Abstract:
Various exemplary embodiments include a cavity having a tuning assembly with tunable capacitive coupling. The tuning assembly may have a recess having a specified depth, designed for a default magnitude of coupling into the cavity. A sleeve may be fully inserted within the recess to have the structure operate at that default coupling magnitude. If a different amount of coupling is desired, the sleeve may be inserted to a particular depth that only includes part of the recess, enabling repeatable tuning of a plurality of cavities.

Description:
TECHNICAL FIELD 
     Various exemplary embodiments relate generally to capacitive input coupling and, more particularly, to tuning a capacitive input coupler. 
     BACKGROUND 
     Many systems use cavity filters to define resonant frequencies for microwave or radio frequency (RF) signals. Such cavities may have an enclosed space surrounded by at least one electrically conductive wall. The dimensions of this enclosed space and the interaction of the electromagnetic waves that embody the signals with the at least one electrically conductive wall define particular frequencies. 
     A cavity filter is not useful without means for coupling energy into the cavity and from the cavity, so a coupler may be added to transfer a portion of the energy from the cavity filter to an external location. A simple coupler could be a direct metal to metal connection, such that the coupler directly taps energy from the conductive walls of the cavity. 
     However, such DC-grounded tapping has a number of drawbacks. For example, due to non-linearity in the electromagnetic waves at the metal-to-metal contacts, Passive Inter-Modulation (PIM) signals may appear when signals pass from the cavity walls into a conductive junction. Such degradation in performance is particularly likely when a conductive wall of a cavity is directly linked to a metallic coupler. PIM signals raise a number of issues, including distortion of a desired signal that may potentially degrade system performance. 
     PIM may be avoided, to some extent, by high quality workmanship, such that the metallic conductor is precisely soldered to a cavity wall. However, even one skilled in metallurgy may be unable to perfectly shape the junction, so some PIM signals will persist. Thus, an alternative solution may be needed that does not involve a metal-to-metal junction. 
     One alternative is to place a dielectric between the metallic wall of the cavity and the external conductor. Fixed capacitive tapping may use a coaxial structure. However, such a structure is not easily tunable, so it can only tap a set amount of energy from a cavity filter. 
     Another conventional method requires insertion of tuning screws into a microwave cavity. While rotating a screw to vary the depth of its penetration into the cavity does achieve tuning, it may be difficult to duplicate such tuning when the environment requires adjustment of a very large coupling range with a single design. Thus, it would be beneficial to have a tuning technique for a cavity that was repeatable, resulting in identical coupling each time the technique was used in the same way in a cavity having the same dimensions. 
     For the foregoing reasons and for further reasons that will be apparent to those of skill in the art upon reading and understanding this specification, there is a need for a capacitive coupling technique that is both easily tunable and adequately reduces PIM. 
     SUMMARY 
     In light of the present need for an improved technique for capacitive coupling from a cavity filter, a brief summary of various exemplary embodiments is presented. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections. 
     In various exemplary embodiments, a filter may provide tunable capacitive input coupling, the filter including one or more of the following: a housing having at least one conductive wall that defines a cavity operating at a default frequency; a conductive element extending inside the cavity from the at least one conductive wall along an axis; and a tuning assembly disposed adjacent the at least one conductive wall and separated from the conductive element by a tunable distance. The tuning assembly may include: a hollow sleeve inserted into a recess having a specified depth along the at least one conductive wall parallel to the axis, the hollow sleeve comprising a non-conductive material and having a particular depth; a wire having a first end inserted fully within the hollow sleeve to the particular depth and a second end bent in a direction orthogonal to said axis, thereby having the capacitive input coupling fixed to a value that is proportional to the particular depth; and a dielectric disposed circumferentially around the first end of the wire, the dielectric retaining the first end of the wire within the hollow sleeve at the particular depth. 
     In addition, in various exemplary embodiments, the particular depth may be determined through manual testing. Furthermore, in various exemplary embodiments, the wire may be L-shaped, having a bend so that the first end and the second end are orthogonal. 
     In various exemplary embodiments, the cavity may have a rectangular shape. Alternatively, the cavity may have a cylindrical shape. In various exemplary embodiments, the conductive element may have a cylindrical shape. 
     In various exemplary embodiments, the dielectric may compress the first end of the wire, thereby holding the wire in a fixed position. In various exemplary embodiments, the specified depth of the hollow sleeve may correspond to a default level of capacitive coupling for the cavity and the default frequency of the cavity. 
     In various exemplary embodiments, the sleeve may be inserted to the particular depth to tune a cavity to operate at a new level of coupling different from the default coupling, the particular depth being less than the specified depth. In various exemplary embodiments, the sleeve may further comprise a locking portion, the locking portion protruding outside of the recess and holding the sleeve in a fixed position within the recess. 
     In various exemplary embodiments, a tuning assembly may comprise: a hollow sleeve inserted into a recess having a specified depth along the at least one conductive wall parallel to an axis, the hollow sleeve comprising a non-conductive material and having a particular depth; a wire having a first end fully inserted within the hollow sleeve to the particular depth and a second end bent in a direction orthogonal to said axis, thereby having the capacitive input coupling fixed to a value that is proportional to the particular depth; and a dielectric disposed circumferentially around the first end of the wire, the dielectric retaining the first end of the wire within the hollow sleeve at the particular depth. 
     In various exemplary embodiments, the particular depth may be determined through manual testing. In various exemplary embodiments, the wire may be L-shaped, having a bend so that the first end and the second end are orthogonal. 
     In various exemplary embodiments, the dielectric may compress the first end of the wire, thereby holding the wire in a fixed position. In various exemplary embodiments, the specified depth of the recess may correspond to a default level of capacitive coupling. 
     In various exemplary embodiments, the sleeve may be inserted to the particular depth to tune a cavity to operate at a new level of coupling different from the default coupling, the particular depth being less than the specified depth. In various exemplary embodiments, the sleeve may further comprise a locking portion, the locking portion protruding outside of the recess and holding the sleeve in a fixed position within the recess. 
     In various exemplary embodiments, a method of assembling a filter includes one or more of the following steps: providing a housing with at least one conductive wall that defines a cavity; placing a conductive element within the cavity, the conductive element mounted on the at least one conductive wall and extending from the at least one conductive wall into the cavity along an axis; mounting a tuning assembly on the at least one conductive wall, the tuning assembly separated from the conductive element and having an internal recess with a specified depth parallel to the axis; inserting a non-conductive sleeve into the internal recess to a particular depth; inserting a first end of a wire fully into the sleeve to the particular depth, the wire having a second end bent in a direction orthogonal to the axis; and placing a dielectric around the first end of the wire to maintain the wire at the particular depth in the sleeve, thereby defining a tuned distance for capacitive coupling between the wire and the conductive element. 
     In various exemplary embodiments, the method may further comprise performing manual testing to determine the particular depth. In various exemplary embodiments, the method may further comprise inserting the sleeve to the particular depth to tune the cavity to operate at a new coupling different from a default coupling, the particular depth being less than the specified depth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand the various exemplary embodiments, reference is made to the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an exemplary cavity filter; 
         FIG. 2  is a sectional view of an exemplary tuning assembly within the filter of  FIG. 1 ; 
         FIG. 3  is a detailed view of the tuning assembly of  FIG. 2 , showing partial removal of a sleeve from a recess in the exemplary tuning assembly; and 
         FIG. 4  is a flowchart for a method of assembling a cavity filter with a tuning assembly for capacitive coupling. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments. 
       FIG. 1  is a perspective view of an exemplary cavity filter  100 . In various exemplary embodiments, filter  100  may include a housing having a bottom portion  110   a , and four side walls  110   b ,  110   c ,  110   d , and  110   e . In operation, the housing may also have a top portion (not shown), but the top portion is absent in  FIG. 1  to permit a view of the interior of filter  100 . Bottom portion  110   a , four side walls  110   b ,  110   c ,  110   d , and  110   e , and the top portion may all be made of conductive material, such as metal. 
     As depicted in  FIG. 1 , filter  100  may be a cavity defined by its conductive walls in the shape of a rectangular solid. However, other suitable shapes will be apparent to those of skill in the art. For example, filter  100  could have a single side wall to define a cylindrical cavity. A cavity with only one wall might have a spherical spheroidal, or ellipsoidal shape. In general, filter  100  has at least one conductive wall defining a cavity that confines electromagnetic waves. 
     Filter  100  also has a conductive element  120  extending orthogonally from bottom portion  110   a  into the cavity. In  FIG. 1 , conductive element  120  is shown as a cylindrical post, but conductive element  120  may be designed to have other shapes, as will be apparent to one having ordinary skill in the art. Conductive element  120  may also act as a source for subsequent transfer of electrical energy. 
     Tuning assembly  130  may be disposed along one side wall  110   b  of the cavity. Although tuning assembly  130  does not physically touch conductive element  120 , it has a virtual connection due to capacitive coupling. As will be described in greater detail below, a designer may vary the distance between conductive element  120  and tuning assembly  130  to change the amount of capacitive coupling. 
     While tuning assembly  130  may be disposed in a corner of a filter, as shown in  FIG. 2 , tuning assembly  130  may be placed in any appropriate place for capacitive coupling in the filter  100  of  FIG. 1 , as will be apparent to one of ordinary skill in the art. The position of tuning assembly  130  within the cavity may permit the distance between tuning assembly  130  and conductive element  120  to be precisely measured. 
       FIG. 2  is a sectional view of an exemplary tuning assembly  200  within the filter  100  of  FIG. 1 . In various exemplary embodiments, tuning assembly  200  may comprise a recess  210 , a sleeve  220 , a locking portion  230 , a wire  240 , and a dielectric  250 . Tuning assembly  200  may be disposed on a corner of a rectangular cavity, as depicted in  FIG. 2 , but its position may be varied to other locations within a cavity resonator, as will be apparent to those having ordinary skill in the art. 
     During manufacture, tuning assembly  200  is fabricated with a hollow recess  210 . Recess  210  may be cylindrical in shape, but other shapes may be applicable, as will be apparent to those having ordinary skill in the art. The specified depth of recess  210  should be designed for subsequent tuning of a cavity resonator. 
     Sleeve  220  fits into recess  210  within tuning assembly  200 . Sleeve  220  may be pushed fully into recess  210 , corresponding to a specified depth set during manufacture, or sleeve  220  may be inserted only to a particular depth within the recess. This procedure may permit repeated use of identical sleeves  220  in cavities to produce similar coupling characteristics. 
     Sleeve  220  may be fabricated from a non-conductive material, such as Teflon™. Sleeve  220  may also be cylindrical in shape, having a long axis that is parallel to the long axis of conductive element  120 , as depicted in  FIG. 1 . Such alignment is exemplary and may keep sleeve  220  at a constant distance from conductive element  120 . However, sleeve  220  may be shaped differently, matching the contour of recess  220 , as will be apparent to those having ordinary skill in the art. 
     Locking portion  230  may ensure that sleeve  220  only reaches a predetermined depth within recess  210 . Exemplary locking portion  230 , as depicted in  FIG. 2 , may comprise two tabs that extend beyond the perimeter of recess  210 . Locking portion  230  may be integral with sleeve  220 . In this case, sleeve  220  may be shaped somewhat like a mushroom, having a thin stem portion within recess  210  and thicker locking portion  230  protruding outside of recess  210  to hold sleeve  220  in position at a particular depth within recess  210 . The particular shape of locking portion  230  may vary, as will be apparent to those having ordinary skill in the art, but locking portion  230  should be manufactured to secure sleeve  220  solidly within recess  210 . 
     A designer may wish to change the coupling from its default level. The default level of capacitive coupling corresponds to the specified depth of recess  210 . Thus, a designer would create a sleeve  220  having a particular depth, using manual testing to determine if that particular depth was appropriate for the desired operating frequency of the resonant cavity. This depth may be specified by determining the proper location of locking portion  230  along sleeve  220 . 
     Wire  240  may be L-shaped, bent so that a first end of wire  240  fits securely within sleeve  220 . A second end of wire  240  may form a right angle, extending orthogonally toward element  120 , as depicted in  FIG. 1 . Wire  240  may be fully inserted into sleeve  220  at the particular depth, thereby defining a constant distance between the second end of wire  240  and conductive element  120 . 
     A specified depth of sleeve  220  may correspond to a particular level of capacitive coupling designed for a cavity. Therefore, a manufacturer may design a plurality of cavities to have identical sleeves, thereby ensuring that those sleeves  220  may produce a default coupling within the cavities when wire  240  is fully inserted into those sleeves  220 . However, it should be apparent to those skilled in art that such determination of an appropriate depth for sleeve  220  may be determined at times other than manufacture. For example, sleeve  220  could be adjusted prior to installation of the cavities in a work environment. 
     In either case, the designer will have flexibility to insert sleeve  220  firmly into recess  210  in tuning assembly  200 . Inserting sleeve  220  further into recess  210  may increase the distance between the second end of wire  240  and conductive element  120 , thereby reducing the capacitive coupling. Conversely, withdrawing sleeve  220  from recess  210  may decrease the distance between the second end of wire  240  and conductive element  120 , strengthening the capacitive coupling. 
     Dielectric  250  may surround the first end of wire  240 . Dielectric  250  may be fabricated from a non-conductive plastic, such as polyethylene terephthalate (PET). When wire  240  is inserted into sleeve  220 , sleeve  220  may exert a compression force on wire  240  and dielectric  250 , thereby holding wire  240  in a fixed position within sleeve  220 . This fixed position may be the position at which wire  240  and dielectric  250  are inserted completely into sleeve  220 , such that the depth of wire  240  is at the particular depth of sleeve  220  within recess  210 . 
     Wire  240  may pass directly through a central axis of dielectric  250 , being aligned with the middle of sleeve  220 . However, it should be apparent to those skilled in the art that wire  240  may be disposed in other positions. Regardless of the actual location of wire  240  relative to dielectric  250 , dielectric  250  should firmly hold wire  240  in place after it has been moved to an appropriate position in sleeve  220 . Thus, locking portion  230  may encompass or otherwise engage the outer perimeter of recess  210 , locking both sleeve  220  and dielectric  250  into recess  210  at a particular depth. 
       FIG. 3  is a detailed view of tuning assembly  300 , showing partial removal of sleeve  320  from recess  310  in tuning assembly  300 . During manual testing, a designer may discover that the capacitive coupling is insufficient. In such a case, sleeve  320  may be built so that it only fills part of recess  310 , reaching a particular depth instead of the specified depth of recess  310 . 
     The designer may perform testing when creating sleeve  320  to correlate the shape of sleeve  310  to the desired capacitive coupling. Locking portion  330  may prevent sleeve  320  from being inserted beyond a particular depth in recess  310 . Dielectric  350  may prevent wire  340  from wobbling within sleeve  320 . Dielectric  350  may fill all space between wire  340  and sleeve  320  or only part of that space. 
       FIG. 4  is a flowchart for a method  400  of assembling a cavity filter with a tuning assembly for capacitive coupling. The method starts in step  405  and proceeds to step  410 . In step  410 , the designer provides a housing having at least one conductive wall that defines a cavity. The wall may be metallic. The cavity may be shaped as a cube, a rectangular cuboid, or a parallelepiped. 
     In step  420 , the designer places a conductive element within the cavity and mounts the conductive element on a wall so that it extends from that wall into the cavity along an axis. The conductive element may, for example, have the shape of a cylindrical post. Like the wall, the conductive element may be made of metal. 
     In step  430 , the designer mounts a tuning assembly on the wall, the tuning assembly being separated from the conductive element and having an internal recess parallel to the axis. The tuning assembly may be cylindrical in shape. The recess may have a specified depth based upon default capacitive coupling levels. 
     In step  440 , manual testing may be performed to determine a particular depth for insertion of the sleeve into the recess. The sleeve may be cylindrical in shape. The sleeve may entirely fill the recess to obtain the default level of capacitive coupling. Alternatively, the designer may shape the sleeve so that it only fills the recess to a particular depth, performing testing to make sure that the sleeve is shaped to match this target. 
     In step  450 , the designer inserts the sleeve into the recess once testing is finished. The locking portion of the sleeve, which may be constructed to match the contour of the outer perimeter, will engage once the sleeve is inserted to the particular depth within the recess having the specified depth. Because the locking portion is wider than the width of the recess, the locking portion will prevent any further insertion, locking the sleeve to the particular depth within the recess. 
     In step  460 , the designer fully inserts a first end of a wire into the sleeve to a particular depth. The wire may have a second end bent in a direction orthogonal to the axis. The wire is fully inserted until it reaches the end of the sleeve. At this point, the locking portion of the sleeve ensures that the wire and the sleeve cannot be pushed any further into the recess, fixing both at their current positions. 
     In step  470 , a dielectric is placed around the first end of said wire to maintain the wire at the particular depth in the sleeve, thereby defining a tuned distance for capacitive coupling between the wire and a conductive element. The method ends in step  475 . 
     Thus, according to the foregoing, various exemplary embodiments provide a reliable and efficient method for capacitively coupling energy into or from a cavity filter. More particularly, the various exemplary embodiments provide a technique for tuning capacitive coupling in a reliable manner. 
     It should be apparent that the foregoing description of a cavity filter is only exemplary. Thus, the teachings of this disclosure are equally applicable to any system where selection of a particular frequency is important. For example, the teachings of this disclosure could be applied to any system that transfers electrical energy in a capacitive manner. Other suitable substitutes will be apparent to those of ordinary skill in the art. 
     Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications may be implemented while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.