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

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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

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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.

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