Abstract:
The invention discloses the utilization of various transmission lines that entail a ferroelectric material as dielectric substrate to introduce an impedance shift by means of an externally applied d.c. bias, which alters the effective length between the input and output signals of the transmission lines of microwave couplers.

Description:
The invention described herein was made in the performance of official duties by an employee of the Department of the Army and may be manufactured, used, licensed by or for the Government for any governmental purpose without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     1.0 Field of the Invention 
     The present invention relates to microwave couplers and, more particularly, to microwave couplers having means for increasing and varying the range of operating frequencies by means of a voltage control (i.e. voltage tunable). Currently, there is a need for frequency agile materials and components used in a wide variety of applications from communication to radar and electronic countermeasures. This invention contributes to the pursuit of this technology. 
     2.0 Description of the Prior Art 
     Microwave couplers are junctions between different sections of transmission lines. They allow microwave radiation to be ducted from one portion of a circuit to another while maintaining amplitude, phase, and modulation integrity. For example, in a microwave transmission line connecting an RF generator to an antenna, a coupler(s) would routinely be placed in the line in order to measure the power being delivered to the antenna. The coupled microwave signal might be only a small fraction of the total power being delivered and might not be at the same phase as the signal transmitted to the antenna, however, these differences should be constant (at a given frequency) and easily characterized. The microwave couplers are fabricated in a predetermined manner so as to provide for a desired center operating frequency with some defined bandwidth, such as f center =10 GHz with a 1 GHz bandwidth (i.e., 9.5 to 10.5 GHz). It is desired to provide means that increases the range of operating frequencies for the microwave couplers and, if desired, altering the characteristic impedance of the microwave couplers, while at the same time maintaining or even reducing the insertion loss or the return loss. All this being done while allowing for tunability of the operating frequency of the microwave coupler. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide means for increasing the range of the operating frequencies of a microwave coupler. 
     It is another object of the present invention to provide means for altering the characteristic impedance of the microwave coupler. 
     It is yet another object of the present invention to provide for tunability of the microwave coupler, while at the same time providing a broader range of operating frequencies for the microwave coupler. 
     It is a further object of the present invention to provide for a tunable branchline coupler, a tunable Wilkinson divider, a tunable backward wave coupler, and tunable Lange couplers each tunable device having increased and adjustable frequency operating ranges. 
     In accordance with these and other objects, the invention provides for a coupler for transferring a RF signal and having at least one input and at least one output with the input receiving the RF signal. The coupler comprises a transmission line and a piece of ferroelectric material. The transmission line has at least one first section having a first effective length defined by a first quarter wavelength and at least one section serving as the output of the coupler. The piece of ferroelectric material is arranged so as to substitute for a normal substrate material under at least one first section and which alters the first effective length by means of an external d.c. bias so as to be defined by a second quarter wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and advantages of the present invention will be more fully understood from the following detailed description having reference to the appended drawings wherein: 
     FIG. 1 schematically illustrates a prior art microwave coupler transferring energy between a RF signal source and a RF signal user: 
     FIG. 2 illustrates one embodiment of the microwave coupler of the present invention that increases the operating frequency of the microwave coupler and that illustrates the use of an external source supplying a d.c. bias; 
     FIG. 3 illustrates a second embodiment of a microwave coupler in accordance with the practice of the present invention; 
     FIG. 4 schematically illustrates the operation of the present invention that is used to alter the characteristic impedance of the microwave coupler in accordance with the practice of the present invention; 
     FIG. 5 illustrates a prior art microwave coupler; 
     FIG. 6 illustrates another embodiment of the present invention related to a backward wave coupler; 
     FIG. 7 illustrates another embodiment of the present invention related to a Lange coupler; 
     FIG. 8 illustrates a schematic depiction of an embodiment of present invention utilizing Ansoft Corp. “Serenade” microwave circuit simulation software (known in the art) for the branchline coupler of FIG. 2 enhanced with ferroelectric material; and 
     FIG. 9 illustrates a schematic depiction of another embodiment of present invention utilizing Ansoft Corp. “Serenade” microwave circuit simulation software (known in the art) for the parallel line or backward wave coupler of FIG. 6 enhanced with ferroelectric material. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to microwave couplers and, more particularly, to microwave couplers having means for increasing and varying their range of operating frequencies by means of a control voltage (i.e., voltage-tunable). This tunability is accomplished by using a control voltage to alter the microwave couplers dielectric permittivity and consequently, their characteristic impedance without severely degrading other characteristics, such as insertion loss and return loss (input matching). To accomplish these improvements, the present invention utilizes a ferroelectric material that has the characteristic of a dielectric constant, which can be varied using an applied d.c. bias. 
     A ferroelectric can be defined as a dielectric with a spontaneous polarization that can be reversed in sign upon the application of an electric field. Many technologically important ferroelectric materials are found in the perovskite oxide, ABO 3  class of materials. Depending upon constituent atoms used in the structure, perovskite oxides have a wide variety of properties, such as superconductors, ferroelectrics, colossal magnetoresistors, dielectrics, conductors, semiconductors, etc. For ferroelectrics, the term ferro has been used because these materials are analogous in some ways to ferromagnetic materials. Ferroelectric materials below the Curie temperature, T c  exhibit a hysteresis loop when plotting polarization (C/cm 2 ) versus applied electric field (V/cm), which is analogous to the hysteresis loop (B versus H) of ferromagnetic materials. The structure of a ferroelectric material becomes less distorted as the temperature increases. Ferroelectrics have many important device applications. Below T c  the ferroelectric effect can be used for radiation-hard memory. The piezoelectric effect can be used for MEMs actuators and sensors. The pyroelectric effect can be used for uncooled IR detectors. Above T c  the ferroelectric material is in the paraelectric regime, where the microwave dielectric losses are minimized while the dielectric constant can be changed with no hysteresis using an applied d.c. bias, such as used in the present invention. Thus, in the paraelectric regime, these materials can be used for phase shifters and other RF-tunable devices. The most popular ferroelectric material for RF phase shifters is Ba 1−x Sr x TiO 3  (BST), where x≦1. By using an appropriate value for x, the Curie temperature can be controlled in a linear manner. Vegard&#39;s type rule (known in the art), between the Curie temperature of BaTiO 2  (T c =393K) and that of SrTiO 3  (T c &lt;70K). Typically, for phase shifters operating at room temperature, 0.4≦x≦0.6 placing the Curie temperature, 175K≦T c ≦250K, for BST. 
     In order that the inventive aspects of the present invention may be more fully appreciated, reference is first made to a prior art microwave coupler by referring to the drawings, wherein the same reference number indicates the same element throughout, and wherein FIG. 1 is a layout of a conventional branchline coupler  10  not having the benefits of the present invention. 
     The branchline coupler  10  is used to divide an input signal, generated by an RF signal source  12 , into two (2) output signals, each at half of the original power, one in phase with the input signal, and one in quadrature (90 deg lag). These output signals may then be transmitted to some other part of the circuit, amplifier, antenna, etc. The branchline coupler  10  has an input stage  16  having a characteristic impedance, such as 50 ohms, which is the impedance that the RF signal source  12  “sees.” The branchline coupler  10  has an output stage  18  that provides the transfer of the microwave energy to the RF signal user  14  as designated by the directional arrow. The branchline coupler  10  has a known insertion loss and a known return loss, wherein directional arrow  20  indicates a current that is returned from the branchline coupler  10  to the RF signal source  12  and directional arrows  24  and  26  indicate the flow of the current from the branchline coupler  10  to the RF signal user  14 . 
     The branchline coupler  10  has two branches comprised of four side members  28 ,  30 ,  32  and  34  that are interconnected to each other by junction members  36 ,  38 ,  40  and  42 . The output stage  18  of the branchline coupler is connected to the RF signal user  14  by conductive transmission lines  44  and  46 . 
     Each of the sides  28 ,  30 ,  32  and  34  acts as a quarter wave (λ/4) transmission line, known in the art. When a transmission line is a quarter of a wavelength, the standing wave developed by feeding in a RF signal into a transmission line varies from a maximum at one end to a minimum at the other end, with no other maximum or minimum therebetween. As the frequency of the applied signal increases, the wavelength decreases. Thus, if a section of a line is a quarter wavelength at one frequency it can not be a quarter wavelength at any other frequency unless its physical length is changed, or the effective length is changed by the practice of the present invention. More particularly, the present invention provides for a branchline coupler  100  having an increased operating frequency yielded by reducing the effective length that defines the quarter wavelength (λ/4) characteristic and which may be further described with reference to FIG.  2 . 
     The branchline coupler  100  of the present invention is quite similar to the branchline coupler  10  of FIG.  1  and utilizes the same reference number to indicate the same elements therebetween, but in addition thereto comprises four pieces of ferroelectric material  102 ,  104 ,  106 , and  108  that are respectively arranged around side members  28 ,  30 ,  32  and  34 . The four (4) pieces of ferroelectric material  102 ,  104 ,  106 , and  108  substitute for a normal substrate material found in the prior art branchline coupler  10 . The ferroelectric material contained in pieces  102  and  106  are essentially the same and, similarly, the ferroelectric material contained in pieces  104  and  108  are essentially the same. Each of the ferroelectric pieces  102 ,  104 ,  106 , and  108  serves as a permittivity/impedance shifter so as to change the effective length of those sections of transmission line between the RF signal appearing at the input stage  16  and existing at the output stage  18 . The effect is that the coupler circuit is retuned to a different frequency because of the change in impedance and effective length. As previously discussed, the magnitude of change is effected by the d.c. bias applied to the ferroelectric pieces  102 ,  104 ,  106 , and  108 . The applied d.c. bias is schematically shown in FIG. 2 as an element  110  having a battery  112  with busses  114  and  116  applied to the ferroelectric pieces  102 ,  104 ,  106 , and  108 . This applied d.c. bias is also applicable to the other embodiments of the present invention, but not shown therefore for the sake of clarity. Further, as seen in FIG. 2 a blocking capacitor C B  is placed at each of the junction members  36 ,  40  and  42 , interchangeably referred to herein as tees, so as to keep the d.c. bias applied to the microwave couplers of the present invention from damaging the R.F. source, that is, RF signal source  12 . The blocking capacitor C B  is used for all embodiments of the present invention in a manner as shown for the respective illustrations thereof. 
     Another embodiment  200  of the present invention may be described with reference to FIG.  3 . The embodiment  200  is a Wilkinson divider, known in the art, and which includes a prior art transmission line element  50  at its input stage  16 A having a characteristic impedance of 50 ohms, a terminating resistor  52  having a typical value of 50 ohms at its output stage  18 A, and two output transmission lines  54  and  56  respectively connected to outputs  14  and  48  respectively labled output #1 and output #2. Further, the Wilkinson divider  200  of the present invention comprises a member  58  which is C-shaped in cross-section and having an upper section  60  and a lower section  62 . 
     The Wilkinson divider  200  comprises a piece of ferroelectric material  202  which surrounds the upper and lower sections  60  and  62  of the C-shaped member  58  as shown in FIG.  3 . The ferroelectric material  202  operates in the same manner as previously described for ferroelectric materials  102 ,  104 ,  106  and  108  of the FIG. 2 embodiment so as to alter the effective length, more particularly, the quarter wavelength (λ/4) transmission line parameter associated with the coupler  200 . The altering of the quarter wavelength is accomplished so as to decrease the operating wavelength which, in turn, increases the operating frequency at which the Wilkinson divider  200  of the present invention performs successfully. 
     The Wilkinson divider  200  of the present invention may also be used to alter the characteristic impedance at which it operates and may be further described with reference to FIG.  4 . FIG. 4 illustrates the terminating resistor  52  connected across a pair of transmission lines, with the first transmission line made up of elements  60  and  202  and the second transmission line made up of elements  62  and  202 . By altering the length of the quarter wavelength (λ/4), that is, by adding the ferroelectric material  202 , the equivalent impedance for transferring energy from the input stage  16 A to the output stage  18 A may also be altered. More particularly, the elements  60 - 202  and  62 - 202  may be treated as legs of a network in which an input signal is applied to the input stage  16 A having an impedance of 50 ohms and the output signal is taken off of the terminating resistor  52  also having an impedance of 50 ohms. The level or amount of the output RF signal appearing across the terminating resistor  52  is determined by the impedance of each of the legs  60 - 202  and  62 - 202  which, in turn, is determined by the effective length of the quarter wavelength (λ/4) which, in turn, is determined by the amount of ferroelectric material of element  202 . 
     A further embodiment of the present invention related to a backward wave coupler, sometimes referred to as a parallel coupler, may be further described by first referring to FIG. 5 showing a prior art backward wave coupler  64 . FIG. 5 schematically illustrates a backward wave coupler  64  as comprised of at least first and second sections  66  and  68 , each of which run in parallel with each other and each of which have an effective length  74  of L 1  The first and second sections are respectively interrelated to transmission lines  70  and  72 . The first section  66  is connected to the RF signal source  12 , the second section  68  is connected to a coupled port element  13 , the transmission line  70  is connected to RF signal user  14 , and the transmission line  72  is connected to a termination element  15 . The effective length  74  of L 1  is of importance and may be increased by the practice of the present invention to a quantity  304  of L 2  which may be described with reference to FIG.  6 . 
     FIG. 6 illustrates a backward wave coupler  300  of the present invention comprised of the elements shown in FIG. 5 having the same reference numbers thereof, but in addition thereto, includes a piece of ferroelectric material  302  that is arranged to surround the first and second sections  66  and  68 . The ferroelectric material  302  operates in the same manner as described for the ferroelectric materials of FIGS. 2 and 4 so as to increase the effective length from L 1 , (shown as  74  in FIG. 5) to L 2  (shown as  304  in FIG.  6 ). The effective length L 2  of FIG. 6 is increased relative to the effective length L 1  of FIG. 5 which, in turn, increases the quarter wavelength dimension of the backward wave coupler  300  which, in turn, decreases the operating frequency of the backward wave coupler  300  relative to that of the backward wave coupler  64 . 
     In the practice of the invention computer aided design (CAD) simulation utilizing Ansoft Corporation “Serenade” microwave circuit simulation software (known in the art) was employed and may be further described with reference to FIGS. 8 and 9. 
     FIG. 8 illustrates a simulation arrangement  500  having an input  501  that is provided with the frequencies starting at 15 GHz and ending at 100 MHz, with incremental size frequencies being 5 GHz. The arrangement  500  has the input  501 , an isolated element  502 , an output  503  having in phase components, and an output  504  having quadrature components respectively connected to ports  1 ,  4 ,  2  and  3  identified by reference numbers  506 ,  508 ,  510  and  512 . 
     FIG. 8 further has an arrangement  514  which represents a schematic for a standard branchline coupler comprised of tees  516 ,  518 ,  520  and  522 , respectively arranged in a clockwise manner, with tee  516  connected to port  1  ( 506 ), tee  518  connected to port  2  ( 510 ), tee  520  connected to port  3  ( 512 ) and tee  522  connected to port  4  ( 508 ). The tees  516 ,  518 ,  520  and  522  serve as junctions, as shown in FIG. 8, between circuit sections  524 ,  526 ,  528 , and  530 . 
     The enhancement provided by the present invention is comprised of the circuit sections  524 ,  526 ,  528  and  530  labeled as “trl” and that use a substrate material different from the normal substrate material. The different substrate material has been previously described as being a ferroelectric material. In the simulation of present invention, the dielectric permittivity was varied between 2.33 and 3.11 in order to simulate the performance of the branchline coupler of FIG. 2 enhanced by the ferroelectric material. 
     FIG. 9 illustrates a simulation arrangement  600  having an input  601  providing the same inputs as input  501  of FIG.  8 . The arrangement  600  has the input  601 , a coupled port  602 , an output  603 , and an isolated element  604  respectively connected to ports  1 ,  4 ,  2  and  3  (already described with reference to FIG. 8) identified by reference number  506 ,  508 ,  510  and  512 . 
     FIG. 9 is analagous to FIG. 8 in most respects except the parallel line or backward wave coupler of FIG. 9, generally indicated by reference number  606  itself consists of a pair of coupled transmission lines  608  and  610  with varying substrate, that is, the varying substrate material, similar to the circuit sections  524 ,  526 ,  528  and  530  of FIG.  8 . Any transmission lines external to this coupler would be assumed to have a constant dielectric as found in a normal substrate. Simulation was again performed in accordance with the embodiment  600  of the present invention and the results of the simulation showed that the maximum coupling (S 21 ) (known in the art) varied from 10.10 GHz to 9.4 GHz while the dielectric constant of transmission lines  604  and  606  was varied from 2.33 to 3.11. 
     In accordance with the practice of the present invention, and with reference to the arrangement  500  of FIG. 8, further computer simulation was performed and the results of the simulation showed a change in center frequency from 9.6 GHz to 8.5 GHz as measured from the point of maximum coupling (i.e. coupling or S 21  was measured and plotted vs the aforementioned frequencies). Likewise, the minimum reflection coefficient (S 11 , and indicator of the quality of the impedance match) varied from 9.8 to 8.7 GHz. This clearly demonstrates proof of the principle of the present invention with a tunability of about 9%. In addition, phase measurements were taken and demonstrated no additional variation in the 90 degree phase difference between ports  2  and  3  after varying the dielectric constant. 
     It should now be appreciated that the practice of the present invention provides for microwave couplings having an increased or decreased operating frequency which are achieved by providing a piece of ferroelectric material around each of the quarter wavelength transmission line members of conventional microwave couplers. 
     Various additional modifications will become apparent to those skilled in the art, all such variations which basically rely on a teaching to which this invention is advanced to the art are properly considered within the scope of this invention.