Microwave Filter Fabrication Method and Filters Therefrom

The present invention pertains to microwave filters and more particularly, to economical fabrication methods therefor and filters resulting therefrom.

Cavity microwave filter design is a well established art and practical designs exist, in particular, for combline and interdigital filters. Such filters, however, often are fabricated of several separately fabricated elements (e.g. cavity walls, resonators, frequency and coupling trimming devices and end resonator taps) which then have to be assembled to exacting tolerances to achieve the desired filter transmission parameters. Such fabrication and assembly is costly in terms of labor man hours. Efforts, therefore, have been made to reduce the fabrication cost. For example, U.S. Pat. No. 4,791,717 discloses a construction method, for an interdigital filter integrated into a down converter, which attempts to reduce the number of separate parts to be assembled by bending a specially cut piece of sheet metal to form part of a cavity housing.

Other U.S. Patents of interest are U.S. Pat. Nos. 2,530,691, 3,737,816, 3,925,883, 4,647,882, 4,802,234, 4,837,535, 4,849,724, 4,963,844 and 4,970,480.

The present invention is directed to microwave filters formed by a specially cut blank of conductive sheet material.

In a preferred embodiment the blank is cut to define wall portions, demarked by bend lines, which can be bent to form a housing defining a substantially closed cavity.

In accordance with a significant feature of a preferred embodiment, extensions, functionally useful as resonator or tuning elements, are formed integral with the blank for projecting into the cavity when the housing is formed.

In accordance with another significant feature, extensions, functionally useful as cavity coupling elements, are formed integral with the blank for external connection to a microstrip circuit.

In accordance with a still further feature, positioning stops are formed integral with the blank to facilitate aligned mounting of the housing on a microstrip circuit board.

In accordance with yet another feature, grounding tabs are formed integral with the blank to facilitate connecting the housing with a microstrip circuit ground plane.

The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.

FIG. 1A is a plan view of a conductive blank embodiment suitable for forming into a microwave filter housing in accordance with the present invention;

FIG. 1B is a view of the area within the line 1B of FIG. 1A illustrating another blank embodiment in accordance with the present invention;

FIG. 1C is a view similar to FIG. 1B illustrating another blank embodiment in accordance with the present invention;

FIG. 2 is an isometric view illustrating a transitional phase in the formation of a microwave combline filter from the blank of FIG. 1;

FIG. 3 is an isometric view illustrating a transitional phase in the formation of a microwave combline filter from the blank of FIG. 1;

FIG. 4 is an isometric view illustrating a transitional phase using capacitance and coupling tabs in the formation of a microwave combline filter from the blank of FIG. 1;

FIG. 5 is an isometric view illustrating a transitional phase in the formation of a microwave combline filter from the blank of FIG. 1;

FIG. 6 is an isometric view of a microwave combline filter embodiment formed from the blank of FIG. 1 in accordance with the present invention;

FIG. 7 is an isometric view illustrating the microwave combline filter of FIG. 6 installed in a microstrip circuit;

FIG. 8 is an enlarged view of the area within the line 8 of FIG. 5;

FIG. 9 is an enlarged view of the area within the line 9 of FIG. 6;

FIG. 10 is an isometric view of another microwave combline filter embodiment in accordance with the present invention;

FIG. 11 is an enlarged view of the area within the line 11 of FIG. 10;

FIG. 12 is an isometric view of a microwave interdigital filter embodiment in accordance with the present invention;

FIG. 13 is a view similar to FIG. 7 illustrating another filter embodiment in accordance with the present invention installed in a microstrip circuit;

FIG. 14 is a view similar to FIG. 7 illustrating another filter embodiment in accordance with the present invention installed in a microstrip circuit;

FIG. 15 is a plan view of another embodiment of a resonator portion of the blank of FIG. 1;

FIG. 16 is an isometric view of another resonator embodiment formed of the resonator portion of FIG. 15; and

FIG. 17 is an isometric view of another embodiment of a tap portion of the blank of FIG. 1.

The present invention is directed to a method for economically fabricating microwave filters and to the filters produced as a result thereof.

In accordance with the invention, a blank 20, fabricated from a sheet of conductive material (e.g. copper, brass or aluminum; brass or aluminum may be silver plated to facilitate soldering and improve conductivity), as illustrated in FIG. 1A, is formed as shown in the transitional steps of FIGS. 2 through 5 into the housing 22 of FIG. 6 defining a cavity 23 therein. The housing 22 and the tuning bracket 24 mounted therein, as shown in FIG. 4, form the the microwave combline filter 30 of FIG. 6 which is suitable for mounting in a microstrip circuit 32 as shown in FIG. 7.

After the blank 20 is bent as shown in the aforementioned figures and the tuning bracket 24 mounted therein, the adjoining edges of the formed blank are bonded in a manner well known in the art (e.g. soldering or welding) to complete fabrication of the filter 30.

Thus the fabrication is simple and economical requiring only manipulative steps similar to the art of paper folding known as origami. Consequently, fabrication labor hours are reduced compared to conventional techniques. The blank 20 may be fabricated by standard methods (e.g. stamping, etching, cutting, or laser milling) in sufficient numbers for a production run and economically stored until needed for assembly. The fabrication method used for the blank may also be used to simultaneously create bend lines such as bend line 36 in FIG. 1A by weakening the material therealong (e.g. partial removal of material or intermittent holes or slots). Forming of the blank 20 into the housing 22 is then reduced to simple bending along the bend lines.

In particular, FIG. 1A is a plan view of a blank 20 fabricated from a sheet of electrically conductive material. The blank 20 defines portions which correspond, respectively, to housing elements and structural elements in the housing 22. The housing elements are housing walls, housing ends, resonators, resonator taps and grounding tabs. The structural elements are fingers, stops and resonator stiffening flaps.

The blank 20 defines a peripheral edge 21 extending around a plurality of planar housing portions including wall portions 40, 42, 44, 46 and 48 and end portions 50 and 52. The planar portions, including leading and trailing wall portions 40 and 48, respectively, and intermediate wall portions 42, 44 and 46 are connected in series, separated by boundaries in the form of bend lines 36, 108, 110 and 112. Other intermediate wall portions 50 and 52 are contiguous with planar portion 42 at opposite ends thereof and respectively separated therefrom by bend lines 114, 116.

As will be explained in detail hereinafter, when the blank 20 is folded about its bend lines to form the housing 22 (as shown in FIG. 6), portions 42 and 46 respectively form bottom and top walls, portion 44 forms a rear wall, portions 40 and 48 respectively form front upper and lower walls, and portions 50 and 52 form left and right end walls. With the blank folded so as to form housing 22, each part of the peripheral edge 21 lies adjacent to another part of the peripheral edge. These adjacent edge parts are preferably sealed by a suitable procedure, e.g., soldering, so that the planar housing portions define a substantially closed internal cavity 23 (as shown in FIG. 6).

With continuing reference to FIG. 1A, note that extensions in the form of planar resonator portions 60, 62, 64 and 66 are formed integrally with the blank 20, extending outwardly from the peripheral edge 21, contiguous with planar portion 40 and respectively separated therefrom by boundaries in the form of bend lines 100, 102, 104 and 106. Note that the height of rear wall portion 44 equals the sum of the heights of front upper and lower wall portions 40, 48.

Thus, with the blank 20 bent around the bend lines to form the housing 22 of FIG. 6, the extensions 60, 62, 64, and 66, bent perpendicular to the front lower wall portions 40, will project into the cavity 23 to be spaced from both the bottom and top wall portions 42, 46 and the rear wall portion 44. Although the height of front upper and lower wall portions 40, 48 are shown in FIGS. 1-6 to be substantially equal, it should be understood that the teachings of the invention include unequal heights which would cause the spacing between the extensions and bottom and top wall portions 42, 46 to also be unequal.

Therefore, the wall portions, end portions and extensions of the blank 20 in FIG. 1A correspond to the housing walls, ends and resonators of the housing 22 of FIG. 6. In a similar manner, the tap portions 70, 72 and grounding tab portions 73 of the blank 20 correspond to the taps and grounding tabs of the housing 22. The tap portions 70, 72 extend from, and are contiguous with, respectively, resonator portions 60, 66. The grounding tab portions 73 are defined by each of the end portions 50, 52. The wall portions 40, 42, 44, 46, 48, 50 and 52 taken together, form a body portion 74.

Similarly, the blank 20 has finger portions 76, stop portions 78 and flap portions 80 corresponding to the structural fingers, stops and flaps on the cavity 22. The blank 20 also defines notches 81, tuning holes 82, 84, 86 and 88 and apertures 90, 92. Bend lines 36, 100, 102, 104, 106, 108, 110, 112, 114, 116, 118 and 119 are defined in a manner similar to that described above relative to bend line 36. For clarity of illustration only exemplary finger portions 76 and notches 81 are designated on FIG. 1. Four notches 81a have a function which differs from that of the notches 81. The function of both will be described in conjunction with FIG. 9.

FIG. 1B illustrates another embodiment of the terminal resonator portions 60, 66 of blank 20. FIG. 1B is a view similar to the area within the line 1B of FIG. 1A showing a capacitance tap portion 70a contiguous with a resonator portion 60a in a blank 20a. The blank 20a is otherwise similar to the blank 20. Partition line 120 is defined in a manner similar to that described above relative to bend line 36. The purpose of partition line 120 will be described below.

FIG. 1C illustrates another embodiment of the terminal resonator portions 60, 66 of blank 20. FIG. 1C is a view similar to FIG. 1B showing a coupling loop portion 70b contiguous with a wall portion 40b and adjacent a resonator portions 60b in a blank 20b. The coupling loop portion has a bend line 121. The blank 20b is otherwise similar to the blank 20. The bend line 121 is defined in a manner similar to that described above relative to bend line 36.

The purpose of those portions of the blank 20 corresponding to housing elements will now be described followed by a description of those portions corresponding to structural elements.

In FIG. 2 the resonator portions 60, 62, 64 and 66 have been bent along bend lines 100, 102, 104 and 106 to form resonator elements 140, 142, 144 and 146 while wall portion 40 has been bent along bend line 108 to form a partial housing wall 147. Resonator tap portions 70, 72 have become, respectively, resonator taps 148, 149.

In FIG. 3 wall portion 44 has been bent along bend line 110 to form housing walls 152 and 154, wall portion 46 has been partially bent along bend line 36 to form housing wall 156 and wall portion 48 has been bent along bend line 112 to form partial wall 158. Partial walls 147 and 158, when joined, will form the housing wall opposite housing wall 154.

In FIG. 4 the tuning bracket 24 has been bonded (e.g. soldered or welded) to the housing wall 154. The tuning bracket 24 has capacitance tabs 160, 162, 164, 166 and coupling tabs 171, 173, 175. Each capacitance tab is spaced above a corresponding resonator (e.g. capacitance tab 160 and resonator 140) and each coupling tab is spaced above and substantially equidistant from a corresponding pair of resonators (e.g. coupling tab 171 and resonators 140, 142).

Capacitance tabs are used to adjust the capacitance of the resonators while coupling tabs are used to adjust the electromagnetic coupling between resonators by bending them in manners well known in the art. Access to each of the capacitance tabs and coupling tabs is gained through a corresponding one of the tuning holes 82, 84, 86 and 88 (e.g. access to capacitance tab 160 and coupling tab 171 is through tuning hole 82). These tuning elements (capacitance tabs and coupling tabs) facilitate adjustment of resonator capacitance and coupling between resonators to tune the transfer function of the filter (30 in FIG. 7).

In FIG. 5 the bend along bend line 36 has been completed so that partial housing walls 147 and 158 adjoin. In FIG. 6, end portions 50 and 52 have been bent along bend lines 114 and 116 to form, respectively, housing ends 178, 179 (wall 156 is broken away to show end 179). Resonator tap 148 now extends through aperture 90 (similarly, but not shown, resonator tap 149 extends through aperture 92). Grounding tab portions 73 are bent along bend lines (118 in FIG. 1A) to form grounding tabs 180. All adjoining housing wall and end edges are bonded to complete the fabrication of the housing 22 defining the cavity 23 therein. Thus the filter 30, illustrated in FIG. 6, is comprised of only two elements, the housing 22 and the tuning bracket 24.

In the transitional steps illustrated in FIGS. 2 through 5 the finger portions 76 become structural fingers 181 which are received in corresponding notches 81 as shown in FIG. 9 which is an enlarged view of the area enclosed by the line 9 in FIG. 6. Each interlocked finger and corresponding notch, when bonded, adds ease of alignment and structural strength to the housing 22. The stop portions 78 of the blank 20 become stops 182 as shown in FIG. 9. The stops 182 fit into the notches 81a of the blank 20. The purpose of the stops 182 will be described below. The flap portions 80 of the blank 20 are bent relative to the resonators 140, 142, 144 and 146 along bend lines 119 as shown in FIG. 2 (designated on resonator 146) to become flaps 183 which add structural strength thereto. The end of each flap 183 nearest the partial wall 147 abuts therewith and the line of abutment is soldered for additional strength.

In FIG. 7 the filter 30 is connected to a microstrip circuit 32. The stops 182 automatically align the resonator tap 148 along the microstrip line 186 when the filter 22 is dropped into a corresponding hole 188 in the circuit board 190 (similarly, but not shown, resonator tap 149 is aligned along microstrip line 187). The grounding tabs 180 are bonded to ground pads 192 of the microstrip circuit board 190. The ground pads 192 are electrically connected to the microstrip ground plane (far side of board 190) by suitable means (e.g. plated through holes). The resonator taps 148, 149 are bonded, respectively, to microstrip lines 186, 187 in manners well known in the art.

FIG. 8, which is an enlarged view of the area enclosed by the line 8 of FIG. 5, illustrates that the blank 20 is bent away from areas 193 of removed material (described relative to bend line 36 above) that define the bend lines of FIG. 1A. Thus the inner housing surface has a smooth transition between walls which prevents creation of electromagnetic discontinuities.

FIG. 10 is a isometric view illustrating another embodiment of the capacitance and coupling adjustments shown in FIG. 4. Capacitance tips 230 terminate resonators 240, 242, 244, 246 and are bent relative thereto to adjust the capacitance thereof similar to the functioning of the capacitance tabs 160, 162, 164 and 166 of FIG. 4. The resonators extend from a wall 247. FIG. 11 is an enlarged view of the area enclosed by the line 11 of FIG. 10 and shows a slot 250 that facilitates bending of the capacitance tips 230. The slots 250 would be defined in a blank similar to the blank 20 of FIG. 1A.

The housing wall 254 has coupling ears 271, 273 and 275 which may be used to adjust resonator coupling similar to tabs 171, 173 and 175 shown in FIG. 4. The ears 271, 273 and 275 would also be defined in a blank similar to the blank 20 of FIG. 1A.

FIG. 12 is an isometric view similar to FIG. 4 illustrating another embodiment in which resonators 341 and 343 are interdigitated with resonators 340, 342 and 344 extending from the partial housing wall 347. The resonators 341 and 343 are defined on a resonator bracket 352 which mounts on housing wall 354.

FIG. 13 is a view similar to FIG. 7 illustrating another filter embodiment installed in the microstrip circuit 32. The filter 30a has a capacitance tap 148a soldered to the microstrip line 186 in capacitive coupling association with the resonator 140a. The capacitance tap 148a and resonator 140a are formed from, respectively, the capacitance tap portion 70a and resonator portion 60a illustrated in FIG. 1B by breaking the capacitance tap portion 70a away from the resonator portion 60a along the partition line 120. A similar cavity coupling arrangement can be used to couple to the microstrip line 187.

FIG. 14 is a view similar to FIG. 7 illustrating another filter embodiment installed in the microstrip circuit 32. The filter 30b has an inductance loop 148b soldered to the microstrip line 186 in inductive coupling association with the resonator 140b. The inductive loop 148b and resonator 140b are formed from, respectively, the inductive loop portion 70b and resonator portion 60b illustrated in FIG. 1C. The inductive loop portion 70b is bent along the bend line 121 (shown in FIG. 1C) to form the inductive loop 148b. A similar cavity coupling arrangement can be used to couple to the microstrip line 187.

FIG. 15 is a plan view of another embodiment of a resonator portion 462 similar to the resonator portion 62 of FIG. 1A. The resonator portion 462 has sub-portions 463, 464, 465, 466 and 467. The resonator portion 462 is formed and bonded into the resonator embodiment 542 (similar to resonator 142) shown in FIG. 16 in which the location of the sub-portions is indicated. The resonator 542 may be bonded at its base to the housing wall 547.

An impedance matching tab 630 extending from a tab embodiment 648 on a resonator 640 and a housing wall 647 is illustrated in the isometric view of FIG. 17. The tab 630 would be used to match the line impedance of a stripline circuit in which the resonator 640 would be integrated. The tab 630 would be defined in a blank similar to the blank 20 of FIG. 1A.

From the foregoing it should now be recognized that a fabrication method has been disclosed herein utilizing a blank fabricated from a conductive sheet and configured to be formed and bonded into a microwave filter housing. Apparatus in accordance with the present invention can be assembled easily and are thus economical to produce.

It should also be recognized that, although resonators 60, 62, 64 and 66 and taps 70, 72 were defined in the blank 20 of FIG. 1 and capacitance tabs 160, 162, 164 and 166 and coupling tabs 171, 173 and 175 were attached to a housing wall 154 in FIG. 4, the element locations could be interchanged. Thus, in general, extensions (whether resonators, taps, capacitance tabs or coupling tabs) could be formed integral with the blank 20 and folded to extend into a cavity formed therewith.

Although the present invention has been described with reference to preferred embodiments, numerous modifications and rearrangements can be made with the equivalent result still embraced within the scope of the invention.