JP2008311964A - Common mode potential adjustment circuit and transmission line structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a common mode potential adjustment circuit for reducing a common mode voltage generated at a transmission line connecting point in order to suppress unnecessary electromagnetic wave radiation. <P>SOLUTION: A common mode potential adjustment circuit 10 is inserted at a connecting point between a transmission line 11 and a transmission line 12 both of which have different vertical cross-sectional shapes with respect to a signal propagation direction. At the transmission line connecting portion, the common mode voltage adjustment circuit 10 comprises impedance inserted in series between signal lines 14, 16, impedance inserted in series between return path lines 15, 17, and impedance inserted between the signal lines 14 and 16 and the return path lines 15 and 17. The common mode potential of each transmission line are continued at the transmission line connecting portion. Thereby, the common mode potential adjustment circuit 10 suppresses unnecessary electromagnetic wave radiation resulting from a common mode potential difference. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technology of a common mode potential adjustment circuit and a transmission line structure, and more particularly to an EMC (electromagnetic) of an apparatus for transmitting a high-speed signal by a transmission line or a cable formed on a printed circuit board in an electronic apparatus and an information processing apparatus. (Compatibility) It relates to a technology that is effective for improving performance.

  A device incorporating an electronic circuit such as an electronic device or an information processing device has a transmission line for signal transmission / reception. For example, signal wiring for signal transmission between ICs and LSIs is wired on a printed circuit board, and a cable is connected for signal transmission between printed circuit boards or between devices.

  The signal speed to be transmitted has been increasing year by year with the speeding up of electronic devices in recent years, and unwanted electromagnetic radiation EMI (Electro-Magnetic Interference) generated due to these high-speed signals has become a problem.

  As one of unnecessary electromagnetic radiation sources generated from the transmission line, there is a common mode current generated in the line. The common mode current is generated by mode conversion of electromagnetic waves propagated at discontinuous points of the transmission line.

  There are an odd mode and an even mode as electromagnetic field modes propagating through the transmission line. Usually, for signal transmission, an odd mode electromagnetic field is applied to the transmission line as a transmission signal at the transmission end. Since this odd-mode electromagnetic field generates voltages and currents of opposite phases to the signal line and return line of the transmission line, the electromagnetic field generated by the odd-mode voltage and current far enough from the transmission line is the electromagnetic field generated from the signal line. The electromagnetic fields generated from the field and the return line are canceled out because they are out of phase with each other, and are usually sufficiently small. Such a reverse-phase voltage / current is called a normal mode voltage / current.

  On the other hand, the even-mode electromagnetic field generates in-phase voltage and current in the signal line and return line of the transmission line, so that the electromagnetic field generated outside the transmission line is not canceled because of the in-phase. The electromagnetic field remaining without cancellation is the main cause of unnecessary electromagnetic radiation generated from the transmission line, and the common-mode current that generates this radiation is the common mode current.

FIG. 14 is a diagram showing the current in the transmission line. The normal mode current 23 flowing in the signal line 21 is I _NS , the common mode current 24 is I _CS , and the normal mode current 25 flowing in the return line 22 is I _NG , common The mode current 26 is _ICG . The normal mode currents flowing in the signal line and the return line have the same strength and are opposite to each other. On the other hand, the common mode current flows in the same direction in the signal line and the return line, and the current intensity is determined by the current distribution ratio h determined by the cross-sectional shape of the transmission line.

The current distribution ratio is defined by the ratio of the common mode current flowing in the signal line to the total common mode current, and becomes h = I _CS / (I _CS + I _CG ), and the current distribution ratio h takes a value between 0 and 1. . The current distribution ratio h can be calculated by a one-dimensional electrostatic field analysis shown in Non-Patent Document 1.

  In the coaxial cable, all common mode currents flow through the outer conductor, so that h = 0. In parallel two lines, the common mode current flows evenly through the signal line and the return line, so h = 0.5. In the case of an ideal microstrip line with a wide ground width, h = 0.

  Next, as shown in FIG. 15, the connection of two transmission lines will be considered. When the current distribution ratios of the connected transmission lines are different, the potential diagram at the transmission line connection portion is as shown in FIG. In the figure, the vertical axis indicates the line potential, and the horizontal axis indicates the position of the transmission line.

The potential of the reference ground 13 is zero, the signal line potential 31 and the return line potential 32 of the transmission line 11 are V _S1 and V _G1 , respectively, and the signal line potential 35 and the return line potential 36 of the transmission line 12 are V _S2 and V _S2 , respectively. V _{G2 is} assumed. The difference between the signal line potential and the return line potential of each line is the normal mode voltages 34 and 38 in each transmission line, and V _N1 = V _S1 −V _G1 and V _N2 = V _S2 −V _G2 , respectively. At this time, if the common mode potentials V _C1 and V _C2 of each line use the current distribution ratios h ₁ and h ₂ of each line, the product of the normal mode voltage and the current distribution ratio V _C1 = h ₁ V _N1 , V _C2 = It can be expressed as h ₂ V _N2 .

When the current distribution ratios of the transmission lines 11 and 12 are different, the normal mode voltages V _N1 and V _N2 of each line are equal at the connection point, so the common mode potentials V _C1 and V _C2 of each line are different. This common mode potential difference 30 generates a common mode voltage that causes unnecessary radiation. In order to suppress such generation of common mode voltage, it is necessary to equalize the current distribution ratio of each transmission line.

  In a single-ended transmission system, ideally, the current distribution ratio of all the systems is desired to be zero. In order to bring the current distribution ratio of the transmission line close to zero, the ground of the transmission line may be strengthened.

  Patent Documents 1 and 2 are techniques for suppressing unnecessary electromagnetic radiation by installing a ground guard pattern adjacent to a signal wiring for the purpose of strengthening the ground of a transmission line.

  Patent Document 3 is a technique for reducing unnecessary electromagnetic radiation by making a common mode potential continuous using a transformer in a differential transmission system.

In general, there is a method of suppressing unnecessary electromagnetic radiation by inserting a common mode filter or the like into a transmission line as in Patent Document 4.
JP 2003-347893 A JP 2006-202964 A JP 2006-191544 A Japanese Patent No. 3003067 IEICE Trans. Commun. Vol. E87-B, no. 8, Aug. 2004

  However, the unnecessary electromagnetic radiation suppression technology by installing the ground guard pattern described in Patent Documents 1 and 2 has a drawback of increasing the mounting area of the transmission line. Further, the method using the transformer of Patent Document 3 and the method using the filter of Patent Document 4 have a limitation in the frequency band in which unnecessary electromagnetic radiation can be suppressed because the characteristics of the transformer and the magnetic material deteriorate at high frequencies.

  Therefore, the present invention aims to solve these problems and provide a technique for suppressing unnecessary electromagnetic radiation that reduces the common mode voltage generated at the connection point when connecting transmission lines having different common mode potentials. is there.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  Of the inventions disclosed in the present application, the outline of typical ones will be briefly described as follows.

  In order to achieve the above-described object, the present invention inserts a common mode potential adjustment circuit at a connection point between a transmission line and a transmission line having different vertical cross-sectional shapes with respect to the signal propagation direction. The common mode potential adjustment circuit includes an impedance inserted in series between the signal lines, an impedance inserted in series between the return lines, and an impedance inserted in parallel between the signal line and the return line in the transmission line connection portion. A common mode potential adjusting circuit that suppresses unnecessary electromagnetic wave radiation caused by the common mode potential difference by providing the common mode potential of each transmission line at the transmission line connecting portion by using these impedances.

  Among the inventions disclosed in the present application, effects obtained by typical ones will be briefly described as follows.

  ADVANTAGE OF THE INVENTION According to this invention, the common mode electric potential adjustment circuit which reduces the common mode voltage generation | occurrence | production in the transmission line connection point which becomes the cause of the unnecessary electromagnetic wave radiation which generate | occur | produces from a transmission line can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment, and the repetitive description thereof will be omitted.

(Concept of the embodiment)
In the common mode potential adjustment circuit of this embodiment, this main circuit has a plurality of transmission lines having different vertical cross-sectional shapes with respect to the signal propagation direction (in this embodiment, two examples will be described, but three (T-shaped Shape), four (cross shape), or even more) transmission line structures are installed between the first transmission line and the second transmission line. This main circuit has a function of suppressing unwanted electromagnetic radiation caused by a common mode potential difference between the first and second transmission lines.

  In each of the following embodiments, this main circuit is substantially equivalent to the common mode potential adjustment circuit itself, and will be referred to as a common mode potential adjustment circuit here. In addition, although Embodiments 1 to 4 describe circuit configuration examples of common mode potential adjustment circuits, and Embodiments 5 to 9 describe transmission line structure examples in which common mode potential adjustment circuits are installed, these are arbitrarily combined. Needless to say, this can be achieved.

(Embodiment 1)
A common mode potential adjustment circuit according to the first embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 is a diagram showing a configuration of a common mode potential adjustment circuit according to the first embodiment of the present invention. The common mode potential adjusting circuit 10 according to the present embodiment is provided at a connection point of a transmission line structure formed by connecting a transmission line (1) 11 and a transmission line (2) 12 having different vertical cross-sectional shapes with respect to the signal propagation direction. It is connected. Each transmission line 11, 12 is composed of each signal line 14, 16 and each return line 15, 17.

The common mode potential adjustment circuit 10 converts the signal intensity applied to the transmission line 12 so that the common mode potentials of the transmission line 11 and the transmission line 12 are equal. The common mode potential adjusting circuit 10 includes impedances Z ₁ and Z ₂ connected in series between the signal line 14 of the transmission line 11 and the signal line 16 of the transmission line 12, the return line 15 of the transmission line 11, and the transmission line 12. Are formed by impedances Z ₃ and Z ₄ connected in series with the return line 17 and impedance Z ₅ connected in parallel with the signal line and the return line.

In the present embodiment, when the current distribution rate h ₂ of the transmission line 12 is larger than the current distribution rate h ₁ of the transmission line 11, the potential diagram at the line connection point is as shown in FIG. As shown in FIG. 16, the common mode potential 37 of the transmission line 12 is larger than the common mode potential 33 of the transmission line 11. In such a case, the common mode potential adjustment circuit 10 has a circuit configuration as shown in FIG.

  FIG. 2 is a diagram illustrating a common mode potential adjusting circuit in which a resistor is provided between the signal lines of the transmission line. As shown in FIG. 2, a common mode potential adjusting resistor 20 is inserted between the signal lines 14 and 16 of the two transmission lines 11 and 12.

  The resistance value R of the resistor 20 to be inserted is determined by the following equation using the current distribution ratio of the transmission lines 11 and 12.

R = Z ₀₂ (h ₂ −h ₁ ) / h ₁
Here, Z ₀₂ is the characteristic impedance 19 of the transmission line 12.

  At this time, the potential diagram at the transmission line connection point is as shown in FIG.

  FIG. 3 is a diagram illustrating a potential diagram at a transmission line connection point when a common mode potential adjusting circuit in which a resistor is installed between signal lines is installed. As shown in FIG. 3, the signal line potential 35 input to the transmission line 12 is lowered by the voltage drop 39 generated by the inserted resistor 20, and the common mode potential 37 of the transmission line 12 is also lowered accordingly. Since the resistor 20 is inserted so that the common mode potentials of the transmission line 11 and the transmission line 12 are equal to each other, the common mode potential difference 30 is zero, and unnecessary electromagnetic radiation is not generated.

  As described above, according to the present embodiment, the configuration in which the common mode potential adjustment circuit 10 formed by the impedance is connected between the transmission line 11 and the transmission line 12, specifically, the common mode potential adjustment circuit. 10, by using the voltage drop of the common mode potential adjusting resistor 20 connected between the signal lines, the common mode potential of each transmission line is made continuous at the transmission line connecting portion, and an unnecessary electromagnetic wave caused by the common mode potential difference Radiation can be suppressed. As a result, it is possible to reduce generation of common mode voltage at the transmission line connection point, which causes unnecessary electromagnetic radiation generated from the transmission line.

(Embodiment 2)
A common mode potential adjusting circuit according to the second embodiment of the present invention will be described with reference to FIGS.

Contrary to the first embodiment, when the current distribution ratio h ₁ of the transmission line 11 is larger than the current distribution ratio h ₂ of the transmission line 12, the potential diagram at the transmission line connection point is as shown in FIG. .

FIG. 4 is a diagram showing a potential diagram at a transmission line connection point when h ₁ > h ₂ . As shown in FIG. 4, the common mode potential 33 of the transmission line 11 is higher than the common mode potential 37 of the transmission line 12. In such a case, the common mode potential adjustment circuit 10 has a circuit configuration as shown in FIG.

  FIG. 5 is a diagram illustrating a common mode potential adjustment circuit in which a resistor is provided between the return lines of the transmission line. As shown in FIG. 5, a common mode potential adjusting resistor 20 is inserted between the return lines 15 and 17 of the two transmission lines 11 and 12.

  The resistance value R of the resistor 20 to be inserted is determined by the following equation using the current distribution ratio of the transmission lines 11 and 12.

R = Z ₀₂ (h ₁ −h ₂ ) / (1−h ₁ )
Here, Z ₀₂ is the characteristic impedance 19 of the transmission line 12.

  At this time, the potential diagram at the transmission line connection point is as shown in FIG.

  FIG. 6 is a diagram illustrating a potential diagram at a transmission line connection point when a common mode potential adjustment circuit in which a resistor is installed between return lines is installed. As shown in FIG. 6, the return line potential 36 input to the transmission line 12 increases due to the voltage drop 39 generated by the inserted resistor 20, and the common mode potential 37 of the transmission line 12 also increases accordingly. Since the resistor 20 is inserted so that the common mode potentials of the transmission line 11 and the transmission line 12 are equal to each other, the common mode potential difference 30 is zero, and unnecessary electromagnetic radiation is not generated.

  As described above, according to the present embodiment, the common mode potential adjusting circuit 10 uses the voltage drop of the common mode potential adjusting resistor 20 connected between the return lines, so that Similar effects can be obtained.

(Embodiment 3)
A common mode potential adjustment circuit according to Embodiment 3 of the present invention will be described with reference to FIG.

  As a configuration of the common mode potential adjustment circuit 10, a circuit configuration as shown in FIG. 7 may be employed.

FIG. 7 is a diagram illustrating a common mode potential adjustment circuit with impedance connected in a π-type. As shown in FIG. 7, the configuration of the common mode potential adjustment circuit 10 may be a π-type circuit configuration inserted between the transmission lines 11 and 12. At this time, the common mode potential difference between the transmission lines is reduced by the inserted impedances (Z ₁ , Z ₂ , Z ₃ , Z ₄ ).

  Therefore, also in the present embodiment, the same effect as in the first embodiment can be obtained.

(Embodiment 4)
A common mode potential adjustment circuit according to a fourth embodiment of the present invention will be described with reference to FIG.

  In addition to the circuit configuration of the third embodiment, a circuit configuration as shown in FIG. 8 is also possible.

FIG. 8 is a diagram showing a common mode potential adjusting circuit with impedance connected to a T-type. As shown in FIG. 8, the common mode potential adjustment circuit 10 may have a T-type or inverted T-type circuit configuration inserted between the transmission lines 11 and 12. At this time, the common mode potential difference between the transmission lines is reduced by the impedance (Z ₁ , Z ₂ , Z ₅ or Z ₃ , Z ₄ , Z ₅ ) to be inserted.

  Therefore, also in the present embodiment, the same effect as in the first embodiment can be obtained.

(Embodiment 5)
A common mode potential adjustment circuit and transmission line structure according to Embodiment 5 of the present invention will be described with reference to FIG.

  As a transmission line structure in which the common mode potential adjusting circuit 10 is installed, a structure as shown in FIG. 9 can be used.

  FIG. 9 is a diagram illustrating a common mode potential adjustment circuit in transmission line connections with different return line widths. As shown in FIG. 9, among the transmission lines installed on the printed circuit board, in the connection of the transmission line (the return line width is wide) 11 and the transmission line (the return line width is narrow) 12 whose return line width changes, A common mode potential adjustment circuit 10 is provided at the transmission line connection point. The circuit configuration of the first to fourth embodiments can be applied to the common mode potential adjustment circuit 10.

  Therefore, the same effects as those of the first embodiment can be obtained even when the present invention is applied to a transmission line structure in which the return line width is changed, as in the present embodiment.

(Embodiment 6)
A common mode potential adjustment circuit and transmission line structure according to Embodiment 6 of the present invention will be described with reference to FIG.

  In addition to the transmission line structure of the fifth embodiment, a structure as shown in FIG. 10 is also possible.

  FIG. 10 is a diagram showing a substrate cross section for explaining the common mode potential adjusting circuit at the layer change point of the signal wiring. As shown in FIG. 10, among the transmission lines installed on the printed circuit board, in the transmission line in which the signal wiring is arranged from the inner layer to the surface layer, the common mode potential adjustment circuit 10 is provided at the transmission line connection point.

  Therefore, even when applied to a transmission line structure in which the signal wiring of the printed circuit board is arranged from the inner layer to the surface layer as in the present embodiment, the same effect as in the first embodiment can be obtained. In addition, in the transmission line structure as in the present embodiment, conversely, in the case of having a common mode potential adjustment circuit at the connection point of the transmission line arranged from the surface layer to the inner layer, or in a through hole having a common mode potential adjustment function Can also be applied.

(Embodiment 7)
A common mode potential adjusting circuit and a transmission line structure according to Embodiment 7 of the present invention will be described with reference to FIG.

  In addition to the transmission line structure of the sixth embodiment, a structure as shown in FIG. 11 is also possible.

  FIG. 11 is a diagram illustrating a common mode potential adjusting circuit at a connection point between the printed circuit board and the cable. As shown in FIG. 11, a common mode potential adjusting circuit 10 is provided at a connection point between a transmission line formed on a printed circuit board and a transmission line using a cable. This cable connects the printed circuit boards.

  Therefore, when the present invention is applied to a connection structure of a transmission line formed on a printed circuit board and a transmission line using a cable as in the present embodiment, the same effect as in the first embodiment can be obtained. Note that, in the structure of a transmission line using a cable as in this embodiment, the present invention can also be applied to a cable connecting between electronic devices, a cable connecting a printed circuit board and an electronic device, and the like.

(Embodiment 8)
A common mode potential adjusting circuit and a transmission line structure according to the eighth embodiment of the present invention will be described with reference to FIG.

  In addition to the transmission line structure of the seventh embodiment, a structure as shown in FIG. 12 may be used.

  FIG. 12 is a diagram showing a connector with a built-in common mode potential adjustment circuit. As shown in FIG. 12, the connection point shown in the seventh embodiment is formed by a connector, and a common mode potential adjusting circuit 10 is provided in the connector.

  Therefore, even when applied to a structure having the common mode potential adjustment circuit 10 in the connector as in the present embodiment, the same effect as in the first embodiment can be obtained. Note that the transmission line structure as in this embodiment can also be applied to a structure having a common mode potential adjustment circuit in the vicinity of the connector.

(Embodiment 9)
A common mode potential adjusting circuit and a transmission line structure according to the ninth embodiment of the present invention will be described with reference to FIG.

  FIG. 13 is a diagram illustrating a common mode potential adjustment circuit in the bus wiring of the printed circuit board. As shown in FIG. 13, in the wiring such as a bus connecting between LSIs (including ICs) installed on a printed circuit board, the transmission line has different transmission line cross-sectional shapes due to slits in the power supply layer or the ground layer. A common mode potential adjustment circuit 10 is provided at the connection points 11 and 12.

  Therefore, even when applied to a transmission line structure having a different cross-sectional shape in the wiring of the printed circuit board as in the present embodiment, the same effect as in the first embodiment can be obtained.

  As mentioned above, the invention made by the present inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and various modifications can be made without departing from the scope of the invention. Needless to say.

  The common mode potential adjusting circuit of the present invention can suppress generation of common mode voltage that causes unnecessary electromagnetic radiation simply by inserting a circuit element having an impedance component such as a resistance at a connection point of the transmission line, and the circuit element to be inserted. The value can also be determined by a one-dimensional analysis of the cross-sectional shape of the transmission line, which is useful in electronic equipment having a transmission line structure.

It is a figure which shows the structure of the common mode electric potential adjustment circuit of Embodiment 1 of this invention. In Embodiment 1 of this invention, it is a figure which shows the common mode electric potential adjustment circuit which installed resistance between the signal lines of a transmission line. In Embodiment 1 of this invention, it is a figure which shows the electric potential diagram in the transmission line connection point at the time of the common mode electric potential adjustment circuit which installed resistance between the signal lines. In a second embodiment of the present invention, illustrating a potential diagram in the transmission line connecting point when the h _1> h _2. In Embodiment 2 of this invention, it is a figure which shows the common mode electric potential adjustment circuit which installed resistance between the return lines of a transmission line. In Embodiment 2 of this invention, it is a figure which shows the electric potential diagram in the transmission line connection point at the time of the common mode electric potential adjustment circuit which installed resistance between the return lines. In Embodiment 3 of this invention, it is a figure which shows the common mode electric potential adjustment circuit by the impedance connected to (pi) type. In Embodiment 4 of this invention, it is a figure which shows the common mode electric potential adjustment circuit by the impedance connected to the T type. In Embodiment 5 of this invention, it is a figure which shows the common mode electric potential adjustment circuit in the transmission line connection from which a return line width differs. In Embodiment 6 of this invention, it is a figure which shows the board | substrate cross section for demonstrating the common mode electric potential adjustment circuit in the layer change point of a signal wiring. In Embodiment 7 of this invention, it is a figure which shows the common mode electric potential adjustment circuit in the connection point of a printed circuit board and a cable. In Embodiment 8 of this invention, it is a figure which shows the common mode electric potential adjustment circuit built-in connector. In Embodiment 9 of this invention, it is a figure which shows the common mode electric potential adjustment circuit in the bus wiring of a printed circuit board. It is a figure explaining the normal mode current and common mode current of a general transmission line. It is a figure which shows the connection of two general transmission lines. Is a diagram showing a potential diagram in the transmission line connecting point when a general h ₁ <h _2.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Common mode electric potential adjustment circuit, 11 ... Transmission line (1), 12 ... Transmission line (2), 13 ... Reference ground, 14 ... Signal line of transmission line (1), 15 ... Return line of transmission line (1) , 16 ... signal line of transmission line (2), 17 ... return line of transmission line (2), 18 ... characteristic impedance of transmission line (1), 19 ... characteristic impedance of transmission line (2), 20 ... common mode potential Resistance for adjustment, 21 ... signal line, 22 ... return line, 23 ... normal mode current of signal line, 24 ... common mode current of signal line, 25 ... normal mode current of return line, 26 ... common mode current of return line , 30 ... common mode potential difference, 31 ... signal line potential of transmission line (1), 32 ... return line potential of transmission line (1), 33 ... common mode potential of transmission line (1), 34 ... transmission line (1) No Mull mode voltage, 35 ... Signal line potential of transmission line (2), 36 ... Return line potential of transmission line (2), 37 ... Common mode potential of transmission line (2), 38 ... Normal mode of transmission line (2) Voltage, 39 ... Voltage drop across the resistor.

Claims (10)

  It is installed between the first transmission line and the second transmission line among the plurality of transmission lines having different vertical cross-sectional shapes with respect to the signal propagation direction, and is caused by the common mode potential difference in the first and second transmission lines. A common mode potential adjustment circuit comprising a circuit for suppressing unnecessary electromagnetic radiation. The common mode potential adjustment circuit according to claim 1,
As the circuit, an impedance inserted in series between the signal lines of the first and second transmission lines and / or between the return lines, and parallel between the signal lines and the return lines of the first and second transmission lines. A common mode potential adjustment circuit using an impedance inserted in the circuit. The common mode potential adjustment circuit according to claim 1,
A common mode potential adjusting circuit, wherein a voltage drop of a resistor inserted between the signal lines of the first and second transmission lines is used as the circuit. The common mode potential adjustment circuit according to claim 1,
A common mode potential adjusting circuit, wherein a voltage drop of a resistor inserted between return lines of the first and second transmission lines is used as the circuit. The common mode potential adjustment circuit according to claim 1,
As the circuit, a voltage drop of a resistor inserted between the signal lines of the first and second transmission lines is used,
The common mode potential adjustment circuit, wherein the resistor is provided in a connector. The common mode potential adjustment circuit according to claim 1,
As the circuit, a voltage drop of a resistor inserted between return lines of the first and second transmission lines is used,
The common mode potential adjustment circuit, wherein the resistor is provided in a connector.   It consists of a plurality of transmission lines having different vertical cross-sectional shapes with respect to the signal propagation direction, and the first and second transmission lines are between the first transmission line and the second transmission line among the plurality of transmission lines. A transmission line structure comprising a circuit for suppressing unnecessary electromagnetic radiation caused by a common mode potential difference. In the transmission line structure according to claim 7,
Each of the plurality of transmission lines includes a signal wiring formed on a printed circuit board. In the transmission line structure according to claim 7,
Each of the plurality of transmission lines is formed by a cable connecting between printed circuit boards and / or between electronic devices. In the transmission line structure according to claim 7,
Each of the plurality of transmission lines is formed by a transmission line formed on a printed circuit board and a cable connecting between the printed circuit boards and / or between electronic devices.

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