Voltage tunable coplanar phase shifters

BACKGROUND OF INVENTION

This invention relates generally to electronic phase shifters and, more particularly to voltage tunable phase shifters for use at microwave and millimeter wave frequencies that operate at room temperature.

Tunable phase shifters using ferroelectric materials are disclosed in United States Patents No. 5,307,033, 5,032,805, and 5,561,407. These phase shifters include a ferroelectric substrate as the phase modulating elements. The permittivity of the ferroelectric substrate can be changed by varying the strength of an electric field applied to the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal passes through the phase shifter. The ferroelectric phase shifters disclosed in those patents suffer high conductor losses, high modes, DC bias, and impedance matching problems at K and Ka bands.

One known type of phase shifter is the microstrip line phase shifter. Examples of microstrip line phase shifters utilizing tunable dielectric materials are shown in United States Patents No. 5,212,463; 5,451,567 and 5,479,139. These patents disclose microstrip lines loaded with a voltage tunable ferroelectric material to change the velocity of propagation of a guided electromagnetic wave.

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. Patent No. 5,312,790 to Sengupta, et al. entitled "Ceramic Ferroelectric Material"; U.S. Patent No. 5,427,988 to Sengupta, et al. entitled "Ceramic Ferroelectric Composite Material-BSTO-MgO"; U.S. Patent No. 5,486,491 to Sengupta, et al. entitled "Ceramic Ferroelectric Composite Material - BSTO-ZrO₂"; U.S. Patent No. 5,635,434 to Sengupta, et al. entitled "Ceramic Ferroelectric Composite Material-BSTO-Magnesium Based Compound"; U.S. Patent No. 5,830,591 to Sengupta, et al. entitled "Multilayered Ferroelectric Composite Waveguides"; U.S. Patent No. 5,846,893 to Sengupta, et al. entitled "Thin Film Ferroelectric Composites and Method of Making"; U.S. Patent No. 5,766,697 to Sengupta, et al. entitled "Method of Making Thin Film Composites"; U.S. Patent No. 5,693,429 to Sengupta, et al. entitled "Electronically Graded Multilayer Ferroelectric Composites"; and U.S. Patent No. 5,635,433 to Sengupta, entitled "Ceramic Ferroelectric Composite Material-BSTO-ZnO". These patents are hereby incorporated by reference. A copending, commonly assigned United States patent application titled "Electronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phases", by Sengupta, filed June 15, 2000, discloses additional tunable dielectric materials and is also incorporated 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.

Adjustable phase shifters are used in many electronic applications, such as for beam steering in phased array antennas. A phased array refers to an antenna configuration composed of 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. Phase shifters play key role in operation of phased array antennas. The electronic beam steering concept applies to antennas used with both a transmitter and a receiver. Phased array antennas are advantageous in comparison to their mechanical counterparts with respect to speed, accuracy, and reliability. The replacement of gimbals in mechanically scanned antennas with electronic phase shifters in electronically scanned antennas increases the survivability of antennas used in defense systems through more rapid and accurate target identification. Complex tracking exercises can also be maneuvered rapidly and accurately with a phased array antenna system.

United States Patent No. 5,617,103 discloses a ferroelectric phase shifting antenna array that utilizes ferroelectric phase shifting components. The antennas disclosed in that patent utilize a structure in which a ferroelectric phase shifter is integrated on a single substrate with plural patch antennas. Additional examples of phased array antennas that employ electronic phase shifters can be found in United States Patents No. 5,079,557; 5,218,358; 5,557,286; 5,589,845; 5,617,103; 5,917,455; and 5,940,030.

United States Patents No. 5,472,935 and 6,078,827 disclose coplanar waveguides in which conductors of high temperature superconducting material are mounted on a tunable dielectric material. The use of such devices requires cooling to a relatively low temperature. In addition, United States Patents No. 5,472,935 and 6,078,827 teach the use of tunable films of SrTiO₃, or (Ba, Sr)TiO₃ with high a ratio of Sr. ST and BST have high dielectric constants, which results in low characteristics impendence. This makes it necessary to transform the low impendence phase shifters to the commonly used 50 ohm impedance.

Low cost phase shifters that can operate at room temperature could significantly improve performance and reduce the cost of phased array antennas. This could play an important role in helping to transform this advanced technology from recent military dominated applications to commercial applications.

There is a need for electrically tunable phase shifters that can operate at room temperatures and at K and Ka band frequencies (typically 18 GHz to 27 GHz and 27 GHz to 40 GHz, respectively), while maintaining high Q factors and have characteristic impedances that are compatible with existing circuits.

SUMMARY OF INVENTION

In one aspect the present invention provides a reflective termination coplanar waveguide phase shifter including a substrate, a tunable dielectric film having a dielectric constant between 70 to 600, a tuning range of 20 to 60 %, and a loss tangent between 0.008 to 0.03 at K and Ka bands, the tunable dielectric film being positioned on a surface of the substrate, first and second open ended coplanar waveguide lines positioned on a surface of the tunable dielectric film opposite the substrate, a microstrip line for coupling a radio frequency signal to and from the first and second coplanar waveguide lines, and a connection for applying a control voltage to the tunable dielectric film.

The conductors forming the coplanar waveguide operate at room temperature. The coplanar phase shifters of the present invention can be used in phased array antennas at wide frequency ranges. The devices herein are unique in design and exhibit low insertion loss even at frequencies in the K and Ka bands. The devices utilize low loss tunable film dielectric elements.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a top plan view of a reflective phase shifter constructed in accordance with the present invention;</li><li>FIG. 2 is a cross-sectional view of the phase shiver of FIG. 1, taken along line 2-2;</li><li>FIG. 3 is a schematic-diagram of the equivalent circuit of the phase shifter of FIG. 1;</li><li>FIG. 4 is a top plan view of another phase shifter;</li><li>FIG. 5 is a cross-sectional view of the phase shifter of FIG. 4, taken along line 5-5;</li><li>FIG. 6 is a top plan view of another phase shifter;</li><li>FIG. 7 is a cross-sectional view of the phase shifter of FIG. 6, taken along line 7-7;</li><li>FIG. 8 is a top plan view of another phase shifter;</li><li>FIG. 9 is a cross-sectional view of the phase shifter of FIG. 8, taken along line 9-9;</li><li>FIG. 10 is a top plan view of another phase shifter;</li><li>FIG. 11 is a cross-sectional view of the phase shifter of FIG. 10, taken along line 11-11;</li><li>FIG. 12 is an isometric view of a phase shifter constructed in accordance with the present invention; and</li><li>FIG. 13 is an exploded isometric view of an array of phase shifters constructed in accordance with the present invention.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally coplanar waveguide voltage-tuned phase shifters that operate at room temperature in the K and Ka bands. The devices utilize low loss tunable dielectric films. In the preferred embodiments, the tunable dielectric film is a Barium Strontium Titanate (BST) based composite ceramic, having a dielectric constant that can be varied by applying a DC bias voltage and can operate at room temperature.

FIG. 1 is a top plan view of a reflective phase shifter constructed in accordance with the present invention. FIG. 2 is a cross-sectional view of the phase shifter of FIG. 1, taken along line 2-2. The embodiment of FIGs. 1 and 2 is a 20 GHz K band 360° reflective coplanar waveguide phase shifter 10. The phase shifter 10 has an input/output 12 connected to a 50-ohm microstrip line 14. The 50-ohm microstrip line 14 includes a first linear line 16 and two quarter-wave microstrip lines 18, 20, each with a characteristic impedance of about 70 ohm. The microstrip line 14 is mounted on a substrate 22 of material having a low dielectric constant. The two quarter-wave microstrip lines 18, 20 are transformed to coplanar waveguides (CPW) 24 and 26 and match the line 16 to coplanar waveguides 24 and 26. Each CPW includes a center strip line 28 and 30 respectively, and two conductors 32 and 34 forming a ground plane 36 on each side of the strip lines. The ground plane conductors are separated from the adjacent strip line by gaps 38, 40, 42 and 44. The coplanar waveguides 24 and 26 have a characteristic impedance of about Z₂₄ = 15 ohms and Z₂₆ = 18 ohms, respectively. The difference in impedances is obtained by using strip line conductors having slightly different center line widths. The coplanar waveguides 24 and 26 work as resonators. Each coplanar waveguide is positioned on a tunable dielectric layer 46. The conductors that form the ground plane are connected to each other at the edge of the assembly. The waveguides 24 and 26 terminate in at open ends 48 and 50.

Impedances Z₂₄ and Z₂₆ correspond to zero bias voltage. Resonant frequencies of the coplanar waveguide resonators are slightly different and are determined by the electrical lengths of λ₂₄ and λ₂₆. The slight difference in the impedances Z₂₄ and Z₂₆ is helpful in reducing phase error when the phase shifter operates over a wide bandwidth. Phase shifting results from dielectric constant tuning that is controlled by applying a DC control voltage 52 (also called a bias voltage) across the gaps of the coplanar waveguides 24 and 26. Inductors 54 and 56 are included in the bias circuit 58 to block radio frequency signals in the DC bias circuit.

The electrical lengths of λ₂₄ and λ₂₆ and bias voltage across the coplanar waveguide gaps determine the amount of the resulting phase shift and the operating frequency of the device. The tunable dielectric layer is mounted on a substrate 22, and the ground planes of the coplanar waveguide and the microstrip line are connected through the side edges of the substrate. A radio frequency (RF) signal that is applied to the input of the phase shifter is reflected at the open ends of the coplanar waveguide. In the preferred embodiment, the microstrip and coplanar waveguide are made of 2 micrometer thick gold with a 10 nm thick titanium adhesion layer by electron-beam evaporation and lift-off etching processing. However, other etching processors such as dry etching could be used to produce the pattern. The width of the lines depends on substrate and tunable film and is adjusted to obtain the desired characteristic impedances. The conductive strip and ground plane electrodes can also be made of silver, copper, platinum, ruthenium oxide or other conducting materials compatible to the tunable dielectric films. A buffer layer for the electrode may be necessary, depending on electrode-tunable film system and processing techniques used to construct the device.

The tunable dielectric used in the preferred embodiments of phase shifters of this invention has a lower dielectric constant than conventional tunable materials. The dielectric constant can be changed by 20 % to 70 % at 20 V/µm, typically about 50 %. The magnitude of the bias voltage varies with the gap size, and typically ranges from about 300 to 400 V for a 20 µm gap. Lower bias voltage levels have many benefits, however, the required bias voltage is dependent on the device structure and materials. The phase shifter in the present invention is designed to have 360° phase shift. The dielectric constant can range from 70 to 600, and typically from 300 to 500. In the preferred embodiment, the tunable dielectric is a barium strontium titanate (BST) based film having a dielectric constant of about 500 at zero bias voltage. The preferred material will exhibit high tuning and low loss. However, tunable material usually has higher tuning and higher loss. The preferred embodiments utilize materials with tuning of around 50 %, and loss as low as possible, which is in the range of (loss tangent) 0.01 to 0.03 at 24 GHz. More specifically, in the preferred embodiment, the composition of the material is a barium strontium titanate (Ba_xSr_1-xTiO₃, BSTO, where x is less than 1), or BSTO composites with a dielectric constant of 70 to 600, a tuning range FROM 20 to 60 %, and a Joss tangent 0.008 to 0.03 at K and Ka bands. The tunable dielectric layer may be a thin or thick film. Examples of such BSTO composites that possess the required performance parameters include, but are not limited to: BSTO-MgO, BSTO-MgAl₂O₄, BSTO-CaTiO₃, BSTO-MgTiO₃, BSTO-MgSrZrTiO₆, and combinations thereof. FIG. 3 is a schematic diagram of the equivalent circuit of the phase shifter of FIGs. 1 and 2.

The K and Ka band coplanar waveguide phase shifters of the preferred embodiments of this invention are fabricated on a tunable dielectric film with a dielectric constant (permittivity) ε of around 300 to 500 at zero bias and a thickness of 10 micrometer. However, both thin and thick films of the tunable dielectric material can be used. The film is deposited on a low dielectric constant substrate MgO in the CPW area with thickness of 0.25 mm. For the purposes of this description a low dielectric constant is less than 25. MgO has a dielectric constant of about 10. However, the substrate can be other materials, such as LaAlO₃, sapphire, Al₂O₃ and other ceramics. The thickness of the film of tunable material can be adjusted from 1 to 15 micrometers depending on deposition methods. The main requirements for the substrates are their chemical stability, reaction with the tunable film at film firing temperature (~1200 C), as well as dielectric loss (loss tangent) at operation frequency. Fig. 5 is a cross-sectional view of the phase shifter assembly 60 of FIG. 4, taken along line 5-5. Phase shifter assembly 60 is fabricated using a tunable dielectric film and substrate similar to those set forth above for the phase shifter of FIGs. 1 and 2. Assembly 60 includes a main coplanar waveguide 62 including a center line 64 and a pair of ground plane conductors 66 and 68 separated from the center line by gaps 70 and 72. The center portion 74 of the coplanar waveguide has a characteristic impedance of around 20 ohms. Two tapered matching sections 76 and 78 are positioned at the ends of the waveguide and form impedance transformers to match the 20-ohm impedance to a 50-ohm impedance. Coplanar waveguide 62 is positioned on a layer of tunable dielectric material 80. Conductive electrodes 66 and 68 are also located on the tunable dielectric layer and form the CPW ground plane. Additional ground plane electrodes 82 and 84 are also positioned on the surface of the tunable dielectric material 80. Electrodes 82 and 84 also extend around the edges of the waveguide as shown in FIG. 5. Electrodes 66 and 68 are separated from electrodes 82 and 84 respectively by gaps 86 and 88. Gaps 86 and 88 block DC voltage so that DC voltage can be biased on the CPW gaps. For dielectric constant ranging from about 200 to 400 and an MgO substrate, the center line width and gap are about 10 to 60 micrometers. The tunable dielectric material 80 is positioned on a planar surface of a low dielectric constant (about 10) substrate 90, which in the preferred embodiment is MgO with thickness of 0.25 mm. However, the substrate can be other materials, such as LaAlO₃, sapphire, Al₂O₃ and other ceramic substrates. A metal holder 92 extends along the bottom and the sides of the waveguide. A bias voltage source 94 is connected to strip 64 through inductor 96.

The coplanar waveguide phase shifter 60 can be terminated with either another coplanar waveguide or a microstrip line. For the latter case, the 50-ohm coplanar waveguide is transformed to the 50-ohm microstrip line by direct connection -of the central line of coplanar waveguide to microstrip line. The ground planes of the coplanar waveguide and the microstrip line are connected to each other through the side edges of the substrate. The phase shifting results from dielectric constant tuning by applying a DC voltage across the gaps of the coplanar waveguide.

FIG. 6 shows a 20 GHz coplanar waveguide phase shifter 98, which has a structure similar to that of FIGs. 4 and 5. However, a zigzag coplanar waveguide 100 having a central line 102 is used to reduce the size of substrate. FIG. 7 is a cross-sectional view of the phase shifter of FIG. 6, taken along line 7-7. The waveguide line 102 has an input 104 and an output 106, and is positioned on the surface of a tunable dielectric layer 108. A pair of ground plane electrodes 110 and 112. are also positioned on the surface of the tunable dielectric material and separated from line 102 by gaps 114 and 116. The tunable dielectric layer 108 is positioned on a low loss substrate 118 similar to that described above. The circle near the middle of the phase shifter is a via 120 for connecting ground plane electrodes 110 and 112.

FIG. 8 is a top plan view of the phase shifter assembly 42 of FIG. 4 with a bias dome 130 added to connect the bias voltage to ground plane electrodes 66 and 68. FIG. 9 is a cross-sectional view of the phase shifter assembly 60 of FIG. 8, taken along line 9-9. The dome connects the two ground planes of the coplanar waveguide, and covers the main waveguide line. An electrode termination 132 is soldered on the top of the dome to connect to the DC bias voltage control. Another termination (not shown) of the DC bias control circuit is connected to the central line 64 of the coplanar waveguide. In order to apply the bias DC voltage to the CPW, small gaps 86 and 88 are made to separate the inside ground plane electrodes 66 and 68, where the DC bias dome is located, to the other part (outside) of the ground plane (electrodes 82 and 84) of the coplanar waveguide. The outside ground plane extends around the sides and bottom plane of the substrate. The outside or the bottom ground plane is connected to an RF signal ground plane 134. The positive and negative electrodes of the DC source are connected to the dome 130 and the center line 64, respectively. The small gaps in the ground plane work as a DC block capacitors, which block DC voltage. However, the capacitance should be high enough to allow RF signal through it. The dome electrically connects ground planes 66 and 68. This connection should be mechanically strong enough to avoid touch anything. The dome is one of these connections. It should be noted that the widths of ground planes 66 and 68 are about 0.5 mm.

A microstrip line and the coplanar waveguide line can be connected to one transmission line. FIG. 10 is a top plan view of another phase shifter 136. FIG. 11 is a cross-sectional view of the phase shifter of FIG. 10, taken along line 11-11. FIGs. 10 and 11 show how the microstrip 138 line transforms to the coplanar waveguide assembly 140. The microstrip 138 includes a conductor 142 mounted on a substrate 144. The conductor 142 is connected, for example by soldering or bonding, to a central conductor 146 of coplanar waveguide 148. Ground plane conductors 150 and 152 are mounted on a tunable dielectric material 154 and separated from conductor 146 by gaps 156 and 158. In the illustrated embodiment, solder 160 connects conductors 142 and 146. The tunable dielectric material 154 is mounted on a surface of a non-tunable dielectric substrate 162. Substrates 144 and 162 are supported by a metal holder 164.

Since the gaps in the coplanar waveguides (&lt; 0.04 mm) are much smaller than the thickness of the substrate (0.25 mm), almost all RF signals are transmitted through the coplanar waveguide rather than the microstrip line. This structure makes it very easy to transform from the coplanar waveguide to a microstrip line without the necessity of a via or coupling transformation.

FIG. 12 is an isometric view of a phase shifter constructed in accordance with the present invention. A housing 166 is built over the bias dome to cover the whole phase shifter such that only two 50 ohm microstrip lines are exposed to connect to an external circuit. Only line 168 is shown in this view.

FIG. 13 is an exploded isometric view of an array 170 of 30 GHz coplanar waveguide phase shifters constructed in accordance with the present invention, for use in a phased array antenna. A bias line plate 172 is used to cover the phase shifter array. The electrodes on the dome of each phase shifter are soldered to the bias lines on the bias line plate through the holes 174, 176, 178 and 180. The phase shifters are mounted in a holder 182 that includes a plurality of microstrip lines 184, 186, 188, 190, 192, 194, 196 and 198 for connecting the radio frequency input and output signals to the phase shifters. The particular structures shown in FIG. 13, provide each phase shifter with its own protective housing. The phase shifters are assembled and tested individually before being installed in the phased array antenna. This significantly improves yield of the antenna, which usually has tens to thousands phase shifters.

The coplanar phase shifters of the preferred embodiments of this invention are fabricated on the voltage-tuned Barium Titanate (BST) based composite films. The BST composite films have excellent low dielectric loss and reasonable tunability. These K and Ka band coplanar waveguide phase shifters provide the advantages of high power handling, low insertion loss, fast tuning, loss cost, and high anti-radiation properties compared to semiconductor based phase shifters. It is very common that dielectric loss of materials increases with frequency. Conventional tunable materials are very lossy, especially at K and Ka bands. Coplanar phase shifters made from conventional tunable materials are extremely lossy, and useless for phased array antennas at K and Ka bands. It should be noted that the phase shifter structures of the present invention are suitable for any tunable materials. However, only low loss tunable materials can achieve good, useful phase shifters. It is desirable to use low dielectric constant material for microstrip line phase shifter, since high dielectric constant materials easily generate high EM modes at these frequency ranges for microstrip line phase shifters. However, no such low dielectric constant conventional materials (&lt;100) are available.

The preferred embodiments of the present invention provide coplanar waveguide phase shifters, which include a BST based composite thick film having a tunable permittivity. These coplanar waveguide phase shifters do not employ bulk ceramic materials as in the microstrip ferroelectric phase shifters above. The bias voltage of the coplanar waveguide phase shifter on film is lower than that of the microstrip phase shifter on-bulk material. The thick film-tunable dielectric layer can be deposited by standard thick, film process onto low dielectric loss and high chemical stability subtracts, such as MgO, LaAlO₃, sapphire, Al₂O₃, and a variety of ceramic substrates.

This invention encompasses reflective coplanar waveguide phase shifters. Reflective coplanar waveguide phase shifter constructed in accordance with the invention can operate at 20 GHz.Transmission coplanar waveguide phase shifters can operate at 20 GHz and 30 GHz. Both types of phase shifters can be fabricated using the same substrate with a tunable dielectric film on the low dielectric loss substrate. A ground plane DC bias and DC block are used. The bias configuration is easy to manufacture, and is not sensitive to small dimensional variations. The phase shifters can have ports with either coplanar waveguide or microstrip lines. For microstrip ports, a direct transformation of the coplanar waveguide to a microstrip is possible. The bandwidth of phase shifters in the present invention is determined by matching sections (impedance transform sections). The use of more matching sections or longer tapered matching sections permits operation over a wider band. However, it results in more insertion loss of the phase shifters.

The preferred embodiment of the present invention uses composite materials, which include BST and other materials, and two or more phases. These composites show much lower dielectric loss, and reasonable tuning, compared to conventional ST or BST films. These composites have much lower dielectric constants than conventional ST or BST films. The low dielectric constants make easy to design and manufacture phase shifters. Phase shifters constructed in accordance with this invention can operate at room temperature (~300°K). Room temperature operation is much easier, and much less costly than prior art phase shifters that operation at 100°K.

The phase shifters of the present invention also include a unique DC bias arrangement that uses a long gap in the ground plane as a DC block. They also permit a single method for transforming the coplanar waveguide to a microstrip line.

While the 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 changes can be made to the preferred embodiments without departing from the scope of the invention, which is defined by the claims.