JP2011090959A - Differential signal harness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a differential signal harness having reduced skew and reduced EMI (electromagnetic interference) or crosstalk. <P>SOLUTION: The differential signal harness includes a signal conductor 2 for transferring differential signals, and connector parts 12 provided at both ends of the signal conductor 2. The connector part 12 has a common mode impedance-matching transfer line 19 for matching common mode impedance on the system side to be connected to the signal conductor 2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a differential signal harness used for transmitting a high-speed digital signal of several Gbps or more, and more particularly to a differential signal harness with small signal waveform deterioration.

  In servers, routers, storage products, and the like that handle high-speed digital signals of several Gbps or more, signal transmission using differential signals is used for signal transmission between electronic devices or between substrates in electronic devices. The electronic devices or the substrates in the electronic device are electrically connected by a differential signal harness having connectors at both ends of the differential signal cable.

  In signal transmission using a differential signal, two signals having inverted phases are used, and the difference between the two signals is synthesized and output on the receiving side. The differential signal cable includes two signal conductors (conductive wire and core wire) for transmitting two signals whose phases are inverted.

  In the differential signal harness, the currents flowing through the two signal conductors in the differential signal cable flow in opposite directions, so that electromagnetic waves radiated to the outside are reduced. Also, in the differential signal harness, noise received from the outside by the differential signal cable is equally superimposed on the two signal conductors, so by combining and outputting the difference between the two signals on the receiving side, The effects of noise can be counteracted. For these reasons, signal transmission using differential signals is suitable for high-speed digital signal transmission.

  As a differential signal cable used in a conventional differential signal harness, there is a twisted pair cable in which two insulated wires each having a signal conductor covered with an insulator are twisted and paired. Twisted pair cables are widely used for medium-distance signal transmission because they are inexpensive, excellent in balance, and easy to bend.

  However, since this twisted pair cable has no conductor corresponding to the ground, it is easily affected by the metal placed in the vicinity of the cable, and the characteristic impedance is not stable. For this reason, the twisted pair cable has a problem that the signal waveform tends to collapse in a high frequency region of several GHz. For these reasons, twisted pair cables are rarely used for transmission lines of several Gbps or more.

  On the other hand, as another differential signal cable used for the differential signal harness, there is a twinax cable in which two insulated wires are arranged in parallel without being twisted and covered with a shield conductor. A twinax cable has a smaller difference in physical length between two conductors than a twisted pair cable, and a shield conductor is provided so as to cover two insulated wires. The characteristic impedance does not become unstable and noise resistance is high. Therefore, the twinax cable is used for signal transmission at a relatively high speed and a short distance (several meters to several tens of meters).

  The shield conductor of the twinax cable includes a conductor using a tape with a conductor (metal foil tape), a conductor using a braided element wire, a conductor attached with a grounding drain wire, and the like.

  As an example, a cross-sectional view of the twinax cable disclosed in Patent Document 1 is shown in FIG.

  A twinax cable 121 shown in FIG. 12 is a shield made of a metal foil tape in which a metal foil such as aluminum is bonded to a polyethylene tape around two insulated wires 124 in which a signal conductor 122 is insulated by an insulator 123. A conductor 125 is wound or vertically attached, and a jacket 126 for protecting the inside of the cable is covered around the shield conductor 125.

  Between the shield conductor 125 and the insulated wire 124, the drain wire 127 is vertically attached so as to be in contact with the conductive surface (metal foil) of the shield conductor 125, and the drain wire 127 is grounded. ing.

  By the way, in order to transmit a high-speed signal of several Gbps or more, it is necessary to reduce a difference in propagation time of two signals in two signal conductors, that is, a skew. This is because when the skew increases, the waveform of the digital signal obtained by synthesizing and outputting the difference between the two signals on the receiving side is destroyed. For example, in high-speed signal transmission corresponding to 10 Gbps, signal quality is deteriorated even with a skew of several ps. Recently, since it is necessary to reduce EMI (Electromagnetic Interference), it is also required to reduce the amount of differential / in-phase conversion.

  In the twinax cable 131 shown in FIG. 13, two signal conductors 132 are covered at once with an insulator 133, and a shield conductor 134 made of a metal foil tape is wound around the insulator 133 or vertically attached. Further, a jacket 135 for protecting the inside of the cable is covered around the shield conductor 134 (Patent Document 2).

  In this twinax cable 131, two signal conductors 132 are collectively covered with one insulator 133, so that the dielectric constant difference of the insulator 133 is suppressed and the skew is reduced.

  In the twinax cable 141 shown in FIG. 14, the periphery of two insulated wires 144 in which the signal conductor 142 is covered with the insulator 143 is covered with a foaming agent tape 145, and the foaming agent tape 145 is made of a metal foil tape. A shield conductor 146 is covered, and a jacket 147 is covered around it. Between the foaming agent tape 145 and the shield conductor 146, a drain wire 148 is vertically attached so as to come into contact with the conductor surface (metal foil) of the shield conductor 146 (Patent Document 3).

  In this twinax cable 141, before covering the two insulated wires 144 with the shield conductor 146, the foaming agent tape 145 as an insulator is wound, and the distance between the signal conductor 142 and the shield conductor 146 is relatively set. By separating them, the electromagnetic coupling (electromagnetic coupling) of both signal conductors 142 is strengthened, and the skew is reduced.

  A twinax cable 151 shown in FIG. 15 has a shield conductor 155 made of metal foil tape wrapped around or vertically attached around two insulated wires 154 in which a signal conductor 152 is covered with an insulator 153 made of foam. Further, a jacket 156 is covered around the shield conductor 155 (Patent Document 4).

  In this twinax cable 151, when the insulator 153 is formed of a foam and the two insulated wires 154 are covered with the tape-shaped shield conductor 155, the insulator 153 is wound tightly so that it is slightly crushed. The distance between the conductors 152 is reduced. As a result, the electromagnetic coupling between both signal conductors 152 is strengthened, and the skew is reduced.

  By using such twinax cables 131, 141, 151 as differential signal cables of the differential signal harness, electromagnetic coupling between the signal conductors is strengthened, and propagating electromagnetic waves are confined between the signal conductors. And skew can be reduced.

JP 2002-289047 A JP 2001-35270 A JP 2007-26909 A US Pat. No. 5,283,390

  However, as the differential signal cable of the differential signal harness, as in the above-described twinax cables 131, 141, 151, the distance between the signal conductors is relatively narrowed to increase the electromagnetic coupling between the signal conductors. Although the use of the electromagnetic wave can confine propagating electromagnetic waves between the signal conductors and reduce the skew, there is a problem that unnecessary noise (EMI; electromagnetic interference) and crosstalk due to electromagnetic radiation increase.

  More specifically, the skew can be reduced by increasing the electromagnetic coupling between the signal conductors, but the effect of reducing the skew is that if the differential mode impedance (hereinafter referred to as differential mode impedance) is 100Ω, the common mode impedance is reduced. This appears when the impedance (hereinafter referred to as common mode impedance) is about 40Ω. In order to increase the differential mode impedance to 100Ω while strengthening the electromagnetic coupling between the signal conductors, the signal conductor must be made thinner if the cable outer diameter is not changed, and the common mode impedance is increased. Inevitable.

  On the other hand, in the transmission line on the system side, the characteristic impedance is determined by the microstrip line structure provided on the printed circuit board, and two 50Ω single-ended wirings are often arranged in parallel. In other words, the common-mode impedance on the system side is often 50/2 = 25Ω, and if the common-mode impedance of the differential signal harness increases, there will be a deviation from the common-mode impedance on the system side. Even if it is removed, the in-phase component becomes inconsistent, and a slight signal reflection occurs.

  Since most of the input transmission signals are in the differential mode, there is almost no influence on the waveform of the transmission signal, but the transmission signal contains only a part of the in-phase component. The in-phase component included in this transmission signal or the in-phase component generated during transmission causes partial reflection due to the mismatch of common mode impedance at the connection part of the differential signal harness. Problems such as crosstalk and EMI occur.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems, and in a differential signal harness used for high-speed transmission of several Gbps or more, a differential signal harness with reduced skew and reduced EMI and crosstalk. It is to provide.

  The present invention has been devised to achieve the above object, and includes at least a pair of signal conductors for transmitting differential signals, and connector portions provided at both ends of the signal conductors. In the differential signal harness, the connector portion is provided with a common mode impedance matching transmission line for matching the common mode impedance of the signal conductor and the common mode impedance of the system to be connected. It is a harness for motion signals.

  The common-mode impedance matching transmission line has an electrical length of ¼ wavelength of the fundamental frequency of the differential signal to be transmitted or a frequency that is an integral multiple of the fundamental frequency, and the differential mode impedance is the differential mode on the system side. It is preferable that the common mode impedance is equal to the impedance and is larger than the common mode impedance on the system side and smaller than the common mode impedance of the signal conductor.

  The signal conductor is composed of a pair of single wires or twisted metal wires arranged in parallel, and the periphery of the pair of metal wires is collectively covered with an insulator, and the periphery of the insulator is a shield conductor. A covered twinax cable is configured, and the connector portion is provided at both ends of the twinax cable and is electrically connected to the pair of metal wires, and the connection substrate The common mode impedance matching transmission line may be a microstrip line provided on the connection substrate of the connector portion.

  The drain line is disposed between the insulator and the shield conductor and vertically arranged in parallel with the pair of metal lines, and the drain line includes the shield conductor and the shield conductor. It may be electrically connected to the ground pattern of the connection substrate.

  The signal conductor comprises a signal wiring pattern provided on a flexible printed circuit board, and the common mode impedance matching transmission line comprises a microstrip line provided on the connector portion at both ends of the flexible printed circuit board. May be.

  A ground pattern may be provided on the surface of the flexible printed board opposite to the surface on which the signal wiring pattern is provided.

Common mode impedance of the common mode impedance matching transmission line, the common mode impedance of the system side Z _O, the common mode impedance of the signal conductors When Z _C, equal to the square root of Z _O · Z _C May be.

The common mode impedance matching transmission line is formed by connecting n divided common mode impedance matching transmission lines having different common mode impedances in series. The common mode impedance on the system side is Z _O , and the signal conductor side is When the common mode impedance and Z _C, common mode impedance of the k-th divided common mode impedance matching transmission line from the signal conductor side is equal to n + 1 th root of ^{_{Z O k · Z C n-}} k + 1 May be.

  According to the present invention, it is possible to provide a differential signal harness with reduced skew and reduced EMI and crosstalk.

It is a see-through | perspective perspective view of the connector part of the harness for differential signals which concerns on one embodiment of this invention. It is a perspective view which shows the whole differential signal harness of FIG. It is a figure which shows the twinax cable used for the harness for differential signals of FIG. 1, (a) is a cross-sectional view, (b) is a perspective view when mounting a twinax cable on a connection board | substrate. It is a figure explaining the impedance of the harness connection part in the harness for differential signals of FIG. In the differential signal harness of FIG. 1, it is a figure _showing the characteristic of the reflection loss _Scc11 in the common mode with respect to the frequency of the differential signal to transmit. It is a see-through | perspective perspective view of the connector part of the harness for differential signals which concerns on one embodiment of this invention. It is a figure explaining the impedance of the harness connection part in the harness for differential signals of FIG. In the differential signal harness of FIG. 6, it is a figure _showing the characteristic of the reflection loss _Scc11 in the common mode with respect to the frequency of the differential signal to transmit. 1 is a perspective view of a differential signal harness according to an embodiment of the present invention. FIG. 10 is an enlarged perspective view of a connector portion of the differential signal harness of FIG. 9. In this invention, it is a perspective view when the transmission line for common mode impedance matching is formed on the system side board | substrate. It is a cross-sectional view of a differential signal cable used in a conventional differential signal harness. It is a cross-sectional view of a differential signal cable used in a conventional differential signal harness. It is a cross-sectional view of a differential signal cable used in a conventional differential signal harness. It is a cross-sectional view of a differential signal cable used in a conventional differential signal harness.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a perspective view of a connector portion of a differential signal harness according to the present embodiment, and FIG. 2 is a perspective view showing the entire differential signal harness.

  As shown in FIGS. 1 and 2, the differential signal harness 1 includes a differential signal cable 11 for transmitting a differential signal, and connector portions 12 provided at both ends of the differential signal cable 11. With.

  Here, a case where a total of two twinax cables 13, one for transmission and one for reception, are used as the differential signal cable 11. Two twinax cables 13 are bundled, and a jacket 11a for protecting both the twinax cables 13 is provided around the two twinax cables 13. The number of twinax cables 13 is not limited to this, and may be one or three or more.

  As shown in FIG. 3 (a), the twinax cable 13 includes a pair of signal conductors 2 for transmitting differential signals arranged in parallel and the periphery of both signal conductors 2 in a lump. An insulator 3 having a predetermined dielectric constant to be covered, a shield conductor 4 provided on the outer periphery of the insulator 3, and a drain wire 5 for ground connection vertically provided between the insulator 3 and the shield conductor 4 And a jacket 6 for protecting the cable provided on the outer periphery of the shield conductor 4.

  As the signal conductor 2, a good electric conductor such as copper, or a single wire or a stranded metal wire obtained by plating the good electric conductor is used.

  The insulator 3 is formed by collectively covering both signal conductors 2 with an insulating resin supplied by an extruder. The insulator 3 is formed in a cross-sectional shape, that is, two substantially straight sides and these two sides. It is formed in a shape composed of two curved sides to be connected. Here, the insulator 3 is formed in an oval shape in sectional view, but may be formed in an elliptical shape in sectional view. Both signal conductors 2 are arranged in the center of the insulator 3 in the thickness direction (vertical direction in FIG. 3A).

  In this embodiment, since two twinax cables 13 for transmission and reception are used as one set, the width and thickness of the insulator 3 are made close to a circle when the two are combined. The ratio is preferably 2: 1. As the insulating resin used for the insulator 3, one having a low dielectric constant and dielectric loss tangent is desirable. For example, polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), polyethylene, or the like may be used. In order to reduce the dielectric constant and the dielectric loss tangent, a foamed insulating resin may be used for the insulator 3.

  A drain line 5 is vertically connected in parallel with both signal conductors 2 at one end (left end in FIG. 3A) of one side of the insulator 3 in the width direction (left and right in FIG. 3A). Arranged alongside. That is, the drain line 5 and both signal conductors 2 are arranged in a straight line along the width direction of the insulator 3. As the drain wire 5, similarly to the signal conductor 2, a good electric conductor such as copper or a single wire or a stranded metal wire obtained by plating the good electric conductor is used.

  As the shield conductor 4, a metal foil tape in which a metal foil such as aluminum is bonded to a polyethylene tape is used. The shield conductor 4 is not limited to this, and a shield conductor 4 may be used. The shield conductor 4 is wound around the insulator 3 and the drain wire 5 so that the conductor surface (metal foil) of the shield conductor 4 is in contact with the drain wire 5.

In the twinax cable 13, in order to reduce skew, the signal conductors 2 are arranged close to each other and the electromagnetic coupling between the signal conductors 2 is strengthened. Specifically, the twinax cable 13 has a common mode impedance Z _C of about 40Ω with respect to the differential mode impedance of 100Ω, and the skew is reduced to an average of about 2 ps per meter.

Returning to FIGS. 1 and 2, connector portions 12 are provided at both ends of the twinax cable 13, respectively. The connector part 12 is fitted and electrically connected to a receptacle 22 provided on the system-side board 21. The differential mode impedance on the system side (system side substrate 21 side) is 100Ω, and the common mode impedance Z _O on the system side is 25Ω.

  The connector portion 12 is provided at both ends of the twinax cable 13, and includes a connection substrate (built-in substrate) 14 electrically connected to the signal conductor 2, and a connector housing 15 that accommodates the connection substrate 14. Become. The connector unit 12 is composed of, for example, an SFP + transceiver (an optical module connector).

  The connection board 14 is made of a rigid board, for example, a glass epoxy board. A contact portion (electrode) 16 is formed at one end of the connection board 14 (on the left front side in FIG. 1), and is a card edge connector 17 that fits into a receptacle 22 provided on the system side board 21. A land 18 for solder-connecting the ends of the two twinax cables 13 is formed at the other end (right rear side in FIG. 1).

  As shown in FIGS. 1 and 3B, when the twinax cable 13 is mounted on the connection board 14, the jacket 6, the shield conductor 4, and the insulator 3 are sequentially stripped to form the twinax cable 13. In this state, the signal conductor 2 and the drain wire 5 are exposed at the terminal portion of the twinax cable 13, and the signal conductor 2 is placed on the signal electrode 23 of the land 18 and the drain wire 5 is placed on the land. According to 18 ground electrodes (ground pattern) 24, they are fixed by soldering. In the twinax cable 13, since the signal conductor 2 and the drain line 5 are arranged in a straight line, even if the distance between the signal conductors 2 is narrow, the drain conductor 5 does not disturb the signal conductor. 2 and the drain wire 5 can be soldered to the land 18 as they are.

  As described above, in the differential signal harness 1, the differential mode impedance of the twinax cable 13 is set to 100Ω, which is the same as the differential mode impedance on the system side, so there is no mismatch in the differential component. It is in a state.

However, the common mode impedance Z _C of the twinax cable 13 is 40Ω, whereas the common mode impedance Z _O on the system side is 25Ω. For this reason, when the twinax cable 13 is directly connected to the system-side board 21, the reflection loss _Scc11 in the common mode is about −12 dB, which causes a standing wave of the in-phase component or EMI from the cable sheath.

Therefore, in the differential signal harness 1 according to the present embodiment, the connector portion 12 includes the common mode impedance (common mode impedance on the signal conductor 2 side) Z _{C of the} twinax cable 13 and the system side to be connected. forming a common mode impedance matching transmission line 19 for matching the common mode impedance Z _O of.

  The common mode impedance matching transmission line 19 is formed of a microstrip line formed on the connection substrate 14 of the connector portion 12 so that the differential and in-phase components are not mismatched between the system side and the twinax cable 13 side. The characteristic impedance and electrical length are set.

  Specifically, the common-mode impedance matching transmission line 19 has an electrical length of ¼ wavelength of the fundamental frequency of the differential signal to be transmitted or ¼ wavelength of an integral multiple of the fundamental frequency. For example, if the transmission rate of the differential signal to be transmitted is 10 Gbps, the fundamental frequency is 5 GHz, so the quarter wavelength of the fundamental frequency is 15 mm. Since the length of 15 mm, which is a quarter wavelength of the fundamental frequency, is reduced to about 10 mm on a general glass epoxy substrate, for example, the connection substrate 14 used for an SFP + transceiver or the like is sufficient. It is possible to fit.

  Further, the common mode impedance matching transmission line 19 has a differential mode impedance equal to the differential mode impedance (100Ω) on the system side.

Further, the common mode impedance matching transmission line 19 has a common mode impedance Z _L larger than the common mode impedance Z _O on the system side and smaller than the common mode impedance Z _C of the twinax cable 13.

More specifically, as shown in FIG. 4, the twinax cable 13 of the common mode impedance Z _C, when the common mode impedance matching transmission line 19 of the common mode impedance Z _L to electrical length l connected, viewed from the system The common mode impedance Z (l) is expressed by the equation (1) shown in [Equation 1].

It is represented by

In equation (1), if the electrical length l is a quarter wavelength of the fundamental frequency (or a frequency that is an integer multiple thereof), tan βl = ∞, and equation (1) is expressed by the following equation (2)
Z (l) = Z _L ² / Z _C (2)
It is represented by

If Z _L ² = Z _O · Z _C is selected from the equation (2), the common mode impedance viewed from the system side, that is, the common mode impedance Z (l) of the differential signal harness 1 is set to the common mode impedance on the system side. Can match Z _O. That is, by setting the common mode impedance Z _L of the common mode impedance matching transmission line 19 to be equal to the square root of Z _O · Z _C , the common mode impedance can be matched.

In the present embodiment, since Z _O = 25Ω and Z _C = 40Ω, the common mode impedance Z _L of the transmission line 19 for common mode impedance matching is expressed by the following equation (3):

Set to

FIG. 5 shows the calculation result of the reflection loss S _cc11 in the common mode when Z _L = 31.5Ω.

As shown in FIG. 5, the reflection loss S _cc11 in the common mode is greatly reduced in the vicinity of the fundamental frequency of 5 GHz, and the bandwidth is about 700 MHz. That is, by forming the transmission line 19 for matching the common mode impedance, the common mode impedance of the differential signal harness 1 near the fundamental frequency of the differential signal to be transmitted is reduced, and the common mode impedance near the fundamental frequency of the differential signal is reduced. It becomes possible to match the components.

As described above, in the differential signal harness 1 according to the present embodiment, the connector portion 12 has the common mode impedance Z _C of the twinax cable 13 and the common mode impedance Z _O on the system side to be connected. A common-mode impedance matching transmission line 19 for matching is formed, and the electrical length of the common-mode impedance matching transmission line 19 is ¼ of the fundamental frequency of the differential signal to be transmitted or a frequency that is an integral multiple of the fundamental frequency. The differential mode impedance is equal to the differential mode impedance on the system side, and the common mode impedance Z _L is larger than the common mode impedance Z _O on the system side and smaller than the common mode impedance Z _C of the twinax cable 13 to the wavelength. Yes.

  Conventionally, when the interval between the signal conductors 2 is narrowed to reduce the skew, the common mode impedance is inevitably increased, and countermeasures for reflection of in-phase components, EMI, and crosstalk must be separately implemented.

  On the other hand, in the differential signal harness 1, even when the type in which the electromagnetic coupling between the signal conductors 2 is strong is selected for the differential signal cable 11 in order to reduce the skew, the common mode impedance matching is performed. The transmission line 19 reduces the common mode impedance in the vicinity of the fundamental frequency of the differential signal to be transmitted, so that the common mode impedance can be matched between the system side and the differential signal harness 1. In-phase component reflection and the like are reduced, and EMI, crosstalk, and the like can be reduced. Since the skew is small and EMI, crosstalk, and the like can be reduced, high-speed signals of several Gbps or more can be transmitted between electronic devices or within electronic devices, which contributes to improved performance of electronic devices.

  In the differential signal harness 1, since the twinax cable 13 in which the insulator 3 is collectively covered by extrusion molding around the signal conductor 2 is used as the differential signal cable 11. Variations can be reduced and variations in characteristic impedance can be suppressed. In the twinax cable 13, the strength of electromagnetic coupling of the signal conductor 2 can be easily adjusted by changing the interval between the signal conductors 2 at the time of extrusion molding. This eliminates the need for a difficult manufacturing method such as tightly wrapping a tape-shaped shield conductor and crushing the insulator, thereby enabling stable production.

  In addition, since the differential signal harness 1 uses the twinax cable 13 in which the drain line 5 is disposed beside the signal conductor 2, even if the interval between the signal conductors 2 is narrow, the connection to the connection substrate 14. Mounting becomes easy, and furthermore, since the stripping length of the shield conductor 4 can be small, the deterioration of electrical characteristics at the mounting portion is also small.

  Next, another embodiment of the present invention will be described.

  The differential signal harness 61 shown in FIG. 6 is different from the differential signal harness 1 of FIG. 1 in that the common mode impedance matching transmission line 19 is divided into two divided common mode impedance matching transmission lines 19a, 19b is connected in series.

  That is, in the differential signal harness 61, the common mode impedance matching transmission line 19 is divided into the split common mode impedance matching transmission line 19a on the twinax cable 13 side (signal conductor 2 side) and the divided common mode on the system side. It is divided into the impedance matching transmission line 19b, and the common mode impedance is changed in two stages in both divided common mode impedance matching transmission lines 19a and 19b.

  The electrical lengths of the divided common mode impedance matching transmission lines 19a and 19b are set to ¼ wavelength of the fundamental frequency of the differential signal transmitted together or an integral multiple of the fundamental frequency. If the transmission rate of the differential signal to be transmitted is 10 Gbps, the fundamental frequency is 5 GHz, so the quarter wavelength of the fundamental frequency is 15 mm. The total length of the transmission lines 19a and 19b for both divided common mode impedance matching is shortened to about 10 mm × 2 = 20 mm on the glass epoxy substrate. For example, the connection substrate 14 is used for an SFP + transceiver and the like. Can also be accommodated.

As shown in FIG. 7, the common mode impedance Z _A of the split common mode impedance matching transmission line 19 a on the twinax cable 13 side is set equal to the third root of Z _O , Z _C, and Z _C. Further, the common mode impedance Z _B of the transmission line 19b for split common mode impedance matching on the system side is set equal to the third root of Z _O , Z _O , Z _C.

Here, since Z _O = 25Ω and Z _C = 40Ω, the common mode impedance Z _A of the split common mode impedance matching transmission line 19a on the twinax cable 13 side is expressed by the following equation (4): The common mode impedance Z _B of the transmission line 19b for dividing common mode impedance matching on the system side is set to Expression (5) shown in [Equation 4].

By setting the common mode impedances Z _A and Z _B of the divided common mode impedance matching transmission lines 19a and 19b as in the equations (4) and (5), the same as the differential signal harness 1 of FIG. The common mode impedance mismatch is eliminated. FIG. 8 shows the calculation result of the reflection loss _Scc11 in the common mode at this time.

As shown by the solid line in FIG. 8, the reflection loss _Scc11 in the common mode of the differential signal harness 61 is greatly reduced near the fundamental frequency of 5 GHz, and the bandwidth is about 2.6 GHz.

  As shown by a broken line in FIG. 8, the differential signal harness 1 in FIG. 1 has a narrow bandwidth of about 700 MHz in which the common mode is matched. According to the differential signal harness 61 provided with 19a, 19b, it is possible to secure a bandwidth about four times that of the differential signal harness 1 of FIG. Crosstalk and the like can be reduced.

Here, the case where the two-stage divided common mode impedance matching transmission lines 19a and 19b are provided has been described. However, the present invention is not limited thereto, and three or more divided common mode impedance matching transmission lines are provided. Also good. For example, when an n-stage divided common mode impedance matching transmission line is provided, the common impedance Z _k of the _kth divided common mode impedance matching transmission line from the cable side is set to the equation (6) shown in Equation 5. That's fine.

  A differential signal harness 91 shown in FIG. 9 and FIG. 10 forms a signal wiring pattern 93 as a signal conductor for transmitting a differential signal on a flexible printed circuit board (FPC) 92, and contact portions at both ends thereof. A common mode impedance matching transmission line 95 made of a microstrip line is formed on a connector portion 94 formed with an (electrode) 94a. Here, a region from the both ends of the flexible printed circuit board 92 to the connection portion between the common mode impedance matching transmission line 95 and the signal wiring pattern 93 is referred to as a connector portion 94.

  As the flexible printed circuit board 92, in order to control the characteristic impedance of the signal wiring pattern 93, it is necessary to form a ground surface (ground pattern) on the surface opposite to the surface on which the signal wiring pattern 93 is formed. Select the double-sided type (type that can form wiring patterns on both the front and back sides).

A ground pattern (not shown) is formed on the surface of the flexible printed board 92 opposite to the surface on which the signal wiring pattern 93 is formed. Thereby, the differential mode impedance of the signal wiring pattern 93 is adjusted to about 100Ω, and the common mode impedance Z _C is adjusted to about 40Ω, and the electromagnetic coupling between the signal wiring patterns 93 is strengthened. It is assumed that the differential mode impedance on the system side is 100Ω and the common mode impedance Z _O is 25Ω.

  The differential mode impedance of the transmission line 95 for common mode impedance matching is set to 100Ω, which is the same as that on the system side, and there is no mismatch in the differential component.

The common-mode impedance matching transmission line 95 is connected to the signal wiring pattern 93 (Z _C = 40Ω) with enhanced electromagnetic coupling and the system side (Z _O = 25Ω) so that they do not become mismatched. The electrical length and the common mode impedance Z _L are set.

Specifically, as in the differential signal harness 1 of FIG. 1, the electrical length of the common mode impedance matching transmission line 95 is ¼ wavelength of the fundamental frequency of the differential signal to be transmitted or an integer multiple of the fundamental frequency. The common mode impedance Z _L is set to be equal to the square root of Z _O · Z _C (Z _L = 31.5Ω).

  According to the differential signal harness 91, the common mode impedance matching transmission line 95 can be integrated and incorporated in the flexible printed circuit board 92 on which the signal wiring pattern 93 is formed. Cost can be reduced. Further, even if the signal wiring pattern 93 having strong electromagnetic coupling is selected, the common mode impedance mismatch near the fundamental frequency can be reduced, so that the reflection of the in-phase component is reduced, and the influence is generated. EMI and crosstalk can be reduced.

  In the above embodiment, the common mode impedance matching transmission line is formed in the connector portion. However, for example, when only a transmission line having a constant impedance such as a flexible flat cable can be produced, the flexible flat cable is formed as shown in FIG. Even if the common mode impedance matching transmission line 111 is formed not on the cable 112 side but on the system side substrate 21, the same effect can be obtained.

  As described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 Differential signal harness 2 Signal conductor 3 Insulator 4 Shield conductor 5 Drain wire 6 Jacket 11 Differential signal cable 12 Connector part 13 Twinax cable 14 Connection board 15 Connector housing 19 Common mode impedance matching transmission line

Claims (8)

In a differential signal harness comprising at least a pair of signal conductors for transmitting differential signals, and connector portions provided at both ends of the signal conductors,
A differential signal characterized in that the connector section is provided with a common mode impedance matching transmission line for matching the common mode impedance of the signal conductor and the common mode impedance of the system to be connected. Harness for. The common mode impedance matching transmission line is:
The electrical length is a quarter wavelength of the fundamental frequency of the differential signal to be transmitted or an integer multiple of the fundamental frequency,
The differential mode impedance is equal to the differential mode impedance on the system side,
The differential signal harness according to claim 1, wherein the common mode impedance is larger than the common mode impedance on the system side and smaller than the common mode impedance of the signal conductor. The signal conductor is composed of a pair of single wires or twisted metal wires arranged in parallel, and the periphery of the pair of metal wires is collectively covered with an insulator, and the periphery of the insulator is a shield conductor. Which constitutes a covered twinax cable,
The connector portion is provided at both ends of the twinax cable, and includes a connection board electrically connected to the pair of metal wires, and a connector housing that accommodates the connection board. The common mode The differential signal harness according to claim 1, wherein the impedance matching transmission line is a microstrip line provided on the connection substrate of the connector portion. The drain wire is disposed vertically between the insulator and the shield conductor, and the drain wire is disposed in parallel with the pair of metal wires,
The differential signal harness according to claim 3, wherein the drain line is electrically connected to the shield conductor and a ground pattern of the connection substrate. The signal conductor comprises a signal wiring pattern provided on a flexible printed circuit board,
The differential signal harness according to claim 1, wherein the common mode impedance matching transmission line is formed of a microstrip line provided in the connector portion at both ends of the flexible printed board.   The differential signal harness according to claim 5, wherein a ground pattern is provided on a surface of the flexible printed board opposite to the surface on which the signal wiring pattern is provided. Common mode impedance of the common mode impedance matching transmission line, the common mode impedance of the system side Z _O, the common mode impedance of the signal conductors When Z _C, equal to the square root of Z _O · Z _C The harness for differential signals according to any one of claims 1 to 6. The common mode impedance matching transmission line is formed by connecting n divided common mode impedance matching transmission lines having different common mode impedances in series.
When the common mode impedance on the system side is Z _O and the common mode impedance on the signal conductor side is Z _C ,
Common mode impedance of the k-th divided common mode impedance matching transmission line from the signal conductor side, any one of claims 1 to 6 is equal to ^{_{Z O k · Z C n-}} k + 1 of the n + 1 th root The described differential signal harness.

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