Patent Publication Number: US-6657521-B2

Title: Microwave waveguide filter having rectangular cavities, and method for its fabrication

Description:
This invention relates to microwave waveguide filters and, more particularly, to a compact design that is readily manufactured. 
     BACKGROUND OF THE INVENTION 
     Satellite communications systems relay the communications signals as microwaves. A microwave communications signal is up-transmitted from a first earth station to the communications satellite, processed on board the satellite, and down-transmitted to a second earth station. Typically, many channels of communications signals are relayed simultaneously. 
     The on-board processing of the communications signals usually involves filtering the microwave communications signals, amplifying the signals, and possibly other signal conditioning. Because many channels are transmitted simultaneously and because the communications are subject to various types of interference, it is important that the microwave signals be filtered to remove noise and any undesirable components, and to ensure separation between the signal bands. 
     On-board microwave signals may be propagated in any suitable fashion. The main approaches are within waveguides, on striplines, and between coaxial conductors. Each of these propagation media has filters available. The present invention is concerned with one of these, the microwave waveguide filter. 
     The usual approach to the microwave waveguide filter is to provide suitably configured and sized cavities in the waveguide. Resonant modes are produced in the cavities, with the result that the microwave energy leaving the microwave waveguide filter is filtered responsive to the configuration and size of the cavity or cavities. Such microwave waveguide filters are operable and are widely used, but they have drawbacks. The existing designs are usually relatively complex structures that are difficult and expensive to manufacture, with high piece counts, resulting in expensive and time-consuming assembly. They may also be difficult, time consuming, and expensive to tune property and to maintain tuned. 
     There is therefore a need for an improved design for a microwave waveguide filter. The present invention fulfills this need, and further provides related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a microwave waveguide filter for quasi-elliptical filtering of microwave signals. The microwave waveguide filter is readily and inexpensively manufactured, and has a low piece count of parts. Additionally, the filtering performance of the design is readily predicted theoretically, reducing the trial-and-error, and thus the time and expense, to tune the filter performance. The design is particularly suited for cross coupled cavity resonator filters for use in the K band and at higher frequencies. 
     In accordance with the invention, a microwave waveguide filter comprises a main-line cavity structure comprising a group of at least two rectangular main-line cavities arrayed along a main propagation path and including a first main-line cavity and a second main-line cavity. Each main-line cavity includes a sidewall. Each pair of adjacent main-line cavities has a common transverse wall therebetween transverse to (and preferably perpendicular to) the main propagation path, and a main-line aperture in the common transverse wall. There is a rectangular first feedback cavity in microwave communication with each of the first main-line cavity and the second main-line cavity through the respective sidewall of the first main-line cavity and the second main-line cavity. Thus, there is a first-cavity feedback aperture between the first feedback cavity and the first main-line cavity, and a second-cavity feedback aperture between the first feedback cavity and the second main-line cavity. 
     Preferably, the main-line cavities and the first feedback cavity have a base wall (i.e., a floor) that lies in a common filter plane. The main-line cavity structure may be linear and unfolded, so that the main propagation path is substantially a straight line. The main-line cavity structure may instead be nonlinear and folded, so that the main propagation path is not substantially a straight line. 
     In one embodiment, the main-line cavity structure includes an input-end main-line cavity at a first end of the main-line cavity structure, and an output-end main-line cavity at a second end of the main-line cavity structure. The main-line cavity structure further includes an input structure in microwave communication with the input-end main-line cavity, and an output structure in microwave communication with the output-end main-line cavity. 
     The size of the feedback cavity is selected to provide the desired filtering. In an example, a first-cavity sidewall of the first main-line cavity and a second-cavity sidewall of the second main-line cavity are parallel (and preferably coplanar) and both of a first-sidewall length. The first feedback cavity has a first-feedback-cavity sidewall that is parallel to the first-cavity sidewall and the second-cavity sidewall. The first-feedback-cavity sidewall has a first-feedback-cavity-sidewall length of about the first-sidewall length in one embodiment, and the first-feedback-cavity-sidewall length of about two times the first-sidewall length in another embodiment. 
     Most conveniently, the main-line cavity structure and the first feedback cavity are formed in a single filter block of material, as by machining and preferably by milling. A single cover is provided to overlie the machined-out main-line cavity structure and to be affixed to the single filter block of material. With this approach, a second microwave waveguide filter may be readily machined into the opposing side of the single filter block of material, in a back-to-back relation to the microwave waveguide filter. 
     The main-line cavity structure may be extended to include a third main-line cavity, a fourth main-line cavity, and additional main-line cavities as desired. One reason to extend the main-line cavity structure is to add one or more additional feedback cavities. For example, the main-line cavity structure may include a rectangular second feedback cavity in microwave communication with each of the third main-line cavity and the fourth main-line cavity through the respective sidewall of the third main-line cavity and the fourth main-line cavity. In this case there would be a third-cavity feedback aperture between the second feedback cavity and the third main-line cavity, and a second-cavity feedback aperture between the second feedback cavity and the fourth main-line cavity. As with the embodiment having a single feedback cavity, it is preferred that each of the main-line cavities, the first feedback cavity, and the second feedback cavity share a base wall that lies in a common filter plane. The base wall is preferably the bottom of the single filter block of material. The second-feedback-cavity-sidewall length is selected in the same manner as described above. The two feedback cavities may be dimensioned similarly for redundant filtering, or differently for filtering different microwave modes. 
     A preferred method for fabricating a microwave waveguide filter comprises the steps of providing a single filter block of material, and fabricating the single filter block of material to have therein a main-line cavity structure comprising a group of at least two rectangular main-line cavities arrayed along a main propagation path and including a first main-line cavity and a second main-line cavity. Each main-line cavity includes a sidewall, and each pair of adjacent main-line cavities has a common transverse wall therebetween transverse to, and preferably perpendicular to, the main propagation path, and a main-line aperture in the common transverse wall. There is a rectangular first feedback cavity in microwave communication with each of the first main-line cavity and the second main-line cavity through the respective sidewall of the first main-line cavity and the second main-line cavity. A first-cavity feedback aperture opens between the first feedback cavity and the first main-line cavity, and a second-cavity feedback aperture opens between the first feedback cavity and the second main-line cavity. This main-line cavity structure is preferably machined, as by numerically controlled milling, into the single filter block of material. Consistent features discussed above may be used in conjunction with the method. 
     The present approach provides sign change coupling between adjacent cavities without any conductive probe extending between the adjacent cavities. In an alternative approach to a microwave waveguide filter that is not within the scope of the invention, a conductive probe extends between adjacent cavities (and without any aperture between the adjacent cavities). The conductive probe usually includes an electrically conductive rod or wire extending between the adjacent cavities, supported in an annular insulator that fills a hole in the wall between the cavities. This conductive probe achieves capacitive coupling between the adjacent cavities, but it requires two parts that must be produced and assembled for each such conductive probe. Additionally, the length of the conductive probe in each of the adjacent cavities must be fine tuned. In the present approach, on the other hand, there is no conductive probe extending between the cavities, and instead the microwave signal is communicated between adjacent cavities by an aperture that provides inductive coupling. Thus, the presently preferred approach vastly simplifies the fabrication time and cost of the microwave waveguide filter both by avoiding the use of conductive probes, and by the ability to fabricate the cavity structure, including both the walls and the apertures, in the single filter block of material. The filter block may be stacked with other filter blocks to form a stacked multichannel filter structure that is efficient from both a weight and a volumetric standpoint. Filter performance, such as for the TE 101  and T 102  modes discussed subsequently, is excellent. 
     The microwave performance of the array of rectangular cavities is readily modeled, so that its performance, and the precise configuration and dimensions required to produce a desired performance, may be predicted. The absolute dimensional lengths of the various walls are determined responsive to the microwave frequencies to be transmitted through the microwave waveguide filter. The present design approach then permits the microwave waveguide filter to be manufactured inexpensively and precisely to the required configurations, dimensions, and tolerances. The amount of fine tuning that is required to achieve the desired performance is therefore minimal, and may be accomplished, for example, by setting one or more tuning screws that extend through the cover of the main-line cavity structure. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a first preferred embodiment of a microwave waveguide filter; 
     FIG. 2 is a schematic sectional view of the first preferred embodiment of FIG. 1, taken on line  2 — 2 ; 
     FIG. 3 is a plan view of a first version of the microwave waveguide filter of FIG. 1, with the cover removed; 
     FIG. 4 is a plan view of a second version of the microwave waveguide filter of FIG. 1, with the cover removed; 
     FIG. 5 is a perspective view, similar to FIG. 1, of a second preferred embodiment of the microwave waveguide filter; 
     FIG. 6 is a schematic sectional view of the second preferred embodiment of FIG. 5, taken on line  6 — 6 ; 
     FIG. 7 is a block diagram of a preferred approach for fabricating and using the microwave filter waveguide; 
     FIG. 8 is a graph of filter response for the microwave waveguide filter of FIG. 3; and 
     FIG. 9 is a graph of filter response for the microwave waveguide filter of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a microwave waveguide filter  26  formed in a single filter block of material  22  with a cover  24  affixed thereto. Extending through the cover  24  are an input structure  26  in the form of a microwave input probe and an output structure  28  in the form of a microwave output probe. A number of tuning screws  30  also extend through the cover  24 , as may be seen in the sectional view of FIG.  2 . 
     When the cover  24  is removed, as in FIGS. 3-4, there is seen in plan view a main-line cavity structure  32  comprising a group of at least two rectangular main-line cavities  34  arrayed along a main propagation path  36 . The main-line cavities  34  include a first main-line cavity  38  and a second main-line cavity  40 . Each main-line cavity  38 ,  40  includes a sidewall  41 . Each pair of adjacent main-line cavities, for example main-line cavities  38  and  40 , has a common transverse wall  42  therebetween transverse to, and preferably perpendicular to, the main propagation path  36 . There is a main-line aperture  44  in the common transverse wall  42  forming an opening between the two adjacent main-line cavities  38  and  40 . (As used herein, an “aperture” is an unfilled opening.) There are additional main-line apertures between adjacent cavities, as for example the aperture  43  in FIG.  4 . In FIGS. 3-4, there are additional cavities arranged along the main propagation path  36 , with a common transverse wall and a main-line aperture between each pair of cavities. 
     The microwave waveguide filter  20  further includes a rectangular first feedback cavity  46  in microwave communication with each of the first main-line cavity  38  and the second main-line cavity  40  through the respective sidewall  41  of the first main-line cavity  38  and the second main-line cavity  40 . Desirably, the walls of the main-line cavities  34  and the feedback cavities such as  46  are arranged rectilinearly, so that all of the walls are either parallel to or perpendicular to the other walls. There is a first-cavity feedback aperture  48  between the first feedback cavity  46  and the first main-line cavity  38 , and a second-cavity feedback aperture  50  between the first feedback cavity  46  and the second main-line cavity  40 . In all cases, it is preferred that each of the main-line cavities  34  and the first feedback cavity  46  have a base wall  51  (that is, the bottom or floor of the cavities in the plan view of FIGS. 3-4) that lies in a common filter plane. 
     In operation with a microwave signal introduced into the first main-line cavity  38 , resonances are established in the first main-line cavity  38 , the second main-line cavity  40 , and the first feedback cavity  46 , according to the absolute and relative dimensions of the cavities  38 ,  40 , and  46 . These resonances determine the nature of the microwave signal that leaves the second main-line cavity  40 . Two embodiments will be discussed in more detail subsequently. 
     The microwave waveguide filter  20  further includes an input-end main-line cavity  52  at a first end  54  of the main-line cavity structure  32 , and an output-end main-line cavity  56  at a second end  58  of the main-line cavity structure  32 . The input structure  26  seen in FIG. 1 is in microwave communication with the input-end main-line cavity  52 , and the output structure  28  seen in FIG. 1 is in microwave communication with the output-end main-line cavity  56 . Any operable type of input structure  26  and output structure  28  may be used. 
     The main propagation path  36  extends through the main-line cavity structure  32  from the input-end main-line cavity  52  to the output-end main-line cavity  56 . FIGS. 3 and 4 illustrate two alternative arrangements of the main-line cavities  34  along the main propagation path  36 . In FIG. 3, the main-line cavity structure  32  is unfolded and linear, and the main propagation path  36  is substantially a straight line. In FIG. 4, the main-line cavity structure  32  is folded and nonlinear, and the main propagation path  36  is not substantially a straight line but instead is jogged. The folded form of the main propagation path is more compact in a lengthwise sense, but it does not allow as complete an access to the sidewalls of all of the main-line cavities  34  as in the unfolded form. 
     As illustrated in FIGS. 3-4, there may be multiple additional main-line cavities  34  between the input-end main-line cavity  52  and the output-end main-line cavity  56 . In the embodiment of FIG. 3, the main-line cavity structure  32  includes a third main-line cavity  60  and a fourth main-line cavity  62 . These main-line cavities  60  and  62  provide access for a rectangular second feedback cavity  64  in microwave communication with each of the third main-line cavity  60  and the fourth main-line cavity  62  through the respective sidewall  41  of the third main-line cavity  60  and the fourth main-line cavity  62 . The access is provided through a third-cavity feedback aperture  66  between the second feedback cavity  64  and the third main-line cavity  60 , and a fourth-cavity feedback aperture  68  between the second feedback cavity  64  and the fourth main-line cavity  62 . 
     The main-line cavity structure  32  permits the use of either a single feedback cavity or more than one feedback cavity. When there is more than one feedback cavity, the feedback cavities may be made identical for redundancy in the filtering, or they may be made different to filter the microwave signal for different modes. FIG. 3 illustrates an embodiment where there are two feedback cavities of different dimensions for different filtering functionality. In this case, the first-cavity sidewall of the first main-line cavity  38  and the second-cavity sidewall of the second main-line cavity  40  are both of a first-sidewall length L 1 . The first feedback cavity  46  has a first-feedback-cavity sidewall that is parallel to the first-cavity sidewall and to the second-cavity sidewall. The first feedback cavity  46  has a first-feedback cavity sidewall length of about the first-sidewall length L 1 . The first main-line cavity  38 , the second main-line cavity  40 , and the first feedback cavity  46  are therefore configured to pass the TE 101  microwave mode. The third-cavity sidewall of the third main-line cavity  60  and a fourth cavity sidewall of the fourth main-line cavity  62  are both of a third-sidewall length L 3 , which in a typical case is equal to L 1  but need not be equal to L 1 . The second feedback cavity  64  has a second-feedback-cavity sidewall parallel to the third-cavity sidewall and the fourth-cavity sidewall. The second-feedback-cavity-sidewall length is about twice the third-sidewall length, or  2 L 3 . The third main-line cavity  60 , the fourth main-line cavity  62 , and the second feedback cavity  64  are therefore configured to pass the TE 102  microwave mode. 
     This microwave filtering performance of this geometrically regular, readily fabricated array of rectangular main-line cavities and feedback cavities may be modeled and predicted for various sizes and geometries. The fabrication procedure is performed largely with a milling machine or similar device that machines the array of cavities to a good degree of precision. Nevertheless, some final fine tuning is often required, and the tuning screws  30  depicted in FIG. 1 are provided for some or all of the cavities to fine tune the resonances. The tuning screws  30  extend downwardly from the cover  24  and into one or more of the main-line cavities and/or the feedback cavities. 
     The basic single-filter block configuration of FIG. 1 may be used to fabricate a second microwave waveguide filter  70  in a back-to-back relation to the microwave waveguide filter  20 , as shown in FIGS. 5-6. The second microwave waveguide filter  70  is formed with a second cavity structure  72  oppositely disposed from the microwave waveguide filter  20 , and closed with a second cover  74 . The second cavity structure  72  may be the same as that discussed above in relation to FIGS. 1-4, or it may be (and usually is) different. The second cavity structure  72  is provided with features like those discussed above, and the prior discussion is incorporated here as applied to the second cavity structure  72 . The microwave waveguide filter  20  and the second microwave guide filter  70  may be microwave isolated from each other, or they may be interconnected with appropriate apertures  76 . 
     A preferred fabrication method for the microwave waveguide filter  20  (and optionally the second microwave waveguide filter  70 ) is illustrated in FIG.  7 . The single filter block is provided as a starting workpiece, step  100 . The starting workpiece is machined with the desired pattern of the microwave waveguide filter or filters, step  102 . The cover (or covers, for the embodiment of FIGS. 5-6) is provided as a starting workpiece, step  104 , and machined, step  106 . The cover  24  is assembled to the single filter block of metal  22 , step  108 , together with tuning screws  30  and the input/output structure  26 ,  28  as desired. The piece count for this final assembled structure is low, and the metal working is desirably accomplished by a standard technique such as milling. The manufacturing cost is therefore low. The assembly is tuned as necessary, step  110 , and operated as a filter, step  112 . Experience has shown that the tuning is much faster and less time consuming than in conventional tuning, leading to reduced cost. No conductive probes extend between adjacent cavities in the preferred structure. 
     The present invention has been reduced to practice for a design like that of FIG.  3 . FIGS. 8-9 illustrate the microwave performance. 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.