A high impedance electrical cable and method of forming same

Abstract: An electrical cable assembly includes a plurality of round electrical conductors. One of the conductors (20) is flattened along a longitudinal surface portion (21). The conductors are arranged in side-by-side transversely spaced position with the flattened portion (21) of the one conductor (20) facing an adjacent conductor (20). An insulative casing (24) is extruded about the conductors and includes a major planar surface (26) permitting mass termination to a connector. The one conductor (20) includes a curved surface (23) portion which faces the major planar surface (26).

Description: FIELD OF THE INVENTION This invention relates generally to high impeded electrical cable and also relates to a method of forming a high impedance cable having conductors being spaced at a given pitch. BACKGROUND OF THE INVENTION Flat multiconductor ribbon cable is used extensively in the electronics industry, especially in the computer field. Cable of this type typically includes a plurality of electrical conductors arranged in side-by-side spaced orientation. These conductors are surrounded by an insulative casing which electrically isolates each of the conductors. Several factors affect the quality and reliability of these cables. The size of each conductor, measured by the cross-sectional area, dictates the amount of signal current that each conductor can carry. The amount of signal current carried is directly proportional to the size of the conductor. In addition, the impedance value of the cable is related, in part, to the spacing between adjacent conductors. In cables having similar dielectric constants, the greater the space between adjacent conductors (i.e. the more insulating mass therebetween) the greater the impedance value of the cable. It is desirable to construct a cable which is capable of carrying high signal currents while also having a high impedance value. Thus, cable having large conductors and ample spacing between adjacent conductors would be ideal. However, in the modern computer environment, a cable assembly of this construction is not practical. In fact, the current state of the computer industry is to require smaller cable, i.e. cable with conductors spaced at a smaller pitch, while maintaining the high signal carrying capabilities of the cable as well as the high impedance value. However, when spacing conductors at a smaller pitch, the insulating mass between facing surfaces of adjacent conductors is reduced. This results in lowering the impedance value of the cable. Side-by-side round conductors, typically used in cables of this type, when spaced at a small pitch, would result in the facing curved surfaces of adjacent conductors being in close proximity. This would cause the impedance value to be lowered beyond tolerability. The art has seen the use of rectangular conductors in flat multiconductor cable assemblies which permit the conductors to be placed on a smaller pitch while maintaining more insulating mass between facing surfaces of adjacent conductors. However, rectangular conductors are difficult to form and are more expensive than conventional round conductors. Further, in most computer applications, mass cable termination to insulation displacing contacts of electrical connectors is desired. Rectangular conductors are inherently difficult to mass terminate in this matter. Wires of non-circular cross-section are known, for example, from GB-A-336 784 which shows condenser wires used as supplementary wires on trunk cables. The two condenser wires in one embodiment are semicircular in cross-section, and planar surfaces face one another. The condenser wires are individually insulated and have a common covering which is essentially oval in cross-section. It is therefore desirable to provide a flat multiconductor electrical cable which permits spacing of electrical conductors at a reduced pitch while maintaining a high degree of signal transmission and a high impedance value. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved multiconductor electrical cable. It is a further object of the invention to form an electrical cable where conductors may be spaced at a small pitch while maintaining high signal carrying capabilities and sufficient insulation between adjacent conductors. According to one aspect of the invention, there is provided a method of forming an electrical cable assembly comprising the steps of: providing a first elongate electrical conductor having a substantially circular cross-section; flattening a first surface portion of said first conductor along its length; providing further electrical conductors; arranging said first conductor in transversely spaced disposition adjacent a second conductor with said flattened first surface portion of first conductor facing said second conductor; and forming an insulative casing over said conductors to place said conductors in mutual electrical isolation, the casing having a first major planar surface lying parallel to longitudinal axes of the conductors. According to a further aspect of the invention, there is provided an electrical cable assembly comprising: a multiplicity of elongate electrical conductors; and an elongate electrically insulative casing continuously surrounding each of said conductors and supporting said conductors in side-by-side, electrically insulated, transversely spaced arrangement, said casing including a first major planar surface lying parallel to the conductor axes; one of said conductors including a first flat surface portion facing an adjacent one of said conductors and a first curved surface portion facing said first major planar surface. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an extend of a conventional round conductor of the type used in accordance with the present invention. Figure 2 shows schematically, the cross-sectional shape of the conductor of Figure 1. Figure 3 shows, partially in section and partially schematically, a portion of a conventional flat multiconductor cable including round conductors of the type shown in Figure 1. Figure 4 shows schematically, an electrical conductor formed in accordance with the present invention. Figure 5 shows, partially in section and partially schematically, a portion of an electrical cable of the present invention employing the conductor shown in Figure 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1 and 2, an electrical conductor 10, used in accordance with the present invention is shown. Conductor 10 is a solid round copper wire of conventional construction used to transmit electrical signals therealong. Conductor 10 has a major longitudianl axis c and a circular cross-sectional shape as shown in Figure 2. Typical wire sizes used in accordance with the present invention include American Wire Gage (AWG) sizes 26 through 30. Round conductors of these sizes have diameters d of between .010 inches and .016 inches. The cross-sectional areas of these conductors range between approximately 100 and 250 circular mils. Electrical resistance of a copper wire is inversely proportional to its cross-sectional area. Therefore, larger wires will have less resistance and can accordingly carry a greater amount of electrical signal therealong. Referring now to Figure 3, a plurality of conductors 10 are arranged in an electrical cable assembly 12. Cable assembly 12 includes an electrically insulative casing 14 formed of extruded plastic such as polyvinyl chloride (PVC). Casing 14 is generally flat having an upper planar surface 16 and a lower planar surface 18 substantially parallel thereto. While planar surfaces 16 and 18 are shown as flat, cable having undulating planar surfaces may also be employed. Cables of this type are commonly referred to as ribbon cables. Conductors 10 are supported within casing 14 in electrical isolation. Conductors 10 are spaced from one another within casing 14 at a given pitch. Conductor pitch is defined by the distance between center line c of adjacent conductors 10. The pitch between conductors of flat ribbon cable is critical as ribbon cable is designed to be mass terminated to electrical connectors (not shown) having insulation displacing contacts fixedly supported in an insulative housing. the pitch of the cable must match the pitch of the connector. In Figure 3, the conductors are spaced at a pitch of P₁. Since conductors 10 are of the round variety, the actual space between facing surfaces of adjacent conductors will be less than P₁. As shown in Figure 3, the distance between tangent points T₁ and T₂ of side-by-side conductors 10′ and 10″ is S₁, which is substantially less than P₁. The impedance value of an electrical cable is determined, in part, by the special separation between facing surfaces of adjacent conductors. As a mass of insulating material increases between adjacent conductors, the impedance value of the cable will correspondingly increase. Thus, as conductor size is increased and/or the pitch between conductors is decreased, the impedance value of the cable will drop. The present invention provides a technique for placing conductors at a closer pitch without either decreasing conductor size or decreasing the impedance value of the cable. Referring to Figure 4, an electrical conductor formed in accordance with the present invention is shown. Conductor 20 is formed from a conventional solid round conductor such as conductor 10 shown in Figure 1. The round conductor 20 is passed through flattening rollers (not shown) to form flat surfaces 21 along the length thereof. The rollers are of the type conventionally used in the metallic forming art to press flat surfaces on metallic objects. Rollers capable of such function are commercially available. Flat surfaces 21 may be placed on conductor 20 either simultaneously or by separate forming steps. As shown in Figure 4, flat surfaces 21 are diametrically opposed and substantially parallel to one another. An important feature of the present invention is that rather than cutting a flat surface on each diametrical side of conductor 20, the conductor is actually flattened in a manner such that opposed upper and lower rounded conductor surfaces 23 and 25 are outwardly deformed from their original condition. Thus, the cross-sectional area of conductor 20 does not change during formation. This permits the conductor to carry the same amount of signal current as was possible prior to the forming steps employed in the present invention. Additionally, upper and lower surfaces 23, 25 also substantially maintain their rounded configuration. This facilitates the ability to mass terminate cable assembly 22 (Fig. 5) with conventional electrical connectors having insulation displacing contacts (not shown). Referring to Figure 5, a cable assembly 22 of the present invention is shown. Cable assembly 22 includes insulative casing 24 similar to casing 14 shown in Figure 3. Casing 24 includes upper and lower major planar surfaces 26 and 28 respectively which support therebetween conductors 20. Cable assembly 22 includes conductors 20 of the type shown in Figure 4. Conductors 20 are arranged within casing 24 so that flattened surfaces 21 are substantially perpendicular to major planar surfaces 26 and 28 and center lines c line in a common plane. Rounded surfaces 23 and 25 face major surfaces 26 and 28 respectively. Cable assembly 22 is typically formed by extruding insulative casing 24 over conductors 20. The conductors 20 of cable assembly 22 are spaced at a pitch P₁ which is less than P₂ the pitch of cable assembly 12 (Fig. 3). Since each of conductors 20 includes flattened surfaces 21, the distance S₂ between facing flattened surfaces 21 of adjacent conductors 20′ and 20″ is not correspondingly reduced. Comparing cable assembly 12 shown in Figure 3, with cable assembly 22 of the present invention shown in Figure 5, this feature is illustrated. While the conductor pitch of the cable assembly 22 of the present invention has been reduced from P₁ to P₂, the actual spacing between facing surfaces of adjacent conductors remains substantially the same. That is, S₁ ≈ S₂. As the amount of insulating mass between facing surfaces of adjacent conductors 20′ and 20″ remains the same, the impedance value of cable assembly 22 would be substantially similar to impedance value of cable assembly 12. Also, as mentioned above, since conductors 20 maintain the same cross-sectional area as conductors 10, the signal carrying capability of cable assembly 22 is not reduced. The present invention, as shown in Figure 5, employs multiple conductors, each identically formed to have diametrically opposed flattened surfaces 21. However, it is contemplated that conductors 20 may be formed to have only one flattened surface. Also, it is contemplated that only selected ones of conductors 20 may be formed to have one or more flattened surfaces. This would permit the cable assembly 22 to have selected different impedance values as between various pairs of conductors. Various changes to the foregoing described and shown structures would now be evident to those skilled in the art. Accordingly, the particularly disclosed scope of the invention is set forth in the following claims.

Claims: A method of forming an electrical cable assembly (22) comprising the steps of: providing a first elongate electrical conductor (20) having a substantially circular cross-section; flattening a first surface portion (21) of said first conductor (20) along its length; providing further electrical conductors (20); arranging said first conductor (20) in transversely spaced disposition adjacent a second conductor (20) with said flattened first surface portion (21) of first conductor facing said second conductor; and forming an insulative casing (24) over said conductors to place said conductors in mutual electrical isolation, the casing having a first major planar surface (26) lying parallel to longitudinal axes of the conductors. A method of claim 1 including the steps of flattening a second surface portion (21) of said first conductor (20) along its length, said first flattened surface portion being spaced from and substantially parallel to said second flattened surface portion. A method of claim 1 or claim 2 wherein said second conductor (20) has a substantially circular cross-section and further including the step of flattening a first surface portion (21) of said second conductor along its length. A method of claim 3 wherein said arranging step further includes arranging said first and second conductors (20) such that said flattened first surface portions (21) of said first and second conductors are in facing disposition. A method of any one of claims 1 to 4 wherein said forming step includes extruding said insulative casing (24) over said first and second conductors. An electrical cable assembly comprising: a multiplicity of elongate electrical conductors (20); and an elongate electrically insulative casing (24) continuously surrounding each of said conductors and supporting said conductors in side-by-side, electrically insulated, transversely spaced arrangement, said casing including a first major planar surface (26) lying parallel to the conductor axes; one of said conductors (20) including a first flat surface portion (21) facing an adjacent one of said conductors and a first curved surface portion (23) facing said first major planar surface (26). An electrical cable assembly of claim 6 wherein said casing (24) includes a second major planar surface (28) spaced from and substantially parallel to said first major planar surface (26) and wherein said conductors (20) are supported between said planar surfaces. An electrical cable assembly of claim 6 or claim 7 wherein said one conductor (20) includes a second flat surface portion (21) spaced from and substantially parallel to said first flat surface portion (21). An electrical cable assembly of any one of claims 6 to 8 wherein said one conductor (20) includes a second curved surface portion (25) facing said second major planar surface (28). An electrical cable assembly of any one of claims 6 to 9 wherein said adjacent one of said conductors includes a first flat surface portion (21) facing said flat surface portion of said one conductor. An electrical cable assembly as claimed in claim 6 wherein each conductor (20) includes first and second parallel flat surfaces (21) and first and second curved surface portions (23, 25) extending between said flat surfaces, said casing having a second major planar surface portion (28) parallel to said first major planar surface portion (26) and the parallel flat surfaces (21) of the conductors lying substantially normal to the first and second planar surfaces (26, 28).

Inventors: STRAUSS RICHARD F, STRAUSS, RICHARD F.
Applicants: THOMAS & BETTS CORP, THOMAS & BETTS CORPORATION
Application year: 1991
Main classification: H01B7
IPC codes: H01B13, H01B7