|Publication number||US3579156 A|
|Publication date||May 18, 1971|
|Filing date||Aug 15, 1968|
|Priority date||Aug 15, 1968|
|Publication number||US 3579156 A, US 3579156A, US-A-3579156, US3579156 A, US3579156A|
|Original Assignee||Worcester Controls Corp|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (6), Referenced by (13), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent Inventor Peter Parfitt Hove, England Appl. No. 752,913 Filed Aug. 15, 1968 Patented May 18, 1971 Assignee Worcester Controls Corporation West Boylston, Mass.
SHIELDED FILTER 10 Claims, 5 Drawing Figs. I US. Cl 333/79, 333/70, 333/78, 336/90, 317/256 Int. Cl H03h 7/04 Field of Search 333/70, 76, 79, 78; 317/242, 261; 323/76; 336/79, 90
References Cited UNITED STATES PATENTS 2,550,244 4/1951 Gusdorf 333/78 7/1958 Sabaroff 333/73C 2,886,788 5/1959 Cushman 336/79 3,267,396 8/1966 Scott 333/70 3,227,973 1/1966 Gray 333/78 3,191,131 6/1965 Adams 333/76 Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. Baraff Attorneys-William D. Hall, Elliott l. Pollock, Fred C. Philpitt, George Vande Sande, Charles F. Steininger and Robert R. Pn'ddy ABSTRACT: An electrical filter comprises an elongated metallic tube having a pair of open-ended cans inserted into the tube ends with the open ends of the cans being joined to the tube adjacent the tube ends. The interior regions of the tube, divided by the closed ends of the cans, contain inductance and capacitance elements electrically interconnected to one another, said interconnections including conductive tabs carried by the capacitance elements. The ends of the tube are hermetically sealed; and the interior of the tube is filled with a potting compound.
PETER PARFITT INVENTOR ATTORNEY SHIELDED rrrxrsn BACKGROUND OF THE INVENTION Filters of various electrical configuration are, in themselves, well known. Such filters may be used for low-pass, high pass band-pass, band elimination, or general filtering purposes; and all of these various known electrical configurations are contemplated by the present invention.
As a practical matter, filters of the general types described comprise a plurality of electrical (inductance, capacitance and/or resistance) components interconnected in repetitive fashion to provide a succession of filter sections between cooperating input and output terminals. in such arrangements, it is often necessary to introduce RF screening means of some type between the various filter sections so that as a signal passes through the filter it is not contaminated by unwanted interference signals or trash percolating from the input end of the filter toward its output end. Such RF screens must be of metal construction, and must completely isolate the filter sections from one another. At the present time, where it is desired to provide a shielded filter of the general type described, it has been normal procedure to assemble the filter components into an elongated metal tube, and, at the same time, to place metal flanges into the tube between selected filter elements. The
metal flanges employed customarily have their edges tinned with a soft solder; and when the parts are assembled in place, a flame is passed around the outside of the tube causing the solder to melt and adhere to the inner wall of the tube so as to anchor the shield flanges in place.
Structures and techniques of the types described above are practical only when the materials constituting the overall filter are carefully selected. The metal parts must be such that they are easily solder-able at relatively low temperatures. Moreover the internal electrical components, and'the potting materials used, must be capable of withstanding the temperatures required to complete the shielding joints. These requirements in turn place considerable limitations on the types of material which can be used in any given filter, as well as upon the electrical characteristics of the filter itself.
By way of example, it is known that a number of metals are difficult to solder. One in particular is stainless steel. Such stainless steel materials require relatively high temperatures to solder, and also ordinarily requires the use of very corrosive fluxes. When it is desired to employ stainless steel in the filter construction, therefore, there is considerable danger that the characteristics and. life of the filter may be severely prejudiced by the high temperatures to which components are subjected during the manufacturing process, and by residual flux remaining in the assembly after the fabrication has been completed. While the present invention is not limited to the use of stainless steel materials, the construction to be described does permit the use of such materials without risking these disadvantages.
The known structures and techniques described are further subject to the disadvantages that it is relatively difficult to-assemble the shielding flanges'and components in place, it is relatively difficult to assure that wiring interconnections between components are not disturbed or broken during potting steps, and it is relatively difficult to eliminate possible shifting of components and shielding elements during the overall filter fabrication. These manufacturing difficulties are complicated by the fact that it is often desired to miniaturize filters of the types contemplated herein; and such miniaturization severely aggravates the problems already mentioned.
The present invention, by employing-a new mechanical arrangement for electrical filters of the types described, obviates all of these disadvantages and permits afilter of superior electrical and mechanical characteristics to be fabricated more easily, and at less cost, than has been the case heretofore.
SUMMARY OF THE INVENTION In accordance with the present invention, a new filter construction is provided wherein an elongated open-ended metallic tube is fitted with a pair of opemended metallic cans inserted respectively into the open ends of the tubes with the open can ends and open tube ends being closely adjacent one another. Each can contains an inductance element and a capacitance element; and the interior tube region defined between the closed can ends contains at least one further filter component (e.g. a capacitor element) or a plurality of inductor and capacitor elements. The closed can ends act as shielding elements extending across the tube interior, so as to subdivide the tube into a plurality of sections having the desired RF isolation therebetween.
The can elements and their associated components are inserted into the tube as subassemblies, and the filter parts are then permanently fixed in place by running appropriate joints (by soldering, welding, or brazing) between the tube and interior cans adjacent the open ends of the tube and cans. Due to the location of the joints thus made, corrosive fluxes may be employed, if needed, since it is possible to completely wash such fluxes away from the assembly before sealing the ends of the tube, the joints being exterior to all electrical parts. Moreover, due to the fact that the joints are disposed closely adjacent to the open ends of the tube, itis possible to join each can to the exterior tube and to a sealing end cap in a single operation, thereby reducing the time and cost of assembly. Further, since the joints being made are located at the extreme ends of the cans and tubes, it is possible to use relatively high temperatures to effect such joints without risking high temperature damage to the delicate electrical components and the encapsulant or potting material, inasmuch as these latter elements are surrounded by metal parts which act as a heat sink during the joining operation.
Further advantages are achieved by the use of inductor and capacitor elements of unique construction disposed within the aforementioned tube and cans. The capacitors are of the extended foil type. While such capacitors are, in themselves well known, it has been customary practice heretofore to attach wires to the exposed layers of foil at either end of the capacitor and to connect these wire leads to other components or to ground. The extended foil capacitors used in the present invention are provided with conductive tags at their opposing ends, and with an insulating sleeve extending through the capacitor between said tags. These constructional aspects of the capacitor facilitate the making of interconnections between components within the tube, and between the capacitor and ground. Moreover the use of such tags provides a wide path for current flow, thereby reducing inductance in the capacitor; and further assures that slack may be removed from any wire runs between filter components thereby reducing the hazard of damage to-loose wires when the completed filter elements are'inserted into the tube.
The inductor arrangements employed comprise turns of insulated wire wound about a toroidal core, with the overall coil then being enclosed within a thin skin of polywrap material shrunk about the inductance. The coil shape and size is related to the geometry and positioning of tag elements on the aforementioned capacitors so that the inductor and capacitor elements can be placed in close proximity to one another, and securely held in desired position during completion of the overall assembly.
The overall arrangement makes it far easier to assemble the filter parts and to assure that they remain in desired positions relative to one another during subsequent fabrication steps; facilitates the making of electrical connections between the filter components; assures that necessary mechanical joints and hermetic sealing can be achieved without limitation in the materials or joining techniques used, and without risking damage to the electrical components or potting compound;
and facilitates the miniaturization of the overall filter when that is required.
BRIEF DESCRIPTION OF THE DRAWlNGS FIG. l is a view, in partial cross section, of a filter constructed in accordance with one embodiment of the present invention;
FlG. RA is an electrical schematic diagram of the filter shown in FIG. l;
FlG. 2 is a perspective view of one form of capacitor employed in the filter of HG. l;
FIG. 3 is a perspective view of another form of capacitor used in the filter of FIG. l; and
FIG. 4 is detail view of an alternative filter end cap construction in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG 1 shows a filter constructed in accordance with the present invention. The filter includes a casing comprising a tube ill) which may be fabricated of any appropriate material, e.g., mild steel, stainless steel, aluminum, brass, etc. Casing tube it) is of elongated hollow configuration (of circular or other desired cross section) and includes a pair of open ends 11 and 12 which are subsequently closed by end caps l3 and M, to be described.
Tube ll encloses a filter assembly (which is normally preassembled and then inserted into said tube) comprising, inter alia, a pair of cans 15 and 16 respectively having open ends 15a, lot: and closed bottom walls 1511, T61). Cans l and 16 are sopositioned relative to one another in tube that the open ends 1511 and lba of the cans are disposed closely adjacent to the ends lll and R2 of tube 10, with bottom walls i512 and 16b of said cans being positioned inwardly in spaced relation to one another and to the ends ill and 12 of tube 110. Cans l5 and 16 sliclably fit the interior of tube lit) (as will be described) and, when said cans are in place, good electrical contact is effected between the cans and tube by solder joints therebetween located closely adjacent the opposite ends of the tube It). in normal practice, tube 10 is grounded, whereby cans l5 and lo are similarly at ground potential. The bottom walls 115!) and lob of the cans are similarly at ground potential and extend across the interior of the tube so as to subdivide the tube into three regions generally designated l7, l8 and 19 which regions are, for all practical purposes, shielded from one another by the intervening grounded walls l5!) and lob.
Filter section l7, comprising the interior of can l5, contains a capacitor 20 and an inductor 21. Capacitor 20 (and, for that matter, the other capacitors to be described hereinafter) is of the the extended foil type and comprises a first plurality of aluminum foil layers 22 separated by appropriate insulation strips (not shown) from a further plurality of aluminum foil layers 23. The several foil layers 22 are interconnected to one another by means of an elongated flat tag 24 soldered to one end of capacitor 20. Tag 24 is provided with end flanges 25 (see FIGS. IA and 2) shaped to conform to the interior of can l5 and eventually soldered thereto by means of solder joints 26. This mechanical arrangement of parts comprising capacitor 20 insures that the capacitor is accurately positioned, and maintained in position, within tube to and can 15, and further assures that, electrically, the several foil layers 22 are at ground potential. The side of capacitor 20, comprising foil layers 22 connected to tag 24, are, by the arrangement described, grounded in a plane extending across the interior of the filter in spaced relation to the additional ground plane supplied by bottom wall b of can 15.
The several foil layers 23, comprising the other side of capacitor 20, are interconnected to one another by a wire 27 extending across and soldered to the exposed foil ends of layer 23. An elongated sleeve 28 of insulating material extends along the axis'of the capacitor with one end protruding through solder tag 24, and wire leads 31 (from inductor 21) and 27 pass axially through the capacitor via sleeve 23. Leads 27 and 31 are soldered, as at 30, to wire lead 29 which extends to terminal 53. of the adjacent end cap 13. By this arrangement of leads, lead 31 can be pulled tight after the inductor and capacitor elements have been assembled into can l5.
Inductor 21, which is also located within can 15 in filter region 17, comprises a toroidal core having insulated wire wound thereon, with the resultant inductor being enclosed within a thin skin of polywrap insulating material, having a thickness of, for example, 0,004 inches, shrunk around the outer diameter of the inductor. This insulating skin need not cover the inductance wires completely. Inductor 21 is provided with a pair of elongated flying leads 31 and 32 a portion of which may be tinned to facilitate the making of electrical connections. Lead 3i is, as previously described, soldered at point 30 to provide electrical continuity between lead 29, one side of capacitor 20, and one end of inductor 21 (see FIG. 11A).
It will be noted that the body of capacitor 20 is positioned offcenter relative to the axis of tube 10 and relative to the center line of inductor 21. This assures that the body of capacitor 20 does not seal off the area around the axis of inductor 21, whereby encapsulant can completely fill all spaced around capacitor 20 and inductor 2i. The offcenter configuration of capacitor 20 further ensures that the live end of capacitor 20 contacts the insulation material surrounding inductor 21.
The central section 18 of the filter comprises a pair of capacitors 35 and 36 and a further inductor 37 physically positioned therebetween. Froman electrical point of view each of the capacitors 35 and as is similar to that already described in reference to capacitor 20, i.e. capacitors 35 and 36 each comprise alternate layers of aluminum foil separated from one another by appropriate insulation strips. A first plurality of the foil layers in capacitor 35 have their exposed ends soldered to an elongated relatively wide wide tag 38 (see FIG. 3), and solder tag 38 is in turn soldered to the outer surface of bottom wall 15b of can 115. As a result, one side of capacitor 35 is grounded (see FIG. 1A).
The second plurality of foil layers in capacitor 35, compris ing the other side of the capacitor, have their exposed end interconnected to a further tag 39 soldered thereto. Tag 39 has opposing, inwardly inclined flanges 12 which are shaped to engage one side of inductor 37. Tag 39 and its associated flanges d2 further provide two soldering points 43 and 44 for wiring, to be described.
A central insulating sleeve 4a extends along the axis of capacitor 35, with the opposing ends of sleeve 40 protruding through the tags 38 and 39. Bottom wall 15b of can 15 is provided with a central aperture 41 adapted to receive a protruding end of insulation sleeve 40 when tag 38 is properly positioned in place for attachment to said bottom wall 15b.
As a result of the arrangement described, lead 32 may be passed from inductor 2i through insulating sleeve 40 for electrical connection with the live side of capacitor 35 at solder point 43; and further connection may be made between said live side of capacitor 35 and one end of inductor 37 by means of lead 45.
Capacitor 36 has a physical and electrical configuration entirely similar to that already described in reference to capacitor 35. To this effect, one side of capacitor 36 is provided with a tag 416 soldered to the exposed foil layers at one end of said capacitor, and provided with inwardly extending flanges 87 engaging inductor 37. Flanges 42 and 47 thus accurately position, and maintain in position therebetween, inductor 37. Flanges 47 provide solder points (similar to points 43 and 44) for leads extending between inductor 37 and capacitor 36. The other end of capacitor 36 has a further tag 48 (like tag 38) soldered to the exposed foil layers and also soldered to the grounded bottom wall lob of can 16.
The third filter region 19 comprises an inductor 49 associated with a capacitor 50. Elements 49 and 50 are arranged, both mechanically and electrically, in the manner already described with refere n :e to elements 21 and 20, and no further description of these elements is therefore necessary.
The several filter components described are assembled in stages as subassemblies, and then inserted into tube 116. The first subassembly comprises can 15 with capacitor 20 and inductor 21 mounted in place, and with capacitor 35 affixed to the exterior of bottom wall 15b. The second subassembly comprises can 16 with components 49 and 50 therein, and with components 36 and 37 attached to the exterior of can 16. The can 16 subassembly is slidably inserted into one end of tube (the right-hand end as viewed in FIG. 1) and then pushed into the interior of tube 10 until bottom wall 16b of the can 16 subassembly engages a location dimple 10a protruding inwardly from tube 10. At this time, inductor 37 will be located closely adjacent the left-hand end of tube 10, as viewed in FIG. 1. Electrical connection 44 in then made between inductor 37 and the can subassembly which, at this time, is outside of tube 10. The can 15 subassembly is then inserted into the lefthand end of tube 10 in engagement with inductor 37, and is slidably pushed into tube 10 to push the can 16 subassembly back down tube 10 toward the right-hand end thereof until bottom wall 15b of can 15 engages location dimple 10a. By this series of manipulative steps, all of the components shown in FIG. 1 (other than the end caps) are properly positioned relative to one another within tube 10, and are properly connected electrically to one another.
Once the several filter components are interconnected in the manner described, the overall assembly may be encapsulated in an appropriate potting compound. Encapsulation takes place while the assembly is in tube 10. In a first stage, potting compound is introduced into the right-hand end of tube 10 so as to fill the interior of can 16; and this first mass of potting compound is then cured. The complete assembly is then moved to the left (as viewed in FIG. 1) so as to push the can 15 subassembly out of tube 10 until central filter section 18 is open; and potting compound is then inserted into the center section in surrounding relation to the elements 35, 36, and 37. The can 15 subassembly is then pushed back into place in tube 10 with can 15 acting as a piston; and further potting compound is thereafter inserted into can 15. The masses of potting compound in can 15, and in center filter section 18, are then cured at the same time.
It will be appreciated that, in the alternative, encapsulation may be effected by providing one or more apertures in the sidewalls of tube 10 adjacent filter section 18 so that potting compound may be introduced into the ends and interior of tube 10 while the filter parts are in place. Such apertures, if provided, can thereafter be closed by solder or the like.
After the several components are assembled and encapsulated, the opposing ends of the overall filter structure may be electrically connected to terminals 51 and 52 in end caps 13 and 14, whereafter said end caps may be hermetically sealed in place across the opposite ends of tube 10. Central terminals 51 and 52 are mounted in, and extend through, ceramic por tions of the end caps acting to insulate said terminals from metallic end cap annuli 53 and 54. Annuli 53 and 54 are shaped to conform to the interior of tube 10, and engage the open ends of cans 15 and 16 as illustrated. When the end caps are in place, a mechanical and hermetic seal may be achieved by means of solder, as at 55 and 56. Solder joints may, at the same time, also be made between tube 10 and cans 15 and 16 at positions closely adjacent the open ends of said cans to assure good ground connections between the tube and cans.
The overall structure thus described has a number of significant assembly and mechanical advantages, some of which have already been discussed. The overall filter, including shield planes 15b and 16b may be held in place within, and electrically connected to, tube 10 by means of solder joints located closely adjacent the open ends of tube 10 in relatively widely spaced relation to said ground planes and to the electrical filter components. These joints may, moreover, be made at the same time that the end caps are sealed in place thereby simplifying overall fabrication of the assembly. Other joining techniques may be employed to similar advantage, including brazing and welding, depending upon the particular materials employed for the filter parts. In any event, the joints between the filter assembly and tube 10 are located in such position relative to the metal parts of the structure that the metal parts of the structure act as a heat sink, thereby preventing electrical components and encapsulant from being subjected to excessive heat during any joint-making steps.
The tag constructions employed of on the several capacitors facilitate the making of interconnections between the filter components, assure that the parts are accurately positioned and maintained in position during assembly and thereafter, and minimize possible damage to wires during the assembly and encapsulation steps. In this latter respect, it will be noted that solder points such as 43 and 44 are positioned on the outside diameter of a capacitor such as 35 or 36 whereby, prior to soldering, the components to be joined can be drawn tightly together so as to remove slack from the wires and reduce the hazard of damaging a loose wire when the completed filter elements are inserted into tube 10.
The use of insulated sleeves extending through the center of each capacitor facilitates the passing of wires between adjacent components, and again makes it possible to removed slack from the wires so as to eliminate the hazard described above. Moreover, the insulating sleeve in each capacitor assists in bringing each filter soldering joint (e.g. 30) into an easily accessible position, which greatly facilitates miniaturization of the filter.
The use of metal 'cans constructed and arranged in the manner described assures that RF shield or screen planes may be disposed between sections of the filter in a far easier manner than has been possible heretofore. One or more such cans may be employed in a given tube, depending upon the number of filter sections to be provided. The use of such cans enables the sealing joints to'be made at the exterior parts of the assembly, and, as previously described, permits the use of corrosive flux (when needed) during the joint making steps without risking damage to the filter or its component parts.
Various modifications may be made to the filter arrangement described without departing from the present invention. By way of example, filter section 18 in the arrangement of FIG. 1 is illustrated as comprising two capacitors and an inductor. However, it will be appreciated that filter section 18 may comprise only a single capacitor. Indeed, it will further be appreciated that the electrical nature of the components which are placed in the several filter sections, and the number of filter sections which are employed, may be varied as necessary to achieve a'filter having any desired electrical characteristics.
It will further be appreciated that the soldered end cap construction shown in FIG. 1 is not mandatory. FIG. 4 shows an alternative construction wherein a tube such as 10 and a can such as 16 are associated with an end cap 60 provided with a metal annulus 61 which is welded in place along joints 62. This welding technique simultaneously secures the end cap in place and physically joins can 16 to tube 10.
Still further arrangements will be suggested to those skilled in the art. It must therefore be understood that the foregoing description is intended to be illustrative only and not limitative of my invention; and all such variations and modifications as are in accord with the principles described are meant to fall within the scope of the appended claims.
1. An electrical filter comprising an elongated hollow tube of conductive material, a can member of conductive material, said can member comprising a bottom wall integral with transversely extending continuous sidewalls, said sidewalls defining a free edge relatively widely spaced form said bottom wall,
said can member being disposed within said elongated tube with its bottom wall positioned transverse to the axis of said tube and extending completely across said tube at a location between and relatively widely spaced from both opposing ends of said tube, the integral sidewalls of said can member being located closely adjacent to and extending along the interior surface of said tube with the free edge of said sidewalls being positioned closely adjacent one of the opposing ends of said tube at a position remote from said bottom wall location, first electrical filter components located within said can member between said bottom wall and said one end of said tube, second electrical filter components located outsideof said can member but within said tube between said bottom wall and the other end of said tube, whereby said bottom wall acts as a shield between said first and second electrical filter components, and means for closing said one end of said tube at a joint located closely adjacent the free edge of said can sidewalls.
2. The structure of claim 1 including means in said tube for fixing the location of said bottom wall relative to the opposing ends of said tube.
3. An electrical filter comprising an elongated hollow tube of conductive material, a first can member comprising a bottom wall integral with upstanding sidewalls defining an open top spaced from said bottom wall; said first can member being disposed within said elongated tube with its bottom wall positioned transverse to the axis of said tube at a location between the opposing ends of said tube, the sidewalls of said first can member being located closely adjacent the interior surface of said tube with the open top of said first can member being positioned closely adjacent one of the opposing ends of said tube, a second can member having a bottom wall, integral sidewalls, and an open top, said second can member being disposed within said tube with its open top positioned closely adjacent the other end of said tube and with its bottom wall positioned transverse to the axis of said tube at a location axially spaced from the bottom wall of said firstcan member, first electrical filter components located within said first can member between its bottom wall and said one end of said tube, second electrical filter components located within said second can member between its bottom wall and the other end of said tube, further electrical filter components located within said tube between the spaced bottom walls of said first and second can members, and closure means closing the opposing ends of said tube.
4. The structure of claim 3 wherein each of said bottom walls includes an aperture, and electrical means extending through said apertures for interconnecting said first, second, and further electrical filter components to one another.
5. The structure of claim 3 wherein said first electrical filter components include a capacitor, a conductive strap electrically connected to one side of said capacitor, said strap extending at least partially across said first can member in a direction generally parallel-to its bottom wall and conductively engaging the sidewalls of said first can member.
6. The structure of claim 5 wherein said capacitor includes a sleeve of insulating material extending therethrough, and an electrical lead connected to one side of said capacitor and extending through said sleeve for electrical connection to a filter component physically located adjacent the other side of said capacitor.
7. The structure of claim 3 wherein said further electrical filter component include a capacitor, a conductive strap electrically connected to one side of said capacitor, said strap extending at least partially across said tube in a direction transverse to the axis of said tube and conductively engaging the bottom wall of a can member.
8. An electrical filter comprising an elongated hollow tube of conductive material, a can member comprising a bottom wall integral with upstanding sidewalls defining an open top spaced from said bottom wall, said can member being disposed within said elongated tube with its bottom wall positioned transverse to the axis of said tube at a location between the opposing ends of said tube, the sidewalls of said can member being located closely adjacent the interior surface of said tube with the open top of said can member being positioned closely adjacent one of the opposing ends of said tube, first electrical filter components located within said can member between said bottom wall and said one end of said tube, second electrical filter components located within said tube between said bottom wall and the other end of said tube, and closure means closing the opposing ends of said tube, said filter com nents includin a capacitor of the extended foil type, an e ongated conduc we strap attached to one side of said capacitor, said strap including positioning flanges at its opposing ends, and an inductor physically engaging said positioning flanges.
9. The structure of claim 3 wherein said closure means comprises end caps hermetically sealing the opposing ends of said tube, said end caps including terminals electrically connected to said filter components.
10. The structure of claim 3 wherein said tube and said can members are each of circular cross section.
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|U.S. Classification||333/181, 336/90, 333/167, 333/185|