Patent Publication Number: US-6985050-B2

Title: Waveguide-finline tunable phase shifter

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of the filing date of U.S. Provisional Application No. 60/198,690, filed Apr. 20, 2000. 

   FIELD OF INVENTION 
   The present invention relates to electronic waveguide devices and more particularly to waveguide-finlines used to control the phase of a guided signal. 
   BACKGROUND OF INVENTION 
   Modern communications systems are using increasingly higher frequencies. At high frequencies, communications utilize higher data transmit/receive rates. When steerable array antennas are used in high frequency communications systems, it is desirable for each antenna element to have fast scan capabilities, small size, low cost and reasonable performance. Phase shifters are critical components for meeting those criteria. 
   Electronic phase shifters are used in many devices to delay the transmission of an electric signal. Waveguide phase shifters have been described in U.S. Pat. Nos. 4,982,171 and 4,654,611. U.S. Pat. No. 4,320,404 discloses a phase shifter using diode switches connected to wire conductors inside a waveguide that are turned on or off to cause a phase shift of the propagating wave. U.S. Pat. Nos. 4,434,409; 4,532,704; 4,818,963; 4,837,528; 5,724,011 and 5,811,830 disclose tuning ferrites, ferromagnetic or ferroelectric slab materials inside waveguides to achieve phase shifting. U.S. Pat. Nos. 4,894,627; 4,789,840 and 4,782,346 disclose devices that use finline structures to build couplers, signal detectors and radiating antennas. These patents either use slab material in a waveguide to construct phase shifters or use finlines for some other application. 
   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. 
   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 to 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-ZrO 2 ”; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material-BSTO-Magnesium Based Compound”; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled “Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled “Electronically Graded Multilayer Ferroelectric Composites”; and U.S. Pat. No. 5,635,433 to Sengupta, entitled “Ceramic Ferroelectric Composite Material-BSTO-ZnO”. These patents are hereby incorporated by reference. Copending, commonly assigned U.S. Pat. No. 6,514,895 to Chiu et al. titled “Electronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phases”, filed Jun. 15, 2000, and U.S. Pat. No. 6,744,077 to Sengupta et al. titled “Electronically Tunable Low-Loss Ceramic Materials Including a Tunable Dielectric Phase and Multiple Metal Oxide Phases”, filed Jan. 24, 2001, disclose additional tunable dielectric materials and are also incorporated by reference. The materials shown in these patents exhibit low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage. 
   U.S. Pat. Nos. 5,355,104 and 5,724,011 disclose phase shifters that include voltage controllable dielectric materials. 
   The prior art does not disclose a finline waveguide structure that is used as a tunable phase shifter. There is a need for tunable phase shifters that are relatively simple in structure, low in cost, and can be rapidly controlled. 
   SUMMARY OF THE INVENTION 
   Tunable phase shifters constructed in accordance with this invention include a waveguide, a finline substrate positioned within the waveguide, a tunable dielectric layer positioned on the finline substrate, a first conductor positioned on the tunable dielectric layer, and a second conductor positioned on the voltage tunable dielectric layer, with the first and second conductors being separated to form a gap. 
   By controlling the voltage applied to the conductors, the phase of a signal passing through the waveguide can be controlled. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded isometric view of a tunable phase shifter constructed in accordance with a first embodiment of the invention; 
       FIG. 2  is a side elevation view of a finline structure the may be used in the phase shifter of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the finline of  FIG. 2  taken along line  3 — 3 ; 
       FIG. 4  is a cross-sectional view of an assembled version of the waveguide phase shifter of  FIG. 1  taken along line  4 — 4 ; 
       FIG. 5  is graph of the phase shift versus bias voltage for a phase shifter constructed in accordance with the invention; 
       FIG. 6  is graph of the losses versus bias voltage for a phase shifter constructed in accordance with the invention; 
       FIG. 7  is an exploded isometric view of another tunable phase shifter constructed in accordance with the invention; 
       FIG. 8  is a side elevation view of a finline structure the may be used in the phase shifter of  FIG. 7 ; 
       FIG. 9  is a cross-sectional view of the finline of  FIG. 8  taken along line  9 — 9 ; 
       FIG. 10  is an exploded isometric view of another tunable phase shifter constructed in accordance with the invention; 
       FIG. 11  is a side elevation view of a finline structure the may be used in the phase shifter of  FIG. 10 ; and 
       FIG. 12  is a cross-sectional view of the finline of  FIG. 11  taken along line  12 — 12 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention provides a waveguide-finline tunable phase shifter that uses a film of voltage tunable material mounted on a finline. When a DC tuning voltage is applied to the tunable film, the dielectric constant of the film changes, which causes a change in the group velocity, and therefore, produces a phase shift in a signal passing through the waveguide. 
   Referring to the drawings,  FIG. 1  is an exploded isometric view of a 30 GHz tunable phase shifter  10  constructed in accordance with a preferred embodiment of the invention. The phase shifter  10  includes a waveguide  12  including side portions  14  and  16 . In one embodiment, the waveguide can be a WR-28, 26 to 40 GHz rectangular waveguide. However, the invention is not limited to a particular type of waveguide or frequency of operation. Side portion  14  includes a longitudinal groove  18  and side portion  16  includes a longitudinal groove  20 . When the side portions are brought together, the grooves form a channel  22 . First and second conductive plates  24  and  26  are positioned between the waveguide portions. Conductive plate  24  includes a connection point  28  for connection to a variable DC voltage source  30  by way of conductor  32 . A finline structure  34  is positioned between the conductive plates, which in the preferred embodiment are made of copper. Insulating sheets  36  and  38  are positioned on opposite sides of conductive plate  24  to insulate it from the conductive waveguide portions. In the preferred embodiment, the insulating sheets are made of mica. Conductive plate  26  is allowed to make electrical contact with the waveguide portions and is connected to an electrical ground either directly, or through the waveguide portions. 
     FIG. 2  is a side elevation view of a finline structure  34  that may be used in the phase shifter of  FIG. 1 , and  FIG. 3  is a cross-sectional view of the finline structure  34  taken along line  3 — 3  in  FIG. 2 . Finline structure  34  includes a low dielectric constant, low loss substrate  40  (see  FIG. 3 ) with a layer of tunable material  42  deposited thereon. The preferred embodiment of this invention utilizes MgO as the substrate material. The tunable material is metalized with conductive material to form electrodes  46  and  48  that define a gap  44 , which separates the electrodes  46  and  48  on the tunable material layer, as best shown in  FIG. 3 . The gap extends longitudinally from a first end  50  to a second end  52  of the structure. The gap includes a central portion  54  and first and second exponentially tapered end portions  56  and  58  respectively (see  FIG. 2 ). The end portions are tapered such that the gap widens near the ends to provide impedance matching. Referring to  FIG. 1 , conductive plates  24  and  26  form exponentially tapered gaps  60  and  62  to provide additional impedance matching. Gaps  60  and  62  lie adjacent to the ends of gap portions  56  and  58  respectively. A plurality of openings, for example  64 ,  66  and  68 , are located in the various components of the phase shifter of  FIG. 1  for receiving fasteners that will be used to hold the phase shifter together. 
   The finline structure is constructed in a unilateral configuration, and in this example, no circuit or metalization is on the rear surface of the substrate  40 . The tunable dielectric film on the front of the finline structure is metalized to form two electrodes  46  and  48  (as shown in  FIGS. 2 and 3 ). In the preferred embodiment, the tunable dielectric film can be a thin film ranging from 0.2 to 2.0 {circumflex over (3)}m in thickness, or a thick film ranging from 2 to 30 {circumflex over (3)}m in thickness, with a dielectric constant ranging from 30 to 2000. The exponentially tapered gaps in the metalization on the tunable dielectric material match the impedance at the ends to that of the center tunable region. The center tunable region includes a gap  54  (see  FIG. 2 ) between two generally parallel edges of the metalized conductors with the width of the gap ranging from about 2 to about 50 {circle around (3)}m to form a capacitor. At each end of the tuning region, the same matching structure is mirrored to convert the impedance to that of the free space waveguide. 
     FIG. 4  is a cross-sectional view of an assembled version of the finline of  FIG. 1  taken along line  4 — 4 . In this view, the transverse orientation of the finline structure within the channel  22  can be seen. In addition, this view shows that conductive plate  26  is electrically connected to the waveguide portions  14  and  16 . 
   DC biasing via the metalized conductors controls the phase shifting. The top conductive plate is isolated using insulating films to prevent voltage breakdown. The bottom part of the finline structure is connected to the waveguide wall or ground. 
     FIG. 5  is graph of the phase shift versus bias voltage for a phase shifter constructed in accordance with the invention. Curve  72  represents data obtained at 300° K. 
     FIG. 6  is graph of the losses versus bias voltage for a phase shifter constructed in accordance with the invention. Curve  74  represents the calculated loss tangent (tanδ) Curve  78  represents the test results of a phase shifter with a calculated conductor loss S 21  dB. Curve  76  represents the measured test results of a phase shifter configured according to the present invention with a biasing voltage applied to yield a conductor loss S 21C  dB. Conductor loss S 21C  dB is less than calculated conductor loss S 21  dB (curve  78 ). 
   The finline mode will propagate through the parallel gap portion of the finline structure. Due to the tunable film dielectric constant decreasing under the biasing voltage, the guided signal will change its phase velocity when passing through this region. For a 360° phase shift, the total length, L, needed is: 
       L   =       λ   g       1   -       1   -   T               
 
where T is the tunability, and λ g  is the wavelength of a signal guided through the device.
 
   Another method for estimating tunability is using the capacitance variance ratio, such as the ratio, K, of C 1 , the tuning section capacitance before biasing, to C 2 , the capacitance after biasing. That is: K=C 1 /C 2 . Since the physical dimensions are not changing, this ratio represents the change of effective dielectric constant K=ε e1 /ε e2 , and K=1/(1−T), where ε e1  represents the dielectric constant at zero bias voltage and ε e2  represents the dielectric constant at a predetermined bias voltage. For example, a finline phase shifter can have a K of about two, or a tunability of about 50%. 
   The biasing voltage required to generate a 360° phase shift is about a few hundred volts.  FIG. 5  shows the phase response versus biasing voltage, which is approximately a linear relationship.  FIG. 6  shows the test results of the phase shifter, indicating that insertion loss is better under the biasing voltage. That is because both the dielectric constant and the loss tangent are decreased under biasing voltage. A way to estimate the performance of the device is using the figure of merit, which is defined as: 
       F   =       Δϕ           S   21     ⁡     (     0   ⁢   v     )       ·       S   21     ⁡     (     400   ⁢   v     )             ⁢           ⁢     (     degree   /   dB     )           
 
where Δφ is the total phase change under biasing voltage and S 21  is the loss in dB.
 
   This invention provides electronic phase shifters that operate at room temperature and include voltage tunable materials. When a DC tuning voltage is applied to the tunable material, the dielectric constant of the material changes, which causes a change in the group velocity and therefore produces a controllable phase shift. 
     FIG. 7  is an exploded isometric view of another tunable phase shifter  80  constructed in accordance with an alternative embodiment of the invention. The phase shifter  80  includes a waveguide  82  including side portions  84  and  86 . Side portion  84  includes a longitudinal groove  88  and side portion  86  includes a longitudinal groove  90 . When the side portions are brought together, the grooves form a channel  92 . A finline structure  94  is positioned between the side portions of the waveguide. 
     FIG. 8  is a side elevation view of a finline structure  94  that may be used in the phase shifter of  FIG. 7 , and  FIG. 9  is a cross-sectional view of the finline structure  94  taken along line  9 — 9 . Finline structure  94  (see  FIG. 7 ) includes a low dielectric constant, low loss substrate  96  (see  FIG. 9 ) with a layer of tunable material  98  (see  FIG. 9 ) deposited thereon. The preferred embodiment of this invention utilizes MgO as the substrate material. The tunable material is metalized with conductive material to form electrodes  100  and  102  that define a gap  104 , which separates the electrodes  100  and  102  on the tunable material layer (as best seen in  FIG. 8 ). The gap extends longitudinally from a first end  106  to a second end  108  of the structure. The gap includes a central portion  110  and first and second exponentially tapered end portions  112  and  114  respectively. The end portions are tapered such that the gap widens near the ends to provide impedance matching. Electrode  102  has a relatively large surface area so that it provides an RF ground to the waveguide structure. In addition, in the embodiment shown in  FIG. 8 , electrode  102  includes and RF choke design  116  to ensure the RF ground and DC isolation. 
   The embodiment shown in  FIGS. 7 ,  8  and  9  uses a spring loaded contact  118  to connect the bias voltage from voltage source  120  to one of the metalized layers on the tunable material (as shown in  FIG. 7 ). This design reduces the size and simplifies the structure. Furthermore, the first electrode  100  is DC grounded, while the second electrode  102  is DC biased and forms an RF ground. The RF ground can be provided via the large area of electrode, or through an RF choke design as shown in  FIG. 8 , on the substrate to ensure an RF ground. 
     FIG. 10  is an exploded isometric view of another tunable phase shifter  122  constructed in accordance with another alternative embodiment of the invention. The phase shifter  122  includes a waveguide  124  including side portions  126  and  128 . Side portion  126  includes a longitudinal groove  130  and side portion  128  includes a longitudinal groove  132 . When the side portions are brought together, the grooves from a channel  134 . A finline structure  136  is positioned between the side portions of the waveguide. 
     FIG. 11  is a side elevation view of a finline structure  136  that may be used in the phase shifter of  FIG. 10 , and  FIG. 12  is a cross-sectional view of the finline structure  136  taken along line  11 — 11  (in  FIG. 11 ). Finline structure  136  includes a low dielectric constant, low loss substrate  138  with a layer of tunable material  140  deposited thereon (as shown in  FIG. 12 ). The preferred embodiment of this invention utilizes MgO as the substrate material. The tunable material is metalized with conductive material to form electrodes  142  and  144  that define a gap  146 , which separates the electrodes  142  and  144  on the tunable material layer (best shown in  FIG. 11 ). The gap extends longitudinally from a first end  148  to a second end  150  of the structure. The gap includes a central portion  152  and first and second exponentially tapered end portions  154  and  156  respectively. The end portions are tapered such that the gap widens near the ends to provide impedance matching. 
   The embodiment shown in  FIGS. 10 ,  11  and  12  uses a spring loaded contact  158  to connect the bias voltage from voltage source  160  to one of the metallized layers on the tunable material (as shown in  FIG. 10 ). This design reduces the size and simplifies the structure. Furthermore, the first electrode is DC grounded, while the second electrode is DC biased with an RF ground. The RF ground can be provided via the large area of the electrode, or by an RF choke design on the substrate to ensure RF ground and DC isolation. 
   Referring to  FIG. 10 , channel forms tapered sections  162  and  164  to provide additional impedance matching. The tapered section lies adjacent to the ends of gap portions  154  and  156 . The embodiment shown in  FIGS. 10 ,  11  and  12  uses a non-standard waveguide to optimize the phase shifter. The non-standard waveguide would then be coupled to a standard waveguide. 
   In the preferred embodiment the tunable dielectric layer is preferably comprised of Barium-Strontium Titanate, Ba x Sr 1-x TiO 3  (BSTO), where x can range from zero to one, or BSTO-composite ceramics. Examples of such BSTO composites include, but are not limited to: BSTO-MgO, BSTO-MgAl 2 O 4 , BSTO-CaTiO 3 , BSTO-MgTiO 3 , BSTO-MgSrZrTiO 6 , and combinations thereof. Other 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 ranges from 0.2 to 0.8, and preferably from 0.4 to 0.6. Additional alternative tunable ferroelectrics include Pb x Zr 1-x TiO 3  (PZT) where x ranges from 0.05 to 0.4, lead lanthanum zirconium titanate (PLZT), lead titanate (PbTiO 3 ), barium calcium zirconium titanate (BaCaZrTiO 3 ), sodium nitrate (NaNO 3 ), KNbO 3 , LiNbO 3 , LiTaO 3 , PbNb 2 O 6 , PbTa 2 O 6 , KSr(NbO 3 ), and NaBa 2 (NbO 3 ) 5  and KH 2 PO 4 . In addition, the present invention can include electronically tunable materials having 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 . The above tunable materials can be tuned at room temperature by controlling an electric field that is applied across the materials. 
   This invention utilizes a finline structure that is disposed within a waveguide. The structure includes a low loss substrate and a tunable dielectric film. The tunable film is metalized to form two conductors. Impedance matching is provided by using exponentially tapered sections of a gap between the conductors. In one embodiment, at the leading edge of the waveguide, two copper plate sections match free-space waveguide to the dielectric substrate, which is sandwiched between the copper plates. On the dielectric substrate, tapered metalized sections on the tunable film match the impedance to the center tunable region. 
   This invention takes advantage of a high dielectric constant of voltage tunable thick film materials, such as BSTO, to build a 360° waveguide-finline phase shifter. 
   The phase shifters of this invention can be electronically tuned to provide repeatable and stable phase shifts. Since the tunable material is a good insulator, the DC power consumption of the tuning voltage supply is very low, with a current typically less than a microampere. The voltage tuned phase shifters have the advantage of fast tuning, good tunability, small size, simple control circuits, low power consumption, and low cost. In addition, the phase shifters show good linear behavior and can be radiation hardened. 
   An example of an application of the phase shifters of this invention is in phased array antennas. An array of radiating elements generates a specified beam pattern, with each element controlled by a phase shifter and the array of elements working together to form a beam in a desired direction. A 360° phase shifter can direct the radiating electromagnetic energy to any specified direction without mechanically moving the radiating element. By assembling a number of antenna elements to form a phased array, the direction of the main lobe of the beam, can be controlled. This is achieved through the adjustment of the signal amplitude and phase of each antenna element in the array. The advantage of phase array antennas is their accurate pointing of the beam in the specified direction that minimizes radiation in unwanted directions, and improves the signal-to-noise ratio and overall efficiency of the system. 
   In phased array antenna applications, the phase control needs to be accurate, reliable and fast. By using the present tunable phase shifter in phased array antennas, an accurate phase shift will be easier to obtain by tuning a DC voltage. The phase shift versus tuning voltage is an approximately linear relationship. In addition, higher power applications can be realized by using waveguide structure phase shifters. 
   While the present invention has been described in terms of what are at present believed to be its preferred embodiments, it will be apparent to those skilled in the art that various changes may be made to the disclosed embodiments without departing from the scope of the invention as defined by the following claims.