Patent Application
20160343474
The subject matter herein relates generally to electrical cables that provide shielding around signal conductors.
Shielded electrical cables are used in high-speed data transmission applications in which electromagnetic interference (EMI) and/or radio frequency interference (RFI) are concerns. Electrical signals routed through shielded cables may radiate less EMI/RFI emissions to the external environment than electrical signals routed through non-shielded cables. In addition, the electrical signals being transmitted through the shielded cables may be better protected against interference from environmental sources of EMI/RFI than signals through non-shielded cables.
Shielded electrical cables are typically provided with a shield layer formed by a metal foil. Signal conductors are typically surrounded by an insulation layer, and the metal foil is subsequently wrapped around the insulation layer to provide shielding for the signal conductors interior of the metal foil. For example, in some known applications a metal foil is spiral wrapped around the insulation layer, such that adjacent loops or revolutions of the metal foil at least partially overlap, which is referred to as overlay, to prevent EMI/RFI leakage across the shield layer. An adhesive polymeric tape, such as Mylar® (a polyester film manufactured by Dupont), may be wrapped around the outside of the metal foil to hold the wrapped metal foil in place.
Wrapping a metal foil as a shield layer in a shielded electrical cable has disadvantages. For example, helically wrapping the foil layer and the tape layer over the foil layer results in discontinuities that affect the signal integrity. The frequency or repetitiveness of the tape overlay causes geometrical changes within the signal pair construction. Tape overlay lengths over the signal conductors play a fundamental role in frequency bandwidth, such that it has a direct effect on attenuation or signal loss. For example, short overlay lengths generally push the attenuation to higher bandwidths, while longer overlay lengths push the attenuation to relatively lower bandwidths. Increasing the overlay may improve insertion loss by pushing the attenuation outside of an operational range of bandwidths, although it may also undesirably increase the rigidity or stiffness of the cable, as well as increase manufacturing time and material usage. Thus, there is a trade-off between signal integrity, flexibility, and manufacturing costs. Furthermore, in some cables, it may be desirable to electrically connect together the shield layers that surround different signal conductors. But, since the adhesive tape on the outside of the shield layer insulates the shield layer, a portion of the tape must be removed or penetrated, or a drain wire must be extracted through the tape layer, in order to access the shield layer.
A need remains for an electrical cable that improves signal performance and simplifies manufacturing.
In an embodiment, an electrical cable is provided that includes at least one conductor assembly. Each conductor assembly includes at least one inner conductor that extends along a length, an insulator, and a shield layer. The insulator engages and surrounds a surface of the at least one inner conductor. The insulator is composed of a dielectric material. The shield layer engages and surrounds an outer perimeter of the insulator. The shield layer is formed of a conductive plastic material to provide electrical shielding for the at least one inner conductor and flexibility.
In another embodiment, an electrical cable is provided that includes an outer jacket and a bundle of plural conductor assemblies. The bundle is surrounded by the outer jacket. The bundle includes at least a first conductor assembly and a second conductor assembly. The first and second conductor assemblies each include at least one inner conductor that extends along a length, an insulator, and a shield layer. The insulator engages and surrounds a surface of the at least one inner conductor. The insulator is composed of a dielectric material. The shield layer engages and surrounds an outer perimeter of the insulator. The shield layer is formed of a conductive plastic material to provide electrical shielding for the at least one inner conductor and flexibility. The shield layer of the first conductor assembly engages the shield layer of the second conductor assembly to electrically common the respective shield layers of the first and second conductor assemblies.
The electrical cable 100 includes at least one conductor assembly 102. The at least one conductor assembly 102 may be held within an outer jacket 104. For example, only one conductor assembly 102 (referred to herein as conductor assembly 102) is shown within the outer jacket 104 in
The outer jacket 104 surrounds the conductor assembly 102 along a length of the conductor assembly 102. In
The conductor assembly 102 includes at least one inner conductor 108 that is configured to convey data signals. The conductor assembly 102 in the illustrated embodiment has a pair 110 of inner conductors 108, although it is recognized that the conductor assembly 102 in other embodiments may include only one inner conductor 108 or more than two inner conductors 108. The inner conductors 108 extend longitudinally along the length of the cable 100. The inner conductors 108 are formed of a conductive material, such as metal. Each conductor 108 may be solid or composed of a combination of multiple strands wound together. The pair 110 of inner conductors 108 may be a differential pair such that the inner conductors 108 carry differential signals. The inner conductors 108 in
The insulator 112 engages and surrounds a surface 114 of each of the inner conductors 108. As used herein, two components are in “engagement” when there is direct physical contact between the two components. The insulator 112 is formed of a dielectric material. An intermediate portion 116 of the insulator 112 extends between the inner conductors 108 such that the inner conductors 108 are separated or spaced apart from one another and do not engage one another. The insulator 112 is configured to maintain separation between the inner conductors 108 along the length of the inner conductors 108 to electrically insulate the inner conductors 108 from one another, preventing an electrical short between the inner conductors 108. The insulator 112 may be one integral insulator member that surrounds and engages both inner conductors 108. Alternatively, the insulator 112 may be two discrete insulator members that engage one another between the inner conductors 108, where each insulator member surrounds a different one of the inner conductors 108. The size and/or shape of the inner conductors 108, the size and/or shape of the insulator 112, and the relative positions of the inner conductors 108 and the insulator 112 may be modified or selected in order to attain a particular impedance for the electrical cable 100. The insulator 112 is surrounded by a shield layer 118.
The shield layer 118 engages and surrounds an outer perimeter 120 of the insulator 112. The shield layer 118 is formed of a conductive plastic material. The shield layer 118 is configured to provide electrical shielding for the pair 110 of inner conductors 108 from external sources of EMI/RFI interference and also, in embodiments of the cable 100 with multiple conductor assemblies 102, to block cross-talk between inner conductors 108 of adjacent conductor assemblies 102. The shield layer 118 is further configured to provide flexibility for the electrical cable 100, allowing the cable 100 to bend at various angles to form a desired signal path between the electrical components. In an embodiment, the conductive plastic material includes a plastic base and metal particles dispersed throughout the plastic base. For example, the metal particles provide electrical conductivity for the electrical shielding properties, and the plastic base provides a flexible medium.
The shield layer 118 may have an integral, one-piece molded body 122. The molded body 122 of the shield layer 118 may lack seams and other irregularities or discontinuities, at least compared to the wrapped metal foil used as a shield in some known shielded cables. The molded body 122 may provide substantially constant, unvarying signal integrity along the length of the shield layer 118. In addition, the molded body 122 of the shield layer 118 does not have gaps or other openings extending through an outer perimeter 124 of the shield layer 118, so no EMI/RFI leak paths can form through the outer perimeter 124, unlike the wrapped metal foil used in some known shielded cables. The consistency provided by the molded body 122 of the shield layer 118 relative to the inner conductors 108 and the insulator 112 may provide enhanced control of the impedance through the electrical cable 100.
The outer jacket 104 surrounds and engages the outer perimeter 124 of the shield layer 118. In the illustrated embodiment, the outer jacket 104 engages the shield layer 118 along substantially the entire periphery of the shield layer 118. In other embodiments in which the cable 100 includes multiple conductor assemblies 102, the outer jacket 104 collectively surrounds the multiple conductor assemblies 102, but may not directly engage each of the conductor assemblies 102.
In the alternative embodiment shown in
In an embodiment, the cross-sectional shape of the outer perimeter 124 of the shield layer 118 may be geometrically similar to the cross-sectional shape of the outer perimeter 120 of the insulator 112. The term “geometrically similar” is used to mean that two objects have the same shape, although different sizes, such that one object is a scaled relative to the other object. For example, as shown in
The shield layer 118 in an embodiment has a uniform radial thickness 132 around the outer perimeter 120 of the insulator 112. The radial thickness 132 is the thickness of the shield layer 118 from an inner surface that engages the outer perimeter 120 of the insulator 112 to an outer surface that defines the outer perimeter 124 of the shield layer 118. Thus, the thickness 132 of the shield layer 118 at a location proximate to the first inner conductor 108A may be approximately equal to the thickness 132 of the shield layer 118 at a second location that is proximate to the second inner conductor 108A. The shield layers 118 shown in
In an embodiment, the inner conductors 108 are each composed of one or more metals, such as copper, aluminum, silver, or the like. The ground conductor 134 may also be composed of one or more metals. The inner conductors 108 and the ground conductor 134 may each be a single solid element or may include a plurality of wound metal strands. The dielectric material of the insulator 112 may be composed of one or more plastics, such as polyethylene, polypropylene, polytetrafluoroethylene, or the like. The insulator 112 may be formed directly to the inner conductors 108 by a molding process, such as extrusion, overmolding, injection molding, or the like. It is recognized that the dielectric material of the insulator 112 may be molded around each of the inner conductors 108 independently, as described above with reference to
In an embodiment, the conductive plastic material of the shield layer 118 includes a plastic base and metal particles dispersed throughout the plastic base. For example, the conductive plastic material may be a colloid or suspension in which the metal particles constitute a dispersed phase, and the plastic base constitutes a continuous phase or medium. The plastic base may be composed at least partially of polyethylene, polypropylene, polytetrafluoroethylene, or one or more other polymers. The metal particles may be composed of copper, aluminum, silver, chromium, nickel, and/or one or more other metals. For example, the metal particles may be stainless steel, which includes chromium. In an embodiment, the metal particles are in the form of powder, flakes, fibers, a combination thereof, or the like. For example, the metal particles may be formed by grinding, milling, chipping, or cutting a block or a strand of metal. The metal particles may have a size on the order of micrometers. The metal particles may include only metals, or may additionally include one or more non-conductive materials, such as carbon. For example, the metal particles may include metal plated carbon fibers. The metal particles may be homogenously dispersed within the plastic base, such that the conductive plastic material of the shield layer 118 has generally uniform conductive properties at different locations along the shield layer 118. The metal particles may be dispersed in the plastic base by adding the metal particles to the plastic base when the plastic base is heated to the liquid phase, and then cooling the plastic base such that the plastic base solidifies with the metal particles therein.
In an embodiment, the conductive plastic material of the shield layer 118 may be applied around the insulator 112 by molding the conductive plastic material on the insulator 112. For example, the shield layer 118 may be formed via an extrusion molding process in which the heated conductive plastic material is applied to the outer perimeter 120 of the insulator 112 as the insulator 112 (and inner conductors 108 therein) is fed axially through an extrusion machine. In another example, the shield layer 118 may be formed by injection molding or overmolding the conductive plastic material around the insulator 112 in a mold. Alternatively, the insulator 112 having the inner conductors 108 therein may be dipped into a container of conductive plastic material. In an alternative embodiment, instead of molding, the shield layer 118 may be applied to the insulator 112 via a physical vapor deposition process or another vacuum deposition process. In another alternative embodiment, the shield layer 118 may be applied to the insulator 112 using an electrostatic deposition process to coat the insulator 112. The molding and other deposition processes described herein are used to provide the shield layer 118 with a generally uniform radial thickness, as described with reference to
The outer jacket 104 is formed of at least one dielectric material, such as one or more plastics (for example, polyethylene, polypropylene, polytetrafluoroethylene, or the like). The outer jacket 104 is not conductive, and is used to insulate the shield layer 118 from objects outside of the cable 100. The outer jacket 104 also protects the shield layer 118 and the other internal components of the cable 100 from mechanical forces, contaminants, and elements (such as fluctuating temperature and humidity). Optionally, the outer jacket 104 may be extruded or otherwise molded around the shield layer 118. Alternatively, the outer jacket 104 may be wrapped around the shield layer 118 or heat shrunk around the shield layer 118.
As shown in
The shield layers 118 of the conductor assemblies 102 extend between the insulators 112 of adjacent conductor assemblies 102. For example, the insulator 112 of the first conductor assembly 102A does not engage the insulator 112 of the second conductor assembly 102B due to the intervening shield layers 118 of the first and second conductor assemblies 102A, 102B. The shield layers 118 provide shielding for the respective inner conductors 108 located interior of the insulators 112. For example, the inner conductors 108 of the first conductor assembly 102A are shielded from the inner conductors 108 of the second conductor assembly 102B by the respective shield layers 118 of the first and second conductor assemblies 102A, 102B which extend between the two pairs of inner conductors 108. The intervening shield layers 118 between the inner conductors 108 of adjacent conductor assemblies 102 may enhance signal integrity by shielding each pair of inner conductors 108 from the other pairs of inner conductors 108 in the cable 100. The shielding may block EMI/RFI emitted from one pair of conductors from interfering with the signal transmission of another pair of conductors in the bundle 140.
It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Dimensions, types of materials, orientations of the various components, and the number and positions of the various components described herein are intended to define parameters of certain embodiments, and are by no means limiting and are merely exemplary embodiments. Many other embodiments and modifications within the spirit and scope of the claims will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.
