LIGHTWEIGHT CAVITY FILTER STRUCTURE

Abstract

Embodiments provide a novel fabrication method and structure for reducing structural weight in radio frequency cavity filters and novel filter structure. The novel filter structure is fabricated by electroplating the required structure over a mold. The electrodeposited composite layer may be formed by several layers of metal or metal alloys with compensating thermal expansion coefficients. The first or the top layer is a high conductivity material or compound such as silver having a thickness of several times the skin-depth at the intended frequency of operation. The top layer provides the vital low loss performance and high Q-factor required for such filter structures while the subsequent compound layers provide the mechanical strength.

Claims

1-20. (canceled)

21. A waveguide structure, comprising: a molded filter body comprising a contoured plastic material coated with an electrically conductive layer, the molded filter body to selectively direct electromagnetic energy; and at least three ports axially aligned for input and output of the electromagnetic energy, wherein the molded filter body is configured to selectively direct the electromagnetic energy between the ports based on a frequency.

22. The structure of claim 21, wherein the molded filter body is mechanically rigid.

23. The structure of claim 21, wherein the conductive layer is at least three skin depths in thickness.

24. The structure of claim 21, wherein the molded filter body has a predetermined maximum thermal expansion coefficient.

25. The structure of claim 21, wherein at least two of the three ports face a same direction.

26. The structure of claim 21, comprising four ports.

27. The structure of claim 21, wherein at least one of the three ports is axially aligned to a waveguide channel.

28. The structure of claim 21, wherein the plastic material is lightweight.

29. The structure of claim 21, wherein the electromagnetic energy is millimeter wave electromagnetic energy.

30. The structure of claim 21, wherein the structure is configured as a diplexer.

31. The structure of claim 21, comprising at least five ports.

32. The structure of claim 21, wherein the conductive layer comprises conformal conductive paint.

33. The structure of claim 21 wherein the molded filter body is configured to selectively direct the electromagnetic energy between the ports based on frequency characteristics of paths between the ports.

34. An apparatus of a base station, the apparatus comprising: transceiver circuitry; and a waveguide structure coupled to the transceiver circuitry, the waveguide structure configured as a filter, wherein the waveguide structure comprises: a molded filter body comprising a contoured plastic material coated with an electrically conductive layer, the molded filter body to selectively direct electromagnetic energy; and at least three ports axially aligned for input and output of the electromagnetic energy, wherein the molded filter body is configured to selectively direct the electromagnetic energy between the ports based on a frequency.

35. The apparatus of claim 34, wherein the waveguide structure is configured as a duplex filter for frequency domain duplex (FDD) mode operation.

36. The apparatus of claim 34 wherein the transceiver circuitry is configured for multiple-input multiple-output (MIMO) operation.

37. The apparatus of claim 34, wherein the apparatus is part of a remote-radio head (RRH) unit associated with the base station.

38. A waveguide apparatus configured as a filter, the apparatus comprising: means for selectively directing electromagnetic energy, the means comprising a molded filter body comprising a contoured plastic material coated with an electrically conductive layer; and means for inputting and outputting the electromagnetic energy, the means comprising at least three ports axially aligned, wherein the molded filter body is configured to selectively direct the electromagnetic energy between the ports based on a frequency.

39. The apparatus of claim 38, further comprising transceiver circuitry coupled to the apparatus to form a base station.

40. The apparatus of claim 38, wherein the waveguide structure is configured as a duplex filter for frequency domain duplex (FDD) mode operation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic diagram of a lumped circuit having a capacitive coupled filter structure.

[0023] FIG. 2 is a schematic diagram of a lumped distributed RF filter.

[0024] FIG. 3 is a top, perspective view of a typical machined or cast aluminum combline duplexer filter structure as fabricated.

[0025] FIG. 4A is a top, perspective view of a metal mold used for the fabrication of a cavity filter structure in an embodiment.

[0026] FIG. 4B is a representation of a cross-sectional view depicting a layer of electroplated metal deposited on a metal mold.

[0027] FIG. 4C is a representation of a cross-sectional view depicting a layer of laminate applied to the surface of the electroplated metal.

[0028] FIG. 4D is a representation of a cross-sectional view of the electroplated metal and laminate after the metal mold has been removed in an embodiment.

[0029] FIG. 4E is a representation of a cross-sectional view depicting multiple layers of laminate applied to the surface of the electroplated metal.

[0030] FIG. 4F is a representation of a cross-sectional view depicting the electroplated metal and the multiple layers of laminate after the metal mold has been removed.

[0031] FIG. 4G is a top, perspective view of the resulting cavity filter structure.

[0032] FIG. 5A is a top, perspective view of an insulating mold used for the fabrication of a cavity filter structure.

[0033] FIG. 5B is a representation of a cross-sectional view depicting a layer of electro-less deposited metal applied to the insulating mold.

[0034] FIG. 5C is a representation of a cross-sectional view depicting a layer of electroplated metal deposited on the electro-less deposited metal.

[0035] FIG. 5D is a representation of a cross-sectional view depicting one or more layers of laminate applied to the surface of the electroplated metal.

[0036] FIG. 5E is a representation of a cross-sectional view depicting the metal layers and the multiple layers of laminate after the insulating mold has been removed.

[0037] FIG. 5F is a top, perspective view of the resulting cavity filter structure.

[0038] FIG. 6A is a top, perspective view of a housing having the shape and contours of a cavity filter structure.

[0039] FIG. 6B is a cross-sectional view of the housing.

[0040] FIG. 6C is a representation of a cross-sectional view depicting an electro-less metal deposited on the surface of the housing.

[0041] FIG. 6D is a representation of a cross-sectional view of electroplated metal deposited on the electro-less deposited metal.

[0042] FIG. 6E is a top, perspective view of the resulting cavity filter structure.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The mechanical structure of a conventional cavity based filter/duplexer housing 101 shown in FIG. 3 would have excessive weight. This is due to its massive and bulky resonator structure forming the cavity walls such as of the walls of cavities 110, 112, and 114 and partitions such as 116 and 118 between various compartments. The main embodiments disclosed herein relate to a manufacturing system and method that reduces the weight of such filter structures.

[0044] Within this disclosure, reference to various metal deposition processes including electro-less deposition and electroplating will be used as specific examples of implementations in one or more embodiments. As used herein and consistent with well known terminology in the art, electro-less plating generally refers to a plating process which occurs without the use of external electrical power. Electroplating generally refers to a process which uses an electrical current to deposit material on a conductive object. However, the use of the these specific plating processes should not be taken as being limited in nature as the methods disclosed herein may be practiced with other metal deposition techniques known in the art. Furthermore, various intermediate processing steps know in the art such as, but not limited to, pretreatment, cleaning, surface preparation, masking, and the use of additional layers to facilitate separation or adhesion between adjacent layers may not have been explicitly disclosed for the purposes of clarity but may be employed in one or more embodiments.

[0045] Moreover, as used throughout this disclosure, the various cross-sectional views of the layered structures during the fabrication process and the resulting cavity filter structures are representations to illustrate the cross-sectional views and may not necessarily be to scale.

[0046] Embodiments relate to novel approaches for the design and fabrication of filters similar, but not limited to the structures described herein and above. Embodiments accordingly also include improved filter structures. The electrical performance of filter structures like those discussed above is very much dependant on the electrical properties of the surface material. Thus, while the surface losses are critical, the cavity wall thickness is of less significance to extent the that, while it helps achieve the desired mechanical rigidity, it is responsible for a disproportionate weight of the finished product. Therefore, in order to reduce the weight of the filter structure, the cavity wall density would need to be reduced substantially. This is to say that the mass per unit volume of the filter structure can be reduced considerably if the filter structure is formed by a controlled electro-deposition process. Details of this process will be discussed in some detail in following sections.

[0047] Embodiments provide a method and apparatus for low cost fabrication of a single or multi-mode cavity filter leading to a lightweight structure. Before a detailed discussion of one or more embodiments is presented, the relevant electrical theory will be described first.

[0048] It is well known to those with ordinary skill in the art that an AC signal penetrates into a conductor by a limited amount, normally penetrating by only a few skin depths. The skin depth by definition is defined as the depth below the surface of the conductor at which the current density has fallen to 1/e (i.e., about 0.37) of the current density. In other words, the electrical energy conduction role of the conductor is restricted to a very small depth from its surface. Therefore, the rest of the body of the conductor, and in the case of a cavity resonator, the bulk of the wall, does not contribute to the conduction.

[0049] The general formulae for calculating skin depth is given in equation (1)

[00001]$\begin{array}{cc}=\sqrt{\frac{2.Math.}{2.Math..Math.f.Math.{}_R.Math.{}_0}}503.Math.\sqrt{\frac{}{{}_R.Math.f}}& \left(1\right)\end{array}$

where [0050] p is resistivity (Ohm-meters), [0051] f=frequency (Hz), and [0052] .sub.0=410.sup.7.

[0053] From equation (1) it is evident that the skin depth is inversely proportional to signal frequency. At RF and microwave frequencies, the current only penetrates the wave-guiding walls by a few skin depths. The skin depth for a silver plated conductor supporting a signal at 1 GHz is 2.01 m. For copper the figure is very close (2.48 m). Hence while the actual wave-guiding walls are a few millimeters thick, the required thickness of the electrical wall is in the order of 10 m.

[0054] Based on the previous discussions, the electrical performance of the filter structure and, indeed, any conducting structure supporting radio frequency signal can have a much reduced conductor thickness without an impact on their electrical characteristics (such as resonator Q-factors and transmission coefficients).

[0055] Embodiments are based on utilizing this property of an electrical conductor. The conventional method of manufacturing cavity filters relies on machining or casting a solid bulk of aluminum or copper and plating the conducting surfaces by electroplating copper or silver. A typical cavity filter is constructed using a structural base metal (e.g., aluminum, steel, invar etc.) plated with copper followed by silver. The plated layer is normally several skin-depths thick. The bulk of the structure serves as a structural support providing mechanical rigidity and thermal stability. It is of course possible to cast the filter structure and electroplate subsequently to achieve the same end result.

[0056] One or more embodiments provide a fabrication method in which the filter structure is formed by electroplating over a mold or a former that is a mirror image of the cavity structure(s). This can be achieved by machining or casting a former out of a metal structure that serves as the cathode in the electroplating process. The plated layer is several skin-depths thick. Beyond what is required to satisfy the electrical conductions, an additional plating laminate will improve the mechanical strength at the expense of added weight. The electroplated cavity structure can include the coaxial resonator, or provision for bolt in resonators (either coaxial or dielectric).

[0057] FIGS. 4A-4D depict an exemplary apparatus and the structures at various steps in the fabrication process. FIG. 4A illustrates a metal mold 201 used for the fabrication of a cavity filter in an embodiment. The mold 201 has a contoured surface having a shape inverse to that of a cavity filter structure 230 shown in FIG. 4G. In general, the fabrication process comprises depositing materials onto the mold 210 and then separating the deposited materials from the mold 210 to result in the desired cavity filter structure 230. For example, the mold 201 has three cylinders 210, 212, and 214 which have an inverse shape to the cavities 240, 242, and 244 of cavity filter 230 shown in FIG. 4G. The metal mold 201 may be coupled to a voltage potential and placed in an electroplating bath which enables metal to be electroplated onto the metal mold 201. Cutaway, cross-sectional views of the structure as built are presented in FIGS. 4B-4G.

[0058] FIG. 4B illustrates an exemplary cross-sectional view depicting the resulting layer of electroplated metal 222 deposited on a metal mold 220. As depicted in FIG. 4G, a laminate 224 may be applied to the electroplated metal 222 to provide additional mechanical rigidity. The laminate 224 may comprise conducting or insulating materials in one or more embodiments. Examples of conducting materials may include metals and metal alloys.

[0059] The electro-plated metal 222 may then be separated from the metal mold 220 to form a shell similar to that shown in cavity filter 230 comprising the electro-plated metal 222 and the laminate 224. While not explicitly described above for the purposes of clarity, additional steps may be employed to enable the separation of the electro-plated metal 222 from the mold 220. Such additional steps may include coating the mold 220 with a sacrificial layer which may be etched, liquefied, or dissolved to facilitate the separation of the electroplated metal 222 from the mold 220. FIG. 4D depicts a cross-sectional view of the electroplated metal 222 and the laminate 224 after the metal mold 220 has been separated from the electroplated metal 222 in an embodiment.

[0060] One or more embodiments provide a method of depositing several different layers with opposing thermal expansion rate to prevent the undesirable thermal expansion of the cavity dimensions.

[0061] FIG. 4E is a representation of a cross-sectional view depicting multiple layers of laminate 226a-226d applied to the surface of the electroplated metal 222. The layers of laminate may comprise metal, metal alloys, or insulating materials with compensating thermal expansion coefficients. For example, multiple layers of laminate may be employed such that each layer of the laminate has a thermal expansion coefficient opposite to that of an adjacent layer of laminate. As discussed above, the electroplated metal 222 may be separated from the mold 220. FIG. 4F illustrates a cross-sectional view depicting the electroplated metal 222 and the multiple layers of laminate 226a-226d after the metal mold 220 has been removed, and FIG. 4G depicts the final cavity filter structure 230.

[0062] As shown in FIG. 4F, the thickness of the electroplated metal 222 has a thickness represented as d.sub.1 and the total thickness of the laminate layers is represented as d.sub.2. The thickness of the electroplated metal 222 d.sub.1 may be on the order of at least one to several times the skin depth associated with the operating radio frequency of the cavity filter structure in one or more embodiments. The thickness d.sub.1 may be approximately 10 micrometers in an embodiment. The total thickness d.sub.2 of the laminate 226a-226d is sufficient to provide mechanical rigidity to the electroplated metal 222 and may approximately one to several millimeters in an embodiment. The thickness d.sub.2 of the laminate may be optimized based on the materials employed.

[0063] Another embodiment provides that the former may be made out of a metal of a non-metallic (insulator) material that is used as the cathode in the electroforming process but after an electro-less deposition process.

[0064] FIGS. 5A-5E depict exemplary structure at various steps in an exemplary fabrication process, and FIG. 5F illustrates the resulting cavity filer structure 330. FIG. 5A illustrates an insulating mold 301 used for the fabrication of a cavity filter. The mold 301 has a contoured surface having a shape inverse to that of a cavity filter structure shown in FIG. 5F. An electro-less deposited metal 321 may be formed on mold 301 using known electro-less deposition processes. FIG. 5B depicts the layer of electro-less deposited metal 321 applied to the insulating mold 320. The electro-less deposited metal 321 may then be connected to a voltage potential and placed in an electro-plating bath as discussed above. FIG. 50 depicts a layer of electroplated metal 322 deposited on the electro-less deposited metal 321.

[0065] In an embodiment, one or layers of laminate 324 are applied to the electroplated metal 322 as illustrated in FIG. 5D. The layers of laminate may comprise metal, metal alloys, insulating materials, or metal alloys interspersed with insulating materials with compensating thermal expansion coefficients. For example, multiple layers of laminate may be employed such that each layer of the laminate has a thermal expansion coefficient opposite to that of an adjacent layer of laminate. The mold 320 may be separated from the electro-less deposited metal 321 as illustrated in FIG. 5E and as discussed above. The final cavity filter structure 330 is shown in FIG. 5F.

[0066] As shown in FIG. 5E, the electro-less deposited metal has a thickness represented as d.sub.1, electroplated metal 322 has a thickness represented as d.sub.2 and the total thickness of the laminate layers is represented as d.sub.3. The thickness d.sub.1 may be in the range of a fraction of micrometer to several micrometers in an embodiment. The thickness d.sub.2 may be in the range of a fraction of a micrometer to several micrometers in an embodiment. The total thickness of the electro less metal 321 and the electroplated metal 322 d.sub.2 (i.e., d.sub.1+d.sub.2) may be on the order of at least one to several times the skin depth associated with the operating radio frequency of the cavity filter structure in one or more embodiments and may be approximately 10 micrometers in an embodiment. The total thickness d.sub.3 of the laminate 324 is sufficient to provide mechanical rigidity to the electro-less deposited metal 321 and the electroplated metal 322 and may be approximately one to several millimeters in an embodiment.

[0067] In an embodiment, yet another fabrication method is to mold the actual filter structure (the negative of what is shown in FIGS. 4A and 5A) out of an insulating compound such as light plastic or polystyrene with a good surface finish. The electrical performance will be achieved by metalizing the surface through electro-less or conductive paint. The thin metal deposit will be electroplated to an appropriate thickness based on the frequency of operation.

[0068] FIG. 6A is a top, perspective view of a housing 401 having the shape and contours of a cavity filter structure. The housing 401 may be formed out of a thin, insulating material which provides sufficient mechanical rigidity with minimal weight. Examples of insulating materials may include lightweight plastics such as, but not limited to, polystyrene. Additional braces and walls may be formed on the housing 401 for additional mechanical support. FIG. 6B depicts a cross-sectional view of the housing 401 in an embodiment, and further illustrates that insulating material 420 is much thinner than that of conventional structures.

[0069] A layer of electro-less deposited metal 421 is deposited on the insulating material 420 as discussed above and shown in FIG. 6C. This layer of electro-less deposited metal 421 may be coupled to a voltage potential to form a cathode in an electroplating process. The resulting cross-section of the electro-plated metal layer 422 deposited to the layer of electro-less metal is shown in FIG. 6D. As a result, the housing 401 now has contoured metal structure which exhibit properties of a conventional cavity filter but at a fraction of the overall weight. FIG. 6E depicts the final cavity filter structure 430. In an embodiment, insulating material 420 may be removed and other structural components may be coupled to the electro-less deposited metal.

[0070] As shown in FIG. 6D, the electro-less deposited metal 421 has a thickness represented as d.sub.1, electroplated metal 422 has a thickness represented as d.sub.2 and the housing insulating material 420 has a thickness represented as d.sub.3. The thickness d.sub.1 may be in a range approximately from a fraction of a micrometer to several micrometers and the thickness d.sub.2 may be approximately in a range from a fraction of a micrometer to several micrometers in an embodiment. The total thickness of the electro-less metal 421 and the electroplated metal 422 d.sub.2 (i.e., d.sub.1+d.sub.2) may be on the order of at least one to several times the skin depth associated with the operating radio frequency of the cavity filter structure in one or more embodiments and may be approximately 10 micrometers in an embodiment. The total thickness d.sub.3 of the housing insulating material 420 is sufficient to provide mechanical rigidity to the electro-less deposited metal 321 and the electroplated metal 322 and may approximately one to several millimeters in an embodiment.

[0071] An embodiment provides related mechanical reinforcement of the electro-deposited filter shell. The ultra light filter structure formed by electroplating may suffer from mechanical rigidity. The structure is then filled by reinforcing foam. A variety of filler options are available for this task. This embodiment is not limited to a filler material and other metal or none metal reinforcement structures are also claimed.

[0072] An embodiment provides the provision of reinforcing the plated cavity structure by insertion of a reinforcement structure before the plating. The reinforcing structure can be fused with the electrodeposited structure, adding mechanical strength and stability.

[0073] An embodiment relates to the method of reinforcing the overall structure by adding, welding, or brazing additional plates or laminates to the structure to achieve mechanical strength while minimizing the added weight.

[0074] An embodiment of invention extends the application of technique described above to other radio subsystems such as antennas, antenna array structures, integrated antenna array-filter/duplexer structures and active antenna arrays.

[0075] The foregoing descriptions of preferred embodiments of the invention are purely illustrative and are not meant to be limiting in nature. Those skilled in the art will appreciate that a variety of modifications are possible while remaining within the scope of the present invention.

[0076] The present invention has been described primarily as methods and structures for fabricating lightweight cavity filter structures. In this regard, the methods and structures for fabricating lightweight cavity filter structures are presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Accordingly, variants and modifications consistent with the following teachings, skill, and knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain modes known for practicing the invention disclosed herewith and to enable others skilled in the art to utilize the invention in equivalent, or alternative embodiments and with various modifications considered necessary by the particular application(s) or use(s) of the present invention.