Signal transmission circuit

Abstract

To provide technique which can easily measure an extraneous noise resistance of a differential transmission line.SOLUTION: A signal transmission circuit (1) comprises: a difference transmission line (10) containing a pair of signal lines; and a pair of coupling lines (20) that is extended in parallel to the difference transmission line at an approach position so as to be electromagnetically coupled to the difference transmission line. On one end side of the difference transmission line, a common mode filter (12) is provided. On the output side of the common mode filter on the one end side of the difference transmission line, a measurement connection part (15) that can perform a connection with a measurement device that can measure an electric feature is provided. On the one end part of the pair of coupling lines, a measurement connection part (25) that can perform the connection with the measurement device is provided.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a signal transmission circuit including a differential transmission line.

Background Art

[0002] In the field of data transmission of information communication devices, differential transmission technology is widely used as a communication method capable of achieving high speed and improved noise resistance. Differential transmission is a communication method in which currents flowing in opposite phases are passed through a pair (two) of signal lines and transmitted by the potential difference between the signal lines. Differential transmission technology is used not only between information communication devices but also between modules within a device and between IC / LSI chips.

[0003] In recent years, RFI (Radio Frequency Interference) has become a major problem in high-speed differential transmission. Since external radio waves in the GHz (gigahertz) band have a short wavelength, the transmission line is likely to become an antenna, and when such external radio waves are mixed into the differential transmission line, common-mode noise is applied to both the transmission side and the reception side. If the common-mode noise is within the allowable range, it can be canceled by the differential transmission method, but RFI occurs when the allowable range is exceeded.

[0004] Patent Document 1 below discloses an arithmetic device capable of measuring an operation margin, which is a margin until communication is affected by noise, during the operation of the arithmetic device. This arithmetic device includes a differential transmission line connecting between a CPU and an IO module, a test wiring arranged in parallel in a predetermined section of the differential transmission line, a noise application unit which is a current-voltage source for applying noise to the differential transmission line using the test wiring, and a margin measurement unit for measuring the occurrence frequency of communication errors between devices connected via the differential transmission line.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] According to the above-described arithmetic unit, a noise signal having a predetermined frequency determined in advance by the noise application unit can be mixed into the differential transmission line via the test wiring, and the occurrence status of communication errors with respect to the noise signal having the predetermined frequency can be monitored during the operation of the arithmetic unit.

[0007] However, since the frequency band of external radio waves that cause RFI can vary depending on the circuit configuration around the differential transmission line and circuit elements such as filters provided in the differential transmission line, there remains a problem in measuring the external noise resistance of the differential transmission line with a method using a signal source that outputs a noise signal having a predetermined frequency.

[0008] As a countermeasure against RFI, there is an example in which a common-mode filter such as a common-mode choke coil is provided at both ends of the transmission circuit side and the reception circuit side in the differential transmission line. In such an example, it is known that the differential transmission line resonates at a high resonance voltage with the common-mode noise due to the characteristics of the common-mode filter, differential-mode noise is generated by mode conversion, and the differential-mode noise is mixed with the differential data signal to cause RFI. Also, due to the characteristics of the common-mode filter provided in the differential transmission line, the occurrence status of RFI associated with external radio waves changes.

[0009] The present invention has been made in view of such circumstances, and provides a technique that enables easy measurement of the external noise resistance of a differential transmission line.

MEANS FOR SOLVING THE PROBLEMS

[0010] In order to solve the above-described problems, the present invention adopts a configuration according to the following aspects. A signal transmission circuit according to an aspect of the present invention includes a differential transmission line including a pair of signal lines, and a pair of coupling lines extending in parallel with the differential transmission line at a proximity position that electromagnetically couples to the differential transmission line. A common mode filter is provided on one end side of the differential transmission line, and a measurement connection portion enabling connection to a measurement device capable of measuring electrical characteristics is provided on the output side of the common mode filter on the one end side of the differential transmission line. A measurement connection portion enabling connection to the measurement device is provided at one end of the pair of coupling lines.

[0011] A method according to another aspect of the present invention is a method for evaluating the external noise resistance of the differential transmission line in the above-described signal transmission circuit using a 4-port vector network analyzer, including electrically connecting a pair of measurement ports of the vector network analyzer to the measurement connection portion of the pair of coupling lines, electrically connecting another pair of measurement ports of the vector network analyzer to the measurement connection portion of the differential transmission line, and measuring the frequency characteristics of the conversion intensity from the common mode noise mixed into the differential transmission line by crosstalk from the pair of coupling lines to the differential mode using the output signal from the vector network analyzer as a noise source.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a technique that enables easy measurement of the external noise resistance of a differential transmission line.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Each of the embodiments described below is an example, and the present invention is not limited to the configurations of the following embodiments.

[0015] [First Embodiment] FIG. 1 is a diagram schematically showing the appearance of a signal transmission circuit 1 according to the first embodiment. The signal transmission circuit 1 according to the first embodiment includes a differential transmission line 10 including a pair of signal lines 10a and 10b, and a pair of coupling lines 20a and 20b that extend in parallel with the differential transmission line 10 at a proximity position so as to electromagnetically couple to the differential transmission line 10. In this embodiment, the signal transmission circuit 1 is formed on a dielectric substrate (printed circuit board), and the differential transmission line 10 and the pair of coupling lines 20a and 20b are arranged on the same substrate surface of the dielectric substrate.

[0016] The differential transmission line 10 includes at least a pair of signal lines 10a and 10b. The pair of signal lines 10a and 10b are arranged in parallel as shown in FIG. 1, connected to a transmission circuit at one end (the end on the side denoted as the TX side in FIG. 1), and connected to a reception circuit at the other end (the end on the side denoted as the RX side in FIG. 1), and transmit differential data signals that are in opposite phases to each other. The differential transmission line 10 may be composed of a plurality of pairs of signal lines 10a and 10b, and is used as a general term for one or more pairs of signal lines that perform differential transmission. In the following description, unless it is necessary to distinguish between the signal line 10a and the signal line 10b, it is generically referred to as the differential transmission line 10.

[0017] In this embodiment, common mode filters (hereinafter, may also be abbreviated as CMF) 12a and 12b are provided on both ends of the differential transmission line 10 for RFI countermeasures. Therefore, the differential transmission line 10 is connected to the transmission circuit via the CMF 12a at one end, and connected to the reception circuit via the CMF 12b at the other end. In the following description, unless it is necessary to distinguish between the CMF 12a and the CMF 12b, it is generically referred to as the CMF 12.

[0018] The CMF 12 is a noise filter that removes common mode noise signals and allows differential mode signals to pass through. The CMF 12 includes, for example, a common mode choke coil. However, the CMF 12 may be any known common mode filter, and its specific configuration is not limited at all.

[0019] Furthermore, a measurement connection portion 15 is provided on the output side of the CMF 12b at one end of the differential transmission line 10. The measurement connection portion 15 is a circuit element that enables connection to a measuring device capable of measuring electrical characteristics. The measurement connection part 15 in this embodiment is provided for each of the signal lines 10a and 10b, and is provided symmetrically in the width direction with respect to the virtual center line between the signal lines 10a and 10b. As shown in FIG. 1, each measurement connection part 15 has a branch part 15a, a lead-out wire part 15b, and a pad part 15c respectively.

[0020] The branch part 15a is provided on the output side of the CMF 12b in the signal lines 10a and 10b respectively, and is a part protruding in opposite directions in the width direction of the signal lines 10a and 10b. The lead-out wire part 15b extends from a position separated by a predetermined interval from the tip of the branch part 15a in a direction away from the signal lines 10a and 10b in a direction orthogonal to the signal lines 10a and 10b respectively. When the branch part 15a and the lead-out wire part 15b are connected to the measuring device, they are configured to be in conduction with the signal lines 10a and 10b by being connected by a jumper therebetween. However, in a state where they are not connected to the measuring device, the branch part 15a and the lead-out wire part 15b are in a non-conductive state. The branch part 15a and the lead-out wire part 15b may be formed of the same conductive material as the signal lines 10a and 10b, or may be formed of a conductive material different from them.

[0021] The pad part 15c is provided for connection to the measuring device, and is provided on both sides in the width direction of the lead-out wire part 15b so as to sandwich the end part on the side opposite to the branch part 15a side in the lead-out wire part 15b. The pad part 15c is connected to the ground plane by a via or the like, and can also be called a ground pad. The connection form between the pad part 15c and the measuring device will be described later.

[0022] Examples of the measuring device that can be connected to the measurement connection part 15 and measure electrical characteristics include a vector network analyzer (hereinafter, may be abbreviated as VNA). However, the measuring device here is not limited to only the VNA, and any device that can measure electrical characteristics such as an oscilloscope may be used.

[0023] A pair of coupling lines 20a and 20b are measurement lines provided to inject external noise into the differential transmission line 10 by crosstalk in order to measure the external noise resistance of the differential transmission line 10. Unlike the differential transmission line 10, the coupling lines 20a and 20b are not connected to the transmission circuit and the reception circuit at both ends. For this reason, the coupling lines 20a and 20b can also be called linear electrodes.

[0024] In the present embodiment, the coupling lines 20a and 20b are constituted by microstrip lines wired on the substrate in the same manner as the differential transmission line 10, and are arranged so as to sandwich the differential transmission line 10 from both sides in the width direction on the same substrate surface. The coupling lines 20a and 20b are formed to be symmetric in the width direction with respect to the virtual center line between the signal lines 10a and 10b, and have the same configuration. In the following description, unless it is necessary to distinguish between the coupling line 20a and the coupling line 20b, one or both of the coupling lines 20a and 20b are collectively referred to as the coupling line 20. Further, an end portion of the coupling line 20 close to the end portion on the transmission circuit side of the differential transmission line 10 is referred to as the transmission side end portion of the coupling line 20, and an end portion of the coupling line 20 close to the end portion on the reception circuit side of the differential transmission line 10 is referred to as the reception side end portion of the coupling line 20.

[0025] A measurement connection portion 25 that enables connection to the above-described measuring device is provided at one end portion of the coupling line 20, and a termination resistor 27 is provided at the other end portion. In the present embodiment, the measurement connection portion 25 is provided at the transmission side end portion of the coupling line 20, and the termination resistor 27 is provided at the reception side end portion of the coupling line 20. However, the measurement connection portion 25 may be provided at the reception side end portion of the coupling line 20, and the termination resistor 27 may be provided at the transmission side end portion of the coupling line 20.

[0026] As shown in FIG. 1, the measurement connection portion 25 has a lead-out portion 25b and a pad portion 25c. The lead-out portion 25b communicates with the coupling line 20 and extends from the end of the coupling line 20 in a direction orthogonal to the coupling line 20 and away from the coupling line 20. That is, the lead-out portion 25b has a shape that bends in a direction in which the end of the coupling line 20 is separated from the differential transmission line 10. The lead-out portion 25b may be formed of the same conductive material as the coupling line 20 or may be formed of a conductive material different from them. The pad portion 25c is provided for connection to the measuring device and is provided on both sides in the width direction of the lead-out portion 25b with the tip portion of the lead-out portion 25b interposed therebetween. The pad portion 25c is connected to the ground plane by a via or the like and can also be called a ground pad. The connection form between the pad portion 25c and the measuring device will be described later.

[0027] The termination resistor 27 is a resistor element provided so as to be electrically connected between the end of the coupling line 20 and the ground plane. In the example of FIG. 1, the termination resistor 27 is attached to the pads on the dielectric substrate whose both ends are connected to one end of the coupling line 20 and the ground plane by a via or the like by soldering or the like. However, the connection form between the coupling line 20 and the termination resistor 27 is not limited to such an example, and various connection forms can be used. For example, it may be a form of connecting from the end of the coupling line 20 to a resistor element built in the substrate by a via or the like.

[0028] Here, the form of connecting the measuring device to the signal transmission circuit 1 will be described. When connecting the measuring device to the signal transmission circuit 1 to measure the electrical characteristics, the signal output port of the measuring device is electrically connected via the measurement connection portion 25 provided on the coupling line 20, and the signal input port of the measuring device is electrically connected via the measurement connection portion 15 provided on the differential transmission line 10. By outputting a predetermined signal from the signal output port of the measuring device, the predetermined signal is transmitted through the coupling line 20, and the noise associated with this signal transmission is mixed into the differential transmission line 10 as crosstalk, which is external noise. By applying external noise evenly to the coupling lines 20a and 20b, the noise mixed into the differential transmission line 10 becomes common-mode noise. However, depending on the conditions of the section of the differential transmission line 10 sandwiched between the CMF12a and CMF12b provided as an RFI countermeasure, this common-mode noise resonates at a high resonance voltage and may propagate to the receiving side as differential-mode noise through mode conversion. Therefore, by measuring the electrical characteristics at the signal input port of the measuring device via the measurement connection portion 15 on the output side of the CMF12b in the differential transmission line 10, it is possible to evaluate the external noise of the differential transmission line 10 (for example, evaluate the frequency characteristics of the conversion strength from common-mode noise to differential mode, etc.). In this embodiment, since the signal output of the measuring device is used instead of the noise source, signals can be applied equally to the coupling lines 20a and 20b, and the frequency of the output signal can be easily changed. For example, by sweeping the frequency of the output signal, the influence on the external noise for each frequency band in the differential transmission line 10 can be easily evaluated.

[0029] FIG. 2 is a diagram showing an example of the connection form of the measuring device to the signal transmission circuit 1 according to the first embodiment. In the example of FIG. 2, a connection form in which the measuring probes 50 and 51 of the measuring device are brought into contact is shown.

[0030] Taking an example where a 4-port VNA is used as the measuring device, the VNA has a pair of measuring probes 50 connected to a pair of measurement ports and a pair of measuring probes 51 connected to the other pair of measurement ports. In FIGS. 2(a) and 2(b), for ease of viewing, the measuring probes 50 and 51 are shown one by one. However, during measurement, a pair of measuring probes 50 connected to a pair of measurement ports constituting the balance port 1 and a pair of measuring probes 51 connected to a pair of measurement ports constituting the balance port 2 are used respectively.

[0031] Specifically, by bringing the tip pins of the pair of measurement probes 50 into contact with the pad portion 25c of the measurement connection portion 25 provided on the coupling line 20 and the tip portions of the lead-out line portions 25b, respectively, it can be connected to the pair of measurement ports set at the signal output port in the VNA (see Fig. 2(a)). On the other hand, in the measurement connection portion 15 provided on the output side of the CMF 12b of the differential transmission line 10, the differential transmission line 10 and the lead-out line portion 15b are made conductive by connecting between the branch portion 15a and the lead-out line portion 15b with a jumper. Then, by bringing the tip pins of the pair of measurement probes 51 into contact with the pad portion 15c of the measurement connection portion 15 and the tip portions of the lead-out line portions 15b, respectively, it can be connected to the pair of measurement ports set at the signal input port in the VNA (see Fig. 2(b)).

[0032] Fig. 3 is an equivalent circuit diagram showing the circuit configuration when a measuring device is connected to the signal transmission circuit 1 according to the first embodiment. In Fig. 3, the pair of measurement ports constituting the balance port 1 of the VNA are indicated by the symbols "I / O-P1" and "I / O-P2", and the pair of measurement ports constituting the balance port 2 of the VNA are indicated by the symbols "I / O-P3" and "I / O-P4". Also, the termination resistor 27 provided on the coupling line 20a is indicated by the symbol R1, and the termination resistor 27 provided on the coupling line 20b is indicated by the symbol R2.

[0033] However, the connection form of the measuring device to the signal transmission circuit 1 is not limited to the example in Fig. 2. For example, a board-mounted connector can be installed on the measurement connection portions 15 and 25 (e.g., by soldering, etc.), and the tip plug of a cable (such as a coaxial cable) connected to the measurement port of the measuring device can be inserted into the connector, so that the measuring device can be connected to the signal transmission circuit 1. Even in such a connection form via a connector, it can be described that the measurement port of the measuring device and the measurement connection portions 15 and 25 are electrically connected. The connection form of the measuring device to the signal transmission circuit 1 may be any form as long as it can be easily connected only during measurement and constitutes an equivalent circuit shown in FIG. 3.

[0034] By connecting the 4-port VNA and the signal transmission circuit 1 in this way, it is possible to measure the frequency characteristics of the conversion strength from the common mode noise mixed into the differential transmission line 10 by crosstalk from a pair of coupling lines 20 with the output signal from the VNA as a noise source to the differential mode. That is, this evaluation method includes a step of electrically connecting a pair of measurement ports of the VNA and the measurement connection part 25 of a pair of coupling lines 20, a step of electrically connecting the other pair of measurement ports of the VNA and the measurement connection part 15 of the differential transmission line 10, and a step of measuring the frequency characteristics of the conversion strength from the common mode noise mixed into the differential transmission line 10 by crosstalk from a pair of coupling lines 20 with the output signal from the VNA as a noise source to the differential mode.

[0035] FIG. 4 is a diagram showing an example of the result of evaluating the external noise resistance of the differential transmission line 10 in the signal transmission circuit 1 according to the first embodiment using a 4-port vector network analyzer. In FIG. 4, the vertical axis represents the magnitude of the differential mode noise, and the horizontal axis represents the frequency of the sine wave signal output from the VNA. Also, in FIG. 4, the length of the section sandwiched between CMF12a and 12b in the differential transmission line 10 is set to 68 mm (millimeters), and the length of the coupling line 20 is approximately the same as the length of the above section of the differential transmission line 10 (corresponding to the first embodiment) (dashed line "Ln / Ld = 1" in FIG. 4(b)), half the length (corresponding to the second embodiment described later) (solid line "Ln / Ld = 0.5" in FIG. 4(b)), one-fourth the length (dashed line "Ln / Ld = 0.25" in FIG. 4(a)), and one-tenth the length (solid line "Ln / Ld = 0.1" in FIG. 4(a)) are prepared. Also, a commercially available common mode choke coil is used as CMF12. Then, the sine wave signal output from the VNA was frequency-swept, and the magnitude of the differential mode noise was measured from Sdc21 (the mode conversion characteristic from the common mode component to the differential mode component) between the balance port 1 and the balance port 2 of the VNA.

[0036] As a result, as shown in Fig. 4(b), for the differential transmission line 10 of the signal transmission circuit 1 according to the first embodiment, it was found that resonance peaks of differential mode noise occurred at external radio waves around 1.3 GHz (gigahertz), around 2.5 GHz, around 3.6 GHz, and around 4.9 GHz. Thereby, it can be evaluated that the differential transmission line 10 of the signal transmission circuit 1 according to the first embodiment has low resistance to external noise at each frequency where the above resonance peak occurred.

[0037] Also, from the other graphs in Figs. 4(a) and 4(b), it can be seen that the length of the coupling line 20 with respect to the differential transmission line 10 does not affect the measurement results. That is, the length of the coupling line 20 is not limited to the example in Fig. 1 and can be any length. Furthermore, it has been verified that when the coupling line 20 is short with respect to the differential transmission line 10, any arrangement of the position of the coupling line 20 with respect to the extending direction of the differential transmission line 10 does not affect the measurement results. That is, the coupling line 20 may be arranged at the center in the extending direction of the differential transmission line 10, may be offset toward the transmission circuit side, or may be offset toward the reception circuit side.

[0038] [Second Embodiment] Fig. 5 is a diagram schematically showing the appearance of the signal transmission circuit 1 according to the second embodiment. As shown in Fig. 5, the signal transmission circuit 1 according to the second embodiment is different from the first embodiment in that the length of the coupling line 20 is about half the length of the differential transmission line 10 and the terminating resistor 27 is not provided in the coupling line 20, and other configurations are the same as those in the first embodiment. Hereinafter, regarding the signal transmission circuit 1 according to the second embodiment, the description will focus on the configuration different from the first embodiment, and the description of the same configuration as the first embodiment will be omitted as appropriate.

[0039] In the second embodiment, a termination resistor 27 is not provided at the receiving end of the coupling line 20, and a resistor connection pad portion 26 is provided. The resistor connection pad portion 26 is provided for connection to a termination resistor and is provided at a position spaced a predetermined distance from the receiving end of the coupling line 20. The resistor connection pad portion 26 is connected to the ground plane by a via or the like and can also be called a ground pad.

[0040] During measurement, a termination resistor is attached between the resistor connection pad portion 26 and the receiving end of the coupling line 20 as in the first embodiment. The termination resistor may be attached so that it can be removed after the measurement is completed, or it may be attached by soldering or the like so that it cannot be easily removed even after the measurement is completed. The resistor connection pad portion 26 is a specific example of a resistor connection portion connectable to a termination resistor.

[0041] By connecting a termination resistor to the receiving end of the coupling line 20 during measurement in this way, it is possible to suppress the signal applied from the VNA from reflecting in the coupling line 20, and it is possible to exclude the component caused by the reflection in the coupling line 20 from the measurement result of the external noise in the differential transmission line 10. When the measurement is performed without connecting the termination resistor, if an inconvenient measurement result is obtained, it becomes impossible to distinguish whether the cause is in the differential transmission line 10 or in the coupling line 20. That is, by connecting a termination resistor to the receiving end of the coupling line 20 during measurement, it is possible to maintain the evaluation accuracy regarding the external noise in the differential transmission line 10.

[0042] Incidentally, in the second embodiment, when the measuring device is not connected, both ends of the coupling line 20 are open. That is, it can also be said that the pair of coupling lines 20 become a floating conductor path in a state where the measuring device is not connected to the measurement connection portion 25 and the terminating resistor is not connected to the resistance connection portion.

[0043] FIG. 6 is a graph showing the magnitude of attenuation of the differential mode signal generated in the differential transmission line 10 of the signal transmission circuit 1 according to the first embodiment in a state where the measuring device is not connected (when not in measurement). Here, the CMF12a and 12b provided at both ends of the differential transmission line 10 are excluded from the configuration of the signal transmission circuit 1 according to the first embodiment, and after the divided section after exclusion is jumper-connected, a configuration in which the terminating resistor 27 is provided in the coupling line 20 (corresponding to the first embodiment) and a configuration in which the terminating resistor 27 is not provided in the coupling line 20 (comparative configuration) are targeted. Then, in each of the configuration corresponding to the first embodiment and the comparative configuration, a pair of measurement ports constituting the balance port 1 of the VNA are connected to the end on the transmission circuit side of the differential transmission line 10, and a pair of measurement ports constituting the balance port 2 of the VNA are connected to the end on the receiving circuit side of the differential transmission line 10. The sine wave signal output from the VNA is frequency-swept, and Sdd21 (the passing characteristic of the differential mode (differential) component) between the balance port 1 and the balance port 2 of the VNA is measured.

[0044] In FIG. 6, the magnitude of attenuation of the differential mode signal generated in the differential transmission line 10 in the configuration corresponding to the first embodiment is shown by a solid line, and the magnitude of attenuation of the differential mode signal generated in the differential transmission line 10 in the comparative configuration is shown by a dotted line. In FIG. 6, the vertical axis represents the magnitude of attenuation generated in the differential transmission line 10 of the differential mode signal, and the horizontal axis represents the frequency of the sine wave signal output from the VNA.

[0045] As described above, when not in measurement, that is, when the measuring device is not connected to the measurement connection portion 25 and the terminating resistor is not connected to the resistance connection portion, the coupling line 20 becomes a floating conductor path. For this reason, there is a possibility that the signal transmission circuit 1 may be affected by the coupling line 20 that has become a floating conductor when performing normal differential transmission.

[0046] According to FIG. 6, in the configuration in which the terminating resistor 27 is provided in the coupling line 20 as in the first embodiment, almost no resonance attenuation peak of the differential mode signal occurs. On the other hand, in the configuration (comparative configuration) in which the terminating resistor 27 is not provided in the coupling line 20, it can be seen that resonance attenuation peaks occur in a plurality of frequency bands. Therefore, when the terminating resistor 27 is not provided in the coupling line 20, there is a high possibility that the coupling line 20 that has become a floating conductor will have an adverse effect on the differential transmission in the differential transmission line 10.

[0047] Therefore, in the above-described comparative configuration in which the terminating resistor 27 is not provided in the coupling line 20, the magnitude of attenuation of the differential mode signal generated in the differential transmission line 10 when the length of the coupling line 20 is changed was investigated by the same method as in FIG. 6. FIG. 7 is a graph showing the results of investigating the magnitude of attenuation of the differential mode signal generated in the differential transmission line 10 with the length of the coupling line 20 changed in a state where the measuring device is not connected (when not in measurement). The result when the length of the coupling line 20 is approximately the same as that of the differential transmission line 10 is shown by the dotted line "Ln / Ld = 1" in FIG. 7(b), the result when the length is half is shown by the solid line "Ln / Ld = 0.5" in FIG. 7(b), the result when the length is one-fourth is shown by the broken line "Ln / Ld = 0.25" in FIG. 7(a), and the result when the length is one-tenth is shown by the solid line "Ln / Ld = 0.1" in FIG. 7(a).

[0048] According to FIG. 7, it can be seen that as the length of the coupling line 20 becomes shorter, the resonance attenuation peak of the differential mode signal is reduced, and conversely, as the length becomes longer, the resonance attenuation peak occurs in the differential mode signal. That is, as shown in the second embodiment, it is preferable that the length of each of the pair of coupling lines 20 is 50% or less of the lengths of the respective signal lines 10a and 10b of the differential transmission line 10, more preferably 25% or less, and still more preferably 10% or less. On the other hand, even if the length of the coupling line 20 is short with respect to the differential transmission line 10, as shown in the graph of FIG. 4, it is possible to sufficiently evaluate the external noise resistance of the differential transmission line 10.

[0049] Therefore, according to the second embodiment, even when the terminating resistor 27 is not provided in the coupling line 20, it is possible to reduce the influence exerted by the coupling line 20 that has become a floating conductor on differential transmission in the differential transmission line 10.

[0050] From the above, the method for evaluating the external noise resistance of the differential transmission line 10 in the signal transmission circuit 1 according to the second embodiment is as follows. That is, the evaluation method includes a step of connecting a terminating resistor to the pair of coupling lines 20 using a resistance connection portion (resistance connection pad portion 26), a step of electrically connecting the pair of measurement ports of the VNA and the measurement connection portion 25 of the pair of coupling lines 20, a step of electrically connecting the other pair of measurement ports of the VNA and the measurement connection portion 15 of the differential transmission line 10, and a step of measuring the frequency characteristics of the conversion strength from the common mode noise mixed into the differential transmission line 10 by crosstalk from the pair of coupling lines 20 to the differential mode using the output signal from the VNA as a noise source.

[0051] [Modification Example] Each of the above embodiments is an example of the signal transmission circuit 1. The signal transmission circuit 1 is not limited to only the above-described configuration, and may be partially and appropriately modified, or may have a further configuration.

[0052] For example, in each of the above-described embodiments, the CMFs 12 are provided at both ends of the differential transmission line 10. However, the CMF 12 may not be provided on the transmission circuit side, and may be provided only on the receiving circuit side. In this case, the transmission circuit side of the differential transmission line 10 may be open-terminated, or may be configured as a bridge termination in which the signal line 10a and the signal line 10b are connected via a resistance element. Even in such a form, similar to each of the above-described embodiments, the external noise resistance of the differential transmission line 10 can be evaluated. Further, the pair of signal lines 10a and 10b of the differential transmission line 10 may be formed of a twisted pair line instead of a microstrip line configuration. For example, on the receiving circuit side of the differential transmission line 10, up to the CMF 12b is formed of a twisted pair line, and the output side of the CMF 12b is formed in the same manner as in each of the above-described embodiments. Even when the pair of signal lines 10a and 10b are formed of a twisted pair line in this way, the coupling line 20 and other configurations may be the same as in each of the above-described embodiments.

[0053] Also, in each of the above-described embodiments, the differential transmission line 10 and the coupling line 20 are arranged on the same substrate surface of the dielectric substrate (printed circuit board), but may be respectively wired in different layers within the multilayer dielectric substrate. In this case, the pair of coupling lines 20 are arranged so as to sandwich from both sides in the width direction of the differential transmission line 10 when viewed from the stacking direction of the multilayer dielectric substrate, or each coupling line 20 is arranged to be line-symmetrical with respect to the virtual center line between the pair of signal lines 10a and 10b of the differential transmission line 10.

[0054] FIG. 8 is a schematic diagram showing an example of the arrangement of the differential transmission line 10 and the coupling line 20 when the signal transmission circuit 1 is formed of a multilayer dielectric substrate. FIG. 8 shows an example in which the signal transmission circuit 1 is formed of a multilayer dielectric substrate including at least three layers of a first-layer dielectric 61, a second-layer dielectric 62, and a third-layer dielectric 63.

[0055] In the example of FIG. 8(a), a pair of coupling lines 20 are arranged on the first dielectric layer 61, and a differential transmission line 10 is arranged on the second dielectric layer 62. The pair of coupling lines 20 are arranged so as to sandwich both sides in the width direction of the differential transmission line 10 when viewed from the stacking direction of the multilayer dielectric substrate (the vertical direction of the paper surface of FIG. 8). In the example of FIG. 8(b), a coupling line 20b is arranged on the first dielectric layer 61, a differential transmission line 10 is arranged on the second dielectric layer 62, and a coupling line 20a is arranged on the third dielectric layer 63. The pair of coupling lines 20 are arranged so as to sandwich both sides in the width direction of the differential transmission line 10 when viewed from the stacking direction of the multilayer dielectric substrate (the vertical direction of the paper surface of FIG. 8). In the example of FIG. 8(c), similar to FIG. 8(b), a coupling line 20b is arranged on the first dielectric layer 61, a differential transmission line 10 is arranged on the second dielectric layer 62, and a coupling line 20a is arranged on the third dielectric layer 63. And the pair of coupling lines 20 are arranged so as to place the virtual center line between the pair of signal lines 10a and 10b of the differential transmission line 10 at the center in the extending direction. That is, the pair of coupling lines 20 are arranged so that each coupling line 20 is line-symmetric with respect to the virtual center line between the pair of signal lines 10a and 10b of the differential transmission line 10.

[0056] Even if the differential transmission line 10 and the coupling lines 20 are respectively wired in different layers in the multilayer dielectric substrate, they are arranged so as to sandwich both sides in the width direction of the differential transmission line 10 when viewed from the stacking direction of the multilayer dielectric substrate or each coupling line 20a and 20b is line-symmetric with respect to the virtual center line between the pair of signal lines 10a and 10b of the differential transmission line 10. If so, the same operational effects as those of the above-described embodiments can be obtained.

[0057] Also, in the above-described first embodiment, a termination resistor 27 is provided at the receiving-side end of the coupling line 20. In the above-described second embodiment, at the time of measurement, the termination resistor 27 is connected to the receiving-side end of the coupling line 20 using the resistance connection portion (resistance connection pad portion 26). However, if a decrease in measurement accuracy is acceptable, a configuration in which the termination resistor 27 is not provided in the first embodiment, a configuration in which the resistor connection portion (resistor connection pad portion 26) is not provided in the second embodiment, that is, a configuration in which the termination resistor 27 is not provided at the receiving end of the coupling line 20 even during measurement can also be adopted.

[0058] FIG. 9 is a diagram comparing the results of evaluating the external noise resistance of the differential transmission line 10 using a four-port vector network analyzer when the termination resistor 27 is connected and when it is not connected in the signal transmission circuit 1 according to the first embodiment. As shown in FIG. 9, when the termination resistor 27 is not connected, the resonance peak of the differential mode noise is measured to be larger. On the other hand, the resonance peak frequencies are approximately the same in any case. Thus, it is possible to evaluate the external noise resistance of the differential transmission line 10 even in a configuration where the termination resistor 27 is not provided at the receiving end of the coupling line 20 during measurement. However, in a configuration where the termination resistor 27 is not provided, since the signal applied from the VNA reflects in the coupling line 20, from the viewpoint of evaluation accuracy, the configurations of the above-described first and second embodiments are more preferable.

[0059] The content of each of the above-described embodiments can also be specified as follows. (Appendix 1) A differential transmission line including a pair of signal lines, A pair of coupling lines extending in parallel with the differential transmission line at a proximity position that electromagnetically couples to the differential transmission line, Comprising, A common mode filter is provided on one end side of the differential transmission line, A measurement connection portion enabling connection to a measuring device capable of measuring electrical characteristics is provided on the output side of the common mode filter on the one end side of the differential transmission line, A measurement connection portion enabling connection to the measuring device is provided at one end of the pair of coupling lines, Signal transmission circuit. (Appendix 2) At the other ends of the pair of coupling lines, a resistance connection part connectable to a termination resistor is provided. The signal transmission circuit according to Supplementary Note 1. (Supplementary Note 3) The length of each of the pair of coupling lines is 50% or less of the length of each signal line of the differential transmission line. The signal transmission circuit according to Supplementary Note 1. (Supplementary Note 4) The differential transmission line and the pair of coupling lines are arranged on the same substrate surface of a dielectric substrate. The pair of coupling lines are arranged so as to sandwich the differential transmission line from both sides in the width direction on the same substrate surface. The signal transmission circuit according to any one of Supplementary Notes 1 to 3. (Supplementary Note 5) The differential transmission line and the pair of coupling lines are respectively wired in different layers within a multilayer dielectric substrate. The pair of coupling lines are arranged so as to sandwich the differential transmission line from both sides in the width direction when viewed from the stacking direction of the multilayer dielectric substrate or are arranged so that each coupling line is line-symmetric with respect to the virtual center line between the pair of signal lines of the differential transmission line. The signal transmission circuit according to any one of Supplementary Notes 1 to 3. (Supplementary Note 6) A method for evaluating the external noise resistance of the differential transmission line in the signal transmission circuit according to Supplementary Note 1 using a 4-port vector network analyzer, electrically connecting a pair of measurement ports of the vector network analyzer and the measurement connection part of the pair of coupling lines, electrically connecting the other pair of measurement ports of the vector network analyzer and the measurement connection part of the differential transmission line, using the output signal from the vector network analyzer as a noise source and measuring the frequency characteristic of the conversion strength from the common mode noise mixed into the differential transmission line by crosstalk from the pair of coupling lines to the differential mode, A method for evaluating the external noise resistance of a differential transmission line including this. (Supplementary Note 7) At the other ends of the pair of coupling lines in the signal transmission circuit, a resistor connection part connectable to a terminating resistor is provided. Before the measurement of the pair of frequency characteristics, connect a terminating resistor to the other ends of the pair of coupling lines using the resistor connection part. A method for evaluating the external noise resistance of the differential transmission line according to supplementary note 6, which further includes this.

Explanation of symbols

[0060] 1 Signal transmission circuit, 10 Differential transmission line, 10a and 10b Signal lines, 12 Common mode filter (CMF), 15 Measurement connection part, 20, 20a and 20b Coupling lines, 25 Measurement connection part, 26 Resistor connection pad part, 27 Terminating resistor, 40 Jumper, 50 Measurement probe, 51 Measurement probe, 61 First layer dielectric, 62 Second layer dielectric, 63 Third layer dielectric

Claims

1. A differential transmission line including a pair of signal lines, A pair of coupling lines extending in parallel with the differential transmission line at a proximity position that electromagnetically couples to the differential transmission line, Comprising, A common mode filter is provided on one end side of the differential transmission line, On the output side of the common mode filter at the one end side of the differential transmission line, a measurement connection part enabling connection to a measuring device capable of measuring electrical characteristics is provided, At one end part of the pair of coupling lines, a measurement connection part enabling connection to the measuring device is provided, A signal transmission circuit.

2. At the other end part of the pair of coupling lines, a resistance connection part connectable to a termination resistor is provided, The signal transmission circuit according to Claim 1.

3. The length of each of the pair of coupling lines is 50% or less of the length of each signal line of the differential transmission line, The signal transmission circuit according to Claim 1.

4. The differential transmission line and the pair of coupling lines are arranged on the same substrate surface of a dielectric substrate, The pair of coupling lines are arranged so as to sandwich the differential transmission line from both sides in the width direction on the same substrate surface, The signal transmission circuit according to any one of Claims 1 to 3.

5. The differential transmission line and the pair of coupling lines are respectively wired in different layers within a multilayer dielectric substrate, The pair of coupling lines are arranged so as to sandwich the differential transmission line from both sides in the width direction when viewed from the stacking direction of the multilayer dielectric substrate or are arranged such that each coupling line is line-symmetric with respect to the virtual center line between the pair of signal lines of the differential transmission line, The signal transmission circuit according to any one of Claims 1 to 3.

6. A method for evaluating the external noise resistance of the differential transmission line in the signal transmission circuit according to Claim 1 using a 4-port vector network analyzer, Electrically connecting a pair of measurement ports of the vector network analyzer and the measurement connection part of the pair of coupling lines, Electrically connecting the other pair of measurement ports of the vector network analyzer and the measurement connection part of the differential transmission line, Using the output signal from the vector network analyzer as a noise source, measuring the frequency characteristic of the conversion strength from the common mode noise mixed into the differential transmission line by crosstalk from the pair of coupling lines to the differential mode, A method for evaluating the external noise resistance of a differential transmission line including the following.

7. At the other ends of the pair of coupling lines in the signal transmission circuit, a resistance connection part connectable to a termination resistor is provided, Before the measurement of the pair of frequency characteristics, a termination resistor is connected to the other ends of the pair of coupling lines using the resistance connection part, The method for evaluating the external noise resistance of the differential transmission line according to claim 6, further including the following.

Images