EP0214276B1

EP0214276B1 - High performance flat cable

Info

Publication number: EP0214276B1
Application number: EP86902117A
Authority: EP
Inventors: Glenn E. Bennett; Raymond Joseph Look; Frank P. Dola; Richard E. Thurman; Paul P. Siwinski
Original assignee: AMP Inc
Current assignee: TE Connectivity Corp
Priority date: 1985-03-04
Filing date: 1986-03-04
Publication date: 1989-08-02
Also published as: DE3664832D1; EP0214276A1; WO1986005311A1

Abstract

A local area network cable assembly includes a flat cable (2), a shielded connector (148) and a cable to connector adapter having a body (150) and a ferrule (146). The cable has a plurality of signal conductors (11, 12, 21, 22) positioned within a common plane. Each conductor is surrounded by a foamed first insulating medium (13, 14, 23, 24) and the insulated conductors forming an associated pair surrounded by a core (15, 25) formed of an extruded second insulating medium having a higher dielectric constant. EMI shields (18, 28) surround each core (15, 25) and are in turn surrounded by an outer insulating body (4). The shields are formed from a foil and the outer body (4) secures the overlapping edges (18a, 18b, 28a, 28b) around the core (15, 25). The ends of the shield are axially slit for interconnection by the adapter to the shielded connector. The cable (2) can be used in under the carpet installations and is suitable for balanced transmission of high frequency signals and exhibits high impedance, low cross talk electrical characteristics.

Description

Background of the Invention

This invention relates to a high performance multiconductor flat cable and more particularly to controlled impedance low loss, low attenuation cable employing a plurality of conductor pairs which can be used in undercarpet installations. The method of manufacturing includes the steps of drawing conductor subassemblies formed of insulated wire pairs surrounded by an intermediate insulator through an extruder; forming conductive shields around the subassemblies prior to extrusion; and extruding an extra insulator around the shielded subassemblies.
Conventional multiconductor cables for transmitting high frequency electrical signals include both shielded twisted pair cables and coaxial cables. Such cables have their greatest utility in transmitting electrical signals between components of electrical systems. Such transmitted signals are normally in digital form although such transmitted signals may also be in analog form.
Shielded twisted pair cables utilize a pair of insulative conductive wires in a twisted pair configuration with a grounded, electrically conductive shield around each twisted wire pair. The shield functions to reduce electromagnetic interference radiation, generally called EMI, which naturally emanates from the signal transmitting wires and which might otherwise adversely affect the performance of adjacent electronic devices. Such shield also functions to minimize cross talk, electrical interference between one pair of wires and an adjacent pair, which would tend to impair the fidelity of the signals being transmitted. Shielded twisted pair cables can be used in a different transmission system where both wires are electrically powered and both constitute signal carrying wires. Information transmitted is a function of the sequential voltage differential between the two wires of the pair. An example of a shielded twisted pair cable is described in US-A-4 404 424. Coaxial cables also use EMI shields to reduce radiation. But in coaxial cables, unlike shielded twisted pair cables, only one electrically powered signal wire is utilized. The signal wire is encased in insulation which is surrounded, in turn, by the grounded, electrically conductive shield. In coaxial cables, the shield also functions as a grounded reference for the voltage of the signal wire. An example of a coaxial cable is described in US-A-3 775 552.
Considerable effort has been extended to develop a flat coaxial cable which would yield the same performance characteristics as conventional coaxial cable but which would also enable the use of conventional mass stripping and termination techniques to thus facilitate the coupling of an electrical connector to the cable. Consider for example US-A-4 488 125. Other flat coaxial cables are disclosed in US-A-4 487 992 and US-A-3 775 552.
One application for a flat cable is an under the carpet wiring situation in which a flat, low profile cable is extended beneath the carpet for connection to, and coupling of, components of an electrical system such as a computer system or the like. Shielded twisted pair cables do not have a low profile suited for use in undercarpet applications since twisted wires are continuously and sequentially located above, to one side, below, and to the other side of each other along the length of the cable. As a result, the cable thickness periodically increases to a double wire thickness along the length of the cable. This arrangement of signal thus precludes low profile cable configurations since low profile cable configurations are possible only in cables having their wires spaced parallel to each other in a single, usually horizontal, plane. The configuration and orientation of wires in a shielded twisted pair cable also precludes mass stripping and termination since the positioning of any one wire with respect to another varies as a function of where the cable is cut along its length.
While many methods of manufacturing electrical cables have been proposed in the past, the instant method is particularly well suited for the manufacture of a flat high performance cable, equivalent in performance to a shielded twisted pair cable.
From one aspect, the present invention consists in a multiconductor flat cable comprising a plurality of conductors, each conductor being separately surrounded by a first insulating medium and in turn surrounded by a second insulating medium, the first insulating medium having a dielectric constant lower than the dielectric constant of the second insulating medium, characterised in that the conductors are arranged in a plurality of associated electrically balanced pairs for transmitting high frequency signals, the cable exhibiting high impedance, low cross talk electrical performance, the conductors being spaced side by side in the same plane and being separable at either end of the cable for individual termination; conductive shields surrounding the second insulating medium surrounding each conductor pair and forming a continuous EMI shield around each conductor pair; and a third insulating medium surrounding the plurality of conductor pairs, the conductive shields being encapsulated in the third insulating medium.
The invention enables the production of a controlled impedance, low attenuation, balanced flat cable having the performance characteristics of a shielded twisted pair cable. The second insulating medium may provide dimensional stability to the conductors comprising a pair and precisely position the conductive shield relative to the conductors. The third insulating medium may be extruded over the shields surrounding adjacent conductor pairs and holds the shields in place to prevent radiation.
From another aspect the invention consists in a local area network cable assembly comprising a flat cable comprising a plurality of pairs of associated signal conductors each pair of conductors being surrounded by an EMI shield, a shielded electrical connector at one end of the cable comprising means for electrically interconnecting the signal conductors to electrical components, the local area network cable assembly being characterised in that at least one end of each EMI shield is separated into a plurality of strips, the assembly including a connector to cable adapter for interconnecting the EMI shield to the shielded connector, the adapter comprising a ferrule, at least one signal conductor extending through the ferrule, the EMI shield strips being deployed only on the exterior of the ferrule; and a body secured around the ferrule and engaging the EMI shield strips, the body being secured to the shielded connector.
The cable according to the invention may be produced by drawing at least one conductor subassembly through the die of an extrusion press. Each subassembly contains a pair of insulated conductive wires which are spaced and parallel with respect to one another. An electrically conductive shield is then formed around each subassembly prior to their movement through the die. An insulator is then extruded therearound during their movement through the die. More specifically, the process preferably includes the steps of first forming conductor subassemblies and then drawing the two conductor subassemblies through the die of an extrusion press. Each subassembly contains a pair of insulated conductive wires which are spaced and parallel with respect to one another. An electrically conductive shield is then formed around each subassembly prior to their movement through the die. The subassemblies are held spaced from each other and in an essentially horizontal plane with the wires of the subassemblies also in an essentially horizontal plane as they are drawn through the die. An insulator is then extruded around the shields as they pass through the die. Each subassembly is supported on a separate supply reel and the shield material is supported in flat foil form on additional reel means. The finished cable is drawn through the die of the extrusion press by a power driven take-up reel. Each shield is formed from a flat foil configuration, to an essentially cylindrical configuration surrounding the subassemblies, prior to passage through the die of the extrusion press. The forming of each shield includes the steps of bending the flat foil into an essentially U-shaped configuration by pulling the flat foil through apertures in dies or forming blocks and then sequentially rolling the ends of the U-shaped foil into contact with each other to surround the subassembly by pulling the U-shaped foil through a series of sequential rollers. The step of feeding a subassembly to the foil is performed before its passage to the final forming block. The bending is accomplished by pulling the flat foil through a U-shaped die in a first forming block and then the U-shaped die in a second forming block. The U-shaped foil supports a subassembly prior to movement into the sequential rollers with the curved portion of the U-shaped foil and subassembly located facing downwardly and with the ends of the U-shaped foil and a flat surface of the subassembly facing upwardly. The sequential rollers include the first rollers to contact and bend one upstanding leg of the U-shaped foil into contact with the flat face of the subassembly and the second rollers then bend the second upstanding leg into contact with the first upstanding leg of the U-shaped foil to thereby completely surround the subassembly. The subassembly and shield fed from the sequential rollers is rotated 90 degrees before feeding it to the die of the extruder. The path of travel of the subassembly and foil is essentially a straight line from the forming blocks to beyond the extruder. Prior to the passage of the foil to the forming blocks, the foil is preferably coated with a lubricant.
In the accompanying drawings Figure 1A is a cross-sectional view of a two-pair flat cable with high performance characteristics manufactured in accordance with the teachings of the present invention.

Figure 1B is an alternate embodiment where the planar ends of the insulating core is overlapped and positioned on the exterior of the conductor pairs.
Figure 2 is a perspective showing of an assembly employing apparatus for carrying out the process steps for manufacturing electrical cable in accordance with the teachings of the present invention.
Figure 3 is an enlarged perspective showing of the portion of the assembly line of Figure 2 immediately prior to the extrusion press where the conductive shields are formed around the subassemblies.
Figures 4 through 11 are cross-sectional views of a shield and subassembly in various stages of formation taken along the lines 4 - 4 through 11 - 11 of Figure 3.
Figure 12 is a view demonstrating the final position of the EMI shield encircling both conductors of the conductor pair, after completion of the process shown in Figure 11.
Figure 13 is a cross-sectional view showing the single conductor pair surrounded by an EMI shield encircled by an outer insulating body.
Figure 14 is a perspective showing of the cable of Figure 1 during the stripping of the edges of the EMI shield and the exterior or third insulator from the cable.
Figures 15A and 15B are perspective showings of a tool used for the slitting operation resulting in the stripped edges of Figure 14.
Figure 16 is a plan view of a cable constructed in accordance with the preferred embodiment of this invention showing the remowal of the respective layers of insulation from the four conductors comprising two conductor pairs.
Figure 17 shows an electrical connector attached to a flat cable by a cable to connector adapter.
Figure 18 is an exploded view of the attachment of the adapter to the cable.
Figure 19 is an exploded view showing attachment of the adapter to the connector.
Figure 20 shows connectors attached to each end of the cable to form a local area network cable assembly.

The multilayer shielded pair cable in accordance with the teachings of this invention provides a controlled, impedance, low attenuation balance multiconductor flat cable suitable for use in transmitting digital or other high frequency signals. The cable will be described in terms a flat conductor cable having two separate pairs of associated wire conductors, four conductors in all. It should be understood, however, that some applications may require a cable having more than just two pairs of conductors. This invention is consistent with the use of any number of pairs of conductors and can be employed with a single pair of conductors or with a large number of pairs. Indeed, this invention is intended for use in applications requiring three or more pairs of conductors or even one pair in a manner similar to the use of the two-pair cable.
The principal embodiment of this invention depicted herein is intended for use in installations in which the flat cable is to be installed along the floor of an office building and under the carpet to enable connections to be made with portions of a network arbitrarily distributed in an office building. It should be understood however that this high performance cable, having conductors located within the same plane, is not limited to use in undercarpet installations. Indeed, constant orientation of the conductors in the same plane renders this cable quite suitable to applications in which it is desirable that the conductors be simultaneously mass terminated to the connector position at the end of the cable. Indeed this cable is quite suitable for use as a preterminated cable assembly in which connectors may be assembled at each end of precise lengths of cable in a factory environment. As can be seen in the drawings, particularly with reference to Figure 1 the cable is fabricated with a common symmetrical cross-sectional profile along its entire length. By virtue of weakened sections 30 and 32 and inherent flexibility, it can rest on the floor in a flat condition regardless which side is placed on the floor.
The cross-sectional configuration shown in Figure 1 demonstrates the relative positioning of four wire conductors 11, 12, 21 and 22 in a flat cable assembly 2. Each of the conductors 11, 12, 21 and 22 employed in the preferred embodiment of this invention comprises a conventional round wire conductor. Conductors 11 and 12 comprise one associated pair of conductors while conductors 21 and 22 comprise a similar pair of associated conductors. Although each of the conductors 11, 12, 21 and 22 is positioned in the same plane, thus facilitating the low profile necessary for use in undercarpet installations, the two conductor pairs are nevertheless electrically balanced. Both of the conductor pairs are embedded in an outer insulating body 4 which comprises the central longitudinally extending portion or region of the cable 2. Similarly shaped wings or ramps 6 and 8 are bonded longitudinally in along the opposite sides of the central body 4. Each of the wings 6 and 8 comprises an inclined surface to provide a smooth transition laterally of the axis of the cable, thus eliminating any sharp bump when the cable is positioned beneath a carpet. In the preferred embodiment of this invention, the insulating ramps 6 and 8 are formed from the same material as the insulating material which forms insulating body in. Wings 6 and 8 are joined to body 4 along weakened longitudinally extending sections 30 and 32. In the preferred embodiment of this invention, the insulating material forming the body 4 and the insulating material forming wings 6 and 8 comprises an extruded insulating material having generally the same composition. A conventional polymer such as polyvinyl chloride, PVC, would be an insulation which comprises one material suitable for use in the jacket or body 4 and in the wings 6 and 8.
The surfaces or faces of the opposed central regions of the cable are parallel to each other. A continuation of such parallelism extends to a limited degree into the wings of the cable. This extending of the parallelism into the wings provides for an extended thicker, horizontal section of the cable between the tapered regions of the wings when the cable is placed on the floor beneath a carpet. This design has been found to further distribute the forces from the carpet through the cable to the floor uniformally and reduce the external forces which would otherwise detrimentally act upon the wires and shield within the cable. As can be seen in Figure 1 the transverse profile of the cable is low, and it is symmetrical about both its central horizontal plane and its central vertical plane so that it may be employed with either face up, reducing the chance for operator error during installation.
The opposed faces of the central region of the body are essentially flat and are as thin as possible consistent with known fabrication techniques while allowing for the high electrical performance of the cable. In the preferred embodiments of the invention the greatest thickness does not exceed 70 mils. The width of the cable should be of such a dimension so that when employed under a carpet it will allow a smooth transition from the floor to the center of the cable and then thereacross. The presence of the cable should not be discernible. A preferred dimension for the width of the cable has been found to be about 2.00 inches. Such a dimension will allow the above described smooth transition but will not enlarge the taper of the wings to the extent of being wasteful of material constituting their body. Each shielded cable pair is separately embedded within the insulating body 4. As shown in Figure 1, the conductors 21 and 22 form one pair 20 of associated conductors encapsulated within a separate insulating core 25 which is in turn embedded within the body 4 of the cable 2. Each conductor 21 and 22 is however encircled by a first insulation 23 and 24 respectively which comprises a foam-type insulation having relatively low dielectric constant. Foam-type insulation such as polypropylene or polyethylene each of which contain a large percentage of air trapped within the material comprise a suitable dielectric material for use around the conductors in areas of relatively high dielectric field. These foam covered conductors can then be embedded within an insulating material 25 which completely surrounds the foam insulation 23 and 24 in the immediate vicinity of the conductors. The insulating material 25 need not have as low a dielectric constant as the foam insulation 23 and 24, since the insulating material 25 is located in areas of relatively low electric fields. The insulating material 25 thus has less effect on the cable impedance than the foam insulation 23 and 24. The insulating material 25 must however be suitable for imparting dimensional stability to conductors 21 and 22. In fact, in this invention the dielectric material 25 holds the conductors 21 and 22 in a parallel configuration along precisely spaced center lines. The insulating material forming the core 25 also comprises a material having greater strength when subjected to compressive forces than the foam- type insulation 23 and 24 surrounding conductors 21 and 22. A material suitable for forming core 25 is a conventional polyvinyl chloride which can be extruded around the foam insulation 23 and 24 surrounding conductors 21 and 22. It is desirable that the foam- type insulation 23 and 24 do not become adhered to the extruded insulating material forming the core 25, since the conductors must be removed from the core 25 for conventional termination into a connector. In the preferred embodiment of this invention, longitudinally extending notches 26, 27 are defined along the upper and lower surfaces of the core 25. These notches, which can be conveniently formed as part of the extrusion process, are located in areas of relatively low dielectric fields and define a weakened section of insulating core 25 to permit separation of conductors 21 and 22 for termination purposes.
Cross talk and noise performance of each pair of conductors is greatly enhanced by the use of EMI shields 18 and 28 encircling the cores 15 and 25 of the conductors within each conductor pair 10 and 20. As shown in Figure 1A, EMI shield 28 can be positioned in partially encircling relationship to conductors 21 and 22 within insulating core 25. The ends 28a and 28b of the EMI shield extend beyond the lateral edge of core 25 during fabrication of the cable. Figure 12 shows that these ends 28a and 28b can then be folded into overlapping relationship along one end or edge of the core 25. In the preferred embodiment of this invention, the one edge of core 25 comprises a planar edge extending transversely, and preferably perpendicular to the plane in which the conductors 21 and 22 are positioned. This planar edge facilitates an assembly of the shield 28 in overlapping relationship along the edge of core 25. Furthermore, by providing the sharp corners of the upper and lower extent of this planar surface good contact is maintained between the overlapped portions 28a and 28b of the cable at these two points. Thus, gaps which can act as an antenna in the shielding are prevented. As shown in Figure 13, the overlapped ends 28a and 28b of the EMI shield 28 are secured in a tightly held configuration by the insulating material extruded around the EMI shield and comprising the insulating body 4. Thus the ends 28a and 28b would not be subject to movement upon flexure of the cable to create a gap or radiating antenna. In the preferred embodiment of this invention, an annealed metallic foil is employed as the EMI shields 18 and 28. For example, an annealed copper foil having a 2 mil thickness is suitable for use an as EMI shield in the preferred embodiment of this invention. Figure 1 B shows an alternate embodiment of this invention in which the planar ends of the insulating core, where the EMI shield is overlapped and positioned on the exterior of the conductor pairs. Figure 1A shows the two ends of the separate EMI shields positioned adjacent to each other within the body 4. Since the invention is suitable for use with more than two pairs of conductors, it is apparent from the relative positioning of the flat overlapping ends of the cable as a matter of choice. For example, if three pairs are employed, the flat ends of all three shields cannot be adjacent if all conductors are positioned within the same plane.
Cable manufactured in accordance with this invention has been constructed which achieves sufficiently high levels of performance to permit its use in applications of the type described herein. The cable is rated at 300 volts D.C. or RMS at 0.5 amperes D.C. or RMS at an operating temperature of -40° to 60°C. The conductor resistance is less than 45 ohms per 304 meters D.C. at approximately 20°C. The cable has a characteristic impedance on the order of 150 ohms and a maximum propagation delay of 2.8 nanoseconds per foot. The maximum attenuation is .72 D.B. per 100 feet at 1 MHz and 7.2 D.B. per 100 feet at 100 MHz. The maximum rise time for an impulse of 250 pecoseconds would be 16 nanoseconds and a 1 percent maximum cross talk between adjacent conductors is also achieved.
Not only is this cable suitable for use in applications in which high electrical performance is required, this cable is also easily adaptable to termination of the separate conductors to an electrical connector at the end of the cable. Figures 16 and 17 illustrate the ease in which the conductors may be presented for termination. Initially the wings 6 and 8 can be removed adjacent the ends. Weakened sections 30 and 32 facilitate the perforation of the ends of the cable since the wings can be removed by simply tearing along the weakened sections 30 and 32. The insulating material comprising the insulating body 4 can then be easily removed from the shielded cable pairs by means of a slitting tool as shown in Figures 15a and 15b. The use of annealed copper foil to which the insulating material forming the body 4 does not adhere permits the simple remowal of this insulating material from the two conductor pairs. The shields 18 and 28 can then be cut and bent away from the extruded insulating core 15 and 25. The extruded insulating material forming core 25 can in turn be simply removed from the foam insulation surrounding conductors 21 and 22, since the foam insulation 23 and 24 as readily adhered to the extrusion insulating material forming core 25. At this point, conductors 21 and 22 within foam insulation 23 and 24 are suitable for sol- derless mass termination by conventional insulation displacement techniques. Both figures 16 and 17 however show the conductors 21 and 22 extending beyond the foam insulation 23 and 24. It should be appreciated that conductors 21 and 22 are shown primarily for illustrative purposes since it will normally not be necessary to remove insulation 23 and 24 from the bare conductors 21 and 22. However, it may be desirable in certain installations to remove the insulation 23 and 24 before terminating conductors 21 and 22 and this invention is suitable for use in this manner.
The cylinders of insulation 23 and 24 for the conductors are preferably extruded around the conductors. The extrusion is preferably polyethylene resin with a predetermined percentage of a foaming agent planted with the polyethylene to be heated and extruded. It is the foaming agent which forms the air within the extruded product when subjected to heat and pressure. In accordance with the nonextrusion techniques, the materials, their compositions and proportions, the heat and speed of extrusion, the post extrusion quenching, etc. are selected so as to form the insulation around the wire to exact dimensional tolerances and as a closed cell foam with about 40 to 60 percent air by volume. It has been found that the maximum amount of air within the dielectric will improve the electrical performance of the system. However, excess air beyond the range as identified herein may degrade the dimensional stability of the integrity of the foam.
These foam covered conductors may then be embedded within an insulating material 25 as by extrusion, which completely surrounds the foam insulation 23 and 24 in the immediate vicinity of the conductors. The insulating material 25 need not have as low a dielectric constant as the foam insulation 23 and 24, since the insulating material 25 is located in areas of relatively lower electric fields. The insulating material 25 must, however, be suitable for imparting dimensional stability and integrity to conductors 21 and 22 as well as to their surrounding insulation 23 and 24. In fact, in this invention the dielectric material 25 holds the conductors 21 and 22 in a parallel configuration along precisely spaced surfaces, edges and center lines with respect to the cable and with respect to each other. The insulating material forming the core 25 also comprises a material having greater strength when subjected to compressive forces than the foam- type insulation 23 and 24 surrounding the conductors 21 and 22. A material suitable for forming core 25 is preferably a conventional flexible polyvinyl chloride, PVC, which can be extruded around the foam insulation 23 and 24 surrounding conductors 21 and 22. It is desirable that the foam- type insulation 23 and 24 not adhere to the extruded insulating material forming the core 25 to facilitate separation of the conductors from the core 25 for conventional termination into a connector.
Longitudinally extending notches 26 and 27 are defined along the upper and lower surfaces of the core 25. These notches, which can be conveniently formed as part of the extrusion process through the appropriate design of the die are located in areas of relatively low dielectric field and define a weakened section of insulating core 25 to permit separation of conductors 21 and 22 for termination purposes. Formed into the upper and lower surfaces of the body 4 are central notches 35 and 36 extending the length of the core along the centerline. Similar to the notches 26 and 27 in the core 25, central notches 35 and 36 constitute weakened sections in the insulating body 4 to permit an operator to separate, by hand, one conductor pair from another. These central notches are naturally formed during the cooling process following the extrusion since a greater quantity of shrinkable PVC is located in the body 4 between upper and lower notches as compared with the quantity of insulator immediately to either side thereof.
The electrical performance of each pair of conductors is greatly enhanced by the use of EMI shields 18 and 28 encircling the cores 15 and 25 of the conductors within each conductor pair 10 and 20. As shown in Figure 4, and EMI shield 28 can be positioned in partially encircling relationship to conductors 21 and 22 within insulating core 25. The ends 28A and 28B of EMI shield extend beyond the lateral edge of core 25 during fabrication of the cable.
Reference is now made to Figure 7 which illustrates machinery capable of carrying out the method of fabricating or manufacturing the cable as disclosed herein. The invention anticipates the utilization of separate supply reels 44 for supporting flat, electrically conductive strips 46, such as of copper, for the forming of the EMI shields. Separate supply reels 48 are also provided, each being adapted to support a supply of the two conductor subassemblies 50.
Each subassembly is formed of two laterally spaced conductive wires surrounded separately by the first, or internal insulating material which is preferably a closed cell polyethylene foam. The polyethylene foam may be extruded onto the wires in a conventional manner. The insulating wires may then be fed in separated pairs through an extrusion die, also in an essentially conventional manner, to form the two conductor subassemblies shown on the supply reels of Figure 2 and within the EMI shield of Figure 1.
Figure 2 is an overview of the apparatus employed in carrying out the method of the present invention. It is adapted to bring together separate strips of copper from the two foil supply reels and the pair of two conductor subassemblies from their two supply reels. The arrangement of components of the apparatus is such as to position the subassemblies and strips for proper orientation along their paths of movement for final extrusion of the able body material around the subassemblies and surrounding shields and for final take up to create the finished cable.
In addition to the various supply reels at the supply station, the significant functioning components of the fabrication system, along the path of travel of the workpiece, include the oiler 52 for lubricating the flat copper foil strips; the V-shaped die or former-block 54 for shaping the copper strips; the U-shaped die or former-block 56 for shaping the copper strips with a conductor subassembly 50 contained therein; the rolling mill station 58 for the final shaping of the copper strips into the EMI shields the orienting block 60 for the pre-extrusion positioning of the EMI shields and their surrounded subassemblies; the extrusion press 62 and the receiving station 64 including the power driven take up reel 66 for receiving the finished cable 68.
The pre-extrusion components of the apparatus are more readily seen in Figure 3 which shows these components enlarged as compared with Figure 2. The copper foil strips are originally in a flat orientation as they rest and then are fed from the supply reels. Their shaping begins as they are fed through a set of forming-blocks. The first, or primary, forming-block is provided with two V-shaped slits 72 through which the strip may pass and which will deform the foils into a V-shaped configurations corresponding to the shape of the slits in the first forming-block. The V-shaped foil strips are next fed through a second, or secondary, U-shaped forming block 56 having two openings 74, aligned with the slits of the V-shaped forming block, of such size and shape so as to receive the V-shaped foil and deform it into a U-shaped configuration. The U-shaped openings are sufficiently large so as to also receive the subassemblies which pass through the openings with the foil. It is immediately prior to the U-shaped forming-block that the supply of two conductor subassemblies 50 are brought into contact with the foil strips 46. The flat portions of the subassemblies 76 preferably face upwardly as are the edges 78 of the foil.
The operation of the forming blocks has been found to be improved by lubricating the strips prior to their bending at the forming-blocks. This is achieved at the lubrication assembly. The lubrication assembly includes an aperture 80 in a block 82. The upper and lower surfaces of the aperture are provided with felt pads 84, closely spaced to contact the foil strips passing therebetween. A hole 86 in the top of the block supports a bottle 88 with a supply of lubricant such as mineral oil. The mineral oil of the bottle is in flow communication with the felt pads to continuously moisten the felt pads with the lubricating mineral oil. Moistening of the lower felt pad occurs through the contact between the upper and lower pads between the foil strips and beyond the edges of the foil strips.
The composite subassemblies of U-shaped foils are then fed into apertures 90 of the rolling mill assembly prior to passage to and through the extrusion press 62. The rolling mill assembly includes the plurality, as for example five in number, of precisely machined rollers 92, 94, 96, 98 and 100, preferably fabricated of steel, and located in the path of travel of the composite subassemblies with foils. It is at this station that the edges of the foil are finally formed to constitute the EMI shield totally surrounding the subassemblies and to be surrounded and encased by the third or exterior insulator which forms the body of the cable. Each roller of the rolling mill is mounted for free rotation on shafts 104. The shafts are, in turn, supported by holes 106 in the side plates 108 of the station. The side plates are supported on their bottom surfaces by a base plate 110. Support is also provided frontwardly, centrally and rearwardly by cross brace plates or supports 112, 114 and 116 to add rigidity to the station for maintaining the rollers in precise orientations for accurately bending or shaping the EMI shield from the U-shaped copper foil to the final essentially cylindrical shape totally surrounding the subassembly.
The progression of one foil strip through the rolling mill is shown in Figures 4 through 11 which are cross-sectional views of the roller station and foil taken along lines 4 - 4 through 11 - 11 of Figure 3. It should be appreciated and understood that similar but opposite operations are separately simultaneously performed on the adjacent copper strip.
Figure 4 illustrates a single copper foil strip in U-shaped configuration with a subassembly located therein passing through an opening 90 in the front support plate which is actually formed of an upper and lower section. As can be seen, the foil and subassembly enter the roller station with their curved sections downwardly and with the legs of the U-shaped foil strip and flat face of the subassembly. The hole is shaped and located to help position and align the subassembly and strip accurately through the rolling operation and to and through the extrusion press.
Figures 5 and 6 illustrate the rolling action of the two initial rollers 92 and 94 downwardly bending the first edge of the foil. The bending of the strip to the horizontal position in contact with the flat side of the subassembly is completed by passage of the subassembly and strip through an aperture 120 in the central support block 114 as shown in Figure 7. Figures 8 and 9 illustrate the two supplemental rollers 96 and 98 bending the second edge of a copper foil over the first edge of the foil to completely flatten the second edge over the first edge to create the EMI shield totally surrounding the subassembly. The rollers are all machined with precise beveled or angled sections which contact the fed foil strip at a precise location to effect the bending of the foil strip as required.
The subassembly and strip are then fed beneath a non-angled final roller 100 to flattening the outside leg of the foil strip over the inside leg, to ensure its proper operation within the cable. Note Figure 10.
Before exiting from the rolling station, the EMI shield passes through a hole 122 in the rearward support plate 116. See Figure 11. At this point an extra degree of compression is provided to the subassembly and to the EMI shield which are now prepared for being fed to and through the extrusion press. The hole 122, by virtue of its precise size and location, assists in maintaining the subassembly and EMI shield on a straight line path to and through the extrusion press.
Between the roller station and the extrusion press, the two EMI shields with their surrounded subassemblies are passed through an opening 124 in an orienting block 60. This arrangement is such as to locate the subassemblies and EMI shields with their flat faces in spaced relationship with such flat faces facing each other. The insulated wires are thus in spaced parallel relationship in a common horizontal plane as are the subassemblies. The distance between the rolling station and orienting block should be sufficiently long so as not to deform the EMI shield.
The two subassemblies encased in copper, the EMI shield, next enter the extrusion press 62 wherein the third, or exterior, insulation layer is formed surrounding the two EMI shields which are, in turn, surrounding the second insulators of the subassemblies. The extrusion press has a die with a profile of the finished cable as can be seen in Figure 1. The shape of the profile is determined by the shape of the die of the final extrusion die except for the central longitudinal depressions. These depressions are formed upon the cooling of the extruded material due to the larger mass of extruded material therebetween as compared with the mass of extruded material on the adjacent sides thereof.
It is preferred that the subassembly and foil strip be fed through the stations of the apparatus performing the inventive method disclosed herein in an essentially straight line path from at least the last forming-block to a location beyond the extrusion press. In this manner the foil will be stressed as little as possible during fabrication and its strength and integrity maintained.
While the preferred embodiment of the present invention has been disclosed as being carried out on two subassemblies, the method of the present is equally suited for being performed on any number of subassemblies and EMI shields, whether only on a single one or on a plurality.
The take up reel, driven in the conventional manner, pulls or draws the final cable product in their in-line paths of movement through the fabrication machinery and also serves as a storage reel for the cable. An inner segment of EMI shield with a subassembly segment will be found coiled on the interior of the take up reel since the pre-extrusion components of the process must be initially fed through the machinery to the take up reel to effect its pulling operation prior to and immediately following the activation of the extrusion press.
The cable described herein is especially suited for use in a local area network for interconnecting distributed electronic equipment. Figures 18 - 21 show a local area network cable assembly consisting of cable 2, connectors 148 and a connector to cable adapter 144. The adapter 144 includes a ferrule 146 to allow an operational electrical connection between the wires and EMI shield of a stripped cable end and a shielded connector 148 functionally equivalent to, and of essentially the same design as that disclosed in US-Patent 4 449 778. Each shielded connector 148 has a plurality of signal connector terminals 108 adapted to establish an insulation displacement interconnection to the respective signal conductors 11, 12, 21, 22. A connector EMI shield 100 surrounds the terminals and is adapted to be interconnected to the cable shields surrounding the conductors.
The adapter of the present invention includes a body 150 which is formed of two essentially identical halves or portions 152 and 154. According to the disclosed preferred embodiment of the invention, the halves are axially split and are provided on their mating surfaces with male projections 156 and associated female receptors 158, pins and apertures, to effect appropriate and accurate alignment of the halves when joined. The halves are different only in that one half contains, at its front or outboard end 160, the male mating projections and, at its rear or inboard end 162, the female apertures. The other segment has, at its outboard end, the female apertures and, at its inboard end, the male projections.
As used herein, the term inboard is intended to mean that axial end of the adapter away from the connector or toward that end when viewed from the adapter. The term outboard is intended to mean that end of the adapter closer to the connector or toward that end when viewed from the adapter.
In addition to the above described adapter halves, the adapter also includes a ferrule 146 in the form of an essentially cylindrically shaped hollow member. The open ends 164 and 166 of the ferrule, both inboard and outboard, are turned outwardly to form ribs 168. These ribs are thus located at the axial ends of the ferrule to provide rigidity to the ferrule as during handling, installation or use.
Both adapter body segments have undercut central cavities to form a recess 170 axially bounded by slots 172 and 174. The recess and slots are of such size and position as to receive the ferrule which contains, internally therethrough, pairs of insulated wires and their second or intermediate covering. Externally thereof, the ferrule receives crossed pairs of conductive strips 176, extensions of the EMI shields, both above and below.
The exterior surface of the adapter is provided with threads 182 which are tapered to increase in diameter. Adjacent the greatest diameter threads, approximately mid span of the adapter is an abutment shoulder 184 next followed by an enlarged, nut-like surface 186 grippable by a wrench. The abutment shoulder limits the extent that the nut 188 may be rotated along the threads toward the outboard end of the halves. The nut-like surface constitutes an area where an operator may grip, normally mechanically with a wrench, the halves while gripping and rotating the nut along the threads, again normally mechanically with a second wrench.
In assembling the cable end to the adapter, a nut 188 is first axially slid down the free end of the cable. The adapter halves are then mated with a section of the second insulator from adjacent the stripped end of the cable spanning the axial ends of the adapter. Axial movement of the cable within the adapter is precluded due to the frictional holding forces of the apertures acting upon the ends of the cable within the adapter and by the projections extending into the cable. The threads of the adapter are, of course, inboard of the free ends of the wires with the wires and exposed insulators just beyond the outboard end of the adapter. The nut is then screwed onto the adapter threads. Pliers are normally utilized to fixedly hold the grippable portion 186 of the adapter while an operator rotates the nut with another pliers toward the abutment shoulder with sufficient force until the segments of the adapter are in tight mating contact.
As the halves are drawn together with the ferrule therebetween, sufficient force is being exerted to deform the ferrule to essentially conform to the shape of the recess. Compressed between the ferrule and the recess of one half is a first crossed pair of conductive strips of the EMI shields. These conductive strips of the EMI shield are formed as the cable is stripped and prepared in the manner previously described. Compressed between the ferrule and the recess of the other half is a second crossed pair of conductive strips of the EMI shield. This relationship makes for an effective grounding of the EMI shield within the adapter.
The materials of the adapter halves and the ferrule must be sufficiently electrically conductive so as to ground the EMI shield when the apparatus is in use. Die cast zinc aluminum or the equivalent has been found suitable for the halves of the adapter body while copper has been found suitable for the ferrule.
The outboard end of the adapter is provided with inboard and outboard spaced coupling plates 190 and 192 which together constitute the mounting brace. These plates are secured at their bases to intermediate, outboardly extending support plates 194 and 196. These support plates extend from the outboard end of the halves and are located on opposite sides of the aperture 178 so that the wires may extend therepast for connection to the connector.
Note is also taken that the wires of the cable are in a vertical orientation as they pass through the adapter. This is necessitated due to the need of the wires to pass beyond the spaced plates. A horizontal orientation of the wires and cable would cause an interference between the support plates and their associated parts on the connector.
Coupling of the adapter to the connector is effected by sliding the spaced plates over spring urged, electrically grounded projections 100 extending from the base 102 of the connector EMI shield positioned within the connector space 148. Note the direction of the arrows which indicate the direction of movement of the adapter and connector with respect to each other.
Although the invention has been described in terms of one embodiment and additional extensions of this invention have been discussed, it will be appreciated that the invention is not limited to the precise embodiment disclosed or discussed since other embodiments will be readily apparent to those skilled in the art.

Claims

1. A multiconductor flat cable (2) comprising a plurality of conductors (11, 12, 21, 22), each conductor being separately surrounded by a first insulating medium (13, 14, 23, 24) and in turn surrounded by a second insulating medium (15, 25), the first insulating medium (13, 14, 23, 24) having a dielectric constant lower than the dielectric constant of the second insulating medium (15, 25), characterised in that the conductors (11, 12, 21, 22) are arranged in a plurality of associated electrically balanced pairs (11, 12 and 21, 22) for transmitting high frequency signals, the cable exhibiting high impedance, low cross talk electrical performance, the conductors (11, 12, 21, 22) being spaced side by side in the same plane and being separable at either end of the cable (2) for individual termination; conductive shields (18, 28) surrounding the second insulating medium (15, 25) surrounding each conductor pair and forming a continuous EMI shield around each conductor pair; and a third insulating medium (4) surrounding the plurality of conductor pairs, the conductive shields (15, 25) being encapsulated in the third insulating medium.

2. The cable of claim 1 characterised in that the first insulating medium (13, 14, 23, 24) comprises a foam material containing air to decrease the dielectric constant and the second (15, 25) and third (4) insulating mediums are extruded.

3. The cable of claim 1 wherein each conductive shield (18, 28) surrounding each pair of associated conductors comprises a metal foil shield with the ends (18a, 18b and 28a, 28b) positioned in overlapping relationship.

4. The cable of claim 3 wherein the third insulating medium (4) is extruded around each metal foil shield to retain the ends (18a, 18b and 28a, 28b) of the metal foil shield in overlapping relationship.

5. The cable of claim 4 wherein each second insulating medium (15, 25) has one planar edge extending transverse to the common plane of the conductors (11, 12, 21, 22), the ends (18a, 18b, 28a, 28b) of each shield (18, 28) positioned in mutual overlapping relationship along the transverse planar edge.

6. A local area network cable assembly comprising a flat cable (2) comprising a plurality of pairs of associated signal conductors (11, 12, 21, 22) each pair of conductors being surrounded by an EMI shield (18, 28) a shielded electrical connector (148) at one end of the cable comprising means for electrically interconnecting the signal (onductors (11, 12, 21, 22) to electrical components, the local area network cable assembly being characterised in that at least one end of each EMI shield (18, 28) is separated into a plurality of strips (176), the assembly including a connector to cable adapter for interconnecting the EMI shield to the shielded connector, the adapter comprising a ferrule (146), at least one signal conductor extending through the ferrule, the EMI shield strips being deployed only on the exterior of the ferrule; and a body (150) secured around the ferrule and engaging the EMI shield strips, the body being secured to the shielded connector.

7. The assembly of claim 6 characterised in that the cable comprises separately insulated conductors (11, 12, 21, 22) with an insulating core (15, 25) surrounding each pair of associated conductors and wherein an EMI shield (18, 28) surrounds each insulating core (15, 25).

8. The assembly of claim 7 wherein the EMI strips (176) are formed by axially slitting the EMI shield (15, 25) adjacent at least one end.

9. The assembly of claim 6 wherein the EMI strips (176) are crossed on the exterior of the ferrule (146).

10. The assembly of claim 6 wherein the adapter body (150) is formed of an electrically conductive material and the shielded connector (148) comprises a connector EMI shield (100), the body conductively engaging the connector EMI shield.