Patent Publication Number: US-9413054-B2

Title: Miniature wideband quadrature hybrid

Description:
BACKGROUND OF THE INVENTION 
     1. Statement of the Technical Field 
     The present invention is directed to the field of directional couplers, and more particularly, to directional couplers having a miniaturized design. 
     2. Description of the Related Art 
     Directional couplers are four-port circuits typically used for sampling of the input power for use in signal monitoring circuits. The sampled signal is typically measured to determine the power level, frequency, and/or signal shape (modulation) of the input signal. One typical directional coupler configuration is referred to as a hybrid coupler, a 3 dB coupler, a 3 dB hybrid coupler, a quadrature coupler, or a quadrature hybrid coupler, amongst other names. Regardless of how it is referred to, the quadrature hybrid coupler generally has the characteristics of dividing the input signal into two signals having equal powers and separated in phase by 90 degrees when the four ports are properly terminated. 
     Quadrature hybrid couplers are commonly implemented by using two edge coupled transmission lines. However, there are design challenges which arise when implementing quadrature hybrid couplers using planar circuit fabrication technologies, such as integrated circuit technologies, stripline technologies, and printed circuit board technologies. U.S. Pat. No. 7,741,929 to Hash discloses one type of miniature hybrid coupler which seeks to overcome some of these design challenges. 
     The design problems associated with implementing quadrature hybrid couplers using planar circuit fabrication technologies are compounded when wideband performance is a design goal. The reason for this generally relates to the need for additional coupler sections when implementing wideband coupler designs. The additional sections typically each have an electrical length of ¼ wavelength and therefore occupy a significant amount of space on a substrate. Moreover, the additional sections generally need to provide a relatively low amount of coupling. Low coupling is not conducive to compact layouts since it usually involves transmission lines traces having a relatively wide physical width and a relatively large space between coupled lines. Consequently, it has not been practical to implement wideband hybrid couplers in RF integrated circuits, except at millimeter wave. Instead, wideband hybrid couplers have been implemented using surface mount technology (SMT) components. These types of hybrid couplers can provide satisfactory performance, but are prohibitively large for many applications and cannot be practically implemented on RFICs. 
     SUMMARY OF THE INVENTION 
     The invention concerns a radio frequency directional coupler which includes a first, second and third transmission line element. Each of the first and second transmission line elements is disposed on a dielectric substrate, and has a first end and a second end. The first and the second transmission line elements disposed on common plane and at least a portion of the first and second transmission line elements are adjacent along a path. At least a third transmission line element extends substantially coextensive with a length of said first and second transmission line elements. At least the third transmission line element is disposed along the path between the first and the second transmission line elements and separated therefrom by a portion of the dielectric substrate. The third transmission line element is electrically connected to a ground plane disposed on a surface of the dielectric substrate opposed from the first and second transmission line elements. 
     The invention also concerns a wideband radio frequency directional coupler which includes three coupler sections. A first and second coupler section each include a first, second and third transmission line arranged as previously described. The first coupler section and the second coupler section are electrically connected by a third coupler section. The third coupler section includes a fourth transmission line element having a first end and a second end and a fifth transmission line element having a first end and a second end. The fourth and fifth transmission line elements are disposed on the common plane, and at least a portion of the fourth and fifth transmission line elements are disposed adjacent along a second path. A first series of conductive coupling elements is disposed along the second path in a second plane parallel to the first plane and separated from the first plane by a predetermined distance to increase a capacitive coupling between the fourth and fifth transmission line elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which: 
         FIG. 1  is a top view of a radio frequency directional coupler. 
         FIG. 2  is a cross-sectional view of the directional coupler in  FIG. 1 , taken along line  2 - 2 . 
         FIG. 3  is an enlarged perspective view of a ground junction portion of the directional coupler in  FIG. 1 . 
         FIG. 4  is a perspective view of a portion of the directional coupler in  FIG. 1 , enlarged to show detail. 
         FIG. 5  is an enlarged perspective view of a cross-over portion of the directional coupler in  FIG. 1 . 
         FIG. 6  is a simplified schematic drawing which is useful for understanding an alternative embodiment of the radio frequency directional coupler in  FIG. 1 . 
         FIG. 7  is a top view of a wideband multi-section radio frequency directional coupler. 
         FIG. 8  is a top view of a center section of the coupler in  FIG. 7 . 
         FIG. 9  is a perspective view showing a cross-section of the coupler in  FIG. 7 , taken along line  9 - 9 . 
     
    
    
     DETAILED DESCRIPTION 
     The invention is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operation are not shown in detail to avoid obscuring the invention. The invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the invention. 
     There is shown in  FIGS. 1 and 2  a radio frequency directional coupler section  100 . The coupler section can be used alone or in combination with other coupler sections as hereinafter described. The basic operating principle of coupler section  100  is similar to that of a conventional hybrid coupler. Radio frequency energy applied to an input port (e.g. port  126 ) is communicated to an output port (e.g. port  130 ). A portion of the RF energy is also coupled from the input port (e.g. port  126 ) to a coupled port  128 . A terminated port (e.g. port  132 ) is terminated in an impedance that preferably matches the characteristic impedance of the device. A cross-over  124  flips the position of the transmission lines. Consequently output port  130  is directly coupled to the input port  126 , port  132  is a terminated (or isolated) port, and  128  is the coupled port. 
     The coupler section  100  includes a transmission line  108 . The transmission line is comprised of a conductive trace having a predetermined width W 1 , which is disposed on a dielectric substrate  102 . The dielectric substrate  102  can be formed of one or more layers of dielectric material disposed on a ground plane  101 . The transmission line has a first end at ground junction  104  and a second end at ground junction  105 . A transmission line  112  is also formed of a conductive trace which has a width W 1 . The transmission line  112  is disposed on the dielectric substrate, and has a first end at ground junction  104  and a second end at ground junction  105 . The transmission lines  108 ,  112  are disposed on a planar surface defined by the dielectric substrate and it can be observed in  FIG. 1  that at least a portion of the transmission lines are adjacent along a path. As shown in  FIG. 2 , the transmission lines are separated by a distance S 1  and have an electrical length that is substantially equal. 
     A pair of transmission lines  110   a ,  110   b  are each also comprised of a conductive trace and each has a width W 2 , which can be the same or different as compared to W 1 . The transmission lines  110   a ,  110   b  are disposed along the path between the transmission lines  108 ,  112  and are separated from transmission lines  108 ,  112  by a portion of the dielectric substrate. As shown in  FIG. 2 , the transmission line  110   a  is spaced apart from each of the transmission lines  108 ,  112  by a distance S 2 . Transmission line  110   b  is advantageously spaced apart from each of the transmission lines  108 ,  112  by the same distance. A conductive ground plane  101  is disposed on a surface of the dielectric substrate  102  opposed from the surface on which the transmission lines  108 ,  110   a ,  110   b , and  112  are disposed. Notably, the transmission lines  110   a ,  110   b  are each electrically connected to the ground plane  101  at one or more locations along its length. For example, ground connections for transmission line  110   a  can be provided at ground junctions  104  and at crossover  124 . Ground connections for transmission line  110   b  are provided at ground junctions  105  and at crossover  124 . 
     An embodiment of the invention is described herein as having two separate transmission lines  110   a ,  110   b , with each line terminating at cross-over  124 . Such an arrangement can be convenient when transmission lines  108 ,  112  are arranged in a rectangular spiral configuration with a cross-over  124 . In such a scenario, the gap between transmission lines  110   a ,  110   b  is a minor discontinuity and has little effect on the performance of the coupler. Accordingly, the transmission lines  110   a ,  110   b  together effectively function as a single continuous transmission line that is substantially coextensive with the first and second transmission lines. The word substantially is used here to clarify that there will exist in this scenario a small discontinuity as between the two transmission lines at the location of the cross-over. Still, it should be understood that the invention is not limited in this regard and in some embodiments (e.g., where a cross-over  124  is absent), a single continuous transmission line  110  can be used instead of two separate transmission lines  110   a ,  110   b . In such a scenario, the single transmission line will be coextensive in length with transmission lines  108 ,  112 . A ground connection can be provided for the transmission line  110  at mid-length or at some other suitable location(s) along the line. A simplified schematic representation of the coupler section  100  shown with a single continuous transmission line  110  is shown in  FIG. 6 . 
     Adjacent portions of the transmission lines  108  and  112  are configured to each have a pre-defined electrical length which is substantially equal. For example, in an embodiment of the invention the electrical length of each of these transmission lines is designed so that they are approximately equal to ¼ of a wavelength of an input RF signal for which the coupler section has been designed. In other words, the overall electrical length of transmission lines  108  and  112 , extending from ground junction  104  to ground junction  105 , is approximately ¼ wavelength. Transmission lines  110   a ,  110   b  are in combination substantially coextensive with the lengths of transmission lines  108  and  112 . However, each of transmission lines  110   a ,  110   b  respectively terminates and is connected to ground at crossover  124 . Accordingly, transmission lines  110   a ,  110   b  are individually only half as long as transmission lines  108 ,  112 . More particularly, transmission lines  110   a ,  110   b  are each individually only approximately ⅛ of a wavelength in length in an embodiment of the invention. It will be appreciated that the invention is not limited to the foregoing transmission line lengths and other electrical lengths are also possible. As may be observed in  FIGS. 1 and 4 , a width of the transmission lines  108 ,  110   a ,  110   b ,  112  remains substantially unchanged at each point along the length of the pre-defined electrical length in the embodiment shown. Still, the invention is not limited in this regard, and discontinuities may be present in one or more of the transmission lines in some scenarios. 
     The transmission lines  110   a ,  110   b  load the transmission lines  108 ,  112  so as to cause them to have a width that is more narrow for a given characteristic impedance than would otherwise be possible in the absence of transmission lines  110   a ,  110   b . Consequently, the presence of the transmission lines  110   a ,  110   b  facilitates a more narrow width of the transmission lines  108 ,  112 , than would otherwise be possible for a coupler in which the transmission lines  110   a ,  110   b  are not present. This reduction in line width facilitates a coupler section  100  which can be made relatively smaller in size. Also, the transmission lines  110   a ,  110   b  function to reduce a coupling between the transmission lines  108 ,  112 . This reduction in coupling is advantageous when implementing coupler sections where only a minimal amount of coupling is desired between the transmission lines  108 ,  112 . The reduction in line width and reduction in coupling can be particularly useful in certain multi-section coupler applications which shall be described in more detail below. 
     Those skilled in the art will appreciate that the actual characteristic impedance value of lines  108 ,  112  and the amount of coupling between the two transmission lines will vary as a function of changes in the specific device geometry (e.g. will vary with changes in W 1 , W 2 , S 1  and S 2 ). The amount of coupling obtained is a very complex interaction which can be determined for specific geometries by using conventional electromagnetic analysis tools. Suitable electromagnetic analysis tools include commercially available software applications which are well known in the art. Accordingly, these tools will not be described here in detail. However, it will be appreciated that specific performance characteristics for a directional coupler can be obtained by utilizing such electromagnetic analysis tools. For example, an iterative approach can involve varying one or more values of W 1 , W 2 , S 1 , and S 2  until a predefined set of performance criteria has been satisfied. Still, there are some generalities concerning the directional coupler described herein which should be noted. For example, increasing W 1  while holding W 2 , S 1 , and S 2  constant lowers the characteristic impedance of  108  and  112 . Increasing W 2  also lowers the impedance of  108  and  112  but also decreases the coupling between  108  and  112 . Increasing S 1  or S 2  lowers both the impedance of  108  and  112  and the coupling. 
     A thickness t 1  of the transmission lines  108 ,  112  can be the same or different as compared to the thickness t 2  of transmission lines  110   a ,  110   b . For example, there is shown in  FIGS. 2-4  an embodiment in which the transmission lines  108 ,  112  have a greater thickness as compared to the transmission lines  110   a ,  110   b . Such an arrangement can be advantageous under certain circumstances. For example, transmission line thickness has an influence upon the coupling between the transmission lines and loss which occurs as RF signals are communicated through the structure. The arrangement shown in  FIG. 3  has the advantage of providing a high degree of compactness while maintaining low loss. Similar performance can be achieved with the transmission lines  108 ,  110   a ,  110   b ,  112  all having the same thickness but with increased separation between the lines. Still, it will be understood that such an arrangement would increase the overall size of the coupler section  100  with no performance advantage. 
     Referring now to  FIG. 3  there is shown a view of ground junction  104  which is enlarged to show detail. The ground junction includes a conductive cross-bar  138  disposed on the dielectric substrate. The cross-bar extends beneath bridge sections  134 ,  136  of the transmission lines  108 ,  112 . The bridge sections  134 ,  136  form a conductive bridge which extends over the cross-bar  138 . The bridge sections are isolated from the cross-bar  138  by a suitable dielectric material, such as air. The cross-bar provides an electrical connection between the transmission line  110   a  and ground lug  118 . The via  140  forms an electrical connection between the ground lug  118  and ground plane  101 . Ground junction  105  has a similar structure to that of ground junction  104 . Accordingly, the description of ground junction  104  is sufficient for understanding the arrangement of ground junction  105 . 
     The transmission lines  108 ,  110   a ,  110   b ,  112  can extend along a straight or linear path. However, in order to provide a compact implementation of the coupler section  101 , it is advantageous for the transmission lines to be disposed along a spiral, serpentine, or meandering path. Such an arrangement is shown in  FIGS. 1 and 4 , wherein the transmission lines  108 ,  110   a ,  110   b    112  are disposed along a path which defines a rectangular spiral. As shown in  FIG. 4 , the rectangular spiral formed by transmission lines  108 ,  110   a ,  110   b  can include a plurality of conductive bridge sections  142   a ,  142   b ,  142   c ,  144   a ,  144   b ,  144   c  at locations  114 ,  116  where the transmission lines traverse one another. At such locations, the conductive bridge sections extend over the transmission lines  108 ,  110   a ,  112  which are being traversed. Each conductive bridge section is isolated from the transmission lines which are being traversed by a space which can be occupied by a dielectric material such as air. A similar arrangement is provided at each location where transmission lines  108 ,  110   b ,  112  traverse one another. 
     In embodiments in which the transmission lines follow a spiral path (e.g. a rectangular spiral), it can be advantageous for the transmission lines  108 ,  112  to cross at the mid-point of the predetermined electrical length defining the adjacent sections. Such an arrangement can ensure that the electrical length of the transmission lines is equal despite having followed a non-linear path. Accordingly, there is shown in  FIGS. 1 and 5  that the coupler section  100  can include a cross-over  124  to facilitate the equalization of the electrical length of the transmission lines  108 ,  112 . The cross-over can include a conductive bridge section  160  where the transmission line  112  traverses transmission line  108  as shown. The bridge section  160  extends above and is isolated from transmission line  108  by a suitable dielectric material such as air. Additional ground connections are also provided for the transmission line  110   a ,  110   b  at the cross-over  124 . Accordingly, it can be observed in  FIG. 5  that conductive cross-bars  156 ,  158  connect transmission lines  110   a ,  110   b  to ground lug  120 . Ground lug  120  is connected to ground plane  101  through via  154 . 
     The inclusion of the transmission line  110   a ,  110   b  as described herein advantageously facilitates narrower line widths for a given impedance and provides for a reduced coupling between the transmission lines  108 ,  112 . However, a further adjustment of the characteristic impedance of the transmission lines  108 ,  112  may sometimes be necessary. Accordingly, some embodiments of the present invention provide additional discrete reactive elements in the inventive coupler section to allow adjustment of the characteristic transmission line impedance. In these embodiments, the discrete reactive elements can be connected to the transmission lines  108 ,  112  at one or more selected locations along their length to adjust the total impedance of the inventive coupler section. 
     For example, as shown in  FIG. 6 , a coupler section  100  can include shunt capacitors  151 ,  153  at a location which is at half of the predetermined electrical length of the transmission lines  108 ,  112 . In the scenario where the transmission lines  108 ,  112  are ¼ wavelength in electrical length, these shunt capacitors would be located at ⅛ of a wavelength. In the embodiment shown in  FIGS. 1 and 5  shunt capacitors are coupled to the transmission lines  108 ,  112  at cross-over  124 . The shunt capacitors are comprised of plate members  150 ,  152 , which are spaced apart from a conductive face  162  of ground lug  120  by a small gap to define a capacitance. A thin film dielectric material is advantageously disposed within the gap and supports the plate members. However, the invention is not limited in this regard and other periodic arrangements of discrete reactive components can be used. Depending upon the particular design requirements, these discrete reactive components can be varied with regard to their number and with respect to the type of discrete reactive elements used for the components. The values of the discrete reactive components and their associated geometry can be determined using conventional electromagnetic analysis tools and methods similar to those described above. 
     As a consequence of the inclusion of discrete reactive elements, the inventive coupler section can be further reduced in size. For example, shunt capacitors, as described above, decrease the even mode impedance of the structure which in turn decreases coupling. In the various embodiments of the present invention, the final dimensions of the inventive coupler, including the dimensions of the transmission lines  108 ,  110   a ,  110   b ,  112 , the spacing between each line, and the size, number, and types of discrete reactive elements can vary according to the impedance requirements and/or the operating frequency needed for the inventive coupler. 
     A coupler section  100  as described herein can be particularly useful in the design of a wideband compact multi-section hybrid coupler. U.S. Pat. No. 7,741,929 to Hash disclosed a miniature quadrature hybrid RF direction coupler suitable for operation over about an octave of bandwidth. However, the implementation of a wideband hybrid coupler that is suitable for operation over more than one octave requires multiple additional coupler sections, each of which is conventionally ¼ of a wavelength long. These additional coupler sections must have low coupling as opposed to the high coupling disclosed in the &#39;929 patent. However, the requirement for low coupling creates certain challenges as hybrid coupler devices which have low coupling are not conducive to compact layouts. Such devices typically require transmission line conductive traces that are physically wide and have large spaces between the coupled transmission lines. Consequently, wideband hybrid couplers have no practical implementation in RF integrated circuits, except at millimeter wave frequencies. However, the coupler section  100 , provides low coupling in a very compact implementation. Accordingly, when coupler section  100  is used in conjunction with the coupler section described in the &#39;929 patent, a compact design can be provided for a wideband coupler. In fact, the design can be sufficiently small in size so as to be suited for implementation on an RF integrated circuit (RFIC) or monolithic microwave integrated circuit (MMIC). 
     Referring now to  FIG. 7  there is shown a wideband radio frequency directional coupler  700  which is comprised of coupler sections  100 ,  200 , and  300 . Each coupler section extends along a predefined path which can be linear, meandering, serpentine, spiral or rectangular spiral as shown. Coupler section  200  comprises an arrangement similar to that of coupler section  100 . Accordingly, the foregoing description of coupler section  100  is sufficient for understanding coupler section  200 . Coupler section  200  includes transmission lines which are arranged in a manner similar to transmission lines  108 ,  110   a ,  110   b ,  112  of coupler section  100 . 
     The coupler section  300  is electrically connected to the coupler sections  100 ,  200 . Coupler section  300  includes transmission lines  308 ,  312 . As shown in  FIG. 8 , transmission lines  308 ,  312  are connected at a first end thereof to end portions of transmission line sections  108 ,  112 . Similarly, the transmission lines  308 ,  312  are connected at a second opposing end to transmission lines  208 ,  212  of coupler section  200 . Also shown in  FIG. 8  are features associated with a ground junction  304 . Ground junction  304  has a structure that is similar to ground junction  104  as shown in  FIG. 3 . Transmission line  210   a  associated with coupler section  200  is coupled to ground at ground junction  304  through ground lug  222  in a manner similar to that previously described with regard to ground junction  104 . 
     Coupler section  300  is a miniature quadrature hybrid RF direction coupler having an arrangement similar to that disclosed in U.S. Pat. No. 7,741,929 to Hash. The coupler section includes coupling elements  310  which are formed of a conductive material. The coupling elements are disposed at a location spaced above or below the length of the transmission lines  308 ,  312 . For example, as shown in  FIG. 9 , the coupling elements can be disposed a distance t 2  below the transmission lines  308 ,  312  and can be spaced a distance t 1  above the ground plane  101 . The coupling elements have a width W 3  and a length L. A coupler section  300  as described herein will provide a relatively high degree of capacitive coupling between transmission lines  308 ,  312  while facilitating transmission line widths W 4  and spacing Y that can be reliably mass produced in semi-conductor and thin film circuit photolithography. As such, the overall width X of the combined transmission lines and spacing between such lines is substantially reduced as compared to a scenario in which the coupling elements  310  are not included. The overall length of transmission lines  308 ,  312  can be λ/4 or less, further facilitating the goal of miniaturization as explained in the &#39;929 patent. 
     Coupler section  300  optionally includes one or more discrete reactive elements connected to the transmission lines along their length. For example,  FIG. 8  shows that capacitors  350 ,  352  can be connected to transmission lines  308 ,  312  and coupled to ground through a dielectric film to ground lug  320 . Ground lug  320  is connected to ground plane  101  by way of a conductive via (not shown). Discrete reactive elements such as capacitors  350 ,  352  are useful to facilitate adjustment of the total impedance of the inventive coupler. Additional details relating to the design of coupler section  300  will not be reproduced here as a full description of such coupler section has been provided in the specification of the &#39;929 patent, the disclosure of which is expressly incorporated herein by reference. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.