Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
1998-11-23
2001-12-25
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S1130AS, C174S1170FF, C174S036000
Reexamination Certificate
active
06333465
ABSTRACT:
The present invention relates to a data transmission cable. It relates more particularly to such a cable for use in the field of computing or in the field of telephony.
BACKGROUND OF THE INVENTION
European patent EP-B-0 599 672 describes a high frequency signal transmission cable comprising at least one conductor unit. Each unit has at least two sets of conductors, such as a quad of twisted conductors, each conductor being insulated by a first insulating sheath. The unit, or a plurality of said units, are surrounded by a second sheath constituted by a thin extruded covering of insulating material situated in contact with the conductors or the set of conductors without said conductors being embedded in the sheath constituted by the covering. The insulating material simultaneously provides sufficient stiffness to hold said conductors in a determined position while said cable is being handled, and sufficient elasticity to make such handling possible.
Unfortunately, to obtain a level of cross-talk that is compatible with good transmission in a cable comprising at least one unit of at least four twisted conductors in the form of sets of conductors that are pairs or quads, the twist pitches of said sets of conductors must be different. The problem for data transmission is that given that each set of conductors has a different electrical length, signals applied simultaneously at one end of the cable to each of said sets of conductors reach the other end of the cable with relative time shifts. The problem becomes particularly troublesome when data transmission rates are high.
At present, this difference is compensated by active equipments associated with passive buffer memories or “buffers”. These memories are expensive, and the storage capacity they require is becoming ever greater.
Thus, the problem which arises is that of mitigating the transmission time differences between conductors in a given set of conductors, and the conductors in another set of conductors, said sets of conductors being situated in a unit of a data transmission cable. The object of the present invention is to make a data transmission cable which solves the problem posed in that it enables propagation time differences between the conductors of said sets of conductors to be reduced considerably, i.e. it makes it possible in general to obtain such propagation time differences that are less than two nanoseconds per hundred meter length of cable (2 ns/100 m).
SUMMARY OF THE INVENTION
To this end, the invention provides a data transmission cable comprising at least one unit of at least four conductors twisted into sets of conductors, which sets are quads or pairs, each conductor set being twisted at a pitch that is different from the pitch of any other conductor set, and each conductor being insulated by means of an insulating sheath, said unit being covered by at least one assembly means, wherein the material used for the insulating sheath of any conductor in a given set of conductors has a relative dielectric constant that is different from that of the material used for the insulating sheath of any conductor in another set of conductors, such that the propagation time differences between the conductors of said set of conductors and the conductors of the other set of conductors is less than 2 ns/100 m.
A preferred embodiment is such that said unit comprises eight conductors in the form of two quads or four pairs.
Said cable of the invention is advantageously such that said assembly means comprises at least one “unit” sheath situated in contact with the conductors of the sets of conductors.
Thus, when the unit is in the form of quads of twisted conductors, the material used for the insulating sheath of each conductor in the first quad has a relative dielectric constant that is different from that of the material used for the insulating sheath of each conductor in the second quad.
Likewise, when the unit is in the form of four pairs (or two pairs) of twisted conductors, the material used for the insulating sheath of each conductor in the first pair has a relative dielectric constant that is different from that of the material used for the insulating sheath of each conductor in each of the other three pairs (or the other pair).
Advantageously, the material used for the insulating sheath of any conductor in a set of conductors of a cable unit of the invention, selected from materials having a relative dielectric constant suitable for making the cable of the invention, has low internal molecular mobility. This molecular mobility is known, it is due to greater or lesser dipole polarity within the material, depending on the nature of a given material, and it is represented by the tangent value &dgr;. Typically, this value is less than about 5×10
−4
at 100 kHz for a material used as the insulating sheath of a conductor in a set of conductors present in a cable unit of the invention.
To obtain a cable of the invention, it is necessary to have materials that possess different relative dielectric constants, so as to be capable of making the insulating sheaths of all of the conductors of a given set of conductors out of the same given material, and the insulating sheaths of the conductors in all of the other sets of conductors out of other materials.
The insulating sheath of any conductor in a given set of conductors is thus made out of a given material which is different from the insulating materials used for the insulating sheaths of the conductors in the other sets of conductors: each set of conductors twisted at a given pitch is associated with one and only one insulating sheath material in unique manner. The materials used for making the insulating sheaths of all of the conductors in a given set of conductors are thus different materials, being selected for their different dielectric constants, or else they are materials of the same kind but possessing different relative dielectric constants.
By way of example, the materials can be selected from various types of polyethylene materials which, at present, provide a range of relative dielectric constants extending approximately from 1.6 to 2.3, with higher density polyethylene having a dielectric constant of about 2.3, and expanded materials such as expanded polyethylene or expanded polypropylene having a relative dielectric constant of about 1.6. The tangent &dgr; of this type of material is typically of the order of 10
−7
to 10
−6
at 100 kHz. A selection can also be made of expanded materials that are synthesized in a manner that is conventional to the person skilled in the art using the “foam skin” technique. Said expanded materials possess a cellular insulating material of very low dielectric constant, of about 1.3 to 1.4 approx., such as cellular polyethylene, said cellular insulating material being surrounded by a protective “skin” layer of solid polyethylene having a higher dielectric constant, of about 2.0 approx., and the thickness of such a layer is typically about 50 &mgr;m. Finally, a selection can also be made from by way of example of various fluorine-containing polymers such as polytetrafluoroethylene (PTFE), polychlorotri-fluoroethylene (PCTFE), polyvinylidine fluoride (PVDF), perfluorinated poly(ethylene-propylene) (PEP), ethylene trichlorofluoroethylene (ETCFE) and ethylene-PTFE copolymer (ETFE) which possess relative dielectric constants of about 2 to 2.6 approx., and on average tangent &dgr; values of less than about 5×10
−4
at 100 kHz.
As is known to the person skilled in the art, the sets of conductors present in the cable unit of the invention possessing the property of having a different dielectric constant for the sheath of each conductor in a given set of conductors compared with any conductor in another set of conductors, must in practice nevertheless possess the same impedance in order for it to be possible for any pair of said cable (single pair or part of a quad) to be used in the same application subsequently. That is why a logical consequence of the above property is that the diameter of the insulating
Alcatel
Mayo, III William H
Reichard Dean A.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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