electrical signal from said first antenna and the time delayed electrical signal from said tunable delay line and combining them so as to cancel said interfering signal component from said electrical signal. An embodiment of the present invention also provides an apparatus capable of interference cancellation, comprising an antenna capable of receiving an electrical signal, said electrical signal comprising a main signal component and an interfering signal component; and a feed back loop in which a portion of said electrical signal passes through a cancellation path that includes a tunable delay line capable of time delaying said electrical signal such that when a coupler recombines said electrical signals said interfering signal component from said electrical signal is substantially cancelled.

Description:
CROSS REFERENCED TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Provisional Patent Application Ser. No. 60/586,437, filed Jul. 8, 2004 entitled “Tunable Delay Line”.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Electrically tunable filters have many uses in microwave and radio frequency systems. Compared to mechanically and magnetically tunable filters, electronically tunable filters have the important advantage of fast tuning capability over wide band application. Because of this advantage, they can be used in the applications such as, by way of example and not by way of limitation, LMDS (local multipoint distribution service), PCS (personal communication system), frequency hopping, satellite communication, and radar systems.  
         [0003]     Filters for use in radio link communications systems have been required to provide better performance with smaller size and lower cost. Significant efforts have been made to develop new types of resonators, new coupling structures and new configurations for the filters. In some applications where the same radio is used to provide different capacities in terms of Mbits/sec, the intermediate frequency (IF) filter&#39;s bandwidth has to change accordingly. In other words, to optimize the performance of radio link for low capacity radios, a narrow band IF filter is used while for higher capacities wider band IF filters are needed. This requires using different radios for different capacities, because they have to use different IF filters. However, if the bandwidth of the IF filter could be varied electronically, the same configuration of radio could be used for different capacities which will help to simplify the architecture of the radio significantly, as well as reduce cost.  
         [0004]     Traditional electronically tunable filters use semiconductor diode varactors to change the coupling factor between resonators. Since a diode varactor is basically a semiconductor diode, diode varactor-tuned filters can be used in various devices such as monolithic microwave integrated circuits (MMIC), microwave integrated circuits or other devices. The performance of varactors is defined by the capacitance ratio, C.sub.max/C.sub.min, frequency range, and figure of merit, or Q factor at the specified frequency range. The Q factors for semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly.  
         [0005]     Since the Q factor of semiconductor diode varactors is low at high frequencies (for example, &lt;20 at 20 GHz), the insertion loss of diode varactor-tuned filters is very high, especially at high frequencies (&gt;5 GHz). Another problem associated with diode varactor-tuned filters is their low power handling capability. Further, since diode varactors are nonlinear devices, their handling of signals may generate harmonics and subharmonics.  
         [0006]     Commonly owned U.S. patent application Ser. No. 09/419,219, filed Oct. 15, 1999, and titled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors”, discloses voltage tunable dielectric varactors that operate at room temperature and various devices that include such varactors, and is hereby incorporated by reference. Compared with the traditional semiconductor diode varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed.  
         [0007]     High power amplifiers are also an important part of any radio link. They are required to output maximum possible power with minimum distortion. One way to achieve this is to use feed forward amplifier technology. A typical feed forward amplifier includes two amplifiers (the main and error amplifiers), directional couplers, delay lines, gain and phase adjustment devices, and loop control networks. The main amplifier generates a high power output signal with some distortion while the error amplifier produces a low power distortion-cancellation signal.  
         [0008]     In a typical feed forward amplifier, a radio frequency (RF) signal is input into a power splitter. One part of the RF signal goes to the main amplifier via a gain and phase adjustment device. The output of the main amplifier is a higher level, distorted carrier signal. A portion of this amplified and distorted carrier signal is extracted using a directional coupler, and after going through an attenuator, reaches a carrier cancellation device at a level comparable to the other part of the signal that reaches carrier cancellation device after passing through a delay line. The delay line is used to match the timing of both paths before the carrier cancellation device. The output of carrier cancellation device is a low level error or distortion signal. This signal, after passing through another gain and phase adjustment device, gets amplified by the low power amplifier. This signal is then subtracted from the main distorted signal with an appropriate delay to give the desired non-distorted output carrier.  
         [0009]     Traditionally, delay lines have been used to give the desired delay and provide the above-described functionality. However, delay filters have become increasingly popular for this application because they are smaller, easily integrated with other components, and have lower insertion loss, as compared to their delay line counterpart. A fixed delay filter can be set to give the best performance over the useable bandwidth. This makes the operation of a feed forward amplifier much easier, as compared to the tuning of a delay line, which simulates adjustment of the physical length of a cable. However, fixed delay filters still have to be tuned manually.  
         [0010]     There is a need for high performance, small size tunable bandwidth filters for wireless communications applications, as well as other applications. There is a further need for electronically tunable delay devices.  
       SUMMARY OF THE INVENTION  
       [0011]     An embodiment of the present invention provides an apparatus capable of interference cancellation, comprising a first antenna capable of receiving an electrical signal, said electrical signal comprising a main signal component and an interfering signal component; a second antenna capable of receiving said electrical signal and passing it to a tunable delay line, said tunable delay line capable of varying the time delay of said electrical signal; and a combiner capable of receiving the electrical signal from said first antenna and the time delayed electrical signal from said tunable delay line and combining them so as to cancel said interfering signal component from said electrical signal. An embodiment of the present invention also provides an apparatus capable of interference cancellation, comprising an antenna capable of receiving an electrical signal, said electrical signal comprising a main signal component and an interfering signal component; and a feed back loop in which a portion of said electrical signal passes through a cancellation path that includes a tunable delay line capable of time delaying said electrical signal such that when a coupler recombines said electrical signals said interfering signal component from said electrical signal is substantially cancelled. Further provided in an embodiment of the present invention is a method of interference cancellation, comprising receiving an electrical signal by a first antenna, said electrical signal comprising a main signal component and an interfering signal component; receiving said electrical signal by a second antenna and passing it to a tunable delay line, said tunable delay line capable of varying the time delay of said electrical signal; combining by a combiner the electrical signal from said first antenna and the time delayed electrical signal from said tunable delay line so as to cancel said interfering signal component from said electrical signal. Also provided is a method of interference cancellation, comprising receiving an electrical signal by an antenna, said electrical signal comprising a main signal component and an interfering signal component; and feeding at least a portion of said electrical signal into a feed back loop in which a portion of said electrical signal passes through a cancellation path that includes a tunable delay line capable of time delaying said electrical signal such that when a coupler recombines said electrical signals, said interfering signal component from said electrical signal is substantially cancelled.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.  
         [0013]      FIG. 1  is a schematic representation of a lumped element tunable bandwidth band-pass filter constructed in accordance with this invention.  
         [0014]      FIG. 2  is a schematic representation of an edged coupled microstrip line band-pass filter with tunable varactors.  
         [0015]      FIG. 3  is a top plan view of a varactor that can be used in the filters of this invention.  
         [0016]      FIG. 4  is a cross-sectional view of the varactor of  FIG. 3 , taken along section  4 - 4  of  FIG. 3 .  
         [0017]      FIG. 5  is a schematic representation of feed forward amplifier that uses a tunable delay filter in accordance with this invention.  
         [0018]      FIG. 6  is a flow diagram illustrating the method of the present invention.  
         [0019]      FIG. 7  illustrates an interference cancellation configuration with double antennas of one embodiment of the present invention.  
         [0020]      FIG. 8  illustrates an interference cancellation configuration with single antennas if one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0021]     Referring to the drawings,  FIG. 1  is a schematic representation of a lumped element tunable bandwidth band-pass filter  10  constructed in accordance with this invention. Filter  10  includes an input  12 , an output  13  and a plurality of resonators  14 ,  16 ,  18 . A first voltage tunable dielectric access varactor  20  couples input  12  with resonator  14 . A second voltage tunable access dielectric varactor  22  couples output  13  with resonator  18 . Additional intercavity varactors  24 ,  26  are connected between adjacent resonators  14 ,  16 ,  18 . Each of voltage tunable access varactors  20 ,  22  and each of voltage tunable intercavity or varactors  24 ,  26  includes a voltage tunable dielectric material having a dielectric constant that varies with an applied control voltage, also called a bias voltage. By changing the control voltage for a respective varactor  20 ,  22 ,  24 ,  26 , the capacitance of the respective varactor  20 ,  22 ,  24 ,  26  changes.  
         [0022]     In tunable bandwidth bandpass filter  10  ( FIG. 1 ), the coupling between adjacent resonators  14 ,  16 ,  18  is achieved by a variable intercavity capacitor or varactor  24 ,  26 . By changing the bias voltage of a respective intercavity varactor  24 ,  26  its capacitance value will change which provides a change in coupling factor. Similarly, access coupling of input  12  through access varactor  20  or access coupling of output  13  through access varactor  22  can be controlled by tuning appropriate access varactors  20 ,  22 . Bandwidth of filter  10  is defined by intercavity coupling (i.e., coupling among resonators  14 ,  16 ,  18 ), as well as access coupling through access varactors  20 ,  22  Therefore, by tuning these various couplings the bandwidth of filter  10  can be tuned or changed.  
         [0023]     When varactors  20 ,  22 ,  24 ,  26  are biased, their capacitance values are smaller, resulting in smaller coupling factors. A consequence of such smaller coupling factors is that filter  10  exhibits a narrower bandwidth. Resonators and coupling structures appropriate for employment in filter  10  may be embodied in different topologies. For example, resonators may be configured as lumped elements for high frequency (HF) applications. Coaxial cavities or transmission lines based on coaxial, microstrip, or stripline lines can be used for low frequency RF applications. Dielectric resonators or waveguides can be used for higher frequency applications. The coupling mechanism between resonators can be capacitive or inductive.  
         [0024]      FIG. 2  shows another example of a tunable bandwidth filter  30  constructed in accordance with this invention using microstrip technology. Filter  30  includes two edge coupled microstrip line resonators  32 ,  34 . An input microstrip line resonator  36  is provided for delivering a signal to filter  30 . An output microstrip line resonator  38  is provided for receiving a signal from filter  30 . In order to tune the bandwidth of filter  30 , the coupling factor between resonators, as well as, between input/output transmission lines and the resonators should be changed. Tunable varactors  40 ,  42  and  44  are provided for coupling resonators  32 ,  34 ,  36 ,  38 . Varactors  40 ,  42 ,  44  are coupled between resonators  32 ,  34 ,  36 ,  38 . Changing bias voltage to a respective varactor  40 ,  42 ,  44  changes the capacitance value for the respective varactor  40 ,  42 ,  44  which changes the coupling factor for the respective varactor  40 ,  42 ,  44 . By effecting changes in the coupling factors of respective varactors  40 ,  42 ,  44 , the bandwidth of filter  30  may be altered. Both the access coupling and intercavity couplings are capacitive in this exemplary embodiment illustrated in  FIG. 2 .  
         [0025]     As illustrated by exemplary filters  10 ,  30  ( FIGS. 1 and 2 ), electrically tunable bandwidth filters use electronically tunable varactors to tune intercavity coupling, thus varying the coupling factor between the resonators, as well as, access coupling. The varactor capacitance may be variously changed among respective varactors by applying different bias voltages to different varactors. In such manner the coupling factors of various varactors may be varied, and bandwidth of the filter in which the varactors are employed may be adjusted.  
         [0026]      FIG. 3  is a top plan view of a varactor  50  that can be used in the filters of this invention.  FIG. 4  is a cross-sectional view of the varactor of  FIG. 3 , taken along section  4 - 4  of  FIG. 3 . In  FIGS. 3 and 4 , a varactor  50  includes a layer  52  of voltage tunable dielectric material positioned on a surface  54  of a low loss, low dielectric substrate  56 . A pair of electrodes  58 ,  60  are positioned on layer  52  and separated by a gap  62 . An input line  64  is connected with electrode  58  and an output line  66  is connected with electrode  60 . A variable DC voltage source  68  is connected between electrodes  58 ,  60  to supply a control voltage to varactor  50 . By changing the control voltage provided by voltage source  58 , the capacitance of varactor  50  can be altered.  
         [0027]     Filters configured according to the teachings of the present invention (e.g., filter  10 ,  FIG. 1 ; filter  30 ,  FIG. 2 ; filter  50 ,  FIGS. 3 and 4 ) have low insertion loss, fast tuning speed, high power-handling capability, high IP3 and low cost in the microwave frequency range. Compared to the semiconductor diode varactors, voltage-controlled tunable dielectric capacitors have higher Q factors, higher power-handling and higher IP3. Voltage-controlled tunable dielectric capacitors (e.g., varactors  20 ,  22 ,  24 ,  26 ,  FIG. 1 ; varactors  40 ,  42 ,  44 ,  FIG. 2 ; varactor  50 ,  FIG. 3 ) have a capacitance that varies approximately linearly with applied voltage and can achieve a wider range of capacitance values than is possible with semiconductor diode varactors.  
         [0028]     Filters  10 ,  30 ,  50  described above can also serve as tunable delay filters. Tunable delay filters can be used in various devices, such as feed forward amplifiers.  FIG. 5  is a schematic representation of feed forward amplifier  70  including tunable delay filters in accordance with this invention. A radio frequency (RF) signal is input to an input port  72  and split by a signal splitter  74  into first and second parts. The first part on a line  76  goes to a main amplifier  78  via a gain and phase adjustment device  80 . The output of main amplifier  78  on line  82  is a high level, distorted carrier signal. A portion of this amplified and distorted carrier signal is extracted using a directional coupler  84  and provided to a carrier cancellation device  88  via an attenuator  86 .  
         [0029]     The second part of the RF signal received at signal splitter  74  is directed on a line  90  to carrier cancellation device  88  via a delay device  92 . Delay device  92  is configured to phase match signals arriving at carrier cancellation device  88  from lines  76 ,  90 . The signal arriving at carrier cancellation device  88  goes to a main amplifier  78  via a gain and phase adjustment device  80 .  
         [0030]     The output of carrier cancellation device  88  is a low level error or distortion signal. This signal, after passing through another gain and phase adjustment device  94 , is amplified by a low power amplifier  96 . An output signal from low power amplifier  96  is provided to a subtractor device  98 . A main distorted signal is provided to subtractor  98  from directional coupler  84  via a delay device  100 . Subtractor  98  produces a difference signal at an output  102  representing the difference between signals provided to subtractor  98  from delay device  100  and from low power amplifier  96 . The difference signal appearing at output  102  the desired non-distorted output carrier signal.  
         [0031]     One or both of the delay devices  92 ,  100  in  FIG. 5  can be a tunable delay filter. By changing the bias voltage of varactor  42  in filter  30  ( FIG. 2 ), for example, its capacitance value will change which provides a change in its coupling factor. Similarly the input/output access coupling for filter  30  can be varied by tuning the corresponding varactors  40 ,  42 . Changing the coupling factors of filter  30  changes the bandwidth, which will result in changing the group delay. Therefore, by tuning the coupling varactors  40 ,  42  the group delay of filter  30  can be changed.  
         [0032]     Resonators and coupling structures can be embodied in different topologies. For example, resonators can be lumped elements for HF applications; coaxial cavities or transmission lines based on coaxial lines, microstrip lines, or stripline lines can be used for low frequency RF applications; and dielectric resonators or waveguides can be used for higher frequency applications. Coupling structures can be capacitive or inductive. The above described structures are only examples. Electronically tunable varactors can be used to tune the coupling factors and hence the bandwidth of any bandpass filter design to provide variable group delay.  
         [0033]     The invention also encompasses a method of delaying an electrical signal, the method comprising the steps of: providing first and second resonators, an input, a first tunable dielectric varactor connecting the input to the first resonator, an output, a second tunable dielectric varactor connecting the second resonator to the output, and a third tunable dielectric varactor connecting the first and second resonators; coupling the electrical signal to the input; and extracting a delayed version of the electrical signal at the output.  
         [0034]     The tunable dielectric varactors in the preferred embodiments of the present invention can include a low loss (Ba,Sr)TiO.sub.3-based composite film. The typical Q factor of the tunable dielectric capacitors is 200 to 500 at 2 GHz with capacitance ratio (C.sub.max/C.sub.min) around 2. A wide range of capacitance of the tunable dielectric capacitors is variable, say 0.1 pF to 10 pF. The tuning speed of the tunable dielectric capacitor is less than 30 ns. The practical tuning speed is determined by auxiliary bias circuits. The tunable dielectric capacitor may be a packaged two-port component, in which tunable dielectric material can be voltage-controlled. The tunable film may preferably be deposited on a substrate, such as MgO, LaAlO.sub.3, sapphire, Al.sub.2O.sub.3 and other dielectric substrates. An applied voltage produces an electric field across the tunable dielectric, which produces a change in the capacitance of the tunable dielectric capacitor.  
         [0035]     Tunable dielectric materials have been described in several patents. Barium strontium titanate (BaTiO.sub.3--SrTiO.sub.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.  
         [0036]     Barium strontium titanate of the formula Ba.sub.xSr.sub.1-xTiO.sub.-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.sub.xSr.sub.1-xTiO.sub.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.  
         [0037]     Other electronically tunable dielectric materials may be used partially or entirely in place of barium strontium titanate. An example is Ba.sub.xCa.sub.1-xTiO.sub.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.sub.xZr.sub.1-xTiO.sub.3 (PZT) where x ranges from about 0.0 to about 1.0, Pb.sub.xZr.sub.1-xSrTiO-.sub.3 where x ranges from about 0.05 to about 0.4, KTa.sub.xNb.sub.1-xO.sub.3 where x ranges from about 0.0 to about 1.0, lead lanthanum zirconium titanate (PLZT), PbTiO.sub.3, BaCaZrTiO.sub.3, NaNO.sub.3, KNbO.sub.3, LiNbO.sub.3, LiTaO.sub.3, PbNb.sub.2O.sub.6, PbTa.sub.2O.sub.6, KSr(NbO.sub.3) and NaBa.sub.2(NbO.sub.3).sub.5 KH.sub.2- PO.sub.4, and mixtures and compositions thereof. Also, these materials can be combined with low loss dielectric materials, such as magnesium oxide (MgO), aluminum oxide (Al.sub.2O.sub.3), and zirconium oxide (ZrO.sub.2), and/or with additional doping elements, such as manganese (MN), iron (Fe), and tungsten (W), or with other alkali earth metal oxides (i.e. calcium oxide, etc.), transition metal oxides, silicates, niobates, tantalates, aluminates, zirconnates, and titanates to further reduce the dielectric loss.  
         [0038]     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”; U.S. application Ser. No. 09/834,327 filed Apr. 13, 2001, entitled “Strain-Relieved Tunable Dielectric Thin Films”; 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.  
         [0039]     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.sub.2O.sub.4, MgTiO.sub.3, Mg.sub.2SiO.sub.4, CaSiO.sub.3, MgSrZrTiO.sub.6, CaTiO.sub.3, Al.sub.2O.sub.3, SiO.sub.2 and/or other metal silicates such as BaSiO.sub.3 and SrSiO.sub.3. The non-tunable dielectric phases may be any combination of the above, e.g., MgO combined with MgTiO.sub.3, MgO combined with MgSrZrTiO.sub.6, MgO combined with Mg.sub.2SiO.sub.4, MgO combined with Mg.sub.2SiO.sub.4, Mg.sub.2SiO.sub.4 combined with CaTiO.sub.3 and the like.  
         [0040]     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.sub.3, BaZrO.sub.3, SrZrO.sub.3, BaSnO.sub.3, CaSnO.sub.3, MgSnO.sub.3, Bi.sub.2O.sub.3/2SnO.sub.2, Nd.sub.2O.sub.3, Pr.sub.7O.sub.11, Yb.sub.2O.sub.3, Ho.sub.2O.sub.3, La.sub.2O.sub.3, MgNb.sub.2O.sub.6, SrNb.sub.2O.sub.6, BaNb.sub.2O.sub.6, MgTa.sub.2O.sub.6, BaTa.sub.2O.sub.6 and Ta.sub.2O.sub.3.  
         [0041]     Thick films of tunable dielectric composites can comprise Ba.sub.1-xSr.sub.xTiO.sub.3, where x is from 0.3 to 0.7 in combination with at least one non-tunable dielectric phase selected from MgO, MgTiO.sub.3, MgZrO.sub.3, MgSrZrTiO.sub.6, Mg.sub.2SiO.sub.4, CaSiO.sub.3, MgAl.sub.2O.sub.4, CaTiO.sub.3, Al.sub.2O.sub.3, SiO.sub.2, BaSiO.sub.3 and SrSiO.sub.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.  
         [0042]     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.sub.2SiO.sub.4, CaSiO.sub.3, BaSiO.sub.3 and SrSiO.sub.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.sub.2SiO.sub.3 and NaSiO.sub.3-5H.sub.2O, and lithium-containing silicates such as LiAlSiO.sub.4, Li.sub.2SiO.sub.3 and Li.sub.4SiO.sub.4. Metals from Groups 3A, 4A and some transition metals of the Periodic Table may also be suitable constituents of the metal silicate phase.  
         [0043]     Additional metal silicates may include Al.sub.2Si.sub.2O.sub.7, ZrSiO.sub.4, KalSi.sub.3O.sub.8, NaAlSi.sub.3O.sub.8, CaAl.sub.2Si.sub.2O.sub.8, CaMgSi.sub.2O.sub.6, BaTiSi.sub.3O.sub.9 and Zn.sub.2SiO.sub.4. The above tunable materials can be tuned at room temperature by controlling an electric field that is applied across the materials.  
         [0044]     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.  
         [0045]     The additional metal oxides may include, for example, zirconnates, silicates, titanates, aluminates, stannates, niobates, tantalates and rare earth oxides.  
         [0046]     Preferred additional metal oxides include Mg.sub.2SiO.sub.4, MgO, CaTiO.sub.3, MgZrSrTiO.sub.6, MgTiO.sub.3, MgAl.sub.2O.sub.4, WO.sub.3, SnTiO.sub.4, ZrTiO.sub.4, CaSiO.sub.3, CaSnO.sub.3, CaWO.sub.4, CaZrO.sub.3, MgTa.sub.2O.sub.6, MgZrO.sub.3, MnO.sub.2, PbO, Bi.sub.2O.sub.3 and La.sub.2O.sub.3. Particularly preferred additional metal oxides include Mg.sub.2SiO.sub.4, MgO, CaTiO.sub.3, MgZrSrTiO.sub.6, MgTiO.sub.3, MgAl.sub.2O.sub.4, MgTa.sub.2O.sub.6 and MgZrO.sub.3.  
         [0047]     The additional metal oxide phases are typically present in total amounts of from about 1 to about 80 weight percent of the material, preferably from about 3 to about 65 weight percent, and more preferably from about 5 to about 60 weight percent. In one preferred embodiment, the additional metal oxides comprise from about 10 to about 50 total weight percent of the material. The individual amount of each additional metal oxide may be adjusted to provide the desired properties. Where two additional metal oxides are used, their weight ratios may vary, for example, from about 1:100 to about 100:1, typically from about 1:10 to about 10:1 or from about 1:5 to about 5:1. Although metal oxides in total amounts of from 1 to 80 weight percent are typically used, smaller additive amounts of from 0.01 to 1 weight percent may be used for some applications.  
         [0048]     In one embodiment, the additional metal oxide phases may 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. The high Q tunable dielectric capacitor utilizes low loss tunable substrates or films.  
         [0049]     To construct a tunable device, the tunable dielectric material can be deposited onto a low loss substrate. In some instances, such as where thin film devices are used, a buffer layer of tunable material, having the same composition as a main tunable layer, or having a different composition can be inserted between the substrate and the main tunable layer. The low loss dielectric substrate can include magnesium oxide (MgO), aluminum oxide (Al.sub.2O.sub.3), and lanthium oxide (LaAl.sub.2O.sub.3).  
         [0050]     When the bias voltage or bias field is changed, the dielectric constant of the voltage tunable dielectric material (di-elect cons.sub.r) will change accordingly, which will result in a tunable varactor. Compared to semiconductor varactor based tunable filters, the tunable dielectric capacitor based tunable filters of this invention have the merits of lower loss, higher power-handling, and higher IP3, especially at higher frequencies (&gt;10 GHz). It is observed that between 50 and 300 volts a nearly linear relation exists between Cp and applied Voltage.  
         [0051]     In microwave applications the linear behavior of a dielectric varactor is very much appreciated, since it will assure very low Inter-Modulation Distortion and consequently a high IP3 (Third-order Intercept Point). Typical IP3 values for diode varactors are in the range 5 to 35 dBm, while that of a dielectric varactor is greater than 50 dBm. This will result in a much higher RF power handling capability for a dielectric varactor.  
         [0052]     Another advantage of dielectric varactors compared to diode varactors is the power consumption. The dissipation factor for a typical diode varactor is in the order of several hundred milliwatts, while that of the dielectric varactor is about 0.1 mW.  
         [0053]     Diode varactors show high Q only at low microwave frequencies so their application is limited to low frequencies, while dielectric varactors show good Q factors up to millimeter wave region and beyond (up to 60 GHz).  
         [0054]     Tunable dielectric varactors can also achieve a wider range of capacitance (from 0.1 pF all the way to several .mu.F), than is possible with diode varactors. In addition, the cost of dielectric varactors is less than diode varactors, because they can be made more cheaply.  
         [0055]     High frequency, radio frequency, and microwave bandpass filters of this invention include a number of resonators and some coupling structures. The resonators can be lumped elements, any type of transmission lines, dielectric resonators, waveguides, or other resonating structures. The coupling mechanism between the adjacent resonators as well as the access transmission line and first and last resonators can be tuned electronically by using tunable dielectric varactors. Tuning the coupling factors of the bandpass filter results in tunable bandwidth filter.  
         [0056]     Electronically tunable dielectric varactors may be used to make tunable delay filters. The invention also relates to compact, high performance, low loss, and low cost tunable delay filters. These compact tunable delay filters are increasingly being used in feed-forward or pre-distortion technologies used in high power amplifiers in wireless communication base stations and other communication systems. The high Q varactor using low loss tunable dielectric material films leads to high performance tunable delay filters with significant advantages over fixed delay filters and coaxial cable delay lines.  
         [0057]     The electronically tunable delay filters of this invention use electronically tunable varactors to tune the group delay of the filter. When the varactor capacitance is changed by applying different bias voltages, the coupling factors between the filter resonators are varied, which result in a change in filter group delay value. Electrically tunable delay filters based on dielectric varactors have important advantages such as high Q, small size, lightweight, low power consumption, simple control circuits, and fast tuning capability. Compared with semiconductor diode varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, faster tuning speed, and lower cost.  
         [0058]     The tunable delay filters include a number of resonators and some coupling structures. The resonators can be lumped element, any type of transmission line, dielectric resonator, waveguide, or another resonator structure. The coupling mechanism between the adjacent resonators as well as the access transmission line and first and last resonators can be tuned electronically by using voltage tunable dielectric varactors. Tuning the coupling factors of the bandpass filter will result in tunable delay filter. Some filter examples are provided, but the patent is not limited to those structures.  
         [0059]     This invention provides an effective way of designing a tunable delay filter. When used in a feed forward amplifier the filters provide an easy way of inducing delay as well as tuning delay to obtain distortion free output signals from power amplifiers. Improved tuning delay can result in better modulated signals. Tunable delay filters can reduce the system cost and significantly improve the quality of radio link.  
         [0060]     This invention provides electrically tunable bandwidth and tunable delay filters having high Q, small size, light weight, low power consumption, simple control circuits, and fast tuning capability.  
         [0061]      FIG. 6  is a flow diagram illustrating the method of the present invention. In  FIG. 6 , a method  200  for delaying an electrical signal begins at a START locus  202 . Method  200  continues with providing a plurality of resonator units coupled between an input locus and an output locus, as indicated by a block  204 .  
         [0062]     Method  200  continues with providing a plurality of tunable dielectric varactor units, as indicated by a block  206 . Respective individual varactor units of the plurality of varactor units are coupled between respective pairs of the plurality of resonator units, coupled between the plurality of resonator units and the input locus, and coupled between the plurality of resonator units and the output locus. Each respective individual varactor unit includes a substrate, a layer of voltage tunable dielectric material established in a first land on the substrate, a first electrode structure for receiving an electrical signal established in a second land on the first land, and a second electrode structure for receiving an electrical signal established in a third land on the first land. The first land and the second land are separated by a gap.  
         [0063]     Method  200  continues with applying the electrical signal to the input locus, as indicated by a block  208 . Method  200  continues with applying a respective tuning voltage to the first electrode structure and the second electrode structure of each respective varactor unit, as indicated by a block  210 . Each respective varactor unit exhibits a respective capacitance. The respective capacitance varies in response to the respective tuning voltage.  
         [0064]     Method  200  continues with receiving an output signal at the output locus, as indicated by a block  212 . The output signal is delayed with respect to the electrical signal. Method  200  then terminates, as indicated by an END locus  214 .  
         [0065]     Another application where a tunable delay line may be used in an embodiment of the present invention is in interference cancellation of a communications link. Interference signals may be generated by multiple sources, such as, but not limited to, antenna multi-path, signals generated by adjacent circuits and coupled to the main signal path due to poor isolation, etc.  
         [0066]     In order to achieve interference cancellation, the time (phase) and amplitude transfer functions of the cancellation circuit must match that of the undesired coupling signal.  FIG. 7 , generally at  700 , illustrates an interference cancellation configuration with double antennas  705  and  715  of one embodiment of the present invention and thus shows the concept when the interference is caused by multi-path in the air interface. The signal received at Antenna  1   705  contains the desired signal plus interference  710 . Antenna  2   715  essentially receives the same signal, which will be combined via combiner  725  with the main signal in path  1 , after going through a tunable delay line  720 . By varying the time delay of the tunable delay line  720 , an optimum combination of the two signals  730  can be achieved as a consequence of interference cancellation.  
         [0067]     Turning now to  FIG. 8  is an illustration  800  of an interference cancellation configuration with a single antenna  805  of one embodiment of the present invention. Although not limited in this respect,  FIG. 8  shows where the interference may be caused by poor circuit isolation. This configuration uses a feed back loop in which a portion of the main path signal that contains the desired as well as interference signals  810  will go through the cancellation path that includes a tunable delay line  840 . By adjusting the time delay of the feed back signal, and recombining it with coupler  820  with the signal in the main path, cancellations of the interference signal may be achieved, thereby providing a signal without interference  830 .  
         [0068]     Similar performance can be achieved by using a phase shifter, either transmission line type or reflective type, instead of tunable delay line. For broadband interference cancellation the combination the two, i.e., a tunable delay line plus a phase shifter may work best. Also, by adding a variable attenuator in series with tunable delay line, and/or phase shifter in both configurations shown above, greater cancellation of the interference signal can be achieved.  
         [0069]     It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: