Differential signal transmission cable

A differential signal transmission cable includes a pair of differential signal lines arranged in parallel to each other, an insulation for bundle-covering the pair of differential signal lines, and a shield conductor wound around an outer periphery of the insulation. The insulation is configured such that an outer circumference thereof in a cross section perpendicular to a longitudinal direction thereof has an oval shape formed with a continuous convex arc-curve. The oval shape has a width in a first direction along the arrangement direction of the pair of differential signal lines being larger than a width in a second direction orthogonal to the first direction.

The present application is based on Japanese patent application No 2012-000529 filed on Jan. 5, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a differential signal transmission cable.

2. Description of the Related Art

As a conventional technique, a parallel two-core shielded wire is known in which a shield conductor is formed by winding a metal foil tape around a pair of insulated wires arranged in parallel and at least one drain conductor arranged in parallel thereto all together, and an outer periphery of the shield conductor is covered by a jacket (see, e.g., JP-A-2002-289047).

In the parallel two-core shielded wire described in JP-A-2002-289047, it is possible to reduce manufacturing time since the shield conductor is formed by winding the metal foil tape.

SUMMARY OF THE INVENTION

In the parallel two-core shielded wire according to JP-A-2002-289047, a portion of the metal foil tape is flat in a transverse cross section. Pressure for pressing the metal foil tape based on tension is not generated in the flat portion since a tension direction of the metal foil tape is parallel to the surface of the flat portion, and the metal foil tape is likely to be loosened. The conventional parallel two-core shielded wire has a problem that skew and differential-to-common mode conversion quantity (i.e., conversion quantity from differential mode to common mode) may increase due to the loosening of the metal foil tape.

Accordingly, it is an object of the invention to provide a differential signal transmission cable that allows suppression of an increase in skew and differential-to-common mode conversion quantity.

(1) According to one embodiment of the invention, a differential signal transmission cable comprises:

a pair of differential signal lines arranged in parallel to each other;

an insulation for bundle-covering the pair of differential signal lines; and

a shield conductor wound around an outer periphery of the insulation,

wherein the insulation is configured such that an outer circumference thereof in a cross section perpendicular to a longitudinal direction thereof has an oval shape formed with a continuous convex arc-curve, and

wherein the oval shape has a width in a first direction along the arrangement direction of the pair of differential signal lines being larger than a width in a second direction orthogonal to the first direction.

In the above embodiment (1) of the invention, the following modifications and changes can be made.

(i) The insulation is configured such that the minimum value of a curvature radius of the outer circumference shape is not less than 1/20 and not more than ¼ of the maximum value of the curvature radius of the outer circumference.

(ii) The outer circumference of the insulation has an elliptical shape, and wherein the elliptical shape has a minor axis not less than 0.37 times and not more than 0.63 times a major axis thereof.

(iii) The outer circumference of the insulation comprises a first curved portion with a pair of symmetrical elliptical arcs located at both ends in the first direction and a second curved portion with a pair of symmetrical elliptical arcs located at both ends in the second direction, and

wherein the cable satisfies a condition represented by the following formula (1):

tan⁢⁢ϕ0=a1⁢b2a2⁢b1⁢tan⁢⁢θ0formula⁢⁢(1)
where a minor or major axis of the elliptical arc of the first curved portion in the first direction is2a1, a major or minor axis of the elliptical arc of the first curved portion in the second direction is2b1, a major axis of the elliptical arc of the second curved portion in the first direction is2a2, a minor axis of the elliptical arc of the second curved portion in the second direction is2b2, a phase angle of a connecting point between the elliptical arc of the first curved portion and the second curved portion is θ0and a phase angle of a connecting point between the elliptical arc of the second curved portion and the first curved portion is φ0.

(iv) The a2is larger than any one of the a1, the b1and the b2

(v) The a1, the b1and the b2are a common value.

(vi) The differential signal transmission cable further comprises:

a covering member for covering a shield conductor,

wherein the shield conductor comprises an insulating member and a conductive film on a surface of the insulating member opposite the covering member.

(vii) The shield conductor comprises a joint or an overlapped region along a longitudinal direction of the insulation, and

wherein the covering member comprises a spiral joint or overlapped region on the shield conductor.

(viii) The shield conductor comprises a spiral joint or overlapped region on the insulation, and

wherein the covering member comprises a braid.

(ix) The insulation comprises a foamed material.

(x) The insulation comprises an outer layer having a degree of foaming lower than that of an internal portion.

POINTS OF THE INVENTION

According to one embodiment of the invention, a differential signal transmission cable is configured such that an insulation thereof has an outer periphery of the cross section formed with a combination of plural curves each having different curvature radii (i.e., the cross section of the insulation being formed oval). Thus, pressure P can be constantly applied to the insulation so as to suppress the loosening of a binding tape even if an insulated wire covered by the insulation moves at the time of winding a metal foil tape around the insulation or tension T of the binding tape becomes less than a predetermined tension.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Summary of Embodiments

A differential signal transmission cable in embodiments includes a pair of differential signal lines arranged in parallel to each other, an insulation for bundle-covering the pair of differential signal lines, and a shield conductor wound around an outer periphery of the insulation, wherein the insulation is configured such that an outer circumference thereof in a cross section perpendicular to a longitudinal direction thereof has an oval shape formed with a continuous convex arc-curve, and wherein the oval shape has a width in a first direction along the arrangement direction of the pair of differential signal lines being larger than a width in a second direction orthogonal to the first direction.

First Embodiment

Structural Outline of Differential Signal Transmission Cable1

FIG. 1is a perspective view showing a differential signal transmission cable1in a first embodiment.FIG. 2Ais a cross sectional view showing the differential signal transmission cable1in the first embodiment which is cut in a transverse direction (a direction perpendicular to a longitudinal direction) andFIG. 2Bis a schematic diagram illustrating a cross section of the differential signal transmission cable1which is cut in a transverse direction. Two circles indicated by a dotted line inFIG. 2Bare to facilitate explanations and show cross sectional shapes of insulated wires which are used for making a cable having a transverse cross sectional shape equivalent to that of the differential signal transmission cable1. Hereinafter, a cross section means a cross section which is cut in a transverse direction unless otherwise indicated.

The differential signal transmission cable1is, e.g., a cable for transmitting differential signals between or within electronic devices such as server, router and storage, etc., using a differential signal of not less than 10 Gbps.

The differential signal transmission is that signals having a phase difference of 180° are respectively transmitted in a pair of conductive wires and a difference between the two signals having different phases is extracted at a receiver. Since direction of the currents flowing in the pair of conductive wires are opposite to each other, an electromagnetic wave radiated from the conductive wire as a transmission path in which the current is flowing is small. In addition, since noise induced from the outside is equally superimposed on the two conductive wires in the differential signal transmission, it is possible to eliminate the noise by extracting a difference.

As shown inFIG. 1, the differential signal transmission cable1in the first embodiment is schematically configured to include, e.g., a pair of conductive wires2(differential signal lines) arranged in parallel at a distance, an insulation3covering the pair of conductive wires2so that an outer circumferential shape of a transverse cross section thereof is formed by combining plural curved lines having different curvature radii, and a metal foil tape7as a shield conductor wound around the insulation3so that an inner circumferential shape of a transverse cross section thereof is formed by combining plural curved lines in accordance with the outer circumferential shape of the insulation3.

The pair of conductive wires2are arranged in parallel to each other. The insulation3covers the pair of conductive wires2together. In addition, the metal foil tape7is wound around an outer periphery of the insulation3. The outer circumferential shape of the insulation3on a cross section perpendicular to a longitudinal direction thereof is an oval shape of a continuous convex arc-curve in which a diameter in a first direction along a parallel direction of the pair of conductive wires2is larger than a diameter in a second direction orthogonal to the first direction. In other words, the outer circumferential shape of the insulation3is a shape formed of an entirely smoothly continued convex surface without flat or recessed portions.

In addition, the differential signal transmission cable1in the first embodiment is provided with, e.g., a binding tape8as a covering member for covering the metal foil tape7which is provided with a plastic tape5as an insulating member and a metal foil6as a conducting layer provided on a surface of the plastic tape5opposite to a surface facing the insulation3(i.e., on a surface facing the binding tape8).

The conductive wire2is, e.g., a solid wire of good electrical conductor such as copper or a solid wire of the electrical conductor which is plated, etc. In addition, a diameter2rof the conductive wire2is, e.g., 0.511 mm. Furthermore, a distance L between the conductive wire2and another conductive wire2is, e.g., 0.99 mm. The distance L is a distance in the cross section between the center of the conductive wire2and the center of the other conductive wire2. Alternatively, a twisted wire formed by twisting plural conductive wires may be used as the conductive wire2when, e.g., flexing characteristics are important.

The insulation3is formed of, e.g., a material having small relative permittivity and dielectric loss tangent. The material is, e.g., polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) and polyethylene, etc. Alternatively, the insulation3may be formed of a foam insulation resin as a foamed material in order to reduce the relative permittivity and dielectric loss tangent. When, e.g., the foam insulation resin is used, the insulation3is formed by, e.g., a method in which a foaming agent is kneaded into a resin and a degree of foaming is controlled by a temperature at the time of molding, or by a method in which a gas such as nitrogen is injected at a forming pressure and foams are created by releasing the pressure.

The insulation3has, e.g., a substantially ellipse (oval) cross sectional shape as shown inFIG. 2B, in which, e.g., a width W1in a major axis direction (a first direction along a parallel direction of the pair of conductive wires2) is 2.8 mm and a width W2in a minor axis direction (a second direction orthogonal to the first direction) is 1.54 mm. The width W1is greater than the width W2(W1>W2) and the width W1is about 1.8 times the width W2in the first embodiment.

Meanwhile, the insulation3has a region30(a region indicated by shading) surrounded by, e.g., a surface connecting tops of the two circles (not oval but perfect circles) indicated by a dotted line inFIG. 2Band a portion of an outer periphery of the insulation3. The circle indicated by a dotted line is, e.g., an inscribed circle in contact with the outer periphery of the cross section of the insulation3. When assuming that the two circles indicated by a dotted line inFIG. 2Bare, e.g., insulated wires, the region30indicates a region of the insulation3which is not formed in an insulation covering the two insulated wires. The maximum width-t of the region30is, e.g., 0.07 mm. The cross sectional shape of the insulation3will be further described below in reference to Comparative Examples 1 and 2.

FIG. 3Ais a schematic diagram illustrating a relation between tension T and pressure P when a metal foil tape101is wound around an insulated wire100having a circular cross section in Comparative Example 1 andFIG. 3Bis a schematic diagram illustrating a relation between tension T and pressure P when the metal foil tape101is wound around an insulated wire102having a flat portion103in Comparative Example 2.

Here, it is necessary to reduce skew in the differential signal transmission cable1in order to transmit high-speed signal of several Gbps. The skew means an arrival time difference of differential signals (i.e., intra-pair skew).

When two insulated wires are used to form a cable, skew occurs due to a slight difference of permittivity within an insulation, a slight difference in outer diameter of the insulation, a slight misalignment of a drain wire running side by side in a longitudinal direction of the insulation, or a gap at an interface between the insulation and a metal foil tape caused by looseness of the metal foil tape provided on the outside of the insulation, etc.

In addition, for the necessity of reducing EMI (Electro-Magnetic Interference), differential-to-common mode conversion quantity needs to be suppressed to be low in the differential signal transmission cable1. If a (left-right) symmetric property of the cable is not good, a portion of the inputted differential signals is converted into a common-mode signal. A rate of conversion into a common-mode is called differential-to-common mode conversion quantity. Particularly, a ratio of the common-mode signal in a port2to the differential signal in a port1can be measured as an S-parameter and is represented by “Scd21”.

A known method of reducing skew is to cover two conductors together with an insulation to suppress a difference of permittivity within the insulation. Meanwhile, another method is also known in which a shield is relatively separated from a conductor by winding an insulation tape around two insulated wires before being covered by a shielding conductive material to enhance electromagnetic coupling between the conductors, thereby forming a cable in which skew is less likely to occur.

The method of reducing skew described above has a certain effect on skew caused by the difference of permittivity within the insulation, where skew is reduced by a combination of a certain outer circumferential shape of the insulation and prevention of misalignment of the conductor.

However, influence of the gap generated by looseness of a metal foil tape wound around the insulation still slightly remains even after taking the measures described above. Especially when gaps are generated at positions asymmetric with respect to a pair of conductors, an arrival time difference of common-mode signal occurs, a degree of influence on the arrival time of differential signals becomes different in the pair of conductors, and skew is thus likely to occur. When the differential signal transmission cable1is used as, e.g., a cable for transmitting high-speed signals equivalent to 10 Gbps, there is a problem of a decrease in yield due to the gap.

The looseness of the metal foil tape occurs either, e.g., in the case of winding a metal foil tape around an insulation or in the case of lengthwise disposing a metal foil tape and then winding a binding tape therearound.

The cause of looseness occurred in the wound metal foil tape is that, e.g., a force of pressing the insulation by the metal foil tape, i.e., pressure P applied to the insulation from the metal foil tape is small.

As shown inFIG. 3A, in the case of Comparative Example 1 in which the metal foil tape101is wound around the insulated wire100having a circular cross section, a force acts on the insulated wire100so as to balance out tension T.

This force is the pressure P applied to a side face of the insulated wire100and has a relation represented by P=T/(2wr1) (w: width of the metal foil tape101, r1: radius of the insulated wire100).

On the other hand, in the case of Comparative Example 2 in which the metal foil tape101is wound around the insulated wire102having a cross sectional shape formed by combining the flat portions103and curved portions104as shown inFIG. 3B, the same pressure as P represented by P=T/(2wr1) is applied to the curved portions104. However, since a direction of the tension T of the metal foil tape101is parallel to a surface of the flat portion103, the pressure P applied to the flat portion103based on the tension T is zero.

Here, when the metal foil tape101is wound, a portion in which the metal foil tape101is straight is present both in the cross section formed by arranging two circular insulated wires and in the cross section formed by combining the flat portions103and the curved portions104as is shown inFIG. 3B.

That is, in the case of Comparative Example 2, since the tension T of the metal foil tape101is parallel to the surface of the flat portion103at the time of winding the metal foil tape101, a force does not act on the flat portion103. On the flat portion103, looseness of the metal foil tape101to be wound occurs by slight movement of the differential signal transmission cable at the time of winding the metal foil tape101or slight change in tension of the metal foil tape101, etc. This results in occurrence of skew and an increase in differential-to-common mode conversion quantity.

Accordingly, in the insulation3of the first embodiment, the regions30indicated by shading inFIG. 2Bare provided on upper and lower sides inFIG. 2B. Therefore, since the direction of the tension T of the metal foil tape7is at any portions not parallel to the surface of the flat portion103, vectors of the pressure P generated by winding the metal foil tape7are not zero.

The plastic tape5of the metal foil tape7is formed of, e.g., a resin material such as polyethylene.

The metal foil6of the metal foil tape7is formed by, e.g., adhering copper or aluminum to a surface of the plastic tape5.

In addition, the metal foil tape7has a joint or an overlapped region along a longitudinal direction of the insulation3. The metal foil tape7in the first embodiment is, e.g., tobacco-rolled so as to cover the insulation3of an insulated wire4. The tobacco-rolling is a method in which the metal foil tape7is placed in a longitudinal direction of the insulation3and is wound around the insulation3only once from the longitudinal side thereof. A joint70shown inFIG. 1is created along the longitudinal direction by, e.g., butting a longitudinal edge of the metal foil tape7against another edge. Meanwhile, when the metal foil tape7is longer than the outer periphery of the insulation3in the transverse direction, a region where an edge of the metal foil tape7overlaps another edge is created. Here, the metal foil tape7is wound around the insulation3. Therefore, an inner circumferential shape of the cross section of the metal foil tape7is a similar shape to the insulation3as shown inFIGS. 2A and 2B.

The binding tape8is formed of, e.g., a resin material.

The binding tape8has a spiral joint or overlapped region on the metal foil tape7. The binding tape8in the first embodiment is, e.g., spirally wound so as to cover the metal foil tape7. The binding tape8is wound around the insulation3so that a widthwise edge does not overlap another widthwise edge. Therefore, a joint80shown inFIG. 1is spirally formed on the metal foil tape7. When wound around the metal foil tape7so that one edge of the binding tape8overlaps another edge, an overlapped region on the metal foil tape7is spirally formed.

A method of manufacturing the differential signal transmission cable1in the first embodiment will be described below.

Method of Manufacturing Differential Signal Transmission Cable1

Firstly, the insulated wire4is formed by covering a pair of conductive wires2with the insulation3. In detail, the conductive wires2are arranged in parallel at a distance. As an example, the pair of conductive wires2is arranged in parallel at a distance of 0.99 mm. In addition, a diameter2rof the conductive wire2is, e.g., 0.511 mm. Then, the insulation3is formed by covering the pair of conductive wires2with expanded polyethylene. The insulation3is formed so as to have relative permittivity of, e.g., 1.5 by controlling a degree of foaming.

Meanwhile, the insulation3has a shape consisting of plural curved lines having different curvature radii as shown inFIG. 2Band, for example, the width W1in a major axis direction is 2.8 mm and the width W2in a minor axis direction is 1.54 mm. Here, the maximum width-t of the region30is, e g, 0.07 mm. The curvature radius of the region30is, e g, 7 mm.

For forming the insulation3, for example, an extrusion die of an extruder is formed according to the shape of the insulation3and expanded polyethylene is extruded together with a pair of conductive wires2from the extrusion die.

Next, the metal foil tape7is placed in a longitudinal direction of the insulated wire4and is wound around the insulated wire4. The winding is carried out so that the plastic tape5faces the insulation3and the metal foil6is exposed to the outside. The metal foil6is exposed to the outside since soldering is carried out in a later process.

Then, the binding tape8is spirally wound around the metal foil tape7and predetermined processes are then performed, thereby obtaining the differential signal transmission cable1.

Relation Between Curvature Radius and Looseness of Metal Foil Tape7

FIG. 4is a graph showing a relation between a curvature radius and an occurrence rate of looseness of metal foil tape in the differential signal transmission cable having a shape shown inFIGS. 2A and 2B. InFIG. 4, the horizontal axis is a curvature radius and the vertical axis is an occurrence rate of looseness of the metal foil tape7. The occurrence rate of looseness of the metal foil tape7means a probability that a gap is generated between the insulation3and the metal foil tape7in a cross section over the entire manufactured cable.

The occurrence rate of looseness of the metal foil tape7is measured by the following method. Firstly, samples of the cable are taken from the entire length of the manufactured cable without bias and a cross section of the cable is each observed. Presence of gap between the insulation3and the metal foil tape7in each sample is checked and a ratio of the number of the samples with a gap to the total number of the samples is defined as an occurrence rate of looseness.

According to the measurement result shown inFIG. 4, when the curvature radius of the region30of the insulation3is not more than 14 mm (20 times the curvature radius of the curved line located in a major axis direction), the occurrence rate of looseness of the metal foil tape7is not more than several % and it is possible to maintain performance of the differential signal transmission cable1.

On the other hand, when the curvature radius of the region30is 2.8 mm (4 times the curvature radius of the curved line located in a major axis direction), the thickness of the region30increases about 0.25 mm even though the occurrence rate of looseness of the metal foil tape7is low. The increase in the thickness of the region30increases characteristic impedance of the differential signal transmission cable1. In addition, when the differential signal transmission cable1is manufactured so as to have a curvature radius of 2.8 mm, an outer diameter of a cable which is formed by twisting plural differential signal transmission cables becomes large and it is difficult to handle. Therefore, the preferred range of the curvature radius is 4 times to 20 times.

Effects of the First Embodiment

In the differential signal transmission cable1of the first embodiment, it is possible to suppress skew and differential-to-common mode conversion quantity. In detail, an outer periphery of the cross section of the insulation3is a combination of plural curved lines having different curvature radii, i.e., is configured to include curved lines having a curvature radius of 0.7 mm located in a major axis direction and the regions30having a curvature radius of 7 mm as shown inFIG. 2B. Therefore, in the differential signal transmission cable1, the pressure P is constantly applied to the surface of the insulation3so as to balance out the tension T of the metal foil tape7at the time of winding the binding tape8around the insulated wire4. The pressure P in the region30decreases to about 1/10 of that in the major axis direction, which is considered because the pressure P is inversely proportional to the curvature radius of the outer periphery of the cross section when the tension T is constant, while the pressure P is not applied to the insulation3in a linear portion when the region30is not formed in the insulation3as described above.

In addition, since the region30is formed in the insulation3in the first embodiment, the pressure P is constantly applied to the insulation3and it is possible to suppress occurrence of looseness of the binding tape8even if the insulated wire4moves at the time of winding the metal foil tape7around the insulation3or the tension T of the binding tape8becomes weaker than a predetermined tension. Accordingly, it is possible to suppress looseness of the metal foil tape7and it is thus possible to suppress formation of a gap at an interface between the insulation3and the metal foil tape7. Therefore, a decrease in performance caused by an increase in skew and differential-to-common mode conversion quantity can be suppressed in the differential signal transmission cable1of the first embodiment.

Second Embodiment

The second embodiment is different from the first embodiment in that the outer circumferential shape of the transverse cross section of the insulation3is an ellipse shape.

FIG. 5Ais a transverse cross sectional view showing a differential signal transmission cable1in a second embodiment andFIG. 5Bis a diagram relating to the maximum value and the minimum value of curvature radius. InFIG. 5B, the horizontal axis is the x-axis and the vertical axis is the y-axis. In the ellipse, a major axis is on the x-axis and a minor axis is on the y-axis. It should be noted that, in each of the following embodiments, portions having the same structure and function as those in the first embodiment are denoted by the same reference numerals and explanations thereof will be omitted.

In the differential signal transmission cable1of the second embodiment, the outer circumferential shape of the insulation3is an ellipse shape having foci A and B. Other configurations are the same as the differential signal transmission cable1in the first embodiment.

Meanwhile, the method of manufacturing the differential signal transmission cable1in the second embodiment is different from that in the first embodiment in that the insulation3is formed in an ellipse shape having a major axis (=2a) of 3.20 mm and a minor axis (=2b) of 1.64 mm.

In the differential signal transmission cable1in the second embodiment, the pressure P is constantly applied to the insulation3at the time of winding the binding tape8around the metal foil tape7. In addition, a vector of the pressure P applied to the insulation3by the metal foil tape7is directed to either the focus A or the focus B which are shown inFIG. 5B.

When the tension T of the metal foil tape7is constant, the pressure P is inversely proportional to the curvature radius of the outer periphery of the cross section of the insulation3as described above. Accordingly, when an ellipse having the major axis2aand the minor axis2bas shown inFIG. 5Ais represented by the formula (2), the curvature radius at a given point (x, y) on the elliptical curve line is represented by the formula (3).

According to the formula (3), it is understood that the curvature radius varies in a range of not less than b2/a and not more than a2/b. Therefore, the minimum value of the pressure P is (b/a)3times the maximum value, i.e., the pressure P on the minor axis decreases to about 13% in the shape of the second embodiment.

However, since the metal foil tape7in the differential signal transmission cable1of the second embodiment can be wound so that pressure is constantly applied to the insulation3in the same manner as the first embodiment, it is possible to suppress occurrence of looseness of the binding tape8even if the insulated wire4moves at the time of winding the metal foil tape7around the insulation3or the tension T of the binding tape8becomes weaker than a predetermined tension.

Accordingly, it is possible to suppress looseness of the metal foil tape7and it is thus possible to suppress formation of a gap at the interface between the insulation3and the metal foil tape7. In addition, since a portion in which the curvature radius sharply varies is not present, a rate of generation of a gap is smaller than the first embodiment. Therefore, a decrease in performance caused by an increase in skew and differential-to-common mode conversion quantity can be suppressed in the differential signal transmission cable1of the second embodiment.

A ratio of the minimum to maximum curvature radii is (b/a)3as described above. Therefore, the curvature radius is not less than 1/20 and not more than ¼ when the minor axis of the cross section of the insulation3is in a range of not less than 0.37 times and not more than 0.63 times the major axis, and if the curvature radius is within the above range, it is possible to suppress looseness of the metal foil tape7in the same manner as the first embodiment.

Third Embodiment

The third embodiment is different from the first and second embodiments in that a degree of foaming within the insulation3is different in an internal portion and in an outer peripheral portion.

FIG. 6is a cross sectional view showing a differential signal transmission cable in a third embodiment. InFIG. 6, a region surrounded by an outer periphery of the insulation3and a dotted line is an insulation layer31.

In the differential signal transmission cable1of the third embodiment, a degree of foaming within the insulation3is different in an internal portion and in an outer peripheral portion. Other configurations are the same as the differential signal transmission cable1in the first embodiment. The degree of foaming is, e.g., 50% in the internal portion and several % in the insulation layer31

The insulation layer31of the insulation3has a degree of foaming lower than that of the internal portion of the insulation3. In other words, in the insulation3, the outer peripheral portion is harder than the internal portion since the insulation layer31is formed.

Meanwhile, the method of manufacturing the differential signal transmission cable1in the third embodiment is to cover a pair of conductive wires2using an extruder in the same manner as the first and second embodiments and also includes an extrusion step of further covering the outermost periphery of the insulation3with the insulation layer31having a low degree of foaming. The remaining of the manufacturing method is the same as the first and second embodiments.

In the differential signal transmission cable1in the third embodiment, the shape of the insulation3is more stable than the differential signal transmission cables1in the first and second embodiments since the insulation layer31is formed on the outer peripheral portion, and the pressure P applied by the binding tape8acts on the insulation3more stably. As a result, it is possible to suppress looseness of the metal foil tape7and it is thus possible to suppress formation of a gap at the interface between the insulation3and the metal foil tape7. Therefore, a decrease in performance caused by an increase in skew and differential-to-common mode conversion quantity can be suppressed in the differential signal transmission cable1of the third embodiment.

Fourth Embodiment

The fourth embodiment is different from the second embodiment in that the outer circumferential shape of the insulation3on a cross section perpendicular to a longitudinal direction consists of a first curved portion as a pair of elliptical arcs and a second curved portion as a pair of elliptical arcs which connects between the pair of elliptical arcs of the first curved portion. Here, an elliptical arc is defined as a concept including a circular arc as a portion of a perfect circle. In addition, an ellipse in the following description is a concept including a perfect circle.

FIG. 7Ais a cross sectional view showing a differential signal transmission cable1in a fourth embodiment taken in a transverse direction which is perpendicular to a longitudinal direction andFIG. 7Bis a diagram illustrating an outer circumferential shape in a cross section of an insulation3of the differential signal transmission cable1. InFIG. 7A, portions having the same structure and function as those in the first embodiment are denoted by the same reference numerals and explanations thereof will be omitted. Meanwhile, inFIG. 7B, the x-axis is a straight line passing through the respective centers of the pair of conductive wires2, and the y-axis is a straight line which passes through an origin O (the middle position between the respective centers of the pair of conductive wires2) indicating the center of the insulation3and is orthogonal to the x-axis.

A first curved portion (or first arc portion)41is composed of a pair of elliptical arcs41a,41blocated at both ends in a first direction which is along a parallel direction of the pair of conductive wires2(a horizontal direction inFIGS. 7A and 7B). A second curved portion (or second arc portion)42is composed of a pair of elliptical arcs42a,42blocated at both ends in a second direction (a vertical direction inFIGS. 7A and 7B) which is orthogonal to the first direction. The elliptical arcs41aand41bare line-symmetric with respect to the y-axis. The elliptical arcs42aand42bare line-symmetric with respect to the x-axis.

InFIG. 7B, a portion, other than the elliptical arc41a, of an ellipse which includes the elliptical arc41ais indicated by a dashed line (a line extended from the elliptical arc41a) and a portion, other than the elliptical arc42a, of an ellipse which includes the elliptical arc42ais also indicated by a dashed line (a line extended from the elliptical arc42a). As shown inFIG. 7B, the ellipse including the elliptical arc41ais an inscribed circle in contact with the ellipse including the elliptical arc42a.

The four elliptical arcs41a,41b,42aand42bare continued smoothly at respective connecting points40ato40d, i.e., without forming an angle at the connecting points40ato40d. InFIG. 7Bwhich shows the outline of the insulation3, the x-axis is the first direction and the y-axis is the second direction.

The elliptical arcs41aand41bof the first curved portion41are portions of an ellipse in which a minor or major axis in the first direction is2a1(2a1=a1×2) and a major or minor axis in the second direction is2b1(2b1=b1×2). Although the relation is a1=b1and the elliptical arcs41aand41bare portions of a perfect circle in an example shown inFIG. 7B, the relation may be a1<b1. When the relation is a1<b1, each of the elliptical arcs41aand41bis a portion of an ellipse having a minor axis in the x-axis direction and a major axis in the y-axis direction. On the other hand, when the relation is a1>b1, each of the elliptical arcs41aand41bis a portion of an ellipse having a major axis in the x-axis direction and a minor axis in the y-axis direction.

The elliptical arcs42aand42bof the second curved portion42are portions of an ellipse in which a major axis in the first direction is2a2(2a2=a2×2) and a minor axis in the second direction is2b2(2b2=b2×2).2a2is larger than2b2 (2a2>2b2), and each of the elliptical arcs42aand42bis a portion of an ellipse having a major axis in the x-axis direction and a minor axis in the y-axis direction.

In the fourth embodiment, the major axis2a2of the second curved portion42is larger than any of the major and minor axes2a1and2b1of the first curved portion41and the minor axis2b2of the second curved portion42(a2>a1, a2>b1and a2>b2). In addition, the major and minor axes2a1and2b1of the first curved portion41and the minor axis2b2of the second curved portion42are common values to each other (a1=b1=b2).

In addition, the entire outer circumferential shape of the insulation3in the fourth embodiment is an oval shape in which the width W1in the first direction is larger than the width W2in the second direction.

The elliptical arc41aof the first curved portion41is an elliptical arc drawn by an orbit expressed by the following coordinate (1). In the coordinate (1), θ0is a phase angle indicating one end (the connecting point40a) of the elliptical arc41awhen viewed from a gravity center O1(a center point between two foci) of an ellipse including the elliptical arc41a, and is an angle formed between a line segment connecting the gravity center O1to the connecting point40aand the x-axis. Meanwhile, X is an offset of the elliptical arc41ain the x-axis direction. The gravity center O1is on the x-axis, and a distance between the origin O and the gravity center O1is X.
(a1cos θ+X, b1sin θ)
(−θ0≦θ≦θ0)  coordinate (1)

A locus of coordinate values when θ(°) in the coordinate (1) is varied from −θ0to +θ0is the elliptical arc41a.

Meanwhile, the elliptical arc41bof the first curved portion41is an elliptical arc drawn by an orbit expressed by the following coordinate (2) in which a direction of the offset indicated by X in the coordinate (1) is opposite.
(a1cos θ−X, b1sin θ)
(180°—θ0≦θ≦180°+θ0)  coordinate (2)

A locus of coordinate values when θ(°) in the coordinate (2) is varied from 180°−θ0to 180°+θ0is the elliptical arc41b.

The elliptical arc42aof the second curved portion42is an elliptical arc drawn by an orbit expressed by the following coordinate (3). In the coordinate (3), φ0is a phase angle indicating one end (the connecting point40a) of the elliptical arc42awhen viewed from a gravity center O2(a center point between two foci) of an ellipse including the elliptical arc42a, and an angle formed between a line segment connecting the gravity center O2to the connecting point40aand a straight line parallel to the x-axis is

A locus of coordinate values when θ(°) in the coordinate (3) is varied from φ0to 180°−φ0is the elliptical arc42a.

Meanwhile, the elliptical arc42bof the second curved portion42is an elliptical arc drawn by an orbit expressed by the following coordinate (4) in which a direction of the offset indicated by Y in the coordinate (3) is opposite.
(a2cos φ,b2sin φ+Y)
(180°+φ0≦φ≦360°−φ0)  coordinate (4)

A locus of coordinate values when φ(°) in the coordinate (4) is varied from 180°+φ0to 360°−φ0is the elliptical arc42b.

The conditions of X and Y under which plural elliptical arcs41a,41b,42aand42bexpressed by the coordinates (1) to (4) are continued at each of the connecting points40ato40d, i.e., the conditions for connecting the first curved portion41to the second curved portion42without level difference are represented by the following formulas (4) and (5).
X=a2cos φ0−a1cos θ0formula (4)
Y=b2sin φ0−b1sin θ0formula (5)

In addition, the condition under which the elliptical arcs41aand42aare continued smoothly at the connecting point40a, i.e., the condition for continuing without forming a raised or recessed portion at the connecting point40ais represented by the following formula (6).

In addition, since the elliptical arcs41aand41bas well as the elliptical arcs42aand42bare each symmetrical, continuity between the elliptical arcs42aand41bat the connecting point40b, between the elliptical arcs41band42bat the connecting point40cand between the elliptical arcs42band41aat the connecting point40dare respectively smooth when the formula (6) is satisfied. That is, the following formula (7) is satisfied at each of the connecting points40b,40cand40dwhere θ=180°−θ0as well as φ=180°−φ0, θ=180°+θ0as well as φ=180°+φ0, and θ=360°−φ0as well as φ=360°−φ0.

The insulation3of the differential signal transmission cable1in the fourth embodiment satisfies all of the formulas (4) to (6). As a result, the elliptical arcs41a,41b,42aand42bare continued smoothly at each of the connecting points40ato40d.

Comparative Example 3

FIGS. 8A and 8Bare diagrams illustrating an outer circumferential shape of a cross section of a differential signal transmission cable in Comparative Example 3, whereinFIG. 8Ais an overall view of the outer circumferential shape andFIG. 8Bis a partial enlarged view thereof.

Elliptical arcs44a,44b,45aand45bshown in Comparative Example 3 which are elliptical arcs expressed by the same coordinates as the coordinates (1) to (4) satisfy the conditions represented by the formulas (4) and (5) (the conditions for continuously connecting elliptical arcs) but do not satisfy the condition represented by the formula (6). Therefore, recessed portions46ato46dwhich are depressed inwardly are formed at connecting points43ato43dof the elliptical arcs44a,44b,45aand45b.

Accordingly, in the differential signal transmission cable of the Comparative Example 3, a gap is likely to be formed between the insulation3and the metal foil tape7wound therearound, which is a cause of an increase in skew and differential-to-common mode conversion quantity.

In the differential signal transmission cable1of the fourth embodiment, the outer circumferential shape of the insulation3satisfies the formula (6) in addition to the formulas (4) and (5), and thus, the first curved portion41and the second curved portion42are continued smoothly. In other words, since the outer circumferential shape of the insulation3of the differential signal transmission cable1in the fourth embodiment is formed of a convex curved line over the entire circumference, pressure due to winding is constantly applied to the insulation3at the time of winding the binding tape8around the metal foil tape7in the same manner as the first and second embodiments.

As described above, in the differential signal transmission cable1of the fourth embodiment, it is possible to wind the metal foil tape7no as to constantly apply pressure to the insulation3in the same manner as the first and second embodiments and it is thus possible to suppress looseness at the time of winding the metal foil tape7around the insulation3. As a result, formation of a gap at the interface between the insulation3and the metal foil tape7can be suppressed, which suppresses occurrence of skew and differential-to-common mode conversion quantity.

Meanwhile, since variation (a difference between the maximum value and the minimum value) in the curvature radius can be reduced as compared to the second embodiment, probability of gap formation is much smaller. Therefore, a decrease in performance caused by an increase in skew and differential-to-common mode conversion quantity can be further suppressed in the differential signal transmission cable1of the fourth embodiment.

In addition, in the differential signal transmission cable1of the fourth embodiment, it is easier to ensure a distance between the conductive wire2and the insulation3than the case where the cross section of the insulation3is an ellipse shape as is in the second embodiment. Therefore, if a foamed material used in the third embodiment is used for the insulation3, a degree of foaming is equalized and the yield is improved.

Modification

FIG. 9is a perspective view showing a differential signal transmission cable1in a modification. In the differential signal transmission cable1of the modification, the metal foil tape7has a spiral joint80on the insulation3and a covering member for covering the metal foil tape7is a braid9. The metal foil tape7is formed by adhering a copper metal foil6on a surface of the plastic tape5and the braid9is composed of sixty-four copper strands each having a strand diameter of 0.08 mm.

In the differential signal transmission cable1in the modification, the insulation3has a shape described in any of the first to third embodiments and it is thus possible to suppress occurrence of looseness even if the metal foil tape7is spirally wound therearound. As a result, formation of a gap at the interface between the insulation3and the metal foil tape7can be suppressed. Therefore, a decrease in performance caused by an increase in skew and differential-to-common mode conversion quantity can be suppressed in the differential signal transmission cable1of the modification.

Alternatively, the metal foil tape7may have a spiral overlapped region on the insulation3.

Although the embodiments and modification of the invention have been described, the invention according to claims is not to be limited to the above-mentioned embodiments and modification. Further, please note that not all combinations of the features described in the embodiments and modification are not necessary to solve the problem of the invention.