TIME-MATCHED MULTIVALENT ELECTRICAL SIGNAL CABLES

FIELD OF THE INVENTION

The invention pertains to multivalent shielded electrical signal cables having more than one valence (layer or shell) of insulated and/or shielded electrical conductors arranged inside the cable.

BACKGROUND OF THE INVENTION

In the field of electrical signal cables, a multiplicity of insulated, insulated and shielded, and insulated twisted pair electrical signal conductors are frequently bundled together to form a cable. The multiplicity of conductors are arranged in layers from the center of the cable outwardly, the typical arrangement being one insulated conductor surrounded by a valence or layer of six like (or ^' different) conductors, then a layer of about twelve conductors closely fitted around the valence of six conductors. Drain wires may be interspersed in the spaces between the insulated conductors. The insulated conductors are held together by an insulating polymer jacket, usually extruded around the entire bundle as a whole. An electrical shielding material may be wrapped around the bundle of insulated conductors before the jacket is extended around the bundle. A tape binder of paper, polymer, or the like may be used to hold the bundle together in the desired shape before the application of shielding material and/or the jacket to aid in holding the conductors in place while subsequent shielding and/or jacketing processes take place.

In such bundles of electrical conductors, it has always been a problem to manufacture a cable in which the signals transmitted through the various conductors in different valences within the cable transmit the signal in the same length of time. Since the conductors are usually spiralled around each other, often at

different twist angles, the physical length of the conductors and hence the signal transit time or absolute time delay differ between conductors located in different valences and many solutions to the problem of providing equal signal transit time among the conductors have been tried. One common method known in the art is to modify the helical pitch or lay of each valence of conductors so that the physical lengths of all the conductors of the cable are matched.

SUMMARY OF THE INVENTION

The present invention proposes to equalize the signal transit time of the conductors within the various valences of a multivalent electrical signal cable by varying appropriately the density of the insulation and hence the dielectric constant and the signal time delay of the signal conductors of the different valences within which the conductors are placed within the cable. The more dense insulation with higher dielectric constant (£ ) is used to insulate conductors in the center of the cable. The valence of conductors surrounding, for example a first valence single signal conductor, will have an insulation of lower density and lower dielectric constant, and hence a shorter signal time delay (Td) than the conductor of the first (or inner) valence conductor. Since the second valence of conductors is usually spiralled around the first valence of conductors, the physical length of the second valence conductors will be longer than the first valence conductor and require as shorter signal time delay so that a signal passing through both valences will exit at the same time as the signal being carried by the first valence conductor. The dielectric constant may also be lowered by choosing an insulating material for an outer valence conductor having a different chemical composition which inherently has a lower dielectric constant than that of the insulation of the inner valence of conductors. Then either different densities of the same insulating material or different materials having different dielectric constants may be used alternatively or mixed within the same cable to provide a differing signal time delay associated with conductors of differing lengths to cause the signals within

different lengths of conductor to arrive at the same time at the end of the cable.

The same principles and conditions apply to individually insulated conductors, insulated and shielded conductors, and twisted pairs of insulated conductors.

The cables of the invention preferably have an outside layer of shielding around the conductors as a unit or the cable as a whole and an insulated protective jacket surrounding the shielding. One or more drain wires may be interspersed among the conductors of the cable.

The preferred electric signal conductor insulation may be those materials commonly used in the art for that purpose.

The shielding material used around individual conductors and around the cable as a whole may be metal foil, metal strands or wires, metal-plated polymer tape and the like.

The cable jacketing may be any polymer material commonly used for the purpose, such as rubber, polyethylene, polypropylene, polyester, polyurethane, polyvinyl chloride, fluorinated polymers, including polytetrafluoroethylene, fluorinated ethylene-propylene copolymers, fluorinated ethers of fluorocarbons, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a spiralled cable of the invention in which an outer valence of insulated conductors having a less dense insulation surrounds an inner valence conductor insulated by a more dense insulation.

Figure 2 is a cross-sectional view of a cable of the invention in which a core of insulation is surrounded by an inner valence of six insulated, shielded, and jacketed conductors spiralled around the core which in turn is surrounded by an outer valence of about twelve insulated, shielded, jacketed conductors spiralled around the inner valence of conductors, and the cable as.a whole surrounded by an outer shield and an outer jacket.

Figure 3 is a perspective view of a cable of the invention with layers cut away for better viewing of its structure in which a one conductor inner valence of insulated shielded conductor is

surrounded by a spiralled middle valence of insulated shielded conductors and the middle valence surrounded by a spiralled outer valence of insulated shielded conductors and an outer cable binder strip, outer cable shielding, and outer cable jacket.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now described in detail with reference to the figures and to the materials and methods of making the cables of the invention.

As is seen in Figure 1, since the outer valence 5_ of insulated conductors will have a greater physical length of conductor per unit cable length than the inner valence 3 insulated conductor, the time delay of the signals transmitted through an outer valence conductor would be longer than that of the conductor of the inner valence conductor, which is shorter in length, if the composition of the insulation covering all the conductors was the same. In order to get the signal transmission of all valences of conductors to arrive at the same time at the end of any given length of cable, the cable of the invention has less dense insulation covering the conductors of the outer valence 5 than covering the inner valence 3_ conductor I. A less dense insulation has a lower dielectric constant, which in turn causes the time delay of the signal transmitted through the conductor covered by the less dense insulation to be smaller than that passed through a conductor covered by the more dense (high dielectric constant) insulation. One may easily calculate the relative difference in insulation density and dielectric constant needed for insulations of the same composition, but of differing densities. One might alternatively use two insulations of differing composition having appropriately differing dielectric constants for different valences of insulated conductors.

In Figure 1, a drain wire or ground wire is shown in a groove of the spiralled outer valence 7 of insulated conductors.

Figure 2 shows in a cross-sectional view of a cable of the invention wherein a multiplicity of shielded insulated conductors is wrapped in a binder tape layer .14 to hold the conductors and a

drain wire 6 bundled together for subsequent cabling operations which apply an outer shielding material 1_4_ and then a protective jacket 1J> to the cable.

The cable is constructed around an inner core of insulation 18 around which is spiralled an inner valence 17 of six shielded, insulated conductors in which insulation ϋ is surrounded by shielding 12 and jacket .13. Spiralled around this inner valence 17 of conductors is an outer valence .16 of conductors J_9 which are covered with an insulation 20 of lower density and smaller dielectric constant than that of insulation ϋ of the inner valence of conductors. Surrounding insulation 20 of the outer valence is shielding 2_1 and jacket 22. The signal transmission speed through the cable is thus arranged to be the same for all conductors by having a more dense insulation on the shorter inner valence 17 conductors than the insulation on the longer outer valence 1_6 conductors.

A cable of the invention having three valences of conductors, each valence having differing densities of insulation and different lengths of conductor, but equal signal transmission speeds end to end, is described by Figure 3.

The inner valence 23 is comprised of a single center conductor 20 covered by insulation 2j. and shielding 22. Spiralled around inner valence 23 is an intermediate valence 27 layer of six insulated 2j> and shielded 26 conductors 24 in which the insulation 25 is less dense and has a lower dielectric constant than insulation J. of the inner valence conductor 20.

Surrounding the intermediate valence 27 of conductors is a spiralled outer valence 32 layer of insulated 24 shielded 33. conductors 35 which insulation 34 has a lower density and dielectric constant than insulation 25 of the intermediate valence 27. of conductors underlying it. A drain wire 6 is shown in the space between two of the conductors of the outer valence layer 3_7. Covering the outer valence layer 37 of conductors is shown an optional binder strip 40 wound around the cable to hold the conductors in place for further cabling operations. This is usually a thin plastic strip of suitable strength and insulation properties.

A shield 41 is placed around the binder 40 covered cable by

serving conductive tape or wire strands, braiding tape or wire strands, wrapping a metallized polymer tape spirally or by cigarette wrapping methods around the bound cable. Following the shielding is usually a layer of protective jacket, which is usually extruded onto the cable and is formed of a tough environment resistant polymer material, such as polyvinyl chloride, urethane rubber, or fluorocarbon resin, for example. The shielding material may be copper, copper alloys, or aluminum, for example, as may also be the center conductors. Center conductors are often plated with silver, gold, nickel, or tin, for example, to improve their properties.

EXAMPLE

A cable of structure similar to that of Figure 3 is to be manufactured. The cable is to be made up of multiple shielded insulated conductors with the helical pitch or lay to remain constant in each valence. Each shielded insulated conductor is to have a characteristic impedance of fifty ohms using a 28 AWG stranded center conductor and a metal foil shield. The insulation material is expanded polytetrafluoroethylene (ePTFE) of varying densities. For the purposes of the example, all time delay (Td) numbers are based on a cable construction of physical length 100 feet. The physical length of the inner valence of a single shielded, insulated conductor is the same as the physical length of the cable, 100 feet. The physical length of the intermediate and outer valences is 3% and 8%, respectively, longer than that of the inner valence. By adjusting the density and therefore the dielectric constant of the ePTFE insulation of the conductors, the electrical lengths and therefore the absolute signal time delays of each valence of conductors can be made equal. Signal time delay, Td, in a conductor is a function of the effective dielectric constant, cr , of the insulation surrounding the conductor by the relationship:

Td = 1.01626 ns/ft. X J^~~

In this example the full density base material is polytetrafluoroethylene (PTFE) of a density 2.15 g/cc and an effective dielectric constant of 2.1.

In this example:

Inner Valence

Insulation density = 1.30 g/cc Insulation dielectric constant = 1.57 Signal time delay = 1.27 nanoseconds/foot

Absolute time delay for 100 ft. cable = 127 ns

Intermediate Valence Insulation density = 1.10 g/cc Insulation dielectric constant = 1.46 Signal time delay = ^" 1.23 ns/ft.

Absolute time delay for 100 ft. cable = 127 ns

Outer Valence

Insulation density = 0.86 g/cc Insulation dielectric constant = 1.46 Signal time delay = 1.23 ns/ft.

Absolute time delay for 100 ft. cable = 127 ns

It is seen that the absolute signal time delay for each valence of conductors is equal to that of the other valences within the cable. One may calculate and construct further outer valence layers of insulated conductors out to the lower limit of dielectric constant of the insulation available. In the case of the preferred expanded polytetrafluoroethylene (ePTFE), one may go as low as an effective dielectric constant of about 1.2 which yields a time

delay of about 1.11 nanoseconds/foot.

The ePTFE insulation is that material disclosed in U.S. Patents 3,953,566, 3,962,153, 4,096,227, 4,187,390, 4,902,423, and 4,478,665, assigned to W. L. Gore & Associates, Inc., Newark, Delaware, and is a microporous material formed largely of interconnected nodes and fibrils of PTFE of varying sizes and in varying proportions.

The major advantage if the invention is the provision of coaxial electrical signal cables of a multiplicity of layers of spiralled insulated shielded signal conductors in which the signals passing through each conductor of the cable transit the conductors at a rate to reach an end of the cable at the same time.