Patent Application
20060152303
The present invention generally relates to radio frequency (RF) and microwave notch (bandstop) filters, and more particularly to tunable RF and microwave notch filters.
Electronic filters are widely used in radio frequency (RF) and microwave circuits. Tunable filters may significantly improve the performance of the circuits, and simplify the circuits. There are two well-known kinds of analog tunable filters used in RF applications, one is electrically tuned, usually by diode varactor, and the other is mechanically tuned. Mechanically tunable filters have the disadvantages of large size, low speed, and heavy weight. Diode-tuned filters that include conventional semiconductor varactor diodes suffer from low power handling capacity that is limited by intermodulation of the varactor, which causes signals to be generated at frequencies other than those desired. This intermodulation is caused by the highly non-linear response of conventional semiconductor varactors to voltage control.
Tunable filters for use in radio frequency circuits are well known. Examples of such filters can be found in U.S. Pat. Nos. 5,917,387, 5,908,811, 5,877,123, 5,869,429, 5,752,179, 5,496,795 and 5,376,907.
Varactors can be used as tunable capacitors in tunable filters. Common varactors used today are Silicon and GaAs based diodes. The performance of these varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range and figure of merit, or Q factor (1/tan δ) at the specified frequency range. The Q factors for these 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. At 10 GHz the Q factors for these varactors are usually only about 30.
Another type of varactor is a tunable dielectric varactor, whose capacitance is tuned by applying a control voltage to change a dielectric constant in a tunable dielectric material. Tunable dielectric varactors have high Q factors, high power handling, low intermodulation distortion, wide capacitance range, and low cost.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film dielectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film dielectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194.
Commonly owned U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999, and titled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors”, discloses voltage tunable varactors and various devices that include such varactors. Commonly owned U.S. patent application Ser. No. 09/434,433, filed Nov. 4, 1999, and titled “Ferroelectric Varactor With Built-In DC Blocks” discloses voltage tunable varactors that include built-in DC blocking capacitors. Commonly owned U.S. patent application Ser. No. 09/844,832, filed Apr. 27, 2001, and titled “Voltage-Tuned Dielectric Varactors With Bottom Electrodes”, discloses additional voltage tunable varactors. Commonly owned U.S. patent application Ser. No. 09/660,309, filed Dec. 12, 2000, and titled “Dielectric Varactors With Offset Two-Layer Electrodes”, discloses other voltage tunable varactors. The varactors disclosed in these applications operate at room temperatures to provide a tunable capacitance.
Tunable filters that can utilize the varactors described in the commonly owned patent applications are described in another commonly owned patent application Ser. No. 09/457,943, filed Dec. 9, 1999 and titled “Electrically Tunable Filters With Dielectric Varactors”.
Filters for use in wireless communications products have been required to provide better performance with smaller size. Efforts have been made to develop new types of resonators, new coupling structures and new filter configurations. One of the techniques used to reduce the number of resonators is to add cross couplings between non-adjacent resonators to provide transmission zeros. As a result of these transmission zeros, the filter selectivity is improved. However, in order to achieve these transmission zeros, certain coupling patterns have to be followed. This impairs the size reduction effort. In some cases, it may be more feasible to add a notch filter to improve the attenuation in a certain frequency range, rather than making the filter complicated by adding cross couplings.
There is a need for a tunable notch filter, which can provide improved operation at radio and microwave frequencies.
This invention provides a notch filter including a main transmission line, a coupling mechanism, and at least one electrically tunable resonator coupled to the transmission line through the coupling mechanism. The resonator can be tuned by using tunable dielectric varactors or microelectromechanical varactors. Telephone handsets that include notch filters are also included.
The present invention provides high performance and small size tunable notch filters for wireless communications applications, as well as other applications. The filters include tunable resonators that include tunable capacitors which can be tunable dielectric varactors or microelectromechanical (MEM) varactors. Compared with traditional semiconductor varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed.
Referring to the drawings,
A first end of segment 30 serves as an input 42 and a first end of segment 40 serves as an output 44. The couplers are separated along the main transmission line by a distance equal to about a quarter wavelength. At least one of the resonators includes a tunable varactor that can be controlled to tune the resonant frequency. The resonant frequency of the resonators can be tuned to be in the stop band, but offset from each other. A larger number of resonators can provide a deeper notch or a wider stop band.
The main transmission line and the coupling mechanism can be constructed using numerous different available structures. For example, the main transmission line can comprise a coaxial transmission line, a microstrip line, a stripline line, a rectangular waveguide, a coplanar waveguide, a ridged waveguide, etc. The coupling structures can be any of several known structures, for example, a capacitive probe, an inductive loop, an iris window, an evanescent waveguide piece, a slot, a hole, etc.
The resonators are the critical components in making the notch filter tunable. RF and microwave resonators usually include a transmission line with its two ends shorted or open. When it is shorted or open for both ends, it requires a half wavelength (λ/2) to resonate. Lines having lengths equal to multiple half wavelengths also work. When a line is shorted in one end and open at the other end, a quarter wavelength (λ/4) is required to resonate. Similarly, lines having lengths equal to multiple quarter wavelengths also work. Whether the lines are a half wavelength or a quarter wavelength, an end capacitor can be added to decrease the resonant frequency. By partially or fully replacing the end capacitor with a varactor, such as an electrically tunable dielectric varactor or MEM varactor, the resonant frequency of the resonator becomes electrically tunable. Examples of such resonators are shown in
The notch filter shown in
The tunable dielectric layer 276 can be a thin or thick film. The capacitance of the varactor of
where C is capacitance of the capacitor; εo is permittivity of free-space; εr is dielectric constant (permittivity) of the tunable film; A is overlap area of the electrode 264 that is overlapped by electrode 270; and t is thickness of the tunable film in the overlapped section. An example of these parameters for 1 pF capacitor is: εr=200; A=170 μm2; and t=0.3 μm. The horizontal distance (HD) along the surface of the substrate between the first and second electrodes is much greater than the thickness (t) of the dielectric film. Typically, the thickness of tunable film is <1 micrometer for thin films, and <5 micrometers for thick films, and the HD is greater than 50 micrometers. Theoretically, if HD is close to t, the capacitor will still work, but its capacitance would be slightly greater than that calculated from the above equation. However, from a processing technical view, it is difficult and not necessary to make HD close to t. Therefore, HD mainly depends on the processing used to fabricate the device, and is typically about >50 micrometers. In practice, we choose HD>10t.
The bottom electrode 264 can be deposited on the surface of the substrate by electron-beam, sputtering, electroplating or other metal film deposition techniques. The bottom electrode partially covers the substrate surface, which is typically done by etching processing. The thickness of the bottom electrode in one preferred embodiment is about 2 μm. The bottom electrode should be compatible with the substrate and the tunable films, and should be able to withstand the film processing temperature. The bottom electrode may typically be comprised of platinum, platinum-rhodium, ruthenium oxide or other materials that are compatible with the substrate and tunable films, as well as with the film processing. Another film may be required between the substrate and bottom electrode as an adhesion layer, or buffer layer for some cases, for example platinum on silicon can use a layer of silicon oxide, titanium or titanium oxide as a buffer layer.
The thin or thick film of tunable dielectric material 276 is then deposited on the bottom electrode and the rest of the substrate surface by techniques such as metal-organic solution deposition (MOSD or simply MOD), metal-organic chemical vapor deposition (MOCVD), pulse laser deposition (PLD), sputtering, screen printing and so on. The thickness of the thin or thick film that lies above the bottom electrode is preferably in range of 0.2 μm to 4 μm. Low loss and high tunability films should be selected to achieve high Q and high tuning of the varactor. These tunable dielectric films have dielectric constants of 2 to 1000, and tuning of greater than 20% with a loss tangent less than 0.005 at around 2 GHz. To achieve low capacitance, low dielectric constant (k) films should be selected. However, high k films usually show high tunability. The typical k range is about 100 to 500.
The second electrode 270 is formed by a conducting material deposited on the surface of the substrate and at least partially overlapping the tunable film, by using similar processing as set forth above for the bottom electrode. Metal etching processing can be used to achieve specific top electrode patterns. The etching processing may be dry or wet etching. The top electrode materials can be gold, silver, copper, platinum, ruthenium oxide or other conducting materials that are compatible with the tunable films. Similar to the bottom electrode, a buffer layer for the top electrode could be necessary, depending on the electrode-tunable film system. Finally, a part of the tunable film should be etched away to expose the bottom electrode.
The substrate layer in the described varactors may be comprised of MgO, alumina (AL2O3), LaAlO3, sapphire, quartz, silicon, gallium arsenide, and other materials that are compatible with the various tunable films and the electrodes, as well as the processing used to produce the tunable films and the electrodes.
For a certain thickness and dielectric constant of the tunable dielectric film, the pattern and arrangement of the top electrode are key parameters in determining the capacitance of the varactor. In order to achieve low capacitance, the top electrode may have a small overlap (as shown in
Tunable dielectric materials have been described in several patents. Barium strontium titanate (BaTiO3—SrTiO3), 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. 5,312,790 to Sengupta, et al. entitled “Ceramic Ferroelectric Material”; U.S. Pat. No. 5,427,988 by Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO-MgO”; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO-ZrO2”; 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 materials shown in these patents, especially BSTO-MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Barium strontium titanate of the formula BaxSr1-xTiO3 is a preferred electronically tunable dielectric material due to its favorable tuning characteristics, low Curie temperatures and low microwave loss properties. In the formula BaxSr1-xTiO3, 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 BaxCa1-xTiO3, 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 PbxZr1-xTiO3 (PZT) where x ranges from about 0.0 to about 1.0, PbxZr1-xSrTiO3 where x ranges from about 0.05 to about 0.4, KTaxNb1-xO3 where x ranges from about 0.0 to about 1.0, lead lanthanum zirconium titanate (PLZT), PbTiO3, BaCaZrTiO3, NaNO3, KNbO3, LiNbO3, LiTaO3, PbNb2O6, PbTa2O6, KSr(NbO3) and NaBa2(NbO3)5KH2PO4, and mixtures and compositions thereof. Also, these materials can be combined with low loss dielectric materials, such as magnesium oxide (MgO), aluminum oxide (Al2O3), and zirconium oxide (ZrO2), 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.
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 Serial 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, MgAl2O4, MgTiO3, Mg2SiO4, CaSiO3, MgSrZrTiO6, CaTiO3, Al2O3, SiO2 and/or other metal silicates such as BaSiO3 and SrSiO3. The non-tunable dielectric phases may be any combination of the above, e.g., MgO combined with MgTiO3, MgO combined with MgSrZrTiO6, MgO combined with Mg2SiO4, MgO combined with Mg2SiO4, Mg2SiO4 combined with CaTiO3 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 CaZrO3, BaZrO3, SrZrO3, BaSnO3, CaSnO3, MgSnO3, Bi2O3/2SnO2, Nd2O3, Pr7O11, Yb2O3, Ho2O3, La2O3, MgNb2O6, SrNb2O6, BaNb2O6, MgTa2O6, BaTa2O6 and Ta2O3.
Thick films of tunable dielectric composites can comprise Ba1-xSrxTiO3, where x is from 0.3 to 0.7 in combination with at least one non-tunable dielectric phase selected from MgO, MgTiO3, MgZrO3, MgSrZrTiO6, Mg2SiO4, CaSiO3, MgAl2O4, CaTiO3, Al2O3, SiO2, BaSiO3 and SrSiO3. 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 Mg2SiO4, CaSiO3, BaSiO3 and SrSiO3. 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 Na2SiO3 and NaSiO3-5H2O, and lithium-containing silicates such as LiAlSiO4, Li2SiO3 and Li4SiO4. 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 Al2Si2O7, ZrSiO4, KalSi3O8, NaAlSi3O8, CaAl2Si2O8, CaMgSi2O6, BaTiSi3O9 and Zn2SiO4. 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 Mg2SiO4, MgO, CaTiO3, MgZrSrTiO6, MgTiO3, MgAl2O4, WO3, SnTiO4, ZrTiO4, CaSiO3, CaSnO3, CaWO4, CaZrO3, MgTa2O6, MgZrO3, MnO2, PbO, Bi2O3 and La2O3. Particularly preferred additional metal oxides include Mg2SiO4, MgO, CaTiO3, MgZrSrTiO6, MgTiO3, MgAl2O4, MgTa2O6 and MgZrO3.
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.
The additional metal oxide phases can 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.
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 (Al2O3), and lanthium oxide (LaAl2O3).
Compared to voltage-controlled semiconductor diode varactors, voltage-controlled tunable dielectric capacitors have higher Q factors, lower loss, higher power-handling, and higher IP3, especially at higher frequencies (>10 GHz).
Tunable dielectric capacitors (dielectric varactors) or microelectromechanical (MEM) varactors can be used as the tunable elements in the notch filters of this invention. At least two varactor topologies of MEM varactors can be used, parallel plate and interdigital. In the parallel plate structure, one plate is suspended at a distance from another plate by suspension springs. This distance can vary in response to electrostatic force between the two parallel plates induced by an applied bias voltage. In the interdigital configuration, the effective area of the capacitor is varied by moving the fingers comprising the capacitor in and out and changing its capacitance value. MEM varactors have lower Q than their dielectric counterpart, especially at higher frequencies, but can be used in low frequency applications.
A notch filter can also be constructed in accordance with this invention by converting a bandpass filter with either a circulator or a 3 dB hybrid.
The invention provides compact, high performance, low loss, and low cost tunable notch filters. In the preferred embodiment, the tunable resonators include tunable dielectric varactors or MEM varactors. These compact notch filters are suitable for wireless communication applications to eliminate unwanted signals in communication systems, to make the notch filter electrically tunable, and to reduce system costs. The tunable notch filter can significantly improve the communication system quality.
While the present invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that various other filters can be constructed in accordance with the invention as defined by the claims.
This application is a continuation application of U.S. patent application Ser. No. 10/780,218, entitled, “ELECTRICALLY TUNABLE NOTCH FILTERS”, filed Feb. 17, 2004, which was a divisional of U.S. patent application Ser. No. 10/013,265 entitled, “ELECTRICALLY TUNABLE NOTCH FILTERS”, filed Dec. 10, 2001, which claimed the benefit of U.S. Provisional Application Ser. No. 60/254,841, filed Dec. 12, 2000.
| Number | Date | Country | |
|---|---|---|---|
| 60254841 | Dec 2000 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10013265 | Dec 2001 | US |
| Child | 10780218 | Feb 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10780218 | Feb 2004 | US |
| Child | 11369109 | Mar 2006 | US |
