MICROWAVE CIRCUIT DEVICE AND ITS FABRICATION

Abstract: A microwave device comprises, for example, an interdigital bandpass filter in which a block (10) of a dielectric material is shaped and drilled with a line of parallel holes (19-23) to define the physical configuration of an interdigital filter in which conductive platings on the walls of the drilled holes comprise resonator rods within a microwave cavity formed by conductive platings (11, 12, 13, 16-18) on exterior surfaces of the block. Coating material is removed from end portions of the rods formed by the coated holes in order to fine-tune the filter to a desired center frequency in the band of operation.

Description: MICROWAVE CIRCUIT DEVICE AND ITS FABRICATION Background of the Invention This invention relates to microwave circuit devices of the type utilizing resonator rods disposed within a microwave resonant cavity. One type of known microwave device (see, for example, U.S. patent 4037182) comprises a resonant cavity formed from a box-like structure having walls of conducti material and a plurality of interdigitated resonator rods extending into the interior of the cavity from opposite walls thereof (such devices have become known in the art "interdigital" devices, and such nomenclature is used herein). The free ends of the rods within the cavity are hollow, and tuning of the entire device is obtained by adjusting the axial position of insulating members movabl disposed within the hollow rod ends. A disadvantage of this type of device is that it is made of relatively complex and expensive material parts and is relatively difficult to fabricate. Also, because the size of the device is inversely related to the dielectric constant of the material within the device cavity, it is known to include a solid, high dielectric constant material within the cavity and around the resonator rods. This adds further cost and complexity to the devices. The present invention provides improvements wit respect to the aforementioned disadvantages of prior know devices. Summary of the Invention One device according to the invention comprises block of dielectric material shaped and drilled to define the physical configuration of an interdigital structure. Drilled holes in the block are interiorly coated and comprise resonator "rods" within a microwave cavity forme by electrically conductive platings on exterior surfaces the block. Coating material is removed at one end of OM various rods to tune the device. Brief Description of the Drawing FIG. 1 is a perspective view of a dielectric material block used in the invention; FIG. 2 is a perspective view of a microwave electric signal filter fabricated in accordance with the method of the present invention; and FIG. 3 is a cross-sectional view of the filter o FIG. 2 taken along the lines 3-3 in FIG. 2. Detailed Description For purposes of illustration, the illustrated device is an interdigital bandpass filter for a frequency range of approximately 800 MHz to 900 MHz. However, the invention is not limited to that type of device, or to th frequency range. The device is formed from a solid block 10 of a high dielectric constant material, e.g., barium titanate, provided with a number of holes 19-23, 26 and 2 therein, the block 10 and the hole walls being plated wit a material, such as copper or silver, having an electrica conductivity much higher than that of the material of the block 10. The plated exterior surfaces of the block comprise a resonant cavity for the device, and the plated walls of the holes 19-23 form a plurality of "interdigita resonator "rods" (actually, hollow tubes) extending into the cavity from opposite walls 17 and 18 thereof. In operation of the device, the platings on the block exteri surfaces provide a ground plane for the device. As shown most clearly in FIG. 3, the platings on the walls of holes 19, 21 and 23 are continuous with the plating 18 on the bottom of the block 10, the resonator rods in these holes thus being electrically connected to the cavity wall 18. The bottom ends of these rods are thu referred to as being "short-circuited" by the grounded cavity wall. Conversely, the top ends of the rods in the holes 19, 21, and 23 are spaced from the block top platin 17, and these rod ends are referred to as being "open- circuited". Interdigitated with holes 19, 21 and 23 are th plated holes 20 and 22. As shown, the rods in these hol are short-circuited at one end by the plating 17, the o ends of the rods being open-circuited. Wall platings on holes 26 and 27 interconnect the wall platings on holes 19 and 23 with the end plati 16 and 13, respectively, and provide means for coupling input/output coupling devices, as described hereinafter, the filter. The holes 26 and 27 can also extend to the holes 19 and 23 from the side platings 11 and 12 of the block. Desired filter characteristics can be obtained using known filter design techniques. The outside diameters of the tubes formed by the hole platings comp the outside diameter of the resonator rods. Although circular cross-section tubes or rods are preferred, othe cross-sectional shapes can be used. One advantage of the described device is that t cavity is automatically (to the extent desired) filled a high dielectric constant material. Thus, small devices can be readily made. The materials of the device are relatively inexpensive and the manufacturing process, described hereinafter, is quite simple. The impedance "looking into" the coupling holes and 27 is a function of their axial intersection with th resonator rods within holes 19 and 23, respectively. The closer the intersection with the short-circuited ends of the rods, the smaller is the impedance, impedance selection can be determined in a trial and error basis o by means of computer simulation confirmed by experiments In either event, devices having desired input/output coupling impedance can be easily made without the use of extra resonator rods, or the like, required in prior kno more complex devices. Also, different devices having different coupling impedances can be easily made using basically the same parts and same manufacturing fixtures and facilities. Additional fabrication details are now provided. Using blocks of, for example, barium titanate, the blocks are preferably first heat treated to impart long-term temperature stability of their dielectric constant and quality (Q) factor. Known treatments can be used, such as described in U.S. patent 4,337,446. The various holes in the block are then provided as by drilling. Then, because the block is to be plated, the surfaces thereof are slightly roughened, to improve platin adherence, as by a known etching process. In applying the plating to the block, an initial metallization layer is advantageously applied by standard techniques for electroplating plastics and other nonconductors. Then, th thickness of the conductor layer is built up to a suitable thickness by additional plating operations in a copper sulfate electrolyte. In copper, at the indicated frequenc range of 800-900 MHz, the skin effect is found in approximately the outer 2.5um of the conductor material. Consequently, it has been found that a plating thickness o approximately five skin depths, i.e., 13μm, provides a suitable compromise between the losses in the material if the plating thickness is too thin, and the cost of extra material. After plating, the device is fine-tuned in order to place the filter operation at the desired center frequency. This fine-tuning is done by removing the electrically conductive plating material from appropriate regions of the microwave device to produce the desired tuning effect. For the interdigital filter of the illustrative embodiment, the material removal is at one en of each of the resonator rods resulting in the open- circuited configuration previously described. Plating material removal is advantageously achieved by drilling the appropriate end of the plated hol with an over-sized drill. For example, in the case of a filter having holes 4 mm diamter, an over-sized drill of f OMP for example, 7 mm is utilized to countersink the holes a thereby remove plating material from both the inside of hole and the outside wall of the cavity around the hole. alternately drilling and applying a frequency sweep test signal to the filter, the removal is effected to achieve the desired resonant frequency for each of the resonator rods, respectively. As additional plating material is removed by deeper countersinking or reaming, the resonant frequency the device is shifted upward as the resonator rod become shorter. In removing plating material for tuning purpose it is desirable to remove little, if any, plating materi from the short-circuiting wall 17 or 18 so that the effectiveness of that wall in the overall ground plane function is not substantially reduced. Each of the resonator rods is so tuned in succession, for example, f the input resonator rod in hole 23 to the output resonat rod at the hole 19, until the filter has been tuned. In order to facilitate the connection of the filter in an electric circuit or transmission line, additional cavity wall plating material is removed, e.g. by reaming around each coupling hole 26 and 27, to break electrical connection between the hole platings and the walls 16 or 13, respectively. Sufficient wall plating i removed to provide clearance for a coaxial coupling devi (not shown) which contacts the conductive plating materi within the coupling holes 26 and 27 without touching the surrounding cavity wall plating material. OMP

Claims: Claims 1. A method of fabricating a microwave device characterized by: forming a block (10) of a dielectric material in the configuration of a resonant cavity in the desired frequency range of operation of said device, forming a hole (19) through said block intersecting exterior surfaces (17 and 18) thereof, coating said block including the wall of said hole with a continuous layer of electrically conductive material, and removing the layer from the block at a region adjacent to the intersection of one end of said hole with one (17) of the block exterior surfaces for electrically separating the layer within said hole from the layer on said exterior surface. 2. A microwave device characterized by a block (10) of dielectric material having a hole (19) therethrough intersecting two faces 17 and 18 of said block, and electrically conductive material coated over outer and inner surfaces of said block except at a region of intersection of said hole with one (17) of said faces, said excepted region being dimensioned to control the tuning of said device. 3. The device in accordance with claim 2 including a coupling aperture (26) extending through said block from an outer surface (16) thereof to said hole, sai coating being continuous between the wall of said apertur and the wall of said hole. 4. The device in accordance with claim 3 in which said block includes a plurality of said holes each of which intersects the same said two block faces and each of which is associated with an uncoated surface regio of said block, the uncoated region of successive ones of said holes being at alternating ones of said two faces. OMP 5. The device in accordance with claim 4 including an interiorly-coated aperture (26) in said blo extending to one (19) of said holes, the interior coatin of said aperture being electrically connected to the interior coating of said hole, and the interior coating in said aperture and t outer surface coating on said block being electrically discontinuous at the intersection of said aperture and t block outer surface. 6. The device in accordance with claim 2 in wh said dielectric material is barium titanate, and said conductive material is copper. 7. The device in accordance with claim 6 in wh said conductive material has a thickness approximately equal to five skin-depth thicknesses at the passband cen frequency of operation of said device.

Inventors: JOHNSON ARLEN KENT, JOHNSON, ARLEN, KENT
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1984
Main classification: H01P1
IPC codes: H01P1, H01P5, H01P11