Abstract:
A cable for transmitting information includes a set of conductors or pairs or quads of conductors insulated from each other to limit crosstalk between the conductors, pairs or quads and at least two metalized longitudinal flexible tapes separating the conductors, pairs or quads. The tapes are applied around each conductor, pair or quad by virtue of twisting or torsion of the cable. The tape is preferably folded so that each pair is shielded by a double thickness of tape.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns an electrical conductor cable for transmitting information. It also concerns a method of manufacturing a cable of this kind. 
     2. Description of the Prior Art 
     Nowadays information is transmitted via electrical cables comprising a large number of conductors and at very high frequencies. The increased frequencies and the increased number of streams of information to be transmitted in parallel on separate conductors leads to the necessity of providing effective insulation between the various electrical conductors assembled together in the same cable. The risk of crosstalk increases with the number of conductors and the frequency. Crosstalk is the result of information transmitted on one conductor being radiated toward another conductor. 
     The information is usually transmitted by pairs of conductors and the pairs are insulated and/or shielded from each other to prevent crosstalk, or near-end crosstalk. 
     The most usual solution to the problem of shielding each pair of conductors is to wrap a metal or metalized tape around each pair in a helix, the cable being formed by assembling the shielded pairs into a common protective sheath. Shielding the pairs individually is slow and difficult. To connect the resulting cable the individual shields of the pairs must be removed to obtain access to the conductors, which makes the connection operation, which is usually carried out on site, difficult. 
     To overcome these drawbacks, document FR-2 738 947 proposes a cable in which the electrical shields of the various conductors, in particular the pairs, comprise a central member with radial fins separating the pairs from each other and partially shielding each pair and a peripheral shield around the central member and the pairs, the peripheral shield completing the shielding of each pair. However, the fins of the central member form stiffener ribs and a cable made this way is unsuitable for many applications in which the cable needs to be flexible. Also, a cable equipped with a central member of the above kind is not easy to manufacture as it necessitates an extrusion operation. 
     The invention provides a flexible cable having good insulation between conductors, pairs or quads and which is also particularly simple to manufacture and use. 
     SUMMARY OF THE INVENTION 
     To this end, the present invention proposes a cable for transmitting information including a set of conductors or pairs or quads of conductors insulated from each other to limit crosstalk between the conductors, pairs or quads and at least two metalized longitudinal flexible tapes separating the conductors, pairs or quads and applied around each conductor, pair or quad by virtue of twisting or torsion of the cable. 
     It has been found that the longitudinal tapes largely wrap around each pair by virtue of twisting or torsion of the cable and the shielding obtained is therefore more effective than simple compartmentalization. 
     Also, the cable is more flexible than with cruciform compartmentalization and simpler and faster to manufacture. 
     In the case of a non-twisted cable, the torsion necessary for the tapes to wrap around each pair can be small. It can also be obtained in a virtually natural fashion by winding the cable onto a spool, or unwinding it therefrom, during manufacture. 
     The tapes are preferably folded so that each conductor, pair or quad is shielded by a double thickness of tape. 
     In one embodiment of the invention, each tape is folded along a longitudinal line, the two sections on respective opposite sides of the longitudinal fold are pressed together and each is extended by an end section comprising the longitudinal edges of the tape, which end sections are pressed against the corresponding end sections of the other tape. 
     In this way, the center sections of the two tapes and the end sections of a tape pressed against the corresponding end sections of the other tape form four flexible walls and therefore delimit four compartments into which the conductors, pairs or quads are inserted. Each of the flexible walls wraps around the conductor, pair or quad by virtue of twisting or torsion of the cable. 
     The longitudinal folding line can be central, so that the four longitudinal sections of each tape have the same length, or offset relative to a central folding line, so that the four longitudinal sections of each tape have different widths. 
     The invention applies to a cable having any number of conductors, pairs or quads insulated from each other. 
     In one embodiment, at least one of the two tapes, or each tape, can be formed of a composite material comprising an insulative support onto which a metal is deposited. The composite tape is made of “Aluster”, for example, i.e. polyester insulation covered with aluminum. 
     The tapes can be shaped at the same time as the conductors, pairs or quads are laid in the resulting compartments, laying obviously being effected downstream of shaping. 
     Other features and advantages of the invention will become apparent from the following description of particular embodiments of the invention given with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a first embodiment of a cable in accordance with the invention in section before it is finished by twisting it. 
     FIG. 2 is a view analogous to that of FIG. 1 after the cable is twisted. 
     FIG. 3 is a diagram showing a second embodiment of a cable in accordance with the invention in section before it is finished by twisting it. 
     FIG. 4 is a view analogous to that of FIG. 3 after the cable is twisted. 
     FIG. 5 is a diagram showing a third embodiment of a cable in accordance with the invention in section before it is finished by twisting it. 
     FIG. 6 is a view analogous to that of FIG. 5 after the cable is twisted. 
     FIG. 7 is a side view of part of an installation for manufacturing a cable in accordance with the invention. 
     FIG. 8 is a plan view in section taken along the axes of the rollers of the installation from FIG.  7 . 
     FIG. 9 shows the steps of forming tapes of a cable in accordance with the invention in the installation shown in FIGS.  7  and  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the example shown in FIGS. 1 and 2 the cable includes four pairs of twisted wires  10 ,  12 ,  14 ,  16 . Each pair conventionally comprises two insulated wires  18 ,  20  (for the pair  10 ), each wire comprising a central conductor and an insulative sheath. The two wires  18  and  20  are twisted together. 
     Two “Aluster” composite tapes  22  and  24  are provided to shield the pairs from each other, that is to say to prevent near-end crosstalk, which consists in transmission of signals from one pair to another by induction. An Aluster tape comprises an aluminum film applied to a polyester film. In the remainder of the description a tape of this kind will be generally referred to as a composite tape. 
     During its manufacture, each tape  22 ,  24  is folded so that it has four sections separated by longitudinal folds (see FIGS.  1  and  9 ). 
     The four sections are the same width. Thus the tape  22  has a central fold  26  on respective opposite sides of which are two sections  28  and  30  of equal width pressed together. The section  28  is extended by a section  32  at right angles which ends at a longitudinal edge  34 . Similarly, the section  30  is extended in the opposite direction by a second end section  36  terminating at a second longitudinal edge  38 . 
     In the same way, the tape  24  is folded with two central sections  281  and  30   1  pressed against each other and two end sections  32   1  and  36   1 . The sections  32  and  32   1  are pressed against each other. Similarly, the section  36  is pressed against the section  36   1 . In this way, seen in cross section, the two tapes form a cross with four branches of equal length. The first branch is formed by the sections  28  and  30 , the second by the sections  32  and  32   1 , the third by the sections  28   1  and  30   1  and the fourth by the sections  36  and  36   1 . 
     The sections at right angles, such as the sections  28  and  32 , are joined by a curved part  40 . The combination of the curved parts  40 ,  42 ,  44 ,  46  forms a space  48  whose section is the shape of a curvilinear quadrilateral. The space  48  is filled by a bare conductive wire  50  constituting a continuity wire whose function is to facilitate connecting the cable at its ends and to assure electrical continuity if the tape is interrupted because of deterioration or an accident. 
     The four branches mentioned above delimit four compartments  52 ,  54 ,  56  and  58  each of which receives a respective twisted pair  10 ,  12 ,  14  and  16 . 
     The cable as a whole is twisted after forming the compartments and installing the twisted pairs in the respective compartments. Each flexible wall, which consists of two central sections of the same tape or two end sections of two different tapes, therefore wraps around one pair and is pressed against that pair. 
     Accordingly, FIG. 2 shows the two tape sections  28 ,  30  pressed together and wrapped around the twisted pair  14 ; similarly, the sections  32  and  32   1  wrap around the pair  12 , the sections  28   1  and  30   1  wrap around the pair  10  and the sections  36  and  36   1  wrap around the pair  16 . 
     The resulting assembly is then conventionally surrounded by an outer shield  13  and insulation. 
     Inside the cable the pairs are separated from each other by a shield which prevents any propagation of waves emitted by one pair toward another pair, which minimizes near-end crosstalk. 
     It is not indispensable for the cable to be twisted for the tape sections to be pressed against the pairs. A different type of torsion can be used. 
     In one embodiment, in which the cable has an overall diameter of approximately 5.65 mm, the conductor of each wire has a diameter 0.53 mm, each wire has an outside (insulation) diameter of 1.42 mm, the continuity wire has a diameter of 0.45 mm and the tapes are composite tapes with a 50 μm thick aluminum layer and a 12 μm thick polyester layer, the cable can be used at frequencies up to 600 MHz with limited crosstalk between pairs. 
     In this example the aluminum layer of each tape faces inward, i.e. the aluminum layers of the sections  28  and  30  are pressed together, and similarly the aluminum layers of the sections  32  and  32   1  and  36  and  36   1  are pressed together. 
     It is therefore unnecessary to provide any lubricant where the tape passes over the guide and forming rollers during manufacture because the polyester in contact with the rollers has sliding qualities significantly better than those of aluminum. 
     In FIGS. 3 and 4, all the reference numbers concerning the tapes are identical to those from FIGS. 1 and 2 except they are “primed”. 
     The example shown in FIGS. 3 and 4 is identical to that shown in FIGS. 1 and 2 except that the tapes  22 ′ and  24 ′, corresponding to the tapes  22  and  24  shown in FIGS. 1 and 2, are shorter than the latter. The resulting cable has the same advantages as that shown in FIGS. 1 and 2. In particular, good protection against near-end crosstalk is also obtained in this case. 
     In FIGS. 5 and 6, all the reference numbers concerning the tapes are identical to those from FIGS. 1 and 2 except they are “double-primed”. 
     The example shown in FIGS. 5 and 6 is identical to that shown in FIGS. 1 and 2 except that the tapes  22 ′ and  24 ′, corresponding to the tapes  22  and  24  shown in FIGS. 1 and 2, are shorter than the latter and folded asymmetrically (for which purpose they are offset relative to each other during manufacture). The resulting cable has the same advantages as that shown in FIGS. 1 and 2. In particular, good protection against near-end crosstalk is also obtained in this case. 
     The invention is not limited to a cable made up of pairs. The flexible tapes could be used to provide shielding between single conductors or quads or other combinations of conductors. 
     The manufacturing installation for cables in accordance with the invention shown in FIGS. 7 and 8 includes guide rollers whose periphery is shaped to fold and form the tapes. Thus, for the tape  22 , there are two guide rollers  70 ,  72 , a roller  74  to form the central fold  26  and a series of successive rollers  76 ,  78 ,  80  and  82  for progressively forming the sections  28 ,  30 ,  32  and  36 . FIG. 4 shows that the roller  76  has at its periphery  84  a concave V-shape that forms the tape with two equal sections  86  and  88  (FIG.  9 ). 
     The periphery of the roller  78  begins to separate each longitudinal half-tape  86 ,  88  into two parts that will form the respective sections  30 ,  36  and  28 ,  32 . The folding effected by the roller  78  is shown at  90  in FIG.  9 . 
     The rollers  80  and  82  complete the shaping and folding. The folding at the exit from the roller  80  is shown at  92  in FIG.  9  and the folding effected by the roller  82  is shown by the part  94 . 
     A die  96  downstream of the roller  96  completes the folding of the tape. 
     Guide means  100 ,  102 ,  104 ,  106  are also provided for the twisted pairs  10 ,  12 , etc. The guide means position the pairs in the compartments  52 ,  54 ,  56 ,  58  on the output side of the die  96  (see FIG.  1 ). The cable as a whole is twisted downstream of the die  96 . 
     The installation shown in FIGS. 7 and 8 is used to manufacture the second and third embodiments of cables in accordance with the invention. FIG. 9 shows in particular the situation in which the tapes are folded symmetrically but can be adapted directly to the situation in which the tapes are folded asymmetrically, as shown in FIGS. 5 and 6. 
     Installing the tapes does not slow down the manufacture of the cable because they are installed at the same time as the twisted pairs. The speed with which the cable is manufactured is therefore higher than that of a cable with extruded cruciform compartmentalization, like the cable described in the previously mentioned document FR-2 738 947. 
     The invention also concerns a method of manufacturing a cable in which the two tapes  22 ,  24  are folded and assembled to form a cross with four branches delimiting four quadrants  52 ,  54 ,  56 ,  58 , a conductor, pair or quad is installed in each quadrant after this shaping and assembly, and torsion is then applied to the assembly, for example by twisting the assembly. 
     This method applies equally to the embodiments shown in FIGS. 3 to  6 .