Patent Publication Number: US-7714679-B2

Title: Spiral coupler

Description:
FIELD OF THE INVENTION 
     This invention relates to an improved spiral coupler. 
     BACKGROUND OF THE INVENTION 
     With the dramatic developments of wireless telecommunication and satellite communication systems, there have been increasing demands for highly integrated RF and microwave circuits with better performance, smaller size and lower cost. Many of these systems use couplers, such as a 3-dB coupler and other directional couplers, in their microwave circuitry. Couplers of many types and variations have been developed for circuits processing signals at microwave frequencies. U.S. Pat. No. 3,516,024 was issued on Jun. 2, 1970 to Lange for an Interdigitated Strip Line Coupler. This coupler, also described in Lange, “Interdigitated Strip-Line Quadrature Hybrid”, MTTS Digest of Technical Papers, Dallas, Tex., May 5-7, 1969, pp. 10-13, has become generally known as a Lange coupler. Since this early work on strip line conductors many variations have been developed. 
     These are variously described in Waterman, Jr., et al., “GaAs Monolithic Lange and Wilkinson Couplers”, and Brehm et al., “Monolithic GaAs Lange Coupler at X-Band”, both of IEEE Transactions on Electron Devices, Vol. ED-28, No. 2, February 1981, pages 212-216, and pages 217-218, respectively; Tajima et al., “Monolithic Hybrid Quadrature Couplers (Braided Structures)”, IEEE GaAs IC Symposium, 1982, pages 154 and 155; Kumar et al., “Monolithic GaAs Interdigitated Couplers”, IEEE, 1983, pages 359-362; Kemp et al., “Ultra-Wideband Quadrature Coupler”, IEEE Transactions, 1983, pp. 197-199; Shibata et al., “Microstrip Spiral Directional Coupler”, IEEE Transactions, 1981, pp. 680-689; Lentz, “Compact Transmission Line Consisting of Interleaved Conductor Strips and Shield Strips”, U.S. Pat. No. 3,162,717 issued Dec. 22, 1964; Oh, “Three Conductor Coplanar Serpentine-line Directional Coupler”, U.S. Pat. No. 3,332,039 issued Jul. 18, 1967; Presser et al., “High Performance Interdigitated Coupler with Additional Jumper Wire”, U.S. Pat. No. 4,636,754 issued Jan. 13, 1987; and Podell et al., “Spiral Hybrid Coupler”, U.S. Pat. No. 4,800,345 issued Jan. 24, 1989. All of these above cited references are herein incorporated in their entirety by this reference. 
     These various forms of interdigitated and strip-line conductors provide coupling with various degrees of success for different fabrication technology. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved spiral coupler. 
     It is a further object of this invention to provide such an improved spiral coupler suitable for semiconductor and other planar fabrication processes. 
     It is a further object of this invention to provide such an improved spiral coupler which has the simplicity of a Lange coupler but with reduced size and improved performance. 
     It is a further object of this invention to provide such an improved spiral coupler which improves isolation and directivity. 
     It is a further object of this invention to provide such an improved spiral coupler in which all ports of coupler are external to the spiral path. 
     The invention results from the realization that an improved spiral coupler can be achieved with a plurality of parallel coextensive conductive strips disposed in a planar spiral path, and a cross-over connection for bridging the strips from the inside to the outside of the spiral path to provide all four ports external to the spiral path. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     This invention features an improved spiral coupler including a plurality of parallel, coextensive conductive strips disposed in a planar spiral path, including a first strip having an input port and a direct or through port, a second strip having a coupler port and an isolated port; and a first cross-over connection for bridging said strips from the inside to the outside of said spiral path to provide all four said ports external access to said spiral path. 
     In a preferred embodiment the spiral path may be symmetrical and the first cross-over connection may be on the axis of symmetry. There may be only two the strips. The improved spiral coupler may further include a second cross-over connection for interchanging the relative positions of the first and second strips in the spiral path. The spiral path may be symmetrical and the second cross-over connection may be on the axis of symmetry. The second cross-over connection may be disposed at the midpoint of the spiral path. Each strip may include a plurality of discrete parallel elements interdigitated with those of the other strips. There may be a number of loops in the spiral path, each loop having first and second cross-over connections. The plurality of discrete elements in each strip may be shunted together at the cross-over connection to present a single conductive member for bridging. There may be a plurality of spaced shunts interconnected between the elements of each strip spaced along the spiral path. 
     This invention also features an improved four port spiral directional coupler including first and second parallel, coextensive conductive strips disposed in a planar spiral path, the first strip having an input port and a direct or through port, the second strip having a coupler port and an isolated port; and a first cross-over connection for bridging the strips from the inside to the outside of the spiral path to provide all four the ports external to the spiral path. 
     In a preferred embodiment the spiral path may be symmetrical and the first cross-over connection may be on the axis of symmetry. The improved four port spiral directional coupler may further include a second cross-over connection for interchanging the relative positions of the first and second strips in the spiral path. The second cross-over connection may be on the axis of symmetry. The second cross-over connection may be disposed at the midpoint of the spiral path. Each strip may include a plurality of discrete parallel elements interdigitated with those of the other strips. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic plan diagram of a prior art directional coupler on a substrate; 
         FIG. 2  is a view similar to  FIG. 1  of a prior art Lange type interdigitated directional coupler; 
         FIG. 3  is a schematic plan diagram of a prior art spiral directional coupler; 
         FIG. 4  is a schematic plan diagram of a spiral coupler according to this invention on a substrate. 
         FIG. 5  is a view similar to  FIG. 4  with each strip having discrete elements interdigitated with each other; 
         FIG. 6  is a view similar to  FIG. 5  in which the spiral path contains two loops; 
         FIG. 7  is a view similar to  FIG. 6  in which the elements in each strip are directly connected in parallel at the cross-over without being shunted together; 
         FIG. 8  is a view similar to  FIG. 5  in which the spiral path contains four loops; and 
         FIG. 9  is a view similar to  FIG. 5  in which the spiral path contains a two-turn loop. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     The invention presents a solution that not only reduces the size of directional couplers, especially the size of a Lange coupler, but also improves the isolation and directivity. The proposed topology only requires one layer of metals for strip lines and another layer for bridging the cross-over connections, similar to that in a standard Lange coupler. The dimensions of the resulting couplers can be reduced to one third to one sixth of standard quarter-wave length couplers. Two strip conductor sections of the couplers are wounded parallel to each other to form a complete spiral loop or loops by properly crossing over each other along the symmetrical central line. In this invention, the lengths of the coupled strip lines are equalized to each other and the symmetry of the structure enables the reciprocal responses between input port, coupled port, direct or through port and isolated port. 
     To preserve the structural symmetry in a spiral loop form, all of the four ports of the coupler are connected symmetrically to the outer circle of the spiral loops. In addition, at the interconnection between the inner circle and the outer inner circle loop, the two-conductor-strip pair of the coupler going from outer loop into the inner loop and the strip pair connecting from inner loop to outer loop cross-over each other. This can be realized by standard microstrip technologies both at the PCB board level and at the semi-conductor die level. The cross-over is located along the symmetrical center line of the coupler. 
     To keep the same line length of the coupled strips, the conductor strips are crossed over each other within the coupling pair at the middle point of the spiral loop. Thereby, each conductor strip runs half-way along the inner side of the loop and half-way along the outer side of the loop. For this purpose, cross-overs between the inner loop and the outer loop using conductors at a second layer can be employed. There are two alternative way to realize this: either using a cross-over over bridge for each strip as shown in the two strip coupler example shown in  FIG. 4  or using a cross-over bridge after shunting multiple strips together as shown in the four strip coupler example in  FIG. 5 . The first type of cross-over introduces less parasitics and presents wider bandwidth, while the second type of cross-over helps to reduce the size of the cross-over section. 
     Besides equalizing the line lengths of the strips, the adjacent strips between inner and outer turns are from the same conductor and the electromagnetic wave propagates along the same direction. The couplers of this invention have higher even mode impedance than a regular un-folded coupler with same strip width and spacing, while the odd mode impedance is close to a regular un-folded coupler. With these properly controlled mutual couplings between loops, a high coupling ratio like 3-dB can be readily achieved over wide bandwidth without using small spacing between strips. In addition, high isolation and directivity can be obtained in the proposed spiral coupler. Besides these two cross-over bridges along the symmetrical center loop of the proposed coupler loop, cross-over connections may also be added near the corners of the loops to reduce the phase dispensing, which also helps to increase the coupling. To cover lower frequency or achieve smaller circuit size, several coupling loops can be cascaded in series to form a multiple-loop coupler, as shown in  FIGS. 6-8 , where two to four loops have been connected in series and the orientations between the adjacent loops have been arranged so that their mutual coupling helps to improve the overall performance. Another alternative approach is using multiple-turn spiral configuration by introducing more loops around the same center, as shown in  FIG. 9 , where a second loop with proper cross-over at the symmetric line is employed around the single loop version shown in  FIG. 5 . 
     There is shown in  FIG. 1  a prior art four port directional coupler  10  including two conductor strips  12  and  14 . Strip  12  has an input port  16  and a direct or through port  18 . Strip  14  has a coupled port  20  and an isolated port  22 . The length of strip  12  and  14  are generally equal to ¼ of the wavelength of the center operating frequency, e.g. for couplers designed for 3 GHz applications, the length will be around 1 centimeter long if the circuit is fabricated using high frequency semicoductor process. The width of the strips and the gap between them are designed to optimize the efficiency of the coupling or transfer. The widths w of strips  12  and  14  and the gap g between them are generally uniform and are chosen to optimize the efficiency of transfer. A typical coupling efficiency is in the range of 10%, i.e. a coupling ration of 10-dB. An improvement is shown in the prior art device of,  FIG. 2 , where the four port directional coupler  10   a  is a Lange type wherein each strip  12   a , and  14   a  is formed from a plurality of elements  12   aa , and  12   aaa ,  14   aa , and  14   aaa . This provides much greater coupling with a transfer efficiency in the range of 50%, i.e. a coupling ration of 3-dB. The widths w of the elements  12   aa ,  12   aaa ,  14   aa ,  14   aaa  are generally uniform as are the gaps and all are chosen to optimize transfer efficiency. 
     In another prior art approach a four part directional coupler  30 ,  FIG. 3 , is configured as a planar spiral wherein two conductor strips  32  and  34  extend inwardly in a spiral beginning in input port  36  and coupled port  35  and terminating in direct or through port  40  and isolated port  38 . One disadvantage of this design is that two of the ports, in this case, direct or through port  40  and isolated port  38  end up inside of the spiral where they are not easily accessible. Another shortcoming of the spiral coupler in  FIG. 3 , is that the width of the gap changes, for example, having a width g 1  in one place and a width g 2  in another in order to balance the coupling and equalize the coupling throughout the length of strips  32  and  34 . 
     An improved four port symmetrical spiral directional coupler  50 , according to this invention,  FIG. 4 , may be disposed on a substrate, such as a suitable PCB board, semi-conductor substrate or other planar fabrication material  52 . Spiral coupler  50  includes a plurality of conductive strips, for example, a first strip  54  and a second strip  56 . Strip  54  has an input port  58  at one end and a direct or through port  60  at the other. Strip  56  has a coupled port  62  at one end and an isolated port  64  at the other. Spiral coupler  50  is the form of a single symmetrical loop  66  having a center line of symmetry  68 . A first cross-over connection  70  is disposed at the center line of symmetry  68  and uses shunts  72  and  74  to direct strips  54  and  56  from the outside of spiral path depicted at  76  to the inside, thus allowing both the input and coupled ports  58  and  62  and the direct and isolated ports  60  and  64  to be external of the spiral path  76 . A second cross-over connection  80  can also be employed using for example shunt  82  but in this case the cross-over connection is used to swap the relative position of strips  54  and  56 . Thus, viewing from right to left at the top of the loop in  FIG. 4 , strip  54  is on the top or right and strip  56  is on the bottom or left. Following the swap at cross-over connection  80  strip  54  is on the left or bottom and strip  56  is on the right or top. This is done to equalize the length of the strips in the spiral and to further balance the coupling effect of coupler  50 . While cross-over connection  70  is preferably located on the center line of symmetry  68 , second cross-over connection  80  is preferably on the center line of symmetry  68  and also at the midpoint of strips  54  and  56 . 
     The approach of this invention as shown in  FIG. 4  can be applied in an interdigitated configuration,  FIG. 5 , where coupler  50   a  includes a conductor strip  54   a  which has a plurality of conductive elements e.g. conductive elements  54   aa , and  54   aaa  and strip  56   a  has a plurality of elements, e.g., conductive elements  56   aa  and  56   aaa . Now in cross-over connection  70   a , the ends of elements  56   aa , and  56   aaa  are connected together at shunts  82  and  84  and also at shunts  86  and  88  and are interconnected by cross-over shunts  90  and  92 . Likewise conductor elements  54   aa  and  54   aaa  have their ends connected together at shunts  94  and  96  and  98  and  100  and are interconnected by cross-over conductors  102  and  104 . Similarly, but more simply, at second cross-over connection  80   a  elements  56   aa  and  56   aaa  are connected by cross-over shunts  106  and  108  and conductor elements  54   aa  and  54   aaa  are cross-over connected by shunts  110  and  112 . Also shown in  FIG. 5  are a plurality of auxiliary shunts  114  appearing throughout loop  66   a  of coupler  50   a , most prevalently in the corner areas in order to further balance the coupling and improve transfer efficiency. 
     Although thus far the invention has been shown with a single loop in  FIGS. 4 and 5  this is not a necessary limitation of the invention. As shown in  FIG. 6  there may be two loops  66   a ,  66   b , each with a first cross-over connection  70   a ,  70   b , and a second cross-over connection  80   a ,  80   b .  FIG. 7 , shows an alternative construction for the first cross-over connections  70   a ,  70   b  of  FIG. 6 . In  FIG. 7 , cross-over connections  70 ′ a  and  70 ′ b  do not use anticipatory shunts before entering the cross-over connections. Rather, four separate cross-over shunts  116 ,  118 ,  120 ,  122  are used for elements  54   aa  and  54   aaa  and four cross-over shunts  124 ,  126 ,  128 ,  130  for conductive elements  56   aa  and  56   aaa  and similar cross-over conductors are used in second cross connection  70 ′ b . The coupler in accordance with this invention may be expanded as desired, for example, in  FIG. 8 , there is shown coupler  50 ′″ a  which includes in the spiral four loops,  66   a ,  66   b ,  66   c , and  66   d , each with its own first cross connection  70   a ,  70   b ,  70   c , and  70   d  and second cross connection  80   a ,  80   b ,  80   c ,  80   d . Also, the coupler in accordance this invention may be expanded around the same loop center, for example, in  FIG. 9 , there is shown  50   b  which includes an extra loop  60   a  around the loop  60   b  used in  FIG. 5  and first cross-over connection  70   a , and  70   b  and second cross-over connection  80   a  and  80   b.    
     Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.