Abstract:
A differential signal cable for transmitting high-speed digital differential signals of several Gbit/s or more is provided to minimize characteristic impedance mismatch and to repress increase in the skew, or increase in disturbance due to differential-mode to common-mode conversion. A differential signal cable comprises two insulated wires arranged parallelly in a contact, each of said two insulated wires comprising a conductor and an insulator jacketing the conductor; a fusion layer provided on the surface of each of said two insulated wires; a drain wire placed longitudinally in a recess created in the interstice between said two insulated wires; and a shield tape lapping around said two insulated wires and said drain wire together, wherein a surface of said insulator of each of said two insulated wires is partially deformed so as to have a flat portion and said two insulated wires are fused each other at said flat portions.

Description:
The present application is based on Japanese Patent Application No. 2010-065981 filed on Mar. 23, 2010, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a differential signal cable for transmitting high-speed digital signals of several Gbit/s or more over a distance from several meters to several ten meters with less degradation of signal waveforms. The present invention further relates to a cable assembly and a multi-pair differential signal cable each using such cable. 
     BACKGROUND OF THE INVENTION 
     In servers, routers, or storage devices that handle high-speed digital signals of several Gbit/s or more, a differential signal transmission system is employed for transmitting signals among such equipment or between circuit boards within such equipment. The differential signal transmission system transmits a signal in a two-phase style of signal on two conductors: one normal phase and the other 180-degree inverted phase. On the receiving end, the differential between the received two-phase signals are combined to be outputted. The directions of signal currents flowing along the two conductors are opposite each other; therefore, the electromagnetic wave that the transmission line may emit is small. Further, noise induced from the outside is superposed evenly on each of the two conductors; therefore, the effect of noise can be eliminated by combining the differential between them at the receiving end. For these reasons, high-speed digital signal transmission systems often use the differential signal. 
     As a differential signal cable to be used for transmitting differential signal, a twisted pair cable may be cited. The twisted pair cable is comprised of two insulated wires, which are conductors covered with insulator and twisted into a pair. The twisted pair cable is an economical cable with excellent circuit balance properties and is easy to bend; the cable therefore is used widely by preference for signal transmission over a medium distance. The twisted pair cable however has large signal attenuation. Because of this, a system that uses a twisted pair cable needs large electric power (about 6 to 10 times of that in a twinax cable as will be described later) for signal processing to compensate the signal attenuation. A twisted pair cable of a general style has no metallic layer thereon that will function as a shield. Therefore, the cable tends to be affected by the existence of a nearby-metallic body inviting a problem of unstable characteristic impedance of the cable. Further, the difference of the physical length between insulated conductors in a pair is large because the construction of the twisted pair cable is a strand of two insulated wires. Therefore, the effect of skew becomes large when the twisted pair cable is covered with a metallic conductor layer that works as a shield. Because of these, signals in the high-frequency region of several GHz tend to deform in their waveforms. Therefore, the twisted pair cable has been seldom used as a transmission line that is to convey signals with a rate of several Gbit/s. 
     On the other hand, there is a cable in which two insulated wires are arranged parallel without twisting and are covered with a shield (hereinafter referred to as a twinax cable). The twinax cable has smaller signal attenuation in the high-frequency range compared to twisted pair cables. Since a shield is provided over two insulated wires, the twinax cable is stable in its characteristic impedance even if a metallic body is placed close to the cable and the susceptibility to noise is low. Because of these advantages, the twinax cable has been used for transmission of relatively higher-speed signal over short distances. As the construction of the shield, tapes with conductive layer and coverings of braided wires are applied. Instead of covering with a shield, a drain wire may be incorporated. 
     For example,  FIG. 5  is a sectional view of the twinax cable  50  disclosed in JP 2002-289047 A as an example of twinax cables, in which two insulated wires  533 , each of which is a signal conductor  531  jacketed with an insulator  532 , are lapped or longitudinally lapped with a shield tape  535 , which is a laminate of a polyethylene tape  538  to which a metallic foil  537  such as aluminum is bonded. Between the shield tape  535  and the insulated wire  533 , a drain wire  534  is arranged so that the drain wire will contact the metallic foil  537  of the shield tape  535 , and is grounded. 
       FIG. 6  is a sectional view of an example of the twinax cable  60  disclosed in JP 2003-346566 A. Two insulated wires  633 , each of which is a signal conductor  631  jacketed with an insulator  632 , have fusion layers  636  severally thereon. Two insulated wires  633  are bonded mutually via the fusion layer  636  and covered with a shield tape  635 , which is a laminate of a metallic foil  637  such as aluminum, applied over the bonded configuration. In the twinax cable  50  illustrated in  FIG. 5 , a positional deviation appears between the insulated wires  533  because of slipping caused by a repeated bending or similar handling of the cable. In the twinax cable  60  illustrated in  FIG. 6 , no slipping occurs in contrast since the insulated wires  633  are mutually fused. It is indicated in the literature that the skew, which is the difference of signal propagation time between two signal conductors, will be reduced by such configuration. Increased skew deforms the digital signal waveform obtained by combining the signal differentials at the receiving end. As a result of this, the signal quality degrades even if the skew is several pico seconds when a transmission system handles high-speed signals of 10 Gbit/s or equivalent rate. 
       FIG. 7  is a sectional view of an example of the twinax cable  70  disclosed in JP 2001-035270 A. Signal conductors  731  are overall-jacketed with an insulator  732  and lapped or longitudinally lapped with a shield tape  735 , which is comprised of a metallic foil tape. 
       FIG. 8  is a sectional view of an example of the twinax cable  80  disclosed in JP 2007-026909 A. Insulated wires  833  are covered with a foamed tape  839  and covered with a shield tape with a drain wire  834  longitudinally placed on the foamed tape  839 . 
     SUMMARY OF THE INVENTION 
     Twinax cables are generally used for transmission of high-speed differential signals of several Gbit/s where distance is 1 to 10 meters or so. Both ends of a twinax cable have connectors installed thereon, each of which mates with a connector receptacle on equipment for eased connection. Inside the connector, a miniature printed circuit board of a card-edge type having plural contacts is installed. The miniature printed circuit board is about 0.5 to 1 mm in thickness. In installing the connector on the twinax cable is therefore performed in such a manner that the conductor of the cable is terminated on the signal circuitry face of the printed circuit board and the drain wire is on the grounding face on the other face of the board. 
     In the transmission of high-speed signals of several Gbit/s, characteristic impedance must be maintained uniform as much as possible; otherwise, undesirable signal reflection will appear causing degradation in signal quality. Uniformity of characteristic impedance is easily disturbed particularly at the junction point between the twinax cable and the printed circuit board, because such junction point is discontinuous in physical constructions: one is a cable-style in cross-section and the other is a micro-strip line. This constructional discontinuity easily generates signal reflection due to deviation of characteristic impedance. Further to the above, the physical construction must be such a configuration as reduces the skew. This consequently requires the cable construction not to cause positional deviation of the insulated wires from their original position and of the drain wire when the cable is bent. 
     In such a conventional cable construction as is illustrated in  FIG. 5 , the drain wire  534  is arranged in an interstice (recess) created between two insulated wires. For the cables for high-speed transmission over a distance between 1 m and 10 m, conductors of No. 24 American Wire Gauge (#24 AWG) are used most generally. In a typical cable design of the #24 AWG wire, the diameter of conductor d 1  is 0.51 mm, the thickness of insulator is 0.445 mm, and consequently the diameter of insulated wire d 2  is 1.4 mm. Where the diameter of conductor of the drain wire d 3  is assumed to be 0.40 mm, the distance r between the conductor of the insulated wire and the drain wire across the cable thickness is approximately 0.11 mm as given geometrically by the formula (1) following the schematic illustration given in  FIG. 9 . 
     
       
         
           
             
               
                 
                   
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     In terminating this cable on a printed circuit board having a board thickness of about 0.5 mm, soldering the conductors on the signal terminal pads on the wiring face of the printed circuit board without bending requires the drain wire  534  to be bent by about 0.4 mm in the board-thickness direction to widen the distance from the conductor for soldering on a land  512  of grounding face (a pad for grounding terminal), as illustrated in  FIG. 10 . Under this condition, the drain wire  534  is forced to be bent from the point where the shield tape  535  is removed and to the point where the drain wire  534  is to be soldered. Further, since the bending cannot be sharp, a space is produced between the end of the insulator  532  and the printed circuit board  510 . As a result of this, the characteristic impedance of this junction portion deviates to higher impedance causing undesired signal reflection with the signal transmission quality degraded. Moreover, the positional stability of a portion of the drain wire  534  in the produced space becomes unstable, inviting increase in the skew or degradation in differential-mode to common-mode conversion properties because of such as positional deviation. 
     The cables of conventional type for differential signal transmission illustrated in  FIGS. 6 to 8  (twinax cables  60 ,  70 , and  80 ) have measures for reducing occurring positional deviation of two insulated wires. In the twinax cable  60 , two insulated wires  633  are fused each other ( FIG. 6 ); in the twinax cable  70 , conductors  731  and  732  are jacketed by overall extrusion ( FIG. 7 ); and in the twinax cable  80 , two insulated wires  833  are lapped with the foamed tape  839 . In the case of the twinax cable  60  illustrated in  FIG. 6  however, connecting the drain wire  634  is required to still be bent similarly as explained in  FIG. 10 , because the position of the drain wire in the cable is the same as in the conventional art illustrated in  FIG. 5 . Therefore, the characteristic impedance of the junction portion deviates to higher impedance causing undesired signal reflection with the signal transmission quality degraded. Further, the positional stability of a portion of the drain wire  634  in a produced space like in the case of the twinax cable  50 , becomes unstable, inviting increase in the skew or degradation in differential-mode to common-mode conversion properties because of such as positional deviation. 
     In the case of the twinax cable  70  illustrated in  FIG. 7 , it is difficult to form a recess for accommodation of a drain wire. Without the accommodation recess, the position of the drain wire is unstable. Therefore, such situation causes positional deviation inviting increase in the skew or degradation in differential-mode to common-mode conversion properties. Further, it is also difficult to attain a desired positional accuracy of physical separation between two conductors. 
     In the case of the twinax cable illustrated in  FIG. 8 , the drain wire  834  is positioned relatively apart from signal conductors  831  compared to other differential signal cables (twinax cables  50 ,  60 , and  70 ). Since the drain wire  834  is located outside the foamed tape  839 , the drain wire  834  does not stably sit in place either. Therefore, such configuration causes positional deviation inviting increase in the skew or degradation in differential-mode to common-mode conversion properties. Moreover, the twinax cable  80  illustrated in  FIG. 8  needs a lapping process of the foamed tape  839 . 
     The present invention has been made in view of above-stated problems. It is therefore an object of the present invention to provide a differential signal cable for transmitting high-speed digital differential signals of several Gbit/s or more that permits minimizing characteristic impedance mismatch, which may appear when connecting to a device such as a printed circuit board having a built-in connector, at the connection point between the cable and such printed circuit board, and that at the same time represses increase in the skew, which is the difference of signal propagation time, or increase in disturbance due to differential-mode to common-mode conversion (degradation in differential-mode to common-mode conversion properties); and to provide a cable assembly and a multi-pair differential signal cable both using the same. 
     MEANS FOR SOLVING THE PROBLEMS 
     The present invention has been made to attain the above-stated object. The invention is a differential signal cable, comprising: two insulated wires arranged parallelly in a contact, each of the two insulated wires comprising a conductor and an insulator jacketing the conductor; a fusion layer provided on the surface of each of the two insulated wires; a drain wire placed longitudinally in a recess created in the interstice between the two insulated wires; and a shield tape lapping around the two insulated wires and the drain wire together, wherein a surface of the insulator of each of the two insulated wires is partially deformed so as to have a flat portion and the two insulated wires are fused each other at the flat portions. 
     The flat portion may have a width larger than the radius of the insulated wire. 
     The insulator may comprise a foamed material and each of the two insulated wires should preferably be fused each other through a heat-melt of the fusion layer. 
     The fusion layer comprises the same material as the insulator having a degree of foaming lower than that of the insulator. 
     The drain wire may be placed in each of two recesses created in the interstice between the two insulated wires. 
     The present invention is a cable assembly comprising a differential signal cable and a connector having a built-in printed circuit board connected to the end of the differential signal cable, wherein: the differential signal cable comprises two insulated wires arranged parallelly in a contact, each of the two insulated wires comprising a conductor and an insulator jacketing the conductor, a fusion layer provided on the surface of each of the two insulated wires, a drain wire placed longitudinally in a recess created in the interstice between the two insulated wires, and a shield tape lapping around the two insulated wires and the drain wire together; the conductor is connected to one face of the printed circuit board and the drain wire is connected to the other face of the printed circuit board; a surface of the insulator of each of the two insulated wires is partially deformed so as to have a flat portion and the two insulated wires are fused each other at the flat portions; and a differential between a distance from the conductor to the drain wire across the cable thickness and a thickness of the printed circuit board is reduced. 
     The distance between the conductor and the drain wire across the cable thickness may be made approximately equal to the thickness of the printed circuit board. 
     The flat portion may have a width larger than the radius of the insulated wire. 
     The insulator may comprise a foamed material and each of the two insulated wires may be fused each other through a heat-melt of the fusion layer. 
     The fusion layer may comprise the same material as the insulator having a degree of foaming lower than that of the insulator. 
     The drain wire may be placed in each of two recesses created in the interstice between the two insulated wires. 
     The present invention is a multi-pair differential signal cable, comprising: a strand of a plurality of the differential signal cables; a shield layer of braided conductors provided over the strand; and a jacket provided over the shield layer. 
     The present invention provides a differential signal cable for transmitting high-speed digital differential signals of several Gbit/s or more that permits minimizing characteristic impedance mismatch, which may appear when connecting to a device such as a printed circuit board having a built-in connector, at the connection point between the cable and such printed circuit board, and that at the same time represses increase in the skew, which is the difference of signal propagation time, or increase in disturbance due to differential-mode to common-mode conversion (degradation in differential-mode to common-mode conversion properties). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of a differential signal cable of an embodiment of the present invention. 
         FIG. 2  is a sectional view of arrangement of the drain wire in the differential signal cable illustrated in  FIG. 1 . 
         FIG. 3  is a side view of an aspect of the mounting of the differential signal cable illustrated in  FIG. 1  on a printed circuit board. 
         FIG. 4  is a sectional view of a multi-pair differential signal cable in an embodiment of the present invention. 
         FIG. 5  is a sectional view of a conventional differential signal cable. 
         FIG. 6  is a sectional view of a conventional differential signal cable. 
         FIG. 7  is a sectional view of a conventional differential signal cable. 
         FIG. 8  is a sectional view of a conventional differential signal cable. 
         FIG. 9  is a sectional view of arrangement of the drain wire in the differential signal cable illustrated in  FIG. 5 . 
         FIG. 10  is a side view of an aspect of the mounting of the differential signal cable illustrated in  FIG. 5  on a printed circuit board. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following explains a preferred embodiment of the present invention referring to drawings attached. 
       FIG. 1  is a sectional view of a differential signal cable of an embodiment of the present invention. 
     As illustrated in  FIG. 1 , a differential signal cable  100  in an embodiment of the present invention is comprised of two insulated wires  3 , each of which is comprised of a signal conductor  1  and an insulator  2  jacketing the conductor  1 , arranged parallelly in an intimate contact; a fusion layer  6  provided on the surface of each of the two insulated wires  3 ; a drain wire  4  placed longitudinally in a recess  7  created in the interstice between the two insulated wires  3 ; and a shield tape  5  lapping around the two insulated wires  3  and the drain wire  4  together. 
     The insulated wire  3  is formed by jacketing the conductor  1  with the insulator  2  supplied from an extruder. 
     As the conductor  1  for the insulated wire  3 , a solid wire or stranded wires of good electrical conductor such as copper, or plated one, is used. 
     As the insulator  2 , such a material as has small dielectric constant and dielectric tangent is preferable. For example, foamed material is feasible for such use. A method for forming the insulator  2  includes a method in which foaming agent is kneaded thereinto before forming process and the degree of foaming is controlled by the forming temperature, and a method in which nitrogen gas is injected thereinto at a forming pressure and foams are created by releasing the pressure. 
     On the surface of the insulated wire  3 , the fusion layer  6  is applied. The fusion layer  6  is formed by, for example, covering with the same material as the insulator  2  with its degree of foaming suppressed low. 
     The drain wire  4  uses, similarly as in the case of the signal conductor  1 , a solid wire or stranded wires of good electrical conductor such as copper, or plated one. 
     The shield tape  5  is a laminate of a polyethylene (PET) tape  8  to which a metal foil  9  is bonded. The shield tape  5  is applied in a manner winding around or lapping longitudinally with its metallic face, the metal foil  9 , contacted with the drain wire  4 . 
     In the differential signal cable  100  of this embodiment meantime, the two insulated wires  3 , part of the insulator surface of each of which is deformed so as to have a flat portion  2   a , are fused each other at the flat portions  2   a.    
     Now, details of this construction will be explained together with a manufacturing method of the differential signal cable  100 . 
     The signal conductor  1  is jacketed with the insulator  2  to form the insulated wire  3 , on which the fusion layer  6  is applied. Two insulated wires  6 , each applied with the fusion layer  6  thereon, are arrayed in parallel and paid off at a constant speed to undergo heating while being pressed in right-left directions to be arrayed at a desired separation pitch. The pressing and heating process causes the fusion layers  6  formed on the surfaces of the insulated wires  3  to be surface-fused each other. In this process, the pressing force is controlled to a proper degree so that the insulated wires  3  will fuse each other with a fusion-width (the vertical dimension in the figure) larger than the radius of the insulated wire  3 . This means that the fused-face between two insulated wires  3  has such a width that the central angle of the width with respect to the insulated wire  3  is 60 degrees or more. The temperature and the pressing force are returned to the initial state after the fusion and then the foams in the insulator  2  return to the initial shape, but the section-shape illustrated in  FIG. 1  is retained since the fused portion does not separate. The drain wire  4  is longitudinally arranged in the recess  7  created as a consequence of fusion. Over the fused two insulated wires  3  and the drain wire  4  arranged longitudinally, the shield tape  5 , which is a laminate of the polyethylene (PET) tape  8  to which the metal foil  9  is bonded, is wound around to secure the physical configuration. 
     Where a #24 AWG wire is used as the signal conductor  1 , the diameter of conductor d 1  is 0.51 mm. Let us assume with this conductor that the diameter of the insulated wire  3  d 2  is 1.4 mm and that the diameter of the drain wire  4  d 3  is 0.40 mm. Following this assumption, the fusion is formed so that the fusion-width a will be half the diameter of the insulated wire  3 , which is 0.7 mm. In this configuration, the thickness-wise distance r between the conductor and the drain wire is geometrically given using the formula (2) as illustrated in  FIG. 2 , which is about 0.21 mm. 
     
       
         
           
             
               
                 
                   
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     The depth of the recess  7  created both sides of the fused-face is 0.21 mm, which is deep enough for retaining the drain wire  4 . 
     Where the fusion-width a is 1.0 mm using the same insulated wire  3  and the drain wire  4  as described above, the thickness-wise distance r between the conductor and the drain wire becomes approximately 0.30 mm. 
     Next, the cable assembly, which is a cable assembly comprised of the differential signal cable  100  of this embodiment and a connector installed on the end thereof, will be explained together with installation method therefor referring to  FIG. 3 . 
     The cable end is treated as illustrated in  FIG. 3 . The shield tape  5  is removed with a dedicated cutter or laser irradiation. The insulator  2  is also removed in a tiered-cut style to expose the signal conductor  1  and the drain wire  4 . The conductor  1  for signal transmission and the drain wire  4  thus exposed are fixed and soldered respectively on a signal terminal pad  11  and a grounding terminal pad  12  on a printed circuit board  10  incorporated in a connector (not illustrated) to establish connections. In this connection configuration, connecting the differential signal cable  100 , of which signal conductor  1  is a #24 AWG wire, to a printed circuit board  10  having thickness of about 0.5 mm requires the drain wire  4  to be bent in the board-thickness direction. When, for example, the fusion-width a is 1.0 mm as assumed in the above description, the thickness-wise distance r between the conductor and the drain wire can be made approximately 0.30 mm. It is therefore enough for the widening of distance to bend the drain wire  4  by about 0.2 mm in the board-thickness direction. Consequently, the removal length of the shield tape  5  can be shortened with degradation of the transmission properties reduced. 
     The differential signal cable  100  of this embodiment permits each conductor and wire to be soldered without bending, with their insulator or jacket as-removed; further, the removal length of the shield tape  5  is made short with degradation of the transmission properties reduced. Further, the insulated wires  3  are fixed mutually by fusion. Therefore, the physical configuration of the cable is stable against bending and the positional deviation of the drain wire  4  is hard to occur. Thus, it becomes feasible to realize a cable that offers in its entirety small skew, much stability, and less degradation in transmission properties. In addition, since the insulated wires  3  are bonded mutually by fusion of the fusion layers  6  rendered by heating, the insulators  2  are deformed with their surface bonded. Consequently, no foams will be crushed even if foaming material is used as the insulator  2 , because it is not that the deformation occurs with the insulators  2  pressed. Therefore, the sizes of foams in the insulator  2  are uniform for every portion thereof after the heating was released. This means that the foams in the insulator  2  for deformed portion and for other portion are approximately same in size. Because of this, the dielectric constant of the transmission line is almost uniform and the dispersion that would occur on propagation is small with the skew minimized. 
     In short, the differential signal cable  100  of this embodiment can make the thickness-wise distance r between the conductor and the drain wire longer even though the insulated wires  3  are given the same diameters as the conventional ones. This configuration, when the differential signal cable  100  is connected to a connector to form a cable assembly, makes uniformity of the distance between the signal conductor  1  and the drain wire  4  undisturbed at the connection point between the differential signal cable  100  and the connector to a practicably utmost extent. Therefore, a change in characteristic impedance hardly occurs with reduced degradation of the transmission properties. 
     In this embodiment, the drain wire  4  is provided in one of two recesses created between two insulated wires  3 ; however, two drain wires may be severally placed in both the recesses. 
       FIG. 4  is a sectional view of a multi-pair differential signal cable of an embodiment of the present invention. 
     As illustrated in  FIG. 4 , a multi-pair differential signal cable  200  of an embodiment of the present invention is comprised of two differential signal cables  100 . The differential signal cable  100  is, as illustrated in  FIG. 1 , comprised of the insulated wire  3  comprised of the signal conductor  1  and the insulator  2 ; the drain wire  4 ; and the shield tape  5 . Two differential signal cables  100  are stranded so as not to separate, covered with a shielding layer  13  comprised of braided conductors to cutoff external noise, and sheathed with a jacket  14  for cable protection. 
     The multi-pair differential signal cable  200  of this embodiment has two differential signal cables  100  one for the sending the other for the receiving and is provided with the shielding layer  14  as a measure against the external noise. Therefore, it is feasible to realize a higher transmission speed with this cable compared to the differential signal cable  100  illustrated in  FIG. 1 .