Abstract:
A simplified method for forming passive microwave components, such as a filter, and passive microwave components formed by the method. The method includes forming a ceramic insert having a plurality of resonator regions and then die casting an outer casing of a conductive material about the ceramic insert. Each resonator region has a cavity that may be filled with the conductive material used to die cast the outer casing or, alternatively, may be filled with a resonator rod made of different materials than the encapsulating metal.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to wireless communications networks and similar electronic systems and, in particular, to microwave filter components for wireless communications networks.  
       BACKGROUND OF THE INVENTION  
       [0002]     Wideband, high-data-rate wireless communications networks based on cellular technologies are used worldwide for delivering an ever increasing amount of information to a mobile society. According to fundamental principles of cellular technology, a coverage area is divided into multiple cells that are mutually arranged to communicate with mobile stations or devices with minimal interference. Communications from a mobile station crossing the coverage area is handed-off between adjacent cells according to the location of the mobile station within the coverage area. Each of the cells is generally served by a base station having a transceiver that communicates with the mobile device. The frequency spectrums of the communications signals associated with the cells are divided into multiple different frequency bands. Therefore, filters, such as passive microwave filters, are used to perform band pass and band reject functions for separating the different frequency bands.  
         [0003]     Cell sizes are often reduced as information bandwidth handled by the cells increases. As a consequence, additional cells are required within a coverage area to provide wireless communication service to an increasing number of mobile stations. Increasing numbers of passive microwave filters are included in tower-mounted amplifiers and related equipment to address the bandwidth increases.  
         [0004]     Conventional microwave filters include a metallic shell or filter body having dividing walls that partition an open interior space into recesses and a cover that closes the recesses to define air-filled filter cavities or resonators. The metalworking process forming the filter body must accommodate precise dimensioning of the recesses to achieve satisfactory filter performance. Typically, the filter body is formed by casting and the cover is formed separately by either casting or stamping. After forming, the filter body may require additional machining for tuning the resonators as desired.  
         [0005]     The cover and filter body are assembled together to complete the microwave filter. A seam is defined about the contacting circumferences of the filter body and the cover. After assembly, the cover must have a good electrical contact with the filter body along the entire extent of the seam to ensure proper filter operation. If the microwave filter is exposed to an outdoor environment, the seam must be hermetically sealed against the infiltration of water and other elements so that the resonators remain moisture-free. The presence of moisture in the resonators reduces the long-term reliability of the microwave filter.  
         [0006]     Generally, such conventional microwave filters are relatively expensive to manufacture. In particular, the need to manufacture the precisely dimensioned resonators and a separate cover increases the cost as each component must be individually manufactured and assembled together.  
         [0007]     The physical size of conventional microwave filters may be reduced by loading inserts of a temperature stable ceramic material characterized by a high dielectric constant and a high quality factor into the recesses previously filled with air. However, despite the reduction in size, the manufacturing cost is not significantly reduced as the microwave filter still includes a filter body and cover, and the ceramic inserts must be loaded into the recesses within the filter body.  
         [0008]     Additionally, to address the cost issue, certain microwave filters incorporate commercially-available metallized ceramic resonators into a low-precision, low-cost sheet metal filter body. The presence of the ceramic reduces the size of the microwave filter. However, such composite structures lack the relatively-low insertion losses and relatively-high rejection numbers required for tower-mounted amplifiers currently used in wireless communication networks. Therefore, filter performance suffers.  
         [0009]     Therefore, it would be desirable to provide a microwave filter which addresses the problematic seams and cost issues associated with precision formed filters. It would also be desirable to address the performance disadvantages associated with low-cost conventional microwave filters.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a perspective view of a ceramic insert for a microwave filter in accordance with the principles of the invention;  
         [0011]      FIGS. 2A-2D  are diagrammatic views showing a method for manufacturing the microwave filter of the invention;  
         [0012]      FIG. 3  is a perspective view of the completed microwave filter; and  
         [0013]      FIG. 4  is a cross-sectional view in accordance with an alternative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     With reference to  FIG. 1 , a ceramic element or insert  10  is fashioned from a machinable, castable or extrudable ceramic characterized by being easily shaped with standard manufacturing methods, unaffected structurally by high temperatures and high pressures encountered during a die casting process, and a low dissipation factor. An exemplary ceramic material suitable for forming the ceramic insert  10  is boron nitride, which is stable in inert and reducing atmospheres up to about 3000° C. and in oxidizing atmospheres to about 850° C., and is machinable using ordinary machine tools formed of hardened tool steel. Boron nitride has a high thermal conductivity of 20 W/(m-K) at room temperature and an excellent thermal shock resistance exceeding 1500° C. Boron nitride has a dissipation factor (measured according to ASTM D-150) of about 0.0011.  
         [0015]     The ceramic insert  10  includes a plurality of annular or tubular resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  and a corresponding plurality of cavities  24 ,  26 ,  28 ,  30 ,  32  and  34  each surrounded by a corresponding one of the resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22 . The resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  are electrically connected in series to form a main coupling path for microwave signals through the microwave filter  65  ( FIGS. 2D, 3 ). The electrical response of the microwave filter  65 , formed using the ceramic insert  10  as described below, may be altered by varying the proximity of adjacent resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22 . The number of resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  is not limited, although microwave filter  65  will typically have four to eight distinct resonator regions. The cavities  24 ,  26 ,  28 ,  30 ,  32  and  34  are aligned parallel to one another and each of the illustrated cavities  24 ,  26 ,  28 ,  30 ,  32  and  34  has a generally circular cross-sectional profile. However, the invention is not so limited as the cross-sectional profile of the individual cavities  24 ,  26 ,  28 ,  30 ,  32  and  34  may be, among other examples, elliptical, rectangular or square. The resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  may be dimensioned, shaped, and arranged, as understood by a person of ordinary skill in the art, to provide, for example, a comb-line filter, interdigital filter or a wave guide filter.  
         [0016]     The ceramic insert  10  may be a monolithic structure in which the resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  are joined by individual bridging segments  23  of ceramic, as shown in  FIG. 1 , or may constitute individual components arranged in a side-by-side, contacting relationship after the microwave filter  65  ( FIGS. 3A, 3B ) is formed. In that latter situation, the individual resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  may include side flats that assist in maintaining the mutual arrangement among the resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  during the die casting process that creates the microwave filter  65 . The space between the adjacent pairs of the resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22  normally should not be filled by metal during the die casting operation. The bridging segments  23  fill the inter-resonator spaces.  
         [0017]     An alternative approach for forming the ceramic insert  10  without the necessity of machining of a ceramic block is ceramic injection molding, which would provide, as an end product, a unitary, monolithic structure of a green ceramic in which the individual resonator regions  12 ,  14 ,  16 ,  18 ,  20 , and  22  are interconnected. A slurry of a ceramic powder and a polymeric binder is injected in an injection molding machine into a mold having a shape complementary to the shape of the ceramic insert  10 . The “green” ceramic insert  10  is heated to remove the polymeric binder and then sintered to strengthen the bonds among grains of the ceramic powder.  
         [0018]     With reference to  FIG. 2A , a die casting machine, generally indicated by reference numeral  40 , includes a stationary platen  42  to which a cover die  44  is attached and a movable platen  46  to which an ejector die  48  is attached. A shaped die cavity  50  is defined between the contacting cover die  44  and ejector die  48 . Movement of the movable platen  46  relative to the stationary platen  42  affords access to the die cavity  50 . A plurality of ejectors  52  penetrate through the ejector die  48  and are extendable into the die cavity  50  for ejecting the partially-completed microwave filter  65  from the die cavity  50  when the cover die  44  is spaced apart from the ejector die  48 .  
         [0019]     A metal reservoir  54  is defined in a shot sleeve  56  having one end communicating with the die cavity  50  and an opposite end having an inlet  58  adapted to receive molten metal  60  provided from a metering device  62 , such as a ladle. A piston  64  of a hydraulic cylinder extends into the shot sleeve  56 . The piston  64  is extendable relative to the shot sleeve  56  for injecting molten metal  60  from the shot sleeve  56  into the die cavity  50 .  
         [0020]     With reference to  FIGS. 2A-2D , the manufacture of the microwave filter  65  using the ceramic insert  10  will be described in accordance with the principles of the invention. As described above with reference to  FIG. 1 , the ceramic insert  10  is formed by either casting, extrusion or injection molding. The movable platen  46  is moved relative to the stationary platen  42  to afford access to the die cavity  50 . As shown in  FIG. 2A , the ceramic insert  10  is inserted into the die cavity  50  and the movable platen  46  is moved to close the die cavity  50 . A metered volume of molten metal  60 , typically aluminum or an aluminum alloy, is introduced through the inlet  58  into the reservoir  54  of the shot sleeve  56 . As shown in  FIG. 2B , the piston  64  is moved within the shot sleeve  56  for introducing the molten metal  60  into the die cavity  50  under high pressure. The molten metal  60  fills the open space within the die cavity  50  not otherwise occupied by the ceramic insert  10 , including the resonator regions  12 ,  14 ,  16 ,  18 ,  20  and  22 . After the metal  60  has solidified, the movable platen  46  is moved to again afford access to the die cavity  50  and the ejectors  52  are extended to dislodge and remove a partially-completed microwave filter  65 . With reference to  FIG. 2C , after solidification, the microwave filter  65  has an elongated outer casing  66  of metal  60  that encapsulates the ceramic insert  10 . Metal  60  filling the cavities  24 ,  26 ,  28 ,  30 ,  32  and  34  of the ceramic insert  10  define individual resonator rods.  
         [0021]     With reference to  FIGS. 2D and 3 , the outer casing  66  may be machined, such as by laser machining or electromachining, to add an input port  68  for introducing an electrical signal into the microwave filter  65  and an output port  70  for extracting a filtered signal. The casing  66  may be further machined to provide threaded openings for tuning adjustment elements  72  that are operative for adjusting the resonant frequency of the cavities  24 ,  26 ,  28 ,  30 ,  32  and  34  by adjusting the position of each tuning element relative to the metal  60  to change the volume of a corresponding one of a plurality of air gaps  73 . Although the tuning adjustment elements  72  are depicted as threaded screws, other types of tuning adjustment elements may be added without deparating from the spirit and scope of the invention. The microwave filter  65  is tuned and tested before being deployed for use.  
         [0022]     The microwave filter  65  is a monolithic unit, generally having the shape of a right parallelepiped, that lacks any seams that would otherwise present entry paths for moisture from the surrounding environment. In addition, the absence of a discrete cover and a discrete filter body, as is conventional, eliminates the need to establish a good electrical contact about the entire mutual line-of-contact. A microwave filter in accordance with the principles of the invention is low cost, high performance, seamless and more compact than conventional microwave filters. The microwave filter  65  may be configured as a comb-line filter, interdigital filter or a wave guide filter. The invention contemplates that other passive microwave components may be formed by the method of the invention.  
         [0023]     With reference to  FIG. 4  in which like reference numerals refer to like features in  FIG. 2D , a microwave filter  74  may include a plurality of resonator rods  76 ,  78 , and  80 , of which only three resonator rods are shown, each filling one of the corresponding cavities  24 ,  26 , and  28  of the dielectric insert  10 . In one embodiment, the resonator rods  76 ,  78 , and  80  are shorter than the length of the resonator to create an air gap  79  in the cavities  24 ,  26 ,  28 ,  34 . During the molding, appropriate steps may be taken to keep molten metal out of the cavities  24 ,  26 ,  28 ,  34 . Resonator rods  76 ,  78 , and  80  are coaxially positioned within the corresponding one of the cavities  24 ,  26 , and  28  and  34  before the ceramic insert  10  is positioned in the die cavity  50  ( FIG. 2A ) and molten metal  60  is injected into the die cavity  50 . The cross-sectional profile of each of the resonator rods  76 ,  78 , and  80  closely matches the cross-sectional profile of the corresponding one of the cavities  24 ,  26 , and  28 . The resonator rods  76 ,  78 , and  80  are formed from a metal that differs in composition from the metal  60  injected during the die casting operation ( FIGS. 3A, 3B ). After the microwave filter  74  is die cast and the metal  60  solidifies, each resonator rod  76 ,  78 , and  80  has a strong metallurgical bond with the inwardly-facing cylindrical sidewall of the corresponding one of the cavities  24 ,  26 , and  28  in the ceramic insert  10 . The tuning adjustment elements  72  and the input and output ports  68 ,  70  are added by machining operations, as described in relation to  FIGS. 2C and 2D . Movement of each of the tuning adjustment elements  72  changes the volume of a corresponding one of a plurality of air gaps  79 .  
         [0024]     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in considerable detail in order to describe a preferred mode of practicing the invention, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. The invention itself should only be defined by the appended claims, wherein