JP4292274B2 - Circuit for small amplitude operation signal - Google Patents

Description

The present invention relates to a transmission cable for high-speed communication, and more particularly to a structure of a communication line cable that bundles a plurality of coaxial lines and transmits individual communication channels to the respective coaxial lines.

Conventionally, single-ended signals that have a logical amplitude based on the power supply voltage have been used for high-speed signal cable transmission. However, in response to the recent increase in drive frequency and bus width associated with the demand for high-speed data transfer, radiation noise From the viewpoint of suppression and immunity to external noise, small-amplitude differential signals (
A cable transmission method using LVDS Low Voltage Differential Signaling has come to be used.

FIG. 5 is a block diagram of a conventional LVDS interface, which shows a transmission side IC 23 and a reception side I.
Between C24, the transmission line 25 and the transmission line 2 having an odd mode impedance Zodd of 50Ω.
6, the transmission lines 25 and 26 are terminated with a 100Ω resistor 27 at the input end of the receiving IC. The transmission side IC 23 has a specification that drives a current of about 3.5 mA and generates a voltage of about 350 mV across the 100Ω termination resistor 27. In addition, the transmission line 25 and the transmission line 26 have the same electrical characteristics and form a so-called balanced transmission line. In the LVDS, one signal is transmitted through these two transmission lines. Sending side IC2
3 generates a differential signal that generates a potential difference between the transmission line 25 and the transmission line 26 based on the input signal from the input terminal 21. On the other hand, the receiving side IC 24 has about 350 mV generated at both ends of the 100Ω termination resistor 27 coupled to the receiving end between the transmission line 25 and the transmission line 26.
The differential signal is received, converted into a CMOS level voltage, and this is output from the output terminal 22.

The principle of LVDS is that the signal current Is generated in the transmission side IC 23 is caused to flow through a loop formed by the transmission line 25 and the balanced transmission path of the transmission line 26 and the termination resistor 27 disposed in the vicinity of the reception side IC 24. A signal voltage is generated in the portion of the resistor 27 to transmit the signal. Digital signals “0” and “1” are identified by switching the direction in which the signal current Is flows. The signal current Is flowing through the transmission line 25 and the transmission line 26 has the same magnitude,
Since the directions are reversed, the magnetic fields generated by the currents flowing through each other cancel each other out, resulting in the effect of suppressing the generation of radiation noise and crosstalk noise. In addition, even with respect to external noise, if the influence is relatively the same between the transmission path 25 and the transmission path 26, the noise resistance is excellent in that the signal logic is not affected.

Conventionally, twisted pair lines or parallel two-wire shielded cables have been used as the transmission lines. However, as a result of the experiment, as shown in Table 1, the conventional twisted pair line or the parallel two-wire shielded cable has a large attenuation and a large amount of noise due to cross-talk between channels. It was found that it cannot be used as a signal transmission line.

In Table 1, UTP is an abbreviation for Unsealed Twist Pair, and Category 7 and Category 6 are cable types in international cable standards. For InfiniBand is a cable that bundles eight shielded balanced two wires. As shown in FIG. 4, the characteristics of these balanced two lines are poor, as shown in FIG. 4, the magnetic fields generated by the balanced two lines are different, that is, the centers of the lines are shifted from each other, so the magnetic fields generated by both lines do not match exactly. This is thought to be because the portions that are not canceled out remain.
JP 2002-354053 A

The present invention, the conventional twisted pair line or to remedy the drawbacks of the two parallel lines shielded cable, attenuation is small, noise due to crosstalk is small, the cable section is smaller by eliminating an insulating coating small amplitude differential signal circuit The purpose is to provide.

In order to solve this, the small-amplitude differential signal circuit of the present invention has a common-mode differential circuit in which a small-amplitude differential signal transmission circuit and a small-amplitude differential signal reception circuit are arranged on both ends. a plurality of circuit units forming a communications channel by connecting with the core wire and the outer conductor of the coaxial line via a choke coil, coaxial lines constituting each circuit unit does not have an insulating coating, said The outer conductor of the coaxial line is electrically connected to the outer conductor of the coaxial line of another channel.

Further, an interposing string is provided in the gap of the coaxial line .

The small-amplitude differential signal circuit of the present invention exhibits excellent characteristics as a high-speed signal circuit because of low attenuation and low noise due to crosstalk. In addition, by eliminating the insulation coating of each coaxial line , the diameter of the insulator inside the outer conductor of the coaxial line can be increased by an amount corresponding to the insulation coating as compared with the coaxial wire having the same diameter insulation coating, so that the attenuation amount accordingly. A small-amplitude differential signal circuit with a small amplitude can be provided. Further, Ri by the placing the common mode choke coil on each side of the coaxial line, a parasitic signal path is blocked by the inductance of the common mode choke coil Runode, be prevented generation of unnecessary signals it can. Also, common mode choke coils, because is constituted by winding a coaxial line on a ferrite core, it is possible to make uniform the characteristic impedance of the signal line, Ru can be eliminated impedance mismatch .

Hereinafter, the communication line cable of this invention is demonstrated using figures.
The present invention is directed to a digital transmission system capable of transmitting a total of 10 gigabits per second (Gbps) or more in full duplex by a pulse amplitude modulation (PAM) system using four coaxial cables.

1 and 2 show the structure of a communication line cable 1 of the present invention used in the transmission system. FIG. 2 is a cross-sectional view of FIG. FIG. 2 is a cross-sectional view of FIG. 1 and 2, 2
, 3, 4, 5 are coaxial lines, and as shown in FIG. 2, each of the coaxial lines 2-5 is composed of a core wire 10-13 and braided shields 6-9 on the outside via insulators 14-17. The In FIG. 2, black circles such as 18 are intervening strings, and four coaxial lines are connected to the sheath 19 together with the intervening strings.
The composite communication line cable 1 is covered. A feature of the communication line cable of the present invention is that there is no insulation sheath on each coaxial line. Therefore, each coaxial line is in a state where the outer shield conductors (braided shields) are electrically connected to each other. The inventor of the present invention has confirmed through experiments that no crosstalk occurs between the channels that transmit each coaxial line even if the coaxial line has no insulating sheath. This is a new finding that breaks conventional common sense. Conventionally, it has been considered that the current flowing through the outer conductor also flows through other coaxial cables, resulting in crosstalk.

The advantages of the communication line having the above structure are as follows.
(1) Each coaxial line forms a current path with the core wire as the forward path and the outer shield as the return path, and the currents of the forward path and the return path are opposite in direction and have the same magnitude. Be countered. At this time, the center point of the magnetic field on the concentric circle where the flowing current is generated is the same and the magnitude of the magnetic field generated on the outside of the outer shield conductor is exactly the same. In this case, the magnetic field does not shift due to the shift of the center point of the forward and return currents. Therefore, in principle, no inter-channel crosstalk caused by the magnetic field deviation occurs in the case of balanced two wires.
(2) Since there is no insulation coating for each coaxial line, the cross-sectional area of the cable when the four wires are bundled is reduced. That is, by eliminating the insulation coating of each coaxial cable, the diameter of the insulator inside the outer conductor of the coaxial line can be increased by an amount corresponding to the insulation coating as compared with the coaxial cable having the same diameter insulation coating.

When the characteristic impedance is the same and the insulator and conductor are the same coaxial line, the attenuation is determined by the diameter of the insulator inside the outer conductor. When forming the composite cable having the same diameter (the same cross-sectional area), if the coaxial cable without insulation coating is used as in the present invention, the outer conductor of each coaxial line is compared with the case of using the coaxial cable with insulation coating. Since the diameter of the internal insulator can be increased, it is possible to provide a transmission cable for high-speed communication with a smaller attenuation if a coaxial cable without insulation coating is used when the diameter of the entire composite cable is the same.

By the way, even when the coaxial line is used for the transmission line, as in the case of the balanced line, as shown in FIG. 3, the outer conductor (outer shield) 33 is connected to the inside of each terminal device by the grounding of the transmitting circuit 31 and the receiving circuit 34. Since it is connected to the common grounds 37 and 38, as shown by 39 in FIG. 3, the external conductor 33, the common ground 37 on the transmission circuit side, the ground, the common ground 38 on the reception circuit side, and the external conductor 3
Since the ground loop is formed by the route 3, an unnecessary signal may be generated.

For this reason, the communication cable of the present invention has a common mode choke coil 35, 3 as shown in FIG. 6 before being connected to the transmission circuit 31 or the reception circuit 34, as shown in FIG.
6 is preferably inserted. In particular, in the case of the present invention, since the outer conductor (coating shield) of the coaxial line is in a mutually connected state, an unnecessary signal component due to the ground loop is easily generated between the channels via the outer conductor. The need to insert a common mode choke coil is high.

If a common mode choke coil is inserted, the parasitic signal path through the ground in FIG. 3 is blocked by the inductance of the common mode choke coil, thereby preventing the generation of the unnecessary signal.

Conventionally, the common mode choke coil has a configuration using a pair of twisted wires, but in the present invention, a single coaxial wire is wound around a toroidal ferrite core as shown in FIG. Thus, by using the coaxial line, the characteristic impedance of the signal line can be made uniform, and it is possible to eliminate the impedance mismatch that causes unnecessary signals.

Also, by wrapping a coaxial line around the ferrite core at the end of the communication line cable 1,
Common mode choke coil can be realized without connector connection. In this case, a common mode choke coil can be incorporated in the connector portion between the terminal device and the communication line cable 1.

The material of the insulators 14 to 17 (FIG. 2) used for the coaxial line is preferably foamed polyethylene,
A communication line cable with very little attenuation can be realized at low cost. Although such foamed polyethylene can be easily used for a coaxial line structure, it is difficult to apply to a twisted pair line or a parallel two-line structure. The amount can be reduced.

In order to identify the channel of the coaxial line, the insulators 14 to 17 may be colored, or a thin film tape colored on the braided shields 6 to 9 may be roughly wound.

According to the present invention, it is possible to realize a high-speed signal transmission cable with excellent characteristics that is small in attenuation, small in noise due to crosstalk, and good in workability of facilities. By using this cable,
A 100 m high-speed transmission line in the gigahertz band can be realized.

It is a notch perspective view of the high-speed signal transmission cable of this invention. It is sectional drawing of the cable of FIG. It is circuit explanatory drawing of the high-speed transmission path of this invention. It is explanatory drawing of the next time which generate | occur | produces in a balanced 2-wire cable. It is explanatory drawing of the method of carrying out cable transmission with a small amplitude differential signal (LVDS Low Voltage Differential Signaling). It is a figure which shows the example of a common mode choke coil.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Communication line cable 2-5 Coaxial line 6-9 Braided shield 10-13 Core wire 14-17 Insulator 18 Interposing string 31 Transmission circuit 32, 33 Transmission path 34 Reception circuit 35, 36 Common mode choke coil

Claims (2)

A small-amplitude differential signal transmission circuit and a small-amplitude differential signal reception circuit are connected to each other through a common mode choke coil disposed on both ends of the coaxial wire and an external conductor. a plurality of circuit units forming a communications channel by a crotch,
The small-amplitude characterized in that the coaxial line constituting each circuit unit does not have an insulating coating, and the outer conductor of the coaxial line is electrically connected to the outer conductor of the coaxial line of another channel. Differential signal circuit . The small-amplitude differential signal circuit according to claim 1, wherein a string for interposition is provided in the gap of the coaxial line .

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