DIFFERENTIAL PAIR CABLE

Abstract: A differential pair cable constructed with a conducting hollow shield (2) encircling an insulating hollow buffer (3), and at least one pair of insulated conductors (4) encircled by the buffer (3), each of the insulated conductors (4) being separated by the buffer (3) from the conducting shield (2) by a distance less than another distance of said each of the insulated conductors (4) from a central axis (5) of the shield (2), and the buffer (3) having a lower dielectric constant than that of insulation (6) on said each of the insulated conductors (4).

Description: Differential Pair Cable Field Of The Invention The invention relates to a differential pair cable having at least one quad of parallel insulated conductors extending within an encircling insulating buffer, and the buffer being encircled by a conducting shield. Background Of The Invention According to U.S. Patent 5,574,250, a known, differential pair cable is constructed with signal transmitting, insulated conductors that are equally radially spaced from a central axis of the cable. The insulated conductors are closer to the central axis of the cable than to an encircling shield, and an insulation layer separates the shield by a lengthy distance from the insulated conductors, so as to reduce both signal attenuation, and signal skew, of high frequency signals being transmitted by the known cable. More specifically, the insulation layer is sufficiently thick to space the shield from the insulated conductors by, at least the same distance, and farther, than they are spaced from the central axis of the cable. Summary Of The Invention The invention comprises, a quad cable wherein at lest one quad comprises two differential pairs of signal ternasmitting conductors, according to which, a conducting hollow shield encircles an insulating, hollow buffer, and at least one quad of insulated conductors is encircled by the buffer. For example, the buffer has a lower dielectric constant than that of insulation on each of the insulated conductors, and each of the insulated conductors being separated by the buffer from the shield by a distance less than another distance between said each of insulated conductors and the central axis of the shield. Closer spacing of the shield to the insulated conductors to reduce cable size, enables the cable cross section to be within 10%, within dimensional tolerences, of the size of the cross section of non-buffer covered quad cables with highly expanded insulation, of about 50% expansion, while achieving low signal skew. Another advantage results in a quad cable having signal transmitting conductors insulated by insulation of stable and uniform dielectric constant, and a buffer of lower dielectric constant that is relatively thin, to attain high frequency performance with a cable of minimimized cross section. a cable of and of lower dielectric constant to achieve high frequency performance of the cable save material costs. The invention avoids a layer of relatively large thickness to space the shield relatively far from the insulated conductors . According to an embodiment of the invention, the buffer is an expanded insulation. The insulation on each of the insulated conductors is of lower expansion than the buffer. An advantage resides in the insulation on the insulated conductors being less expensive to produce, due to the insulation having a lower expansion than that of the buffer. Uniformity of dielectric constant, and consequent lower signal skew, are achieved by controlling uniformity in the dielectric constant of the insulation on the insulated conductors. Such uniformity is less costly to attain for insulation requiring less expansion. Thus, another advantage resides in the insulation on the insulated conductors having a uniform dielectric constant. Marking individual conductors of a cable with color coding is desirable to distinguish among the conductors, for example, to prevent undesired, cross over connections at opposite ends of the conductors. In the known cable, the insulated conductors are insulated with a foamed or otherwise expanded insulation to reduce the dielectric constant of the insulation. It has been found that marking the expanded insulation with color coding degrades the improved dielectric constant, which, in turn, degrades the characteristic impedance of the insulated conductors. Further, the marking contributes to nonuniformity of the dielectric constant and increases signal skew. Heretofore, marking of the known cable has been restricted to an exterior jacket 8 of the known cable, leaving the individual conductors unmarked, and, thus, indistinct from one another. According to a further embodiment, the insulation on each of the conductors of a differential pair is suitable for distinctive marking, for example, to provide color coding. Although marking degrades the dielectric constant of the insulation on the conductors, and contributes to signal skew, such disadvantages are compensated for by improved uniformity of the dielectric constant of the insulation on the cable. Further, improved signal attenuation of the cable is preserved by encircling the marked insulated conductors with a buffer material of relatively lower dielectric constant. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, according to which: Figure 1 is a fragmentary perspective view of an electrical cable according to the invention, with parts shown cut away; Figure 2 is an enlarged cross section of the cable as shown in Fig . 1 ; Figure 3 is a fragmentary perspective view of another embodiment of an electrical cable according to the invention, with parts shown cut away; Figure 4 is a fragmentary perspective view of another embodiment of an electrical cable according to the invention, with parts shown cut away. Detailed Description With reference to Figs. 1-3, embodiments of a differential pair cable 1 will now be described. The cable 1 comprises, a conducting hollow shield 2 that encircles an insulating hollow buffer 3, and at least one pair of insulated conductors 4 that are encircled by the buffer 3, the buffer 3 separating the conducting shield 2 from the insulated conductors 4 by a distance less than another distance of the insulated conductors 4 from a central axis 5 of the hollow shield 2. The buffer 3 has a lower dielectric constant than that of insulation 6 on the insulated conductors 4, to reduce signal attenuation of the cable 1. Further with reference to Figs. 1-3, the central axis 5 of the shield 2 extends lengthwise of the cable 1. The cable 1 comprises at least one pair of insulated conductors 4 comprising a differential pair. The insulated conductors 4 of each differential pair are spaced directly across the central axis 5 from each other. Further, the insulated conductors 4 of each pair engage a cylindrical filler 7 comprising, for example, a cylindrical filament of insulating and flexible material, such as solid polyethylene. The filler 7 maintains a desired minimum spacing of each of the insulated conductors 4 from the central axis 5. The filler 7 is engaged by the insulated conductors 4 of each pair to resist movement of any one of the insulated conductors 4 radially inward. For the purposes of illustration, two pairs of insulated conductors 4 are shown, with each pair comprising at least one differential pair. The cable 1, with two differential pairs of insulated conductors 4 is known as a Quad cable . An insulating jacket 8 concentrically encircles the shield 2. The overall diameter of the cable 1 is determined, in part, by the thicknesses of the buffer 3, the shield 6, the jacket 8, and the total number of one or more differential pairs of insulated conductors 4 in the cable 1. Further the overall diameter of the cable 1 is determined, in part, by whether adjacent insulated conductors 4 engage one another side to side, or whether they are spaced apart, side to side, while the insulated conductors 4 are being maintained at equal distances from the central axis 5. According to an embodiment of the invention, Fig. 1, the insulated conductors 4 are insulated by solid insulation 6, meaning, insulation 6 with a very low rate of expansion. An advantage resides in the solid insulation 6 being of less expensive material, as compared with more expensive, expanded insulation 6. Expanded insulation 6 is more expensive for multiple reasons. First, expanded insulation 6 is produced to contain air to reduce substantially the dielectric constant. Further, the expanded insulation 6 is selected from materials with an inherent, low dielectric constant, such as polytetrafluoroethylene, which materials are relatively expensive. Further, the greater the expansion, the more costly is the cost of controlling uniformity of the dielectric constant over the entirety of the insulation 6 on the insulated conductors 4. Nonuniformity of the dielectric constant is undesired, since nonuniformity degrades signal skew performance of each of the differential pairs of insulated conductors 4. A feature of the invention resides in the buffer 3 comprising a dielectric constant less than that of the insulation 6 on the conductors 4. A differential pair cable 1 of this construction advantageously attains an improved signal attenuation without a need for relatively expensive insulation materials on both the buffer 3 and the insulated conductors 4. For example, foamed polypropylene tape is helically wrapped over the insulated conductors 4 and along the length of the cable 1, with adjacent helices 9 abutting and overlapping slightly to provide a concentric hollow shape for the buffer 3. For example, a differential pair cable 1 is constructed with individual, 24 AWG (American Wire Gauge), conductors 4 comprised of, tin plated copper 0.024 inch, (0.61 mm.) diameter. Solid insulation 6 on the individual conductors 4 comprises, for example, 0.030 inch, (0.76 mm.) of solid, low density polyethylene, 0.084 inch, (2.13 mm.) diameter. The foamed polypropylene tape has a thickness of The polypropylene material is less expensive than polytetrafluorethylene material. With reference to Fig. 1, the shield 2 comprises a polyester film coated with a film layer of aluminum on one side facing outward of the central axis 5. The shield 2 is helically wrapped against the buffer 3 and along the length of the cable 1, with adjacent helices overlapping 25%. The film has a thickness of . The shield 2 further comprises a metal conducting braid of 38 AWG tin plated copper strands braided with 85% coverage. Advantageously, the braid encircles and conductively engages the aluminum film layer on the shield 2. With reference to Fig. 3, the shield 2 comprises solely the braid. The buffer 3 is a hollow extrusion. With reference to Fig. 4, the shield 2 comprises solely the polyester film, described with reference to Fig. 1 above. In each of the embodiments, the buffer 3 is selected with a dielectric constant greater than that of the insulation 6 on each of the insulated conductors 4, to enable relatively closer spacing of each of the insulated conductors 4 to the shield 2 than to the center axis 5. The jacket 8 covering the braid comprises, for example, .025 inch, (0.64 mm.) of polyvinyl chloride. The cable 1 has an overall diameter of 0.305 inch, (1.65 mm. ) . The cable 1 constructed with two pairs of insulated conductors 4, according to the embodiment, has the following performance. Requirement Specification Buffered Quad Nonbuffered Quad Impedance 150 +/- 10 Ohms 148.3 151.1 Near End Crosstalk 1.5% max. 0.3 1.3 Attenuation 9dB/30m. max. 6.2 8.2 Rise Time Degradation 310 548 600 ps Max, in 30m. Time Delay Skew 100 500 200 ps/30 m. max. An unexpected result resides in significantly exceeding the performance of prior known, differential pair cables having a typical time delay skew of 200 ps/20 meters. According to the invention, the insulation 6 on the insulated conductors 4 comprises, an insulation 6 having a lower expansion than that of the buffer 3. A solid insulation 6 is an insulation 6 with a very low expansion, and includes, for example, low density polyethylene . The invention avoids the use of an expensive insulation 6 material such as polytetrafluoroethylene for either the insulation 6 on the conductors 4 or the buffer 3. The invention further avoids the need for an insulation 6 on the individual conductors 4 with a rate of expansion equal to or greater than that of the buffer 3, thereby avoiding an increased expense of production, and avoiding the undesired tendency for uniformities in the dielectric to increase with increased expansion of the insulation 6 on the conductors 4. A uniform dielectric constant is important, since nonuniformities in the dielectric constant contribute to signal skew. Solid insulation 6, even for a has a very low amount of expansion. A low density polyeythylene, for example, is considered a solid insulation 6, despite having a low density. According to a further advantage, the insulation 6 on the conductors 4 is suitable for distinctive marking 10, for example, to provide color coded insulated conductors 4. Such marking 10 has been known to degrade the dielectric constant of the insulation 6, both by increasing the dielectric constant, and by contributing to nonuniformity of the dielectric constant. The cable 1 compensates for degradation of dielectric constant caused by the marking 10. The insulation 6 on the conductors 4 has an expansion less than that of the buffer 3 , to attain relative uniformity of dielectric constant in the insulation 6 on the conductors 4, as compensation for degradation of the dielectric constant due to the marking 10. Color coding is desired on insulated conductors 4 within the same cable 1, for example, to avoid undesired cross connection of the insulated conductors 4 at opposite ends of the cable 1. Color coding is especially useful to provide a distinctive marking 10 on each of the insulated conductors 4, especially when the insulated conductors 4 extend helically along the length of the cable 1, as shown in Fig. 2. An unexpected result is obtained by the cable 1 according to which, insulated conductors 4 of a differential pair are adapted for color coding by distinctive marking 10, with consequent degradation of the dielectric constant of the insulation 6 on the conductors 4 being compensated for by the insulation 6 having a lower expansion than that of a buffer 3 of lower dielectric constant. According to an advantage of the invention, the insulated conductors are insulated by insulation 6 that is capable of being marked, for example, with color coding, and the buffer comprises insulation 6 having a lower dielectric constant than the marked insulation.6 on the insulated conductors . A further advantage resides in the marked insulation 6 on the conductors being less expensive, as compared with the known cable that requires more expensive, expanded insulation 6 on the conductors . Although preferred embodiments have been disclosed, other embodiments and modifications of the invention are intended to be covered by the spirit and scope of the appended claims .

Claims: What Is Claimed Is: 1. A differential pair cable comprising: a conducting hollow shield encircling an insulating hollow buffer, and at least one pair of parallel insulated conductors encircled by the buffer, each of the insulated conductors being separated by the buffer from the conducting shield by a distance less than another distance of said each of the insulated conductors from a central axis of the hollow shield, and the buffer having a lower dielectric constant than that of insulation on said each of the insulated conductors . 2. A differential pair cable as recited in claim 1 wherein, the insulated conductors are insulated by solid insulation, and the buffer comprises air entrained insulation. 3. A differential pair cable as recited in claim 1 wherein, the conducting shield comprises a flexible metal foil. 4. A differential pair cable as recited in claim 1 wherein, the conducting shield comprises a flexible metal foil engaged by a hollow metal braid. 5. A differential pair cable as recited in claim 1 wherein, the conducting shield comprises a hollow metal braid. 6. A differential pair cable as recited in claim 1 wherein, the insulated conductors are marked with color coding. 7. A differential pair cable as recited in claim 1 wherein, insulation on the insulated conductors has a lower expansion than that of the buffer.

Inventors: GRANT CHARLES L, PATEL MUKESH M, GRANT, CHARLES, L., PATEL, MUKESH, M.
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1998
Main classification: H01B11
IPC codes: H01B11