Cable for differential transmission

Abstract

To provide a cable for differential transmission which reduces a propagation delay time difference between an in-phase mode and a differential mode, and reduces position variation by bending and reduces an internal skew.SOLUTION: A cable 10 for differential transmission has a two core parallel type insulated wire 4 in which two insulated wires 3 having central conductors 1 and insulators 2 provided on the outer peripheries of the central conductors 1 are aligned in parallel with each other, a film-like external conductor 5 covering the two core parallel type insulated wire 4, a cover conductor 6 covering the film-like external conductor 5, and an externally-covering body 7 covering the cover conductor 6, wherein in a gap part A between the two core parallel type insulated wire 4 and the film-like external conductor 5, and an opposed part B opposed to the two insulated wires and the film-like external conductor 5, an intervening and covering layer 8 composed of a fluorine-based resin which is the same or the same kind as the insulators 2 is provided.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a differential transmission cable having good transmission characteristics and skew characteristics. Specifically, the present invention relates to a differential transmission cable that reduces the propagation delay time difference between the common mode and the differential mode, reduces the positional change due to bending, and suppresses the internal skew to a small value.

Background Art

[0002] In the broadcasting field, higher-definition broadcasting and video content such as 4K and 8K have emerged, and high-speed transmission performance is required for cables in order to recognize high-definition images. As a metal cable for differential transmission used in data centers and supercomputers, a "twinax cable" (also called a 2-core parallel coaxial cable) in which two covered wires are arranged in parallel and a shield is wound around is known. As an example of this twinax cable, two insulated wires each covered with an insulating layer are arranged in parallel, and the two are covered with an external conductor made of a copper PET tape and a cross-wound or braided structure, and an outer covering such as a PPS tape or a PVC sheath is covered thereon (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For 5G antennas, a 28 GHz signal is down-converted and used at 10 GHz. In transmissions exceeding 10 Gbit / s, the time per bit is less than 0.1 ns. Therefore, it is necessary to reduce the "internal skew", which is the deviation of the propagation time between the two wires of the differential transmission cable. For a cable with an internal skew of 9 ps per meter, the internal skew at 3 meters is 27 ps.

[0005] A differential transmission cable capable of withstanding high-speed transmission needs to suppress the internal skew to a small value. Also, when a twinax cable is used for high-speed transmission, it is necessary to suppress the internal skew to a small value. In addition, a change in skew due to bending during use of the cable becomes a problem. As causes of skew, 1) a time shift due to a difference in physical length between two insulated wires covered, 2) a time shift due to asymmetry of the center conductor or the outer conductor, 3) a time shift due to a propagation delay time difference between the common mode and the differential mode, and 4) a change in the position of the insulated wire and the outer conductor due to distortion generated inside the cable by bending can be considered. Regarding the difference in physical length in 1), as described in Patent Document 1 above, by providing an air layer in the insulating layer to reduce the relative permittivity, it is possible to reduce the time shift even if there is a physical length. Regarding the asymmetry in 2), it is considered technically difficult at present. Regarding the propagation delay time difference between the common mode and the differential mode in 3), it is necessary to make the permittivity as uniform as possible with the outer conductor as a waveguide. Regarding 4), it was considered technically difficult to suppress the change in position with the outer conductor due to bending.

[0006] In response to such problems, the inventor considered reducing the propagation delay time difference between the common mode and the differential mode in 3). In conventional products, there was a large gap between the insulating layer and the outer conductor, and a time difference occurred between the common mode that transmitted through that gap and the differential mode that transmitted between the center conductors. Also, regarding the position change in 4), the inventor considered suppressing the position change of the insulated wire and the outer conductor due to bending.

[0007] The present invention has been made to solve the above problems, and an object thereof is to provide a differential transmission cable that reduces the propagation delay time difference between the common mode and the differential mode, reduces the position change due to bending, and suppresses the internal skew to a small value.

Means for Solving the Problems

[0008] The differential transmission cable according to the present invention is a two-core parallel type insulated wire in which two insulated wires having a center conductor and an insulator provided on the outer periphery of the center conductor are arranged in parallel, a film-shaped outer conductor covering the two-core parallel type insulated wire, a cover conductor covering the film-shaped outer conductor, and an outer covering covering the cover conductor. A differential transmission cable, wherein an intervening coating layer made of the same or a similar fluororesin as the insulator is provided at a gap portion A between the two-core parallel type insulated wire and the film-shaped outer conductor, and at an opposing portion B between the two insulated wires and the film-shaped outer conductor.

[0009] According to the present invention, since an intervening coating layer made of the same or a similar fluororesin as the insulator is provided at the gap portion A between the two-core parallel type insulated wire and the film-shaped outer conductor, the dielectric constant of the gap portion A after providing the intervening coating layer becomes the same as or substantially the same as the dielectric constant of the insulator, and the difference in dielectric constant between the insulator constituting the inside and the gap portion A can be eliminated or reduced. As a result, the propagation delay time difference between the common mode and the differential mode can be reduced. Further, since an intervening coating layer made of the same or a similar fluororesin as the insulator is provided at the opposing portion B between the two insulated wires and the film-shaped outer conductor, the positional change between the insulated wire and the outer conductor due to bending can be suppressed, and skew can be reduced.

[0010] In the differential transmission cable according to the present invention, the intervening coating layer covers the opposing portion B with a thickness of 0.1 mm or less. According to the present invention, since the intervening coating layer covers the opposing portion B with a thickness of 0.1 mm or less, the positional change between the insulated wire and the outer conductor due to bending can be suppressed, and skew can be reduced.

[0011] In the differential transmission cable according to the present invention, the intervening coating layer is filled in the gap portion A. According to the present invention, since the intervening coating layer is filled in the gap portion A, the presence of the gap portion A having a significantly different dielectric constant from the insulator can be eliminated, and the dielectric constant difference in each part inside can be reduced and made uniform. As a result, the propagation delay time difference between the common mode and the differential mode can be reduced.

[0012] In the differential transmission cable according to the present invention, it is preferable that the insulator has a foamed structure or a hollow structure.

[0013] In the differential transmission cable according to the present invention, it is preferable that an adhesive layer is provided on the surface of the film-like outer conductor on the side of the two-core parallel type insulated wire. According to this invention, since the adhesive layer is provided on the surface of the film-like outer conductor on the side of the two-core parallel type insulated wire, it is possible to prevent the displacement between each insulated wire and the film-like outer conductor during terminal processing, and it is also possible to adhere to the intervening coating layer. As a result, the lengths of the two insulated wires constituting the two-core parallel type insulated wire do not change due to the terminal processing during connector attachment, and the occurrence of the skew in the above (1) caused by the change in the lengths of the two insulated wires is eliminated, and low delay can be satisfied.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a differential transmission cable that reduces the propagation delay time difference between the common mode and the differential mode, reduces the positional change due to bending, and suppresses the internal skew to a small level.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0016] An embodiment of the differential transmission cable according to the present invention will be described with reference to the drawings. The present invention includes inventions having the same technical idea as the embodiments and forms described in the following embodiments and drawings, and the technical scope of the present invention is not limited only to the descriptions of the embodiments and the drawings.

[0017] [Differential transmission cable] As shown in FIG. 1, the differential transmission cable 10 according to the present invention includes a two-core parallel-type insulated wire 4 in which two insulated wires 3 having a central conductor 1 and an insulator 2 provided on the outer periphery of the central conductor 1 are arranged in parallel, a film-shaped outer conductor 5 covering the two-core parallel-type insulated wire 4, a cover conductor 6 covering the film-shaped outer conductor 5, and an outer covering 7 covering the cover conductor 6. The differential transmission cable 10 is characterized in that an intervening coating layer 8(8a, 8b) made of the same or a similar fluororesin as the insulator 2 is provided in a gap portion A between the two-core parallel-type insulated wire 4 and the film-shaped outer conductor 5 and in an opposing portion B between the two insulated wires and the film-shaped outer conductor 5.

[0018] In this differential transmission cable 10, since the intervening coating layer 8a made of the same or a similar fluororesin as the insulator 2 is provided in the gap portion A between the two-core parallel-type insulated wire 4 and the film-shaped outer conductor 5, the dielectric constant of the gap portion A after providing the intervening coating layer 8a becomes the same as or substantially the same as the dielectric constant of the insulator 2, and the difference in dielectric constant between the insulator 2 constituting the inside and the gap portion A can be eliminated or reduced to make it uniform. As a result, the propagation delay time difference between the common mode and the differential mode can be reduced. Further, since the intervening coating layer 8b made of the same or a similar fluororesin as the insulator 2 is provided in the opposing portion B between the two insulated wires 3, 3 and the film-shaped outer conductor 5, the positional change between the insulated wire 3 and the outer conductor 5 due to bending can be suppressed, and skew can be reduced.

[0019] Hereinafter, each component will be described.

[0020] [Two-core parallel-type insulated wire] The two-core parallel insulated wire 4 is an essential component of the differential transmission cable 10 according to the present invention, and is formed by arranging two insulated wires 3, each of which is composed of a central conductor 1 and an insulator 2 provided on the outer periphery of the central conductor 1, in parallel. As shown in Fig. 1, the two insulated wires 3 that make up the two-core parallel insulated wire 4 are arranged side by side in two adjacent positions in one direction. "Arranged side by side" means arranged side by side, and usually, it is desirable that the adjacent insulated wires 3, 3 are in contact with each other, but they do not necessarily have to be in contact. The two-core parallel insulated wire 4 composed of two insulated wires 3 is preferably used as a differential cable.

[0021] (Central conductor) The central conductor 1 is composed of one strand extending in the longitudinal direction of each insulated wire 3 (for example, see Fig. 2), or is composed of a plurality of strands twisted together (for example, see Fig. 1). The type of the strand is not particularly limited as long as it is made of a highly conductive metal, but highly conductive metal conductors such as copper wire, copper alloy wire, aluminum wire, aluminum alloy wire, copper-aluminum composite wire, etc., or those with a plating layer on their surfaces can be preferably mentioned. Copper wire and copper alloy wire are particularly preferred. As the plating layer, a solder plating layer, a tin plating layer, a gold plating layer, a silver plating layer, a nickel plating layer, etc. are preferred. The cross-sectional shape of the strand is not particularly limited, but it is preferably circular, and may be substantially circular or rectangular.

[0022] The cross-sectional shape of the central conductor 1 is not particularly limited either. It may be circular (including elliptical), rectangular, etc., but is preferably circular. The outer diameter of the central conductor 1 is desirably as large as possible so that the electrical resistance (alternating current resistance, conductor resistance) is small, but in order to reduce the outer diameter of the insulated wire 3, for example, it can be in the range of about 0.09 to 1 mm. Usually, no insulating film is provided on the surface of the central conductor 1, but an insulating film (not shown) may be provided as necessary. The type and thickness of the insulating film are not particularly limited, but for example, those that decompose well during soldering are preferred, and a thermosetting polyurethane film, etc. can be preferably mentioned.

[0023] (Insulator) The insulator 2 is a low dielectric constant insulating layer continuously provided in the longitudinal direction on the outer periphery of the central conductor 1. The material of the insulator 2 is not particularly limited as long as it is a fluororesin, and it is arbitrarily selected according to the required impedance characteristics. For example, low dielectric constant fluororesins with a dielectric constant of 2.0 to 2.5 such as PFA (ε2.1), FEP (ε2.1), ETFE (ε2.5), etc. are preferred, and among them, PFA resin is preferred. In addition, a coloring agent may be contained in the material of the insulator 2. The thickness of the insulator 2 is not particularly limited either and is arbitrarily selected according to the required impedance characteristics, but it is preferably in the range of about 0.15 to 1.5 mm, for example. The forming method of the insulator 2 is not particularly limited, but any of a solid structure, a hollow structure, and a foamed structure can be easily formed by extrusion. In particular, the hollow structure and the foamed structure employing a low dielectric constant material can reduce the thickness of the insulator 2 without degrading the transmission characteristics, so that the outer diameter of the insulated wire 3 can be reduced. As a result, the volume of the differential transmission cable 10 can be reduced, and the thickness of the differential transmission cable 10 can also be reduced.

[0024] Figure 3 shows an example of the insulator 2 with a hollow structure. This hollow structure has a void portion 2A inside the structure body, and for example, the void portion 2A can have a cross-sectional form surrounded by an inner annular portion 2B, an outer annular portion 2C, and a connecting portion 2D. The void portion 2A is continuously provided in the insulator 2, but its form may be round or rectangular and is not particularly limited. Such an insulator 2 with a hollow structure is preferably adopted because it has excellent side pressure strength, is not easily crushed during the manufacturing process, and can make the high-frequency characteristics stable. In addition, the insulator 2 with a hollow structure can be formed by extruding resin on the outer periphery of the central conductor 1 running through the extrusion die. The thicknesses of the inner annular portion 2B, the outer annular portion 2C, and the connecting portion 2D are not particularly limited, but are, for example, in the range of about 0.01 to 0.05 mm, and the outer diameter of the formed insulator 2 with a hollow structure can be, for example, in the range of about 0.4 to 1.0 mm.

[0025] <Film-like external conductor> The film-shaped external conductor 5 (hereinafter sometimes simply referred to as "external conductor 5") is provided on the outer periphery so as to cover the two-core parallel-type insulated wire 4 as shown in FIG. 1. The external conductor 5 is not particularly limited as long as it is in the form of a film that can be vertically attached or wound horizontally so as to cover the two-core parallel-type insulated wire 4. However, as shown in FIG. 4, it is composed of a base film 5C, a metal layer 5B provided directly or via an adhesive layer (not shown) as required on one surface of the base film 5C, and an adhesive layer 5A provided on the other surface of the base film 5C. That is, the adhesive layer 5A is provided on the surface of the external conductor 5 on the side of the two-core parallel-type insulated wire 4. The external conductor 5 is provided so as to cover the two-core parallel-type insulated wire 4 in a manner where the adhesive layer 5A faces the surface on the side of the two-core parallel-type insulated wire 4 and the metal layer 5B faces the cover conductor 6 side. By winding such an external conductor 5 and adhering the adhesive layer 5A to the two-core parallel-type insulated wire 4, the lengths of the two insulated wires 3, 3 do not change or are less likely to change during terminal processing when attaching the connector. As a result, the skew caused by the change in the lengths of the two insulated wires 3, 3 can be suppressed.

[0026] The base film 5C is not particularly limited, but polyester films such as polyethylene terephthalate and polyethylene naphthalate can be preferably used. The thickness of the base film 5C is arbitrarily selected from, for example, those in the range of about 2 to 20 μm. Since the base film 5C made of such a material has a dielectric constant larger than that of the fluororesin constituting the insulator 2 described above, if the base film 5C is thick, the overall dielectric constant will increase and the transmission characteristics will deteriorate. Therefore, the thickness of the base film 5C is preferably in the range of 2 to 10 μm so as not to increase the overall dielectric constant too much.

[0027] The metal layer 5B preferably includes a copper layer, an aluminum layer, and the like. The metal layer 5B is formed on the base film 5C by vapor deposition or plating, or preferably includes a metal foil or the like bonded via an adhesive layer (not shown, such as a polyester-based thermoplastic adhesive resin) provided as necessary. The thickness of the metal layer 5B is not particularly limited and varies depending on the forming method. For a film formed by vapor deposition or plating, it can be arbitrarily selected from within the range of about 2 to 8 μm, and for a bonded metal foil, it can be arbitrarily selected from within the range of about 6 to 16 μm. Even with such a thin metal layer 5B, the external conductor 5 is stably held in a manner such that the adhesive layer 5A side covers the insulating wire 3 by the cover conductor 6 provided thereon. Note that the adhesive layer provided as necessary for bonding the metal layer 5B to the base film 5C is not particularly limited, but is preferably an adhesive layer capable of heat bonding, and examples thereof include urethane-based adhesives, epoxy-based adhesives, and acrylic-based adhesives. Since the metal layer 5B is electrically connected to the cover conductor 6, the shielding effect is stabilized and the transmission loss can be reduced.

[0028] The adhesive layer 5A is not particularly limited, and examples thereof include thermosetting resins such as polyurethane resin, polyester resin, and polyesterimide resin. This adhesive layer 5A is provided by applying the above thermosetting resin on the base film 5C. The external conductor 5 is provided so as to cover the two-core parallel insulating wire 4, and then the cover conductor 6 and the outer covering 7 are provided. When the outer covering 7 is provided, the adhesive layer 5A thermosets at the extrusion temperature, and the thermoset adhesive layer 5A functions to bond the insulating wire 3 and the external conductor 5. The thickness of the adhesive layer 5A is not particularly limited, but from the viewpoint of transmission characteristics, it is preferably within the range of 0.8 to 50 μm, and particularly preferably 0.8 to 5 μm. By providing the external conductor 5 including such a thin resin structure (adhesive layer 5A and base film 5C) on the insulating wire 3, the transmission characteristics of the differential transmission cable 10 as a whole can be made particularly good.

[0029] The external conductor 5 is wound around the two-core parallel insulated wire 4 either longitudinally or transversely. "Longitudinally" means winding the external conductor 5 so as to wrap along the longitudinal direction of the two-core parallel insulated wire 4, and "transversely" means winding the external conductor 5 around the outer circumference of the two-core parallel insulated wire 4. When winding the external conductor 5 either longitudinally or transversely, the adhesive layer 5A side is set to the two-core parallel insulated wire 4 side, and the metal layer 5B side is set to the cover conductor 6 side. By doing so, the adhesive layer 5A adheres the two-core parallel insulated wire 4 and the external conductor 5, preventing the displacement between each insulated wire 3, 3 and the external conductor 5 during terminal processing, and bringing the metal layer 5B into contact with the cover conductor 6 to conduct electricity. Note that both longitudinal and transverse winding are wound such that there is an overlapping portion (not shown) in a part thereof.

[0030] <Void portion, facing portion, and intervening covering layer> As shown in FIG. 1, the void portion A is two substantially inverted triangular gaps formed between the two-core parallel insulated wire 4 composed of the two adjacent insulated wires 3, 3 and the external conductor 5, whether the two adjacent insulated wires 3, 3 are in contact or adjacent without contact. The facing portion B is the portion where the two insulated wires 3, 3 and the external conductor 5 face each other. In other words, the facing portion B is the portion located at the position where the outer circumferences of the two insulated wires 3, 3 and the external conductor 5 are in contact, and the void portion A can be said to be the portion located at a position other than the facing portion B (the position where the outer circumferences of the two insulated wires 3, 3 and the external conductor 5 are not in contact).

[0031] In the present invention, an intervening coating layer 8a made of the same or a similar fluororesin as the insulator 2 is provided in the void portion A. By doing so, the dielectric constant of the void portion A after providing the intervening coating layer 8a becomes the same as or substantially the same as the dielectric constant of the insulator 2, and the difference in dielectric constant between the insulator 2 and the void portion A that constitutes the interior can be eliminated or reduced and made uniform. As a result, the propagation delay time difference between the common mode and the differential mode can be reduced. Further, in the present invention, since an intervening coating layer 8b made of the same or a similar fluororesin as the insulator 2 is provided in the opposing portion B, the positional change between the insulating wire 3 and the external conductor 5 due to bending can be suppressed, and skew can be reduced. Note that "the dielectric constant is the same or substantially the same" can mean a difference within a small range of 0 or more and 0.1 or less.

[0032] The constituent material of the intervening coating layer 8 is the same or a similar fluororesin as the insulator 2. The reason why the same or a similar fluororesin is preferable is that it is easy to eliminate the difference in dielectric constant between the insulator 2 and the void portion A that constitutes the interior covered by the external conductor 5 or it is easy to adjust to reduce the difference. A more preferable constituent material of the intervening coating layer 8 is a low-dielectric-constant fluororesin having a dielectric constant of 2.0 to 2.5, such as PFA (ε2.1), FEP (ε2.1), ETFE (ε2.5), etc., similar to the constituent material of the insulator 2. Among them, PFA resin is preferable.

[0033] First, the intervening coating layer 8b of the opposing portion B will be described. The intervening coating layer 8b provided on the opposing portion B preferably has a thickness of 0.1 mm or less. By providing such a thickness, the difference from the dielectric constant of the insulator 2 can be made the same or smaller, the positional change between the insulating wires 3, 3 and the external conductor 5 due to bending can be suppressed, and skew can be reduced. When the insulator 2 has a solid structure, there is no problem even if the thickness of the intervening coating layer 8b at the opposing portion B exceeds 0.1 mm because it becomes the same as or comparable to the dielectric constant of the insulator 2. On the other hand, when the insulator 2 has a hollow structure or a foamed structure, if the thickness of the intervening coating layer 8 at the opposing portion B exceeds 0.1 mm, a difference from the dielectric constant of the insulator 2 may easily occur. The lower limit of the thickness of the intervening coating layer 8b at the opposing portion B is preferably about 0.05 mm sufficient to suppress the positional change between the insulating wires 3, 3 and the external conductor 5. The intervening coating layer 8b at the opposing portion B can be formed by coating with the same or the same type of fluororesin as the insulator 2 by an extrusion method.

[0034] Next, the intervening coating layer 8a of the void portion A will be described. The intervening coating layer 8a provided in the void portion A becomes the same as or comparable to the dielectric constant of the insulator 2 due to the decrease in the dielectric constant caused by the presence of the gap C remaining after the intervening coating layer 8a is filled. When the insulator 2 has a solid structure, it is preferable to make the gap C existing after the intervening coating layer 8a is provided as small as possible. On the other hand, when the insulator 2 has a hollow structure or a foamed structure, the gap C existing after the intervening coating layer 8a is provided is preferably set to a size such that it is comparable to the porosity of the insulator 2. The size of the gap C is adjusted by the material and porosity of the insulator 2 and the material of the intervening coating layer 8a. However, since the gap C exists with an appropriate size, it can be easily adjusted to have the same or comparable dielectric constant as the insulator 2 by selecting the type of fluororesin. By making the dielectric constant of the void portion A filled with the intervening coating layer 8a the same as or comparable to the dielectric constant of the insulator 2, the propagation delay time difference between the common mode and the differential mode can be reduced.

[0035] The method of filling the intervening coating layer 8a while leaving the gap C is not particularly limited, but it may be a method of extruding a fluororesin so as to cover the entire void portion A and the opposing portion B after disposing the intervening member 9 in the void portion A (see, for example, FIG. 2), or a method of extruding a fluororesin so as to cover the entire void portion A and the opposing portion B without using the intervening member 9 (not shown).

[0036] The intervening member 9 shown in the example of FIG. 2(A) is provided so as to be inserted into the void portion A. After the intervening member 9 is provided in the void portion A, it will be covered with the intervening coating layer 8 so as to cover them. However, the size of the intervening member 9 is adjusted so that the intervening coating layer 8 in the void portion A does not bulge due to the provision of the intervening member 9. The intervening member 9 is made of the same or a similar fluororesin as the insulator 2, similar to the material forming the intervening coating layer 8a. Further, the structure of the intervening member 9 can be selected in consideration of the size of the gap C remaining in the void portion A, and as illustrated in FIG. 2(A), it is preferably triangular so as to fit into two substantially inverted triangular gaps above and below. On the other hand, when the gap C may be slightly larger, it can be rhombic, round (both not shown) or other polygonal shapes. Further, when the size of the gap C is insufficient, the intervening member 9 can also have a hollow structure or a foamed structure, and can be adjusted so that the dielectric constant of the intervening coating layer 8a in the void portion A approaches or becomes the same as the dielectric constant of the insulator 2. The method of forming such an intervening member 9 is not particularly limited, but it can be easily formed by the extrusion method having the above-described cross-sectional shape.

[0037] <Cover Conductor> The cover conductor 6 is formed on the external conductor 5 in a braided structure of fine metal wires or a horizontally wound shield structure of fine metal wires. The cover conductor 6 covers the external conductor 5 and prevents the slack and displacement of the external conductor 5, thus contributing to maintaining stable transmission characteristics. In the case of the horizontally wound shield structure, the horizontally wound fine metal wires may be single-layered or laminated, and are not particularly limited, but a single layer is preferred.

[0038] The metal fine wires constituting the cover conductor 6 are not particularly limited as long as they are highly conductive metal fine wires. For example, various metal fine wires typified by tin-plated copper wires can be preferably used. The outer diameter of the metal fine wires can be, for example, in the range of about 0.04 to 0.1 mm. The number of metal fine wires is arbitrarily selected depending on whether it is a braided structure or a spiral-wound shield structure, and the size of the object to be covered, etc.

[0039] Since the cover conductor 6 makes it difficult for the outer conductor 5 wound around the outer periphery of the two-core parallel-type insulated wire 4 to shift, even when bent or bent back, it is difficult for the distance between the outer conductor 5 and the center conductor 1 to change. As a result, phase fluctuations are less likely to occur, and a decrease in signal transmission characteristics (attenuation amount, skew) can be suppressed. In particular, in the differential transmission cable 10 for transmitting differential signals at high speed, it is possible to prevent the transmission characteristics from deteriorating by suppressing the change in the signal transmission speed between the two insulated wires 3, 3.

[0040] <Outer jacket> The outer jacket 7 is provided on the outer periphery of the cover conductor 6, and its material is not particularly limited as long as it is insulating. It may be provided by spirally winding a resin tape having an adhesive layer on one side, but in the present invention, it is preferably provided by extruding the resin. As the constituent resin of the outer jacket 7, in the case of resin extrusion, various ones applied to general coaxial cables as the outer jacket can be used. For example, fluorine-based resins such as PFA, FEP, and ETFE may be used, vinyl chloride resins may be used, polyolefin resins such as polyethylene may be used, or polyester resins such as polyethylene terephthalate may be used. The thickness of the outer jacket 7 can be, for example, in the range of about 0.1 to 1 mm.

[0041] When using a resin tape, it is possible to prevent the cover conductor 6 from shifting its position (especially in the case of a horizontally wound shield structure) by fusing it with the cover conductor 6. When using a resin tape with a fusion layer, wind it horizontally with the side of the fusion layer facing the cover conductor 6. Examples of the material of the resin tape include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyamide (PA), polyimide (PI), polyphenylene sulfide (PPS), ethylene-tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), fluorinated resin copolymer (perfluoroalkoxy fluorine-based resin: PFA), polyetheretherketone (PEEK), etc. The thickness of the resin tape is not particularly limited as long as it can ensure the required dielectric withstand voltage, but it can be about 0.004 to 0.01 mm. The fusion layer is provided on one side of the resin tape, and examples of its material include thermosetting resins such as polyurethane resin, polyester resin, and polyesterimide resin. The thickness of the fusion layer is not particularly limited either, but it can be about 0.001 mm.

Example

[0042] Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. Note that the present invention is not limited to the following examples.

[0043] [Example 1] As the center conductor 1, seven silver-plated soft copper wires with an outer diameter of 0.203 mm were twisted to have an outer diameter of 0.61 mm. An insulator 2 was formed on the outer periphery of this center conductor 1. The insulator 2 was formed by extruding PFA resin (manufactured by DuPont, ε2.1) at 350 °C using a die nipple for a hollow structure to obtain a hollow structure with a cross-sectional shape in which the void portion 2A was surrounded by an inner annular portion 2B, an outer annular portion 2C, and a connecting portion 2D. This hollow structure had an outer diameter of 1.53 mm, a porosity of 50%, and a dielectric constant ε of about 1.6. Thus, the insulated wire 3 was produced. The outer diameter of the two-core parallel-type insulated wire 4 formed by arranging the two produced insulated wires 3, 3 in close contact with each other was 1.53 mm in the vertical width and 3.06 mm in the horizontal width when viewing FIG. 1 in a plan view.

[0044] An intervening coating layer 8 was provided so as to cover the two-core parallel insulation wire 4. First, triangular intervening members 9 with a base of 1.41 mm and a height of 0.705 mm were arranged in the void portions A existing above and below respectively (see Fig. 2(A)), and then, a PFA resin was extruded to form a covering over the entire void portion A and the opposing portion B where the intervening members 9 were arranged (see Fig. 2(B)). The material of the intervening members 9 is the same as the PFA resin that covers the whole. The thickness of the intervening coating layer 8b at the opposing portion B is 0.10 mm. After providing the intervening coating layer 8, the size was such that the vertical width was 1.63 mm and the horizontal width was 3.06 mm when viewed in plan in Fig. 1. Although a gap C remained after providing the intervening coating layer 8a in the void portion A, the porosity of the void portion A including the gap C was the same as the porosity (50%) of the above-described insulator 2.

[0045] Thereafter, the two-core parallel insulation wire 4 provided with the intervening coating layer 8 was provided so as to be vertically attached and covered with an external conductor 5. The external conductor 5 is composed of a PET base film 5C with a thickness of 12 μm, a metal layer 5B made of a copper foil with a thickness of 9 μm provided on one surface thereof via an adhesive layer (not shown in Fig. 4), and an adhesive layer 5A with a thickness of 1 μm provided by applying a urethane adhesive resin on the other surface of the PET base film 5C. A film-shaped external conductor 5 with a width of 10 mm and a total thickness of 0.022 mm was used. This external conductor 5 was vertically attached such that the adhesive layer 5A side faced the insulation wire 3 side and the overlapping portion had a width of 2 mm.

[0046] Thereafter, a braided shield was provided as a cover conductor 6 to cover the whole. The cover conductor 6 with a braided shield structure used a total of 96 copper wires with an outer diameter of 0.10 mm. After providing the cover conductor 6, the dimensions were such that the vertical width was 2.0 mm and the horizontal width was 3.6 mm when viewed in plan in Fig. 1. Thereafter, as an outer jacket 7, PVC was extruded to form an outer jacket 7 with a thickness of 0.45 mm. Thus, the differential transmission cable 10 of Example 1 with a vertical width of 2.9 mm and a horizontal width of 4.5 mm when viewed in plan in Fig. 1 was obtained.

[0047] [Example 2] The intervening coating layer 8 covering the two-core parallel insulating wire 4 was formed without using the intervening member 9. At this time, the intervening coating layer 8 was formed by extrusion while adjusting the extrusion amount of the PFA resin so as to cover the entire void portion A and the opposing portion B. The material of the intervening member 9 is the same as the PFA resin that covers the whole. By adjusting the extrusion amount, the thickness of the intervening coating layer 8b at the opposing portion B was set to 0.10 mm, and the porosity of the void portion A including the gap C remaining after filling the void portion A with the PFA resin was made the same as the porosity (50%) of the insulator 2 described above. The size after providing the intervening coating layer 8 was such that the vertical width was 1.63 mm and the horizontal width was 3.16 mm when viewing FIG. 1 in plan view. Otherwise, in the same manner as in Example 1, a differential transmission cable 10 of Example 2 having a vertical width of 2.9 mm and a horizontal width of 4.5 mm when viewing FIG. 1 in plan view was obtained.

[0048] [Comparative Example 1] In Example 1, a differential transmission cable of Comparative Example 1 was obtained in the same manner as in Example 1 except that the intervening coating layer 8 was not provided. In this differential transmission cable, the dielectric constant ε of the insulator 2 is about 1.6 as described above, but the dielectric constant ε of the void portion A where the intervening coating layer 8 is not provided is about 1.0, and there is a difference between the two.

[0049] [Evaluation] The skews of the differential transmission cables obtained in Examples 1 and 2 were both 5 ps per meter, but the skew of the differential transmission cable of Comparative Example 1 that does not include the intervening coating layer 8 was 9 ps per meter. These results show that by providing the intervening coating layer 8 made of the same or the same type of fluororesin as the insulator 2 in the void portion A and the opposing portion B, the dielectric constant of the intervening coating layer 8 and the dielectric constant of the insulator 2 become the same or substantially the same, and the difference in dielectric constant between the insulator 2 and the intervening coating layer 8 that constitute the inside can be reduced, and it can be said that the propagation delay time difference between the common mode and the differential mode can be reduced. The skew characteristics were obtained from the transmission characteristics of 1 m of the cable using a network analyzer.

[0050] Furthermore, the result after bending the obtained differential transmission cable in the longitudinal direction (minor axis direction) was measured. In the bending test, a differential transmission cable with a minor diameter of 2.0 mm was wound around a cylinder with a diameter of 20 mm, and then the differential transmission cable was restored to its original state to check the skew characteristics. The skew of the differential transmission cables of Examples 1 and 2 in which the intervening coating layer 8 was uniformly coated over the entire void portion A and the opposing portion B changed only slightly from 5 ps per meter before the mechanical test to 7 ps per meter after the mechanical test. However, the differential transmission cable of Comparative Example 1 without the intervening coating layer 8 changed from 9 ps per meter before the mechanical test to 14 ps per meter after the mechanical test. From this result, by providing the intervening coating layer 8, distortion due to bending can be suppressed and the skew characteristics can be improved.

Explanation of Signs

[0051] 1 Center conductor 2 Insulator 2A Void portion 2B Inner annular portion 2C Outer annular portion 2D Connecting portion 3 Insulated wire 4 Two-core parallel insulated wire 5 Film-like external conductor (metal resin tape) 5A Adhesive layer 5B Metal layer 5C Base film 6 Cover conductor (braided shield or horizontally wound shield) 7 Outer covering 8 Intervening coating layer 8a Intervening coating layer filled in the void portion 8b Intervening coating layer covering the opposing portion 9 Intervening member 10 Differential transmission cable A Void portion B Opposing portion C Gap

Claims

1. A differential transmission cable having two parallel insulated wires each having a central conductor and an insulator provided on the outer periphery of the central conductor, a film-shaped outer conductor covering the two parallel insulated wires, a cover conductor covering the film-shaped outer conductor, and an outer jacket covering the cover conductor, wherein an intervening coating layer made of the same or a similar fluororesin as the insulator is provided in a gap portion between the two parallel insulated wires and the film-shaped outer conductor and in an opposing portion between the two insulated wires and the film-shaped outer conductor. A differential transmission cable characterized by this.

2. The differential transmission cable according to claim 1, wherein the intervening coating layer covers the opposing portion with a thickness of 0.1 mm or less.

3. The differential transmission cable according to claim 1 or 2, wherein the intervening coating layer is filled in the gap portion.

4. The differential transmission cable according to claim 1 or 2, wherein an adhesive layer is provided on a surface of the film-shaped outer conductor on the side of the two parallel insulated wires.

5. The differential transmission cable according to claim 1 or 2, wherein the insulator has a foamed structure or a hollow structure.

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