Abstract:
A radio frequency mechanical filter ( 10 ) of the resonant type employs a plurality of parallel spaced-apart resonators ( 40 ) disposed within a housing ( 16 ) defining a cavity. The resonators are supported at their first ends ( 46 ) to one wall ( 20 ) of the housing ( 16 ), with the remaining ends of the resonators being unsupported. A TEFLON support bracket ( 12 ) is disposed across the resonators ( 40 ) at points between the ends ( 44, 46 ) of the resonators. In one form, the support bracket ( 12 ) is movable along the length of the resonators to change the center frequency and bandwidth response of the filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to combline filters and, in particular, to radio frequency band pass filters. 
     2. Description of the Related Art 
     Various filter designs have been proposed to allow certain frequencies to be transmitted to the output load while rejecting the remaining frequencies. Such filters designed for use in the frequency range of several hundred megahertz to one gigahertz include as an important example, combline strip line filters fixed to a design frequency. These types of filters are sometimes termed “mechanical” filters, a reference to their physical construction. Combline filters of this type, especially when designed for high Q, low insertion loss applications, require demanding high tolerance O-ring and bushing components and correspondingly intricate assembly techniques, requiring substantial labor investment. 
     It is not uncommon, due to the long lead time sometimes required for fabrication and delivery, that the design frequency characteristics of an earlier fabricated filter may no longer precisely match requirements needed at the time of installation. In order to alter the center frequency of the filter, for example, the filter must be returned to the manufacturer for rebuilding. Time delays can be reduced by maintaining an inventory at the manufacturer&#39;s site, although warehousing and storage costs incurred by the manufacturer are substantial and ultimately the purchaser incurs additional charges for the replacement service. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a radio frequency combline filter in accordance with the present invention showing the housing with a cover thereof removed for viewing the resonator and transform rods and support bracket therefor in the housing; 
     FIG. 2 is an exploded perspective view of the filter of FIG. 1; 
     FIG. 3 is an exploded perspective view of the support bracket showing the notched and cover support thereof; 
     FIG. 4 is a side elevational view of the notched support showing recesses formed therein; 
     FIG. 5 is an end elevational view of the notched support showing one of the studs for attaching to the cover support; and 
     FIG. 6 is a fragmentary top plan view of a modification to the radio frequency combline filter of FIG. 1 having tuning adjustment studs for cooperating with the support bracket. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention finds advantages in applications where demanding combline filter performance is required, especially where filters are required to have a high Q and low insertion loss. The present invention provides advantages in this regard by employing relatively long resonators while eliminating losses associated with support bushings and O-rings located at the tuned end of the resonators. At the same time, the filter does not increase the microphonic risk associated with unwanted vibrations of the tuned ends of the resonators. Moreover, the present invention provides further advantages in applications where frequency adjustment is required at the installation site, as this can now be readily accomplished without requiring complex diagnostic equipment and procedures. Further, in certain instances, the present invention allows a shift in filter bandwidth, with a simple mechanical adjustment. 
     As will be seen herein, advantages of the present invention result from elimination of bushing supports and O-rings at the tuned ends of the resonators and the introduction of a continuous support spanning all of the resonators employed, located at mid-portions of the resonators, between their fixed ends and tuned ends. 
     Turning now to the drawings and initially to FIG. 2, an improved combline filter  10  with either fixed or adjustable frequency characteristics (i.e., center frequency and bandwidth) is shown. In the adjustable frequency embodiment, the frequency characteristics are adjusted by merely shifting a resonator support bracket  12  from side to side as indicated by arrows  14  in FIG.  1 . In the preferred embodiments, filter  10  is employed as a pass band filter operating in the 400 MHZ range. 
     As can be seen in FIGS. 1 and 2, filter  10  includes a metal housing  16 , preferably formed from a machined aluminum slab. Housing  16  can include first and second spaced-apart walls  20 ,  22  and a pair of opposed end walls  24 ,  26 , and a back wall  28 . As shown in FIG. 2, the housing includes a sheet metal cover  30  secured to the housing walls by a plurality of threaded fasteners  32 . 
     A plurality of parallel spaced-apart resonators are disposed within a substantially hollow cavity  33  of the housing  16 . Included are seven conventional resonator rods  40  and two conventional transform rods  42 . Preferably, the resonator rods  40  are of solid cylindrical construction, although hollow resonator rods or resonator rods of other construction can be employed, if desired. The preferred resonator rods  40 , although solid, are provided, as indicated in FIG. 2, with hollow ends  44  for tuning the resonating frequency of the rods  40  and accordingly ends  44  will be referred to herein as the tuned ends of the resonator rods  40 . The opposite ends  46  of the resonator rods  40  (herein referred to as support ends) are secured to wall  20  by fasteners  48 . Tuning elements  52  pass through wall  22  and are received in the hollow tuned ends  44  so as to provide conventional tuning for the resonator rods  40 . The transform rods  42  are mounted at their first ends to wall  22  by fasteners  58 . Opposite ends of the transform rods pass through wall  20  and are supported by transform mounts  62  shown in FIG. 2, secured to wall  20  by fasteners  64 . 
     In order to meet high Q, low insertion loss design requirements, the resonator rods  40  and transform rods  42  are relatively long and in the preferred and illustrated embodiment are made to span a distance of approximately six inches. As those skilled in the art will appreciate, if the resonator rods  40  are left unsupported, there would be a significant microphonic risk in the performance and reliability of the filter, and which could ultimately lead to mechanical failure due to vibration of the rods. Heretofore, these drawbacks have been addressed by either shortening the length of the resonator rods or supporting the tuned end of the resonator rods, e.g., with bushings made of dielectric material, such as TEFLON. Certain demanding applications require filters of increased performance which cannot be obtained by either of these conventional methods for providing the necessary support for the longer resonator rods. While the full length of the resonator rods can be physically supported by using TEFLON bushings and associated O-rings located at the tuned ends of the resonators, such arrangements inherently degrade the Q of the filter and have been observed to incur signal losses which are objectionable in more demanding applications. In other words, because the tuned ends of the rods are press fit in the bushings, stresses are introduced which over time can degrade the signal in an inexact and unpredictable manner. Further, the tight tolerances for providing the press fit between the bushings, O-rings and rods generate undesirable manufacturing costs and difficulties. 
     The present invention avoids the above-discussed O-rings and bushings used for support at the tuned end of the resonator rods. The filter  10  herein uses the single support bracket  12  of dielectric, preferably TEFLON material located generally in the mid-portions of the resonator rods, spaced from the opposite rod ends  44 ,  46 , as illustrated. As can be seen for example in FIG. 1, the TEFLON support bracket  12  generally spans the length of the housing  16 . Referring to FIGS. 1-3, the TEFLON support bracket  12  is preferably formed with a pair of mating support members, including a notched support  70  and a covering support  72 . As best seen in FIG. 3, studs  74  projecting from notched support  70  are received in apertures  76  formed in covering support  72 . Preferably, a friction fit between studs  74  and apertures  76  is provided to securely maintain the support members  70 ,  72  in a joined position, once mated. If desired, compressive force applied to covering support  72  by sheet metal cover  30  may be relied upon to maintain support members  70 ,  72  in a joined position, provided the covering support is appropriately dimensioned for this purpose. 
     Notched support  70  has a plurality of arcuate recesses  78  for receiving the resonator rods  40  in the manner indicated, for example, in FIG.  1 . The filter  10  according to the present invention is fabricated at a substantial cost savings since labor intensive fitting of O-rings and bushings at the tuned ends of the resonator rods is no longer required. Rather, the notched support  70  is inserted within the housing cavity in the manner indicated for example in FIG. 2, with its bottom surface  82  engaging wall  28  and with its ends  84  engaging end walls  24 ,  26 . A variety of conventional means may be employed to secure the notched support in a fixed position, such as adhesives or dielectric fasteners, for example. However, it is most preferred that notched support  70  be mounted for lateral movement in the direction of arrows  14  shown in FIG. 1 in order to provide ready adjustment of the filter&#39;s frequency characteristics. To this end, the relative sizing of the openings provided by the attached supports  70  and  72  and the diameter of the rods  40  and transforms  42  cooperate to provide an adjustable mount for the bracket  12  relative to the rods  40  and transforms  42  to allow the bracket  12  to slide laterally thereon. The preferred dielectric TEFLON material of the bracket  12  assists in implementing the adjustable mount as it generally has a low coefficient of friction associated therewith. 
     The introduction of dielectric material of the support bracket in the mid-portions of the resonator rods alters both the coupling co-efficients and the capacitance between the resonators and between the resonators and the ground plane. As will be explained herein, these parameters change with respect to the mounting location of the support bracket  12  within the filter housing in such a way as to shift the tuned frequency of the filter. This shift in frequency occurs fairly, such that the filter remains within specifications if shifted one band lower (such as, between five and six MHZ) associated with movement of the support bracket  12  by a predetermined distance which can be less than a few inches, typically on the order of one inch, in either direction from a support bracket center position. 
     If desired, dielectric studs can project from back wall  28  at spaced intervals in the direction  14  so as to be received in notched stud  70 , in the manner similar to that shown in FIG. 3, for example. In another form, spaced dielectric studs  88  may extend from wall  28  so as to be received in notches formed in the end  84  of support  70 . These types of arrangements allow the notched support  70  to be moved a fixed, predetermined distance along the length of resonator rods  40  and the transform rods  42 , to thereby carry out a precisely known or predetermined tuning adjustment of filter  10 . Variations in the filter design are possible. For example, with support  12  projecting beyond end walls  24 ,  26 , motor driven lead screw arrangements or lever arm arrangements (not shown) can be provided to move the support bracket within the housing cavity without requiring disassembly of the filter. 
     After the notched support  70  is installed within the housing cavity  33 , the resonator rods  40  and transform rods  42  are installed, and are allowed to rest within the recesses  78 . With the resonator and transform rods installed, the covering support  72  is joined to notched support  70 . In this manner, notched support  70  aids commercial manufacture and assembly by providing alignment of the resonator rods  40  within the housing cavity  33 , and so as to stabilize the tuned ends  44  of the resonator rods  40  while the tuning elements  52  are being fitted. Notched support  70  also provides improved accuracy in the positioning and parallelism of the resonator rods  40  and transform rods  42 . The covering support  72  also holds the resonator and transform rods  42  secured during shipping, providing a cushioning at the mid-portions of the resonator and transform rods, preventing their mis-alignment while reducing the stresses imparted to the tuned ends of the resonator rods. 
     In a conventional manner, the slabline housing  16 , made of an assembly of metal components, provides grounding for the electrically active elements, e.g., the resonator and transform rods. As mentioned, by shifting the support  12  along the length of the resonators  40 , both resonator-to-resonator capacitance and resonator-to-ground capacitance are uniformly increased or decreased for each resonator, thus changing the frequency characteristics of the filter in an advantageous manner. 
     As mentioned, filter  10  employs input transform rods  42  in the combline filter construction. As a result, as the filter  10  is tuned up in frequency, the bandwidth of the filter decreased, and as the filter  10  is tuned down in frequency, the bandwidth of the filter increases. In the preferred embodiment, it is possible, with movement of support  12 , to shift the bandwidth of filter  10  by a commercially significant amount. Previously, a replacement housing would have to be constructed, in order to accommodate different spacing between resonator rods and transform rods. For a commercial manufacturer of filters, a substantial cost savings can be enjoyed since a number of different filters can now be replaced with a single adjustable filter according to principles of the present invention, thereby reducing parts inventory and associated costs. 
     By providing the filter with an adequate tuning range (e.g., in both the tuning screws and aperture coupling screws - not shown) the bandwidth of the filter  10  can be adjusted while maintaining filter frequency constant. As mentioned, the filter  10  operates in the 400 MHZ range and the resonator rods span a distance of approximately six inches. In the preferred embodiment, by tuning elements  52 , a filter originally configured with 0.5 dB bandwidth of six MHZ was adjusted to operate as a filter with a 0.5 dB bandwidth of approximately 4.5 MHZ, with a shifting of support  12  approximately one inch toward the support ends  46  of resonator rods  40 . As mentioned, according to observed phenomenon with combline filters having input transform rods, this reduction in bandwidth is associated with an increase in filter frequency, and accordingly the filter frequency was reduced to its original value by adjustment of tuning elements  52 . A similar, but opposite shift in frequency characteristics was also observed when support  12  is shifted a like amount (approximately one inch) toward the tuned ends  44  of resonator rods  40 . 
     The emphasis above was on changing the bandwidth of the combline filter. It is also important to emphasize that the tuned frequency of the combline filter  10  may also be readily changed. With the tuning arrangement of the present invention, movement of the support  12  along the length of the resonators by a distance of one inch, the filter frequency is adjusted up or down one channel, that is, approximately five to six MHZ. That is, when the support  12  is moved approximately one inch closer to the tuned ends of the resonators  40 , the filter frequency is moved up approximately five MHZ. If the support is moved toward the support end  46  of the resonator rods by a distance of approximately one inch, the filter frequency is shifted down approximately five MHZ. 
     It is important to note that the changes in frequency characteristics of the combline filter  10  are path independent and are found to be reliably restored upon return of the support bracket  12  to its original position within the housing cavity  33 . Further, it is possible to alter frequency characteristics of the combline filter  10  under field operating conditions without requiring elaborate diagnostic and test equipment, especially if a frequency change is desired. 
     The drawings and the foregoing descriptions are not intended to represent the only forms of the invention in regard to the details of its construction and manner of operation. Changes in form and in the proportion of parts, as well as the substitution of equivalents, are contemplated as circumstances may suggest or render expedient; and although specific terms have been employed, they are intended in a generic and descriptive sense only and not for the purposes of limitation, the scope of the invention being delineated by the following claims.