Patent Publication Number: US-6710679-B2

Title: Analog rat-race phase shifters tuned by dielectric varactors

Description:
FIELD OF INVENTION 
     This invention relates generally to microwave devices and more particularly to analog phase shifters. 
     BACKGROUND OF INVENTION 
     Phased array antennas include a large number of elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. This electrically steered beam concept applies to both the transmitter and the receiver. Phased array antennas are advantageous in comparison to their mechanical counterparts with respect to their speed, accuracy, and reliability. The replacement of gimbal-scanned antennas by their electronically scanned counterpart increases antenna survivability through more rapid and accurate target identification. Complex tracking exercises can also be accomplished rapidly and accurately with a phased array antenna system. 
     A phase shifter is an essential element, which controls the phase of a microwave signal, in a phased array antenna. A good performance and low cost phase shifter can significantly improve performance and reduce the cost of the phased array, which should help to transform this advanced technology from recent military dominated applications to commercial applications. 
     Previous patents that disclose ferroelectric phase shifters include U.S. Pat. Nos.: 5,307,033, 5,032,805, and 5,561,407. The phase shifters disclosed therein include one or more microstrip lines on a ferroelectric (voltage-tuned dielectric) substrate to produce the phase modulating. Tuning of the permittivity of the substrate results in phase shifting when a radio frequency (RF) signal passes through the microstrip line. Microstrip ferroelectric phase shifters suffer from high conducting losses, high modes, DC bias, and impedance matching problems. Coplanar waveguide (CPW) phase shifters made from voltage-tuned dielectric films, whose permittivity may be varied by varying the strength of an electric field on the substrate have also been disclosed. 
     B. T. Henoch and P. Tamm disclosed a 360° varactor diode phase shifter in “A 360° Varactor Reflection Type Diode Phase Modulator,”IEEE Trans. On Microwave Theory and Tech., Vol. MTT-19, January 1971, pp. 103-105. Their design included two parallel coupled series resonant circuits that were connected to a circulator by means of a quarter-wave transformer. The transformer equalizes the insertion loss. However, the phase shifter showed large frequency dependence at phase shifts between 0° to 360°. 
     Ulriksson has modified the above design to optimize frequency response for all phase shifts up to 180° by introducing a slight change in one of the parallel coupled resonant circuits, see B. Ulriksson, “Continuous Varactor-Diode Phase Shifter With Optimum Frequency Response,” IEEE Trans. On Microwave Theory and Tech., Vol. MTT-27, July 1979, pp. 650-654. 
     There is a need for analog phase shifters that are capable of operating at frequencies in the range of 1 to 18 GHz, wherein the phase shift can be electronically controlled. 
     SUMMARY OF INVENTION 
     Phase shifters constructed in accordance with this invention include a first rat-race ring having four ports, an input coupled to a first one of the ports, an output coupled to a second one of the ports, a first resonant circuit coupled to a third one of the ports, and a second resonant circuit coupled to a fourth one of the ports, each of the first and second resonant circuits including a tunable dielectric varactor. 
     A third resonant circuit can be connected in parallel with the first resonant circuit, and a fourth resonant circuit can be connected in parallel with the second resonant circuit. Each of the third and fourth resonant circuits can also include a tunable dielectric varactor. 
     In one embodiment, the first rat race ring can be connected to another phase shifting stage including a second rat race ring. Additional resonant circuits including tunable dielectric varactors can be connected to ports of the second rat race ring. 
     In another embodiment, the first rat race ring can be connected to a digital switched line phase shifting stage. The digital switched line phase shifting stage can include a first and second microstrip lines coupled to each other by first and second capacitors, an input coupled to the first microstrip line, and an output coupled to the second microstrip line, first and second PIN diodes connected between the first microstrip line and ground, third and fourth PIN diodes connected between the second microstrip line and ground, and means for applying a bias voltage to the first and second microstrip lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a 180° analog dielectric varactor phase shifter constructed in accordance with this invention; 
     FIG. 2 is a schematic representation of a 360° analog dielectric varactor phase shifter with two 180° analog phase shifters constructed in accordance with this invention; 
     FIG. 3 is a schematic representation of another 360° analog dielectric varactor phase shifter with one 180° digital rat-race phase shifter constructed in accordance with this invention; 
     FIG. 4 is a schematic representation of another 360° analog dielectric varactor phase shifter with one 180° digital switched line phase shifter constructed in accordance with this invention; 
     FIG. 5 is a top plan view of a dielectric varactor that can be used in the phase shifters of the present invention; and 
     FIG. 6 is a cross sectional view of the dielectric varactor of FIG. 5 taken long line  6 — 6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, FIG. 1 shows a schematic drawing of a 180° analog phase shifter  10  constructed in accordance with the present invention. The phase shifter  10  includes a rat-race ring  12  having four ports  14 ,  16 ,  18  and  20 , and a characteristic input impedance of Z 0 =50 ohm. Port  14  is connected to an input point  22  by way of the series connection of a microstrip lines  24  and  26 , and capacitor  28 . Port  16  is connected to line  30 , which is in turn connected to a pair of parallel circuit branches  32  and  34  having impedances Z 2  and Z 1 , respectively. Branch  32  includes the series connection of lines  36 ,  38  and capacitor  40 . Branch  34  includes the series connection of lines  42 ,  44  and capacitor  46 . One end of each of the circuit branches is connected to ground. The parallel circuit branches have components with slightly different electrical properties to improve the figure of merit and the frequency response of the phase shifter. 
     Port  18  is connected to line  48 , which is in turn connected to a pair of parallel circuit branches  50  and  52 . Branch  50  includes the series connection of lines  54 ,  56  and capacitor  58 . Branch  52  includes the series connection of lines  60 ,  62  and capacitor  64 . One end of each of the circuit branches is connected to ground. A terminal  66  is provided for connection to an external bias voltage supply. Terminal  66  is connected to line  48  through a circuit branch including the series connection of line  68  and resistor  70 . In operation, an RF signal is input to port  14 , equally divided between port  16  and port  18 , and reflected in their short ends. Since the rat-race ring is an inherent 180° hybrid, an extra quarter-wavelength strip is added on port  18  to compensate for the 180° phase difference. Each termination of port  16  and port  18  has the same resonant circuits, which include two series-tuned circuits in parallel and connected to ground at the short ends. Each of the series-tuned circuits includes a high impendence microstrip line, as an inductor, and connects to a dielectric varactor with shorted end in series. It should be noted these two resonant circuits have slightly different inductance and capacitance to optimize frequency response. A DC voltage is input in port  18  through a resistor  70 , working as a RF chock to avoid RF signal leak into the DC source. Two DC block capacitors  28  and  78  are mounted on input and output respectively to isolate varactor bias voltage from devices outside of the ring. 
     In order to achieve a 360° phase shift, another 180° analog phase shifter can be added. FIG. 2 is a schematic representation of a 360° analog tunable dielectric varactor phase shifter with two 180° analog phase shifters constructed in accordance with the invention. The phase shifter  80  includes a second rat-race ring  82  having four ports  84 ,  86 ,  88  and  90 . Port  90  is connected to port  20  of ring  12  through a circuit branch including the series connection of lines  47  and  92 , and capacitor  78 . Port  84  is connected to line  94 , which is in turn connected to a pair of parallel circuit branches  96  and  98 . Branch  96  includes the series connection of lines  100  and  102 , and capacitor  104 . Branch  98  includes the series connection of lines  106  and  108 , and capacitor  110 . One end of each of the circuit branches is connected to ground. Port  88  is connected to line  112 , which is in turn connected to a pair of parallel circuit branches  114  and  116 . Branch  114  includes the series connection of lines  118  and  120 , and capacitor  122 . Branch  116  includes the series connection of lines  124  and  126 , and capacitor  128 . One end of each of the circuit branches is connected to ground. A terminal  130  is provided for connection to an external bias voltage supply. Terminal  130  is connected to line  112  through a circuit branch including the series connection of line  132  and resistor  134 . Port  86  is connected to an output point  144  by way of the series connection of a microstrip lines  138  and  140 , and capacitor  142 . The two rat-race rings of FIG. 2 are identical and connected in series. The center DC blocking capacitor  78  is used for isolation of the DC bias voltages. 
     FIG. 3 is a schematic representation of another 360° analog dielectric varactor phase shifter  146  constructed in accordance with this invention. The phase shifter of FIG. 3 utilizes the rat-race ring  82  and its associated components and adds a 180° phase shifter  148 . Phase shifter  148  includes a ring  150  having ports  152 ,  154 ,  156  and  158 . Port  152  is connected to an input point  160  through a circuit branch  162  including the series connection of lines  164  and  166 , and capacitor  168 . Port  154  is connected to ground through a circuit branch  170  including the series connection of lines  172  and  174 , and PIN diode  176 . Port  156  is connected to ground through a circuit branch  178  including the series connection of lines  180  and  182 , and PIN diode  184 . A terminal  186  is provided for connection to an external bias voltage supply. Terminal  186  is connected to line  180  through a circuit branch including the series connection of line  188  and resistor  190 . Port  158  is connected to ring  82  through the series connection of lines  192  and  92 , and capacitor  78 . In FIG. 3, the first rat-race ring  150  generates 0 or 180° digital phase shifts by switching the PIN diodes to the on or off states. 
     FIG. 4 is a schematic representation of another 360° analog dielectric varactor phase shifter  194  including a 180° analog rat race ring phase shifter and a 180° digital switch line phase shifter  196  constructed in accordance with this invention. The phase shifter  194  of FIG. 4 utilizes the rat-race ring  82  of FIG. 2, and its associated components and adds a 180° digital switch phase shifter  196 . The digital switch phase shifter  196  includes first and second microstrip lines  198  and  200  connected to each other through capacitors  202  and  204 . Microstrip line  198  serves as a 180° phase shift line and microstrip line  200  serves as a reference line. Terminals  206  and  208  are provided for receiving a bias voltage. The bias voltage on terminal  206  is application to line  198  through resistor  210 . The bias voltage on terminal  208  is application to line  200  through resistor  212 . PIN Diodes  214  and  216  are connected between line  198  and ground. PIN Diodes  218  and  220  are connected between line  200  and ground. An RF input  222  is connected to line  200  through the series connection of lines  224  and  226 , and capacitor  228 . Line  198  is connected to ring  82  through a circuit branch including the series connection of lines  230  and  232 , and capacitor  234 . The digital switch line phase shifter generates 0 or 180° digital phase shifts by selecting PIN diode switch on or off states. A signal from input  222  can go to either line  198  or  200  depending upon the “on” or “off” state of PIN diodes pairs  214  and  216 , or  218  and  220 . Two PIN diodes are usually needed to isolate the off-state line from the signal path. 
     The phase shifters of this invention utilize varactors, which include a low loss, tunable dielectric material having high tuning capabilities. In the preferred embodiments, the material comprises a barium strontium titanate (BST) based composite film. FIG. 5 is a top plan view of a dielectric varactor  236  that can be used in the phase shifters of the present invention; and FIG. 6 is a cross sectional view of the dielectric varactor of FIG. 5 taken along line  6 — 6 . The dielectric varactor  236  includes two planar electrodes  238  and  240  mounted on a surface  242  of a substrate  244 . A film of tunable dielectric material  246  is also positioned in the surface of the substrate. Portions  248  and  250  of electrodes  238  and  240  respectively, extend over a surface  252  of the tunable dielectric material and are separated to form a predetermined gap  254 . The substrate can, for example, comprise MgO, alumna (AL 2 O 3 ), LaAlO 3 , sapphire, quartz, silicon, gallium arsenide, and other compatible materials to the tunable films and their processing. A voltage supplied by an external variable DC voltage source  256  produces an electric field across the gap adjacent to the surface of the tunable dielectric material, which produces an overall change in the capacitance of the varactor. The width of the gap can range from 10 to 40 μm depending on the performance requirements. An input  258  is connected to the first electrode  238  and an output  260  is connected to the second electrode  240 . The electrodes are constructed of conducting materials, for example, gold, silver, copper, platinum, ruthenium oxide or other compatible conducting materials to the tunable films. 
     The typical Q factor of the dielectric varactors is 50 to 100 at 20 GHz, and 200 to 500 at 1 GHz with capacitance ratio (C max /C min ) around 2. The capacitance of the dielectric varactor can vary over a wide range, for example, 0.1 pF to 1.0 nF. The tuning speed of the dielectric varactor is about 30 nanoseconds. The dielectric varactor in the present invention has the advantages of high Q, low intermodulation distortion, high power handling, low power consumption, fast tuning, and low cost, compared to semiconductor diode varactors. 
     Tunable dielectric materials have been described in several patents. Barium strontium titanate (Ba x Sr 1−x TiO 3 , where x is less than 1), 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 varactors of the phase shifters in the preferred embodiments of the present invention can 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 ,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 include at least two Mg-containing compounds. Alternatively, the metal oxide phase may optionally include Mg-free compounds, for example, oxides of metals selected from Si, Ca, Zr, Ti, Al and/or rare earths, or 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 present invention provides varactor-tuned rat-race phase shifters. These rat-race phase shifters do not employ bulk ceramic materials as was used in prior art microstrip ferroelectric phase shifters. The bias voltage of these rat-race phase shifters is lower than that of the microstrip phase shifter on bulk material. The thick or thin films of the tunable dielectric material can be deposited onto low dielectric loss and high chemical stability subtracts, such as MgO, LaAlO 3 , sapphire, Al 2 O 3 , and a variety of ceramic substrates. 
     The analog 180° phase shifter in the preferred embodiments includes two parallel coupled series resonant circuits. The resonant circuits include a high impendence line, as an inductor, and a dielectric varactor in series. Zero to 180° phase shifts are determined by capacitances of the dielectric varactors, which are controlled by DC voltages. 
     In alternative embodiments, the present invention also provides 360° varactor-tuned microstrip rat race phase shifters. The varactors (tunable capacitors) preferably include barium strontium titanate (BST) based composite films. These BST composite films have excellent low dielectric loss and reasonable tunability. 
     While the present invention has been described in terms of what are at present its preferred embodiments, it will be apparent to those skilled in the art that various modifications can be made to the preferred embodiments without departing from the invention as defined by the following claims.