Differential transmission cable and multipair differential transmission cable

Abstract

A differential transmission cable includes a pair of signal lines, an insulation covering the pair of signal lines, and a shielding tape that includes a conductor layer and an insulation layer formed on one surface of the conductor layer and is helically wound around the insulation. The diameter of the signal line is thinner than at least 30 AWG (American Wire Gauge), and differential characteristic impedance is not less than 80 and not more than 120.

Claims

1. A differential transmission cable, comprising: a pair of signal lines; an insulation covering the pair of signal lines; and a shielding tape that comprises a conductor layer and an insulation layer formed on one surface of the conductor layer and is helically wound around the insulation; wherein a diameter of each of the signal lines is not more than 34 AWG (American Wire Gauge), and a differential characteristic impedance of the signal lines is not less than 80 and not more than 120 , and wherein the insulation is formed in an ellipse or racetrack shape in cross-section such that a major axis thereof coincides with an alignment direction of the signal lines and the centers of the major and minor axes coincide with the center point of a line segment connecting between the centers of the signal lines, a major-axis length of the insulation is not more than at least 1.5 mm, and a minor-axis length of the insulation is not more than at least 0.8 mm.

2. The differential transmission cable according to claim 1, wherein Scd21 as an S-parameter indicating mode conversion is not more than 24 dB at least in a frequency range of not more than 12.5 GHz.

3. The differential transmission cable according to claim 1, wherein the insulation layer comprises a polyethylene terephthalate and the conductor layer comprises a copper.

4. A multipair differential transmission cable, comprising a plurality of the differential transmission cables according to claim 1, wherein the plurality of the differential transmission cables are collectively shielded.

5. A differential transmission cable, comprising: a pair of signal lines; an insulation covering the pair of signal lines; and a shielding tape that comprises a conductor layer and an insulation layer formed on one surface of the conductor layer and is helically wound around the insulation; wherein a diameter of each of the signal lines is thinner than at least 30 AWG (American Wire Gauge), and a differential characteristic impedance of the signal lines is not less than 80 and not more than 120 , wherein Scd21 as an S-parameter indicating mode conversion is not more than 24 dB at least in a frequency range of not more than 12.5 GHz.

6. The differential transmission cable according to claim 5, wherein the insulation layer comprises a polyethylene terephthalate and the conductor layer comprises a copper.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Next, the present invention will be explained in more detail in conjunction with appended drawings, wherein:

(2) FIG. 1 is a schematic cross sectional view showing a configuration example of a multipair differential transmission cable in an embodiment of the present invention;

(3) FIG. 2 is a schematic perspective view showing a configuration example of a differential transmission cable in the embodiment of the invention;

(4) FIG. 3 is a cross sectional view showing a shielding tape;

(5) FIG. 4 is a graph showing insulation diameter corresponding to a differential characteristic impedance of 100 respectively when using 30 AWG, 32 AWG and 34 AWG signal lines;

(6) FIGS. 5A to 5C are graphs showing S-parameter measurement results in the embodiment of the invention using 34 AWG signal lines, wherein FIG. 5A shows the measurement result of differential-mode loss Sdd21, FIG. 5B shows the measurement result of mode conversion Scd21 and FIG. 5C shows the measurement result of common-mode loss Scc21;

(7) FIGS. 6A to 6C are graphs showing S-parameter measurement results in Comparative Example using 30 AWG signal lines, wherein FIG. 6A shows the measurement result of differential-mode loss Sdd21, FIG. 6B shows the measurement result of mode conversion Scd21 and FIG. 6C shows the measurement result of common-mode loss Scc21;

(8) FIGS. 7A to 7C are graphs showing S-parameter measurement results in Comparative Example using 34 AWG signal lines and adopting the longitudinal wrapping-type shielding method, wherein FIG. 7A shows the measurement result of differential-mode loss Sdd21, FIG. 7B shows the measurement result of mode conversion Scd21 and FIG. 7C shows the measurement result of common-mode loss Scc21; and

(9) FIGS. 8A to 8C are graphs showing S-parameter measurement results in Comparative Example using 30 AWG signal lines and adopting the longitudinal wrapping-type shielding method, wherein FIG. 8A shows the measurement result of differential-mode loss Sdd21, FIG. 8B shows the measurement result of mode conversion Scd21 and FIG. 8C shows the measurement result of common-mode loss Scc21.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment

(10) An embodiment of the invention will be described below in conjunction with the appended drawings.

(11) FIG. 1 is a schematic cross sectional view showing a configuration example of a multipair differential transmission cable in the present embodiment.

(12) A multipair differential transmission cable 50 has plural bundled differential transmission cables 10, a shielding tape 52 wound around the plural differential transmission cables 10, a braided wire 53 covering the shielding tape 52, and a jacket 54 covering the braided wire 53. The plural differential transmission cables 10 are shielded together by the shielding tape 52 and the braided wire 53.

(13) The number of the differential transmission cables 10 is eight in the example shown in FIG. 1 but is not specifically limited, and may be, e.g., two, eight, or twenty-four, etc. In the example shown in FIG. 1, two differential transmission cables 10 are arranged in the center in the cross section of the multipair differential transmission cable 50, and six differential transmission cables 10 are arranged at substantially equal intervals around the two differential transmission cables 10 via a filler 51.

(14) Materials used to form general cables can be used to form the shielding tape 52, the braided wire 53 and the jacket 54. The filler 51 is formed of, e.g., paper, yarn or foam. The foam is, e.g., polyolefin foam such as polypropylene foam or ethylene foam.

(15) FIG. 2 is a schematic perspective view showing a configuration example of the differential transmission cable 10 in the embodiment of the invention.

(16) The differential transmission cable 10 has a pair of signal lines 11, an insulation 12 covering the pair of signal lines 11, a shielding tape 13 helically wound around the insulation 12, and an outer-layer tape 15 helically wound around the shielding tape 13 and covering the shielding tape 13.

(17) The pair of signal lines 11 are conductor wires formed of copper, etc., and transmit differential signals. The pair of signal lines 11 are covered with a single insulation 12. In other words, the differential transmission cable 10 in the present embodiment has the two cores-in-one insulation structure.

(18) The insulation 12 is formed in an ellipse shape or a racetrack shape (a shape formed of two parallel straight lines of the same length and semi-circular arcs connecting ends of the two straight lines, a rounded rectangular shape) in cross-section such that a major axis thereof coincides with an alignment direction of the signal lines 11 and the centers of the major and minor axes coincide with the center point of a line segment connecting between the centers of the signal lines 11. In this example, the insulation 12 is formed in an ellipse shape.

(19) The insulation 12 is formed of an insulating material such as polyethylene, polytetrafluoroethylene (PTFE) or tetrafluoroethylene-hexafluoropropylene copolymer (FEP), etc. A foamed insulating material such as polyethylene foam can be also used as the insulation 12. The insulations 12 having a permittivity of about 1.5 to 3 can be used.

(20) FIG. 3 is a cross sectional view showing the shielding tape 13. The shielding tape 13 has a strip-shaped conductor layer 13a and an insulation layer 13b formed on one surface of the conductor layer 13a. The conductor layer 13a can be formed of a strip-shaped conductive metal foil such as copper foil or aluminum foil. The insulation layer 13b can be formed of an insulating resin such as PET (polyethylene terephthalate). Alternatively, the insulation layer 13b may be an oxide film formed by oxidizing one surface of the strip-shaped conductive metal foil such as copper foil or aluminum foil which is provided as the conductor layer 13a. The shielding tape 13 used in this example is a copper-PET tape in which the insulation layer 13b formed of PET is provided on one surface of the conductor layer 13a formed of copper.

(21) The shielding tape 13, with the insulation layer 13b facing in and the conductor layer 13a facing out, is wound around the insulation 12 to facilitate connection of the conductor layer 13a to a ground of a circuit board at the time of connecting the differential transmission cable 10 to a connector, etc., provided on the circuit board.

(22) The outer-layer tape 15 is formed of a strip-shaped flexible member and has a laminated structure composed of, e.g., a flexible insulating resin layer such as PET and an adhesive layer containing an adhesive agent. The outer-layer tape 15, with the adhesive layer facing in and the resin layer facing out, is helically wound around the shielding tape 13. It is possible to prevent separation of the shielding tape 13 from the insulation 12 by winding the outer-layer tape 15.

(23) In the differential transmission cable 10 in the present embodiment, the diameter of the signal line 11 is thinner than at least 30 AWG (American Wire Gauge) and differential characteristic impedance is not less than 80 and not more than 120, and is preferably 100.

(24) The size of the insulation 12 in the major and minor axis directions is adjusted according to the diameter of the signal line 11 such that differential characteristic impedance is about 100 (10020). FIG. 4 shows the size of the insulation 12 in the major and minor axis directions corresponding to a differential characteristic impedance of 100 respectively when using 30 AWG, 32 AWG and 34 AWG signal lines 11.

(25) As a result of intense study, the present inventors found that, when the differential characteristic impedance is 100 (10020) in case of using the spiral wrapping-type shielding method, a frequency at which differential-mode suck-out occurs increases with a decrease in the diameter of the signal line 11. After further study, it was found that the diameter of the signal line 11 needs to be thinner than 30 AWG (in other words, the gauge number of the signal line 11 needs to be larger than 30) in order to provide sufficient differential mode bandwidth also in high-speed transmission at not less than 25 Gb/s.

(26) To reliably suppress differential-mode loss due to differential-mode suck-out in high-speed transmission at not less than 25 Gb/s, the diameter of the signal line 11 is desirably not larger than 34 AWG. In the present embodiment, a 34 AWG wire is used as the signal line 11. When the diameter of the signal line 11 is not larger than 34 AWG, the insulation 12 is formed such that the major-axis length is not more than at least 1.5 mm and the minor-axis length is not more than at least 0.8 mm (see FIG. 4).

(27) In the present embodiment, the spiral wrapping-type shielding method is adopted. Therefore, it is possible to reduce mode conversion as compared to when adopting the longitudinal wrapping-type shielding method.

(28) In general, mode conversion increases with an increase in a difference in capacitance between the two signal line 11 and the ground. Therefore, it is possible to reduce mode conversion by providing the two cores-in-one insulation structure in which the two signal line 11 are covered together with the single insulation 12.

(29) In the longitudinal wrapping-type shielding method, a small gap is likely to be formed between the insulation and the shielding tape in the production process especially when the diameter is small, which is considered to be a cause of an increase in a difference in capacitance between the two signal lines 11 and the ground and a resulting increase in mode conversion.

(30) In contrast, in the spiral wrapping-type shielding method, since the insulation layer 13b is sandwiched between turns of the conductor layer 13a in the wound shielding tape 13, it is considered that capacitance occurs and is inserted in series between the capacitance of the two signal lines 11 and the capacitance of the ground, an effective capacitance difference between the two signal lines 11 and the ground decreases and mode conversion is thus reduced.

(31) Furthermore, it is considered that use of the spiral wrapping-type shielding method causes a suck-out to occur in common-mode (referred to as common-mode suck-out) as well as in differential-mode, common-mode signals attenuate due to the common-mode suck-out, and as a result, mode conversion is reduced.

(32) As a result of intense study, the present inventors found that a frequency at which common-mode suck-out occurs is relatively low (a frequency of not more than 12.5 GHz) even when the diameter of the signal line 11 is reduced to 34 AWG or smaller, mode conversion Scd21 is thereby stably reduced to not more than 24 dB and the noise power a can be reduced to a level comparable to or less than the conventional level even in high-speed transmission at not less than 25 Gb/s. As such, in the differential transmission cable 10 in the present embodiment, Scd21 as an S-parameter indicating mode conversion is not more than 24 dB at least in a frequency range of not more than 12.5 GHz which is fundamental frequency in transmission at 25 Gb/s, and the differential transmission cable 10 is suitable for use in high-speed transmission at not less than 25 Gb/s.

(33) FIGS. 5A to 5C show the measurement results of differential-mode loss Sdd21, mode conversion Scd21 and common-mode loss Scc21 when using 34 AWG signal lines 11. A network analyzer (Agilent N5245A, measurement bandwidth of 10 MHz to 50 GHz) was used to measure each S parameter.

(34) In the present embodiment, a frequency at which differential-mode suck-out occurs is not less than 20 GHz as shown in FIG. 5A and sufficient differential mode bandwidth is provided in 25 Gb/s transmission having a fundamental frequency of 12.5 GHz.

(35) In addition, the mode conversion Scd21 is almost entirely as very small as not more than 40 dB in the measurement frequency range as shown in FIG. 5B and the mode conversion Scd21 is stably reduced to not more than 24 dB. In the present embodiment, common-mode suck-out occurs in a frequency range of not more than 10 GHz as shown in FIG. 5C and it is considered that this results in common-mode signal attenuation and reduction in the mode conversion Scd21.

(36) FIGS. 6A to 6C show the measurement results of differential-mode loss Sdd21, mode conversion Scd21 and common-mode loss Scc21 when using 30 AWG signal lines 11 for the purpose of comparison.

(37) In this case, as shown in FIGS. 6A to 6C, while the mode conversion Scd21 is reduced by adopting the spiral wrapping-type shielding method, differential-mode suck-out occurs at a frequency of 12 to 14 GHz since the thick signal lines 11 with a size of 30 AWG are used, and sufficient differential mode bandwidth is not provided in 25 Gb/s transmission having a fundamental frequency of 12.5 GHz.

(38) FIGS. 7A to 7C and FIGS. 8A to 8C show the measurement results of differential-mode loss Sdd21, mode conversion Scd21 and common-mode loss Scc21 when adopting the longitudinal wrapping-type shielding method and using 34 AWG and 30 AWG signal lines 11 for the purpose of further comparison.

(39) As shown in FIGS. 7A to 7C and FIGS. 8A to 8C, when adopting the longitudinal wrapping-type shielding method, differential-mode suck-out does not occur but the mode conversion Scd21 is very large since common-mode suck-out also does not occur. Therefore, it is difficult to stably reduce the mode conversion Scd21 to not more than 24 dB.

(40) Differential mode bandwidth and the amount of mode conversion were evaluated for the examples in FIGS. 5A to 8C. Using the formula (3) shown below, differential-mode loss within the transmission band per unit length of cable, Sdd2112.5 GHz, was defined as a parameter for evaluating the differential mode bandwidth. Sdd2112.5 GHz is very small when the differential mode bandwidth is narrower than 12.5 GHz which is fundamental frequency of 25 Gb/s transmission.

(41) $\begin{array}{cc}.Math.S_{\mathrm{dd}21}12.5\mathrm{GHz}.Math.\underset{f12.5\mathrm{GHz}}{\min }{S}_{\mathrm{dd}21}\left[\mathrm{dB}\right]\mathrm{Cable}\mathrm{length}\left[m\right]& \left(3\right)\end{array}$

(42) Also, as a parameter for evaluating the amount of mode conversion, the maximum value of the common mode, Scd2112.5 GHz, was defined using the formula (4) and noise power (the integrated value of mode conversion), 12.5 GHz, was defined using the formula (5).

(43) $\begin{array}{cc}.Math.S_{\mathrm{cd}21}12.5\mathrm{GHz}.Math.\underset{f12.5\mathrm{GHz}}{\max }{S}_{\mathrm{cd}21}\left[\mathrm{dB}\right]& \left(4\right)\\ .Math.12.5\mathrm{GHz}.Math.\sqrt{2\underset{k=1}{\overset{K}{.Math.}}{.Math.S_{\mathrm{cd}21}.Math.}^2.Math.f}& \left(5\right)\end{array}$

(44) The evaluation results of the differential mode bandwidth and the amount of mode conversion are summarized in Table 1. The cables used for evaluation all had a length of 2 m. For Sdd2112.5 GHz, not less than 8.0 dB/m was regarded as Pass () and less than 8.0 dB/m was regarded as Fail (x). For Scd2112.5 GHz, not more than 24 dB/m was regarded as Pass () and more than 24 dB/m was regarded as Fail (x). For 12.5 GHz, not more than 1.010.sup.4 was regarded as Pass () and more than 1.010.sup.4 was regarded as Fail (x).

(45) TABLE-US-00001 TABLE 1 Invention Comparative Examples Signal line 34 AWG 30 AWG 34 AWG 30 AWG Shielding method Spiral Spiral Longi- Longi- tudinal tudinal Differential-mode loss 7.4 12.1 7.0 5.0 within transmission band X (Sdd21 12.5 GHz) [dB/m] Maximum value of Mode 39.3 38.8 18.5 15.0 conversion X X (Scd21 12.5 GHz) [dB] Noise power 0.16 10.sup.4 0.18 10.sup.4 13.1 10.sup.4 2.34 10.sup.4 ( 12.5 GHz) X X

(46) As shown in Table 1, in the present embodiment which adopts the spiral wrapping-type shielding method and uses the 34 AWG signal lines 11, the cable passed all evaluations of Sdd2112.5 GHz, Scd2112.5 GHz and 12.5 GHz, which shows that it is possible to provide sufficient differential mode bandwidth in 25 Gb/s transmission and also to reduce mode conversion noise.

(47) On the other hand, when using the thick signal lines 11 with a size of 30 AWG, the cable failed the evaluation of Sdd2112.5 GHz, which shows that sufficient differential mode bandwidth is not provided in 25 Gb/s transmission. Meanwhile, when adopting the longitudinal wrapping-type shielding method, the cables failed the evaluations of Scd2112.5 GHz and 12.5 GHz, which shows that mode conversion noise is high.

(48) Functions and Effects of the Embodiment

(49) As described above, the differential transmission cable 10 in the present embodiment is provided with the pair of signal lines 11, the insulation 12 covering the pair of signal lines 11 and the shielding tape 13 having the conductor layer 13a and the insulation layer 13b formed on one surface of the conductor layer 13a and helically wound around the insulation 12, and is configured that the diameter of the signal line 11 is thinner than at least 30 AWG (American Wire Gauge) and differential characteristic impedance is not less than 80 and not more than 120.

(50) Due to such a configuration, it is possible to realize the differential transmission cable 10 which generates only small mode conversion noise, is capable of providing sufficient differential mode bandwidth, and is thereby compatible with high-speed transmission at not less than 25 Gb/s.

(51) The differential transmission cable 10 in the present embodiment has a small diameter and is suitable for, e.g., interconnection between circuit boards in a device.

Summary of the Embodiment

(52) Technical ideas understood from the above described embodiment will be described below citing the reference numerals, etc., used for the embodiment. However, each reference numeral, etc., described below is not intended to limit the constituent elements in the claims to the members, etc., specifically described in the embodiment.

(53) [1] A differential transmission cable (10), comprising: a pair of signal lines (11); an insulation (12) covering the pair of signal lines (11); and a shielding tape (13) that comprises a conductor layer (13a) and an insulation layer (13b) formed on one surface of the conductor layer (13a) and is helically wound around the insulation (12); wherein the diameter of the signal line (11) is thinner than at least 30 AWG (American Wire Gauge), and differential characteristic impedance is not less than 80 and not more than 120.

(54) [2] The differential transmission cable (10) defined by [1], wherein the diameter of the signal line (11) is 34 AWG (American Wire Gauge) or smaller.

(55) [3] The differential transmission cable (10) defined by [2], wherein the insulation (12) is formed in an ellipse or racetrack shape in cross-section such that a major axis thereof coincides with an alignment direction of the signal lines (11) and the centers of the major and minor axes coincide with the center point of a line segment connecting between the centers of the signal lines (11), a major-axis length of the insulation (12) is not more than at least 1.5 mm, and a minor-axis length of the insulation (12) is not more than at least 0.8 mm.

(56) [4] The differential transmission cable (10) defined by any one of [1] to [3], wherein Scd21 as an S-parameter indicating mode conversion is not more than 24 dB at least in a frequency range of not more than 12.5 GHz.

(57) [5] The differential transmission cable (10) defined by any one of [1] to [4], wherein the shielding tape (13) is formed by providing the insulation layer (13b) comprising polyethylene terephthalate on one surface of the conductor layer (13a) comprising copper.

(58) [6] A multipair differential transmission cable (50), comprising: a plurality of the differential transmission cables (10) defined by any one of [1] to [5], wherein the plurality of the differential transmission cables (10) are collectively shielded.

(59) Although the embodiment of the invention has been described, the invention according to claims is not to be limited to the embodiment. Further, please note that all combinations of the features described in the embodiment are not necessary to solve the problem of the invention.

(60) The invention can be appropriately modified and implemented without departing from the gist thereof.