Abstract:
A segmented stub has at least one serpentine signal path that increases the effective electrical length of the stub without increasing the overall physical length or area of the stub. This permits more compact monolithic millimeter-wave and microwave integrated circuit design.

Description:
TECHNICAL FIELD 
     This disclosure relates to high frequency circuitry, such as microwave circuitry, millimeter-wave circuitry, and the like. Specifically, this disclosure relates to improved circuit elements that provide frequency dependent reduced impedances such as short circuits. 
     BACKGROUND 
     In designing monolithic microwave integrated circuits (MMIC&#39;s), a radial stub is often used to provide a frequency selective short circuit. The standard radial stub is a pie shaped metal structure whose radial length is nominally a quarter-wavelength of the desired operational frequency. Such stubs tend to be quite large and consume significant epitaxial substrate real estate leading to large and expensive circuits. There thus is a need to reduce the amount to real estate consumed by the components of an MMIC so that cheaper and smaller MMIC designs can be obtained. 
     SUMMARY 
     This need is met by the provision of a meandering serpentine path that provides an increased electrical length device within the footprint of a conventional stub. In one example of the invention, one or more cutouts are provided in the edges of a stub to create a serpentine conductive layer on a substrate. In another example of the invention, one or more cut outs are provided in the edges of a radial stub. The one or more cut outs increase the electrical length of the stub without increasing the radial length or surface area of the stub. This allows a smaller stub for a given frequency of operation leading to smaller and more compact and economical MMIC designs. As discussed below, there are other examples of the invention that provide this characteristic. Additional examples will readily occur to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional radial stub. 
         FIG. 2  shows an example of a segmented radial stub in accordance with the invention. 
         FIG. 3  is a cross sectional view of the device shown in  FIG. 2  taken along section line  3 - 3  in  FIG. 2 . 
         FIGS. 4 and 5  are additional embodiments of the invention involving multiple stub structures, like the one in  FIG. 2 , connected to a common input terminal. 
         FIG. 6  is another embodiment of the invention involving a ground plane on the same side of a substrate supporting a stub structure like the one in  FIG. 2 . 
         FIG. 7  is yet an additional embodiment of the invention involving a stub structure defined by the absence of metal in a layer of conductive material. 
         FIG. 8  shows an embodiment of the invention involving a circular shaped stub. 
         FIG. 9  shows an embodiment of the invention involving multiple circular stub structures joined at a common input terminal. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a conventional radial stub used to provide a frequency dependent short circuit in microwave circuitry. The nominal short circuit frequency is centered within a relatively narrow band of frequencies determined by the size and shape of the stub, most notably by its radial length in this instance. The radial stub  10  is a layer of conductive material, such as gold, silver, or copper, deposited on one major surface of a non-conductive dielectric substrate having a conductive ground plane on the other major surface of the substrate. The substrate has a predetermined thickness to provide a desired separation between the radial stub and the ground plane. The thickness of the stub  10  preferably is at least the skin depth or more. 
     The layer of conductive material is pie-shaped and comprises a narrow end  12  and a wider end  14 . The stub  10  includes two radially directed edges  16  and  18  between ends  12  and  14 . The edges  16  and  18  each form an angle of approximately 5° to approximately 85° with respect to the center line  20  of the stub  10 . The radial length of the stub  10  is nominally a quarter wavelength of the nominal operational frequency at which a short circuit is desired. 
     This type of structure in  FIG. 1  can consume too much surface area on an MMIC. 
       FIGS. 2 and 3  show an example of a radial stub in accordance with the invention. It is a structure that provides a reduced impedance, such as a short circuit, at a certain nominal operational frequency, but for a given operational frequency, it is radially shorter than the configuration of  FIG. 1  and takes up less space in an MMIC application. Like the stub of  FIG. 1 , the stub  10  is a generally pie shaped conductive layer formed on one major surface of a non-conductive substrate  11  having a conductive ground plane  13  on its other major surface. The stub of  FIG. 2  has ends  12  and  14  and radially extending edges  16  and  18  each forming an angle of approximately 5° to approximately 85° with respect to the center line  20  of the device. As in the structure of  FIG. 1 , the thickness of the stub  10  in  FIGS. 2 and 3  is at least the skin depth or more. 
     A series of notches or cutouts  22 ,  24 ,  26 ,  28 , and  30  are formed in the edge  18  at predetermined locations along that edge  18 . Another series of notches or cut outs  32 ,  34 ,  36 , and  38  are formed in the edge  16 . The cut outs  32 ,  34 ,  36 , and  38  are spaced along the edge  16  so that they are radially staggered with respect to the cut outs  22 ,  24 ,  26 , and  30  in edge  18 , thus creating a meandering serpentine structure within the general footprint of a conventional radial stub like the one in  FIG. 1 . The effective electrical length of the device thus is increased for a given overall radial length. This reduces the size of such components used in MMIC applications which reduces fabrication costs and provides a competitive advantage. 
     The dimensions of the cut outs increase with increasing radial distance from the narrow end  12  of the stub in  FIG. 2 . The spacing between adjacent cut outs also increases with increasing radial distance from the narrow end  12  of the stub. The cross sectional area of the stub normal to current flow likewise increases with increasing radial distance from the narrow end  12  of the stub. 
     Stubs in accordance with the invention can be dimensioned for use in circuitry operating, for example, above about 10 GHz. Such stubs can also be dimensioned for use in circuitry operating below 10 GHz. The overall radial dimensions of such stubs can be as much as about 50% less than the overall radial dimensions of conventional radial stubs operating at comparable frequencies. 
     The cut outs shown in  FIG. 2  have curved edges each extending to a respective point on the center line  20  of the stub  10 . The shape, depth, and spacing of the cut outs in  FIG. 2  are illustrative, however. Any shape, depth, and spacing of the cut outs that produce a desired stub size and frequency of reduced impedance may be used. The nature of the cut outs to achieve a desired result can be determined by experimentation. 
     Although the footprint of the stub of  FIG. 2  is a sector of a circle, other flared shapes are possible for the footprint, such as triangles and structures with curved sides, such as horn shapes, as long as cut outs can be formed in edges of the device so that a meandering serpentine arrangement of increased electrical length and reduced operational short circuit frequency is achieved. Other shapes are also possible as long as cutouts along one or more edges can be appropriately included so as to result in a serpentine structure that increases the electrical length of the device without increasing the size of its footprint. 
       FIG. 4  shows a stub structure involving two stubs  10  formed on the substrate  11  like the one shown in  FIG. 2 . The narrow end  12  of each stub  10  is joined to a common input terminal  40  on the substrate. The centerlines  20  of the stubs  10  form a 180° angle. Any number of stubs  10  can be connected together to a common input terminal  40 .  FIG. 5  shows an example of three stubs  10  connected to a common input terminal  42 . 
       FIG. 6  shows a variation of the embodiment of  FIG. 2  in which a radial stub  10  is coplanar with a ground plane  44  on one major surface of a non-conductive substrate  11 . The stub  10  is located in a sector shaped opening  46  and is spaced a predetermined distance from the edges of the ground plane  44  around the periphery of the stub  10 . In addition to the ground plane  44  on the same side of the substrate as the stub  10 , another ground plane like ground plane  13  in  FIG. 3  may be located on the back side of the substrate  11 . 
     Stubs in accordance with this invention can be defined by the absence of conductive material in a predetermined region of conductive layer or ground plane.  FIG. 7  shows such a variation of the invention whereby the stub is defined by a shaped opening  48  in a conductive layer  50  situated on a non-conductive substrate  11 . The opening  48  may have the same general shape as a stub composed of a shaped conductive layer. The example shown in  FIG. 7  is shaped like the radial stub of  FIG. 2 . Other shapes for opening  48  also are possible, such as those shown in  FIGS. 4-6 , and  8 . The meandering serpentine shape of the stub in  FIG. 7  is created by the provision of protrusions  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 , and  68  extending into opening  48 , instead of cut outs. 
       FIG. 8  shows a circular ball stub  70  in accordance with the invention comprising a circular patch of conductive material on a non-conductive substrate. The ball stub  70  has a plurality of concentric rings of cut outs  72  formed in the circular structure. Radially adjacent rings of cut outs  72  are circumferentially staggered to provide meandering serpentine electrical paths from the center of the ball stub  70  to the periphery of the ball stub  70 . The dimensions of the cut outs  72  increase with increasing radial distance from the center of the ball stub  70 . The distance between radially adjacent rings of cut outs  72  also increases as radial distance from the center of the ball stub increases. The ball stub  70  of  FIG. 8  is connected to a common input terminal  74  on the substrate. Although the ball stub  70  in  FIG. 8  is shown to be a unitary structure, it may be plurality of  FIG. 2  radial stubs laid radial edge to radial edge around the entire circumference of the circular structure. Any number of ball stubs  70  may be connected to a common input terminal  74 . As shown in  FIG. 9 , for example, two ball stubs  70  may be connected to a common input terminal  74 . As in the case of the radial stub shape of  FIG. 2 , stubs like the ones shown in  FIGS. 4-7  are possible for the ball stub shape. For example, a ball stub embodiment like the radial stub embodiment of  FIG. 7  can be implemented by a circular opening in a layer of conductive material with conductive islands corresponding to the cut outs  72  in the circular opening. 
     This invention can be used, for example, in any microwave or millimeter-wave MMIC design that requires the use of frequency dependent reduced impedance or short circuit elements. A specific example of circuitry in which stubs in accordance with the invention can be advantageously used is the dc decoupling circuitry in a multi-stage W-band antimonide based compound semiconductor low-noise amplifier. 
     Stubs in accordance with this invention can be fabricated using any technique that is able to create a conductive layer or film on a substrate having the desired shape and dimensions. For example, patterned metallization can be created by lithographic techniques used in the semiconductor industry. 
     The Title, Technical Field, Background, Summary, Brief Description of the Drawings, Detailed Description, and Abstract are meant to illustrate the preferred embodiments of the invention and are not in any way intended to limit the scope of the invention. The scope of the invention is solely defined and limited in the claims set forth below.