Differential transmission circuit

Abstract

To provide a differential transmission circuit that suppresses reflections that may occur during signal transmission.SOLUTION: A differential transmission circuit according to an embodiment includes: a main line having a pair of signal transmission lines and a pair of termination resistors; and multiple branch lines connected to the main line and each having a pair of signal transmission lines. The pair of termination resistors include a first resistor and a second resistor, and the pair of signal transmission lines in the main line include a first transmission line and a second transmission line. The first resistor is connected to one end of the first transmission line and one end of the second transmission line. The second resistor is connected to the other end of the first transmission line and the other end of the second transmission line. A differential impedance of the pair of signal transmission lines in the main line is at least twice a resistance value of the termination resistor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments relate to a differential transmission circuit. [Background technology]

[0002] Techniques have been developed for transmitting signals between devices in which multiple modules are connected to the same bus line. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3407469 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the embodiments is to provide a differential transmission circuit that suppresses reflections that may occur during signal transmission. [Means for solving the problem]

[0005] A differential transmission circuit according to an embodiment includes a main line having a pair of signal transmission lines and a pair of termination resistors, and multiple branch lines connected to the main line and each having a pair of signal transmission lines. The pair of termination resistors includes a first resistor and a second resistor, and the pair of signal transmission lines in the main line includes a first transmission line and a second transmission line. The first resistor is connected to one end of the first transmission line and one end of the second transmission line, and the second resistor is connected to the other end of the first transmission line and the other end of the second transmission line. The differential impedance of the pair of signal transmission lines in the main line is at least twice the resistance value of the termination resistor. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a schematic diagram of a differential transmission circuit according to an embodiment; [Figure 2]10A and 10B are schematic diagrams illustrating the operation of a differential transmission circuit according to a modified example. [Figure 3] 4 is a graph showing the operation of the differential transmission circuit in the example. [Figure 4] 4 is a graph showing the operation of the differential transmission circuit in the example. DETAILED DESCRIPTION OF THE INVENTION

[0007] <1. Embodiment> (1.1. Configuration of differential transmission circuit 1) A differential transmission circuit 1 according to an embodiment will be described below. As shown in Fig. 1, the differential transmission circuit 1 includes a main line 10 and a branch line 20. The main line 10 includes a pair of signal transmission lines 13 consisting of a first transmission line 13a and a second transmission line 13b, and a pair of termination resistors 12 consisting of a first resistor 12a and a second resistor 12b.

[0008] The first resistor 12a and the second resistor 12b each function as a termination resistor. Specifically, the first resistor 12a is connected to one end of the first transmission line 13a and one end of the second transmission line 13b. The second resistor 12b is connected to the other end of the first transmission line 13a and the other end of the second transmission line 13b.

[0009] A plurality of branch lines 20, each having a pair of signal transmission lines 23, are connected to the main line 10. The pair of signal transmission lines 23 in the branch line 20 includes a first branch transmission line 23a and a second branch transmission line 23b. The first branch transmission line 23a is connected to the first transmission line 13a of the main line 10 at a first line branch point 11a. The second branch transmission line 23b is connected to the second transmission line 13b of the main line 10 at a second line branch point 11b. In the following description, the first line branch point 11a and the second line branch point 11b are also collectively referred to as branch points 11.

[0010] A transmitting / receiving device 40 is provided at the end of the branch line 20. The transmitting / receiving device 40 has a transmitting IF (Interface) 41 and a receiving IF 42 that are connected to a circuit board (not shown). A signal transmitted from a board circuit is transmitted to the branch line 20 through the transmitting IF 41, transmitted from the main line 10 to another branch line 20, and received by another board circuit through the receiving IF 42.

[0011] The first resistor 12a and the second resistor 12b may have the same resistance value. The differential impedance Zd of the pair of signal transmission lines 13 in the main line 10 is at least twice the resistance value of the termination resistor 12. If the first resistor 12a and the second resistor 12b have different resistance values, the differential impedance Zd of the signal transmission line 13 may be at least twice the smaller of the resistance values ​​of the first resistor 12a and the second resistor 12b.

[0012] In addition, in the first transmission line 13a of the main line 10, the adjacent line length Lb, which is the length from the first line branch point 11a closest to the first resistor 12a to the adjacent first line branch point 11a, is less than or equal to the terminal line length Lt, which is the length from the second resistor 12b to the first line branch point 11a closest to the second resistor 12b.

[0013] Similarly, in the second transmission line 13b of the main line 10, the adjacent line length Lb, which is the length from the second line branch point 11b closest to the first resistor 12a to the adjacent second line branch point 11b, is less than or equal to the terminal line length Lt, which is the length from the second resistor 12b to the second line branch point 11b closest to the second resistor 12b.

[0014] (1.2. Operation of differential transmission circuit 1) 1(b), the operation of the differential transmission circuit 1 will be described. In the differential transmission circuit 1 in which multiple branch lines 20 are connected to a main line 10, reflections occur at branch points 11 of the main line 10. For example, when a signal is transmitted from a transceiver device 40a provided at the end of the branch line 20 closest to the first resistor 12a to a transceiver device 40b provided at the end of an adjacent branch line 20, a signal reflected at another branch point 11, as in the case of path R1, is transmitted to the transceiver device 40b. Such a signal reflected at another branch point 11 can cause distortion of the signal waveform.

[0015] In contrast, in the differential transmission circuit 1 according to the present embodiment, as described above, the differential impedance of the pair of signal transmission lines 13 in the main line 10 is set to be at least twice the resistance value of the termination resistor 12, and the adjacent line length Lb is set to be equal to or less than the termination line length Lt. With this configuration, when a signal is transmitted from the transceiver device 40a provided at the end of the branch line 20 closest to the first resistor 12a to a transceiver device 40 provided at the end of another branch line 20, an opposite-phase signal reflected by the termination resistor 12 can be generated so as to pass through the path R2. As a result, the signal that has passed through the path R1 and the opposite-phase signal that has passed through the path R2 cancel each other out, thereby suppressing distortion of the signal waveform.

[0016] (1.3. Variation 1) A differential transmission circuit 1 according to a modification of this embodiment will be described, focusing on differences from the above embodiment, with reference to Fig. 2. In a differential transmission circuit 2 according to modification 1, in addition to the features of the above embodiment, as shown in Fig. 2(a), in the first transmission line 13a, an adjacent line length Lb', which is the length from the first line branch point 11a closest to the second resistor 12b to the adjacent first line branch point 11a, is equal to or shorter than a termination line length Lt', which is the length from the first resistor 12a to the first line branch point 11a closest to the first resistor 12a.

[0017] Similarly, in the second transmission line 13b, the adjacent line length Lb', which is the length from the second line branch point 11b closest to the second resistor 12b to the adjacent second line branch point 11b, is less than or equal to the terminal line length Lt', which is the length from the first resistor 12a to the second line branch point 11b closest to the first resistor 12a.

[0018] With this configuration, even when a signal is transmitted from the transceiver device 40c provided at the end of the branch line 20 closest to the second resistor 12b to a transceiver device 40 provided at the end of another branch line 20, it is possible to suppress signal waveform disturbances due to signal reflections that may occur at the branch point 11 of the main line 10, as in the above embodiment.

[0019] (1.4. Variation 2) In the differential transmission circuit 3 according to the second modification, as shown in FIG. 2(b), the lengths between the branch points 11 on the first transmission line 13a and the second transmission line are substantially equal. That is, the lengths between the first line branch points 11a and the second line branch points 11b are both the adjacent line length Lb. Moreover, the lengths from the first resistor 12a to the branch point 11 closest to the first resistor 12a and from the second resistor 12b to the branch point 11 closest to the second resistor 12b are both the termination line length Lt. This configuration facilitates the design of a differential transmission circuit 3 that can achieve the effects of the above-described embodiment.

[0020] <2. Example> Below, examples of the differential transmission circuit 1 according to this embodiment will be described. In the following examples, the differential impedance of the signal transmission line 13 in the main line 10 is set to Zd. The resistance values ​​of the first resistor 12a and the second resistor 12b are set to Rt.

[0021] (2.1. Signal waveform when differential impedance Zd is a variable) 3, a simulation of a signal waveform when the differential impedance Zd is a variable will be described. In this example, the waveform of a signal transmitted from the transceiver 40a, which is provided at the end of the branch line 20 closest to the first resistor 12a, to the transceiver 40b, which is provided at the end of the adjacent branch line 20, was detected while the differential impedance Zd of the signal transmission line 13 was changed. In the first transmission line 13a and the second transmission line, the adjacent line length Lb, which is the length from the branch point 11 closest to the first resistor 12a to the branch point 11 adjacent to that branch point 11, was set to 20 mm. Furthermore, the termination line length Lt, which is the length from the second resistor 12b to the branch point 11 closest to the second resistor 12b, was set to 120 mm.

[0022] FIG. 3(a) shows the signal waveform when Zd=Rt. In region P1, it can be seen that the signal waveform fluctuates so as to straddle the threshold value TH. The threshold value TH indicates the signal strength required to determine whether the circuit board is on or off. When the signal waveform fluctuates so as to straddle the threshold value TH, the on / off determination on the circuit board becomes unstable. In other words, when Zd=Rt, it was found that the signal waveform was distorted due to the influence of reflections that occur at the branch point 11 of the main line 10.

[0023] 3(b) shows the signal waveform when Zd=1.5Rt. In this case, too, it can be seen that the signal waveform fluctuates across the threshold TH in region P2. In other words, even when Zd=1.5Rt, it was found that the signal waveform was disturbed by the influence of reflections occurring at branch point 11 of main line 10.

[0024] 3(c) shows the signal waveform when Zd=2Rt. In this case, it can be seen that the signal waveform fluctuates in the region P3 where it exceeds the threshold value TH. In other words, when Zd=2Rt, it was found that the influence of the reflection occurring at the branch point 11 of the main line 10 was suppressed so that the behavior of the circuit board was stabilized.

[0025] Figure 3(d) shows the signal waveform when Zd = 3Rt. Figure 3(e) shows the signal waveform when Zd = 4Rt. Even in these cases, it can be seen that the signal waveform fluctuates in areas P4 and P5 where it exceeds the threshold TH. In other words, when Zd = 3Rt and when Zd = 4Rt, it was found that the effects of reflections occurring at branch point 11 of main line 10 were suppressed so that the behavior of the circuit board was stabilized.

[0026] From the above, in a simulation of the signal waveform when the differential impedance Zd is a variable, it was found that the effect of reflection occurring at the branch point 11 of the main line 10 can be suppressed when the differential impedance of the signal transmission line 13 is set to be at least twice that of the termination resistor 12.

[0027] (2.2. Signal waveform when the termination line length Lt is a variable) 4, a signal waveform will be described when the termination line length Lt is a variable. In this example, the waveform of a signal transmitted from the transceiver device 40a provided at the end of the branch line 20 closest to the first resistor 12a to the transceiver device 40b provided at the end of the adjacent branch line 20 was detected while the termination line length Lt was changed. Note that, in the first transmission line 13a and the second transmission line, the adjacent line length Lb, which is the length from the branch point 11 closest to the first resistor 12a to the adjacent branch point 11, was set to 40 mm. The differential impedance Zd of the signal transmission line 13 was set to twice the resistance value Rt of the first resistor 12a and the second resistor 12b.

[0028] 4(a) shows the signal waveform when Lt = 1 mm. In region P6, it can be seen that the signal waveform fluctuates across the threshold TH. In other words, when Lt = 1 mm, it was found that the signal waveform was disturbed by the influence of reflections occurring at branch point 11 of main line 10.

[0029] Figure 4(b) shows the signal waveform when Lt = 40 mm. In region P7, it can be seen that the signal waveform fluctuates in the region above threshold TH. In other words, when Lt = 40 mm, it was found that the influence of reflections occurring at branch point 11 of main line 10 was suppressed so that the behavior of the circuit board was stabilized.

[0030] Figure 4(c) shows the signal waveform when Lt = 100 mm. In region P8, it can be seen that the signal waveform fluctuates in the region above threshold TH. In other words, even when Lt = 100 mm, it was found that the influence of reflections occurring at branch point 11 of main line 10 was suppressed so that the behavior of the circuit board was stable.

[0031] From the above, in a simulation of a signal waveform when the termination line length Lt is a variable, it was found that when the adjacent line length Lb is set to be less than the termination line length Lt, the effect of reflection occurring at the branch point 11 of the main line 10 can be suppressed so that the behavior of the circuit board is stable.

[0032] <3. Other embodiments> Although the embodiments of the present disclosure have been described above, they are not limited to the above. The above embodiments merely exemplify the case where the differential impedance Zd of the signal transmission line 13 is at least twice that of the termination resistor 12. However, any configuration may be added or modified as long as this condition is met.

[0033] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

[0034] The present invention includes the following aspects. (Appendix 1) a main line having a pair of signal transmission lines and a pair of termination resistors; and a plurality of branch lines connected to the main line and having the pair of signal transmission lines, wherein the pair of termination resistors includes a first resistor and a second resistor, the pair of signal transmission lines in the main line includes a first transmission line and a second transmission line, the first resistor is connected to one end of the first transmission line and one end of the second transmission line, the second resistor is connected to the other end of the first transmission line and the other end of the second transmission line, and the differential impedance of the pair of signal transmission lines is at least twice the resistance value of the termination resistors. (Appendix 2) the pair of signal transmission lines in the branch line includes a first branch transmission line and a second branch transmission line, the first branch transmission line is connected to the first transmission line, and the second branch transmission line is connected to the second transmission line, and in the first transmission line, an adjacent line length, which is the length from a branch point closest to the first resistor to a branch point adjacent to the branch point, is equal to or shorter than a termination line length, which is the length from the second resistor to the branch point closest to the second resistor. (Appendix 3) 3. The differential transmission circuit according to claim 2, wherein an adjacent line length, which is the length from the branch point closest to the first resistor to the branch point adjacent to the branch point, in the second transmission line is equal to or less than a termination line length, which is the length from the second resistor to the branch point closest to the second resistor. (Appendix 4) 3. The differential transmission circuit according to claim 2, wherein an adjacent line length, which is the length from a branch point closest to the second resistor to a branch point adjacent to the branch point, in the first transmission line is equal to or shorter than a termination line length, which is the length from the first resistor to a branch point closest to the first resistor. (Appendix 5) 5. The differential transmission circuit according to claim 4, wherein in the second transmission line, an adjacent line length, which is the length from the branch point closest to the second resistor to the branch point adjacent to the branch point, is equal to or shorter than a termination line length, which is the length from the first resistor to the branch point closest to the first resistor. (Appendix 6) 6. The differential transmission circuit according to claim 1, wherein the lengths between branch points of the first transmission line and the second transmission line are substantially equal. [Explanation of symbols]

[0035] 1: differential transmission circuit, 2: differential transmission circuit, 3: differential transmission circuit, 10: main line, 11: branch point, 11a: first line branch point, 11b: second line branch point, 12: termination resistor, 12a: first resistor, 12b: second resistor, 13: signal transmission line, 13a: first transmission line, 13b: second transmission line, 20: branch line, 23: signal transmission line, 23a: first branch transmission line, 23b: second branch transmission line, 40: transmitting / receiving device, 41: transmitting IF, 42: receiving IF

Claims

1. a main line having a pair of signal transmission lines and a pair of termination resistors; a plurality of branch lines connected to the main line and each having a pair of signal transmission lines; the pair of termination resistors includes a first resistor and a second resistor, the pair of signal transmission lines in the main line includes a first transmission line and a second transmission line, the first resistor is connected to one end of the first transmission line and one end of the second transmission line; the second resistor is connected to the other end of the first transmission line and the other end of the second transmission line; A differential transmission circuit, wherein the differential impedance of a pair of signal transmission lines in the main line is at least twice the resistance value of the termination resistor.

2. the pair of signal transmission lines in the branch line includes a first branch transmission line and a second branch transmission line; the first branch transmission line is connected to the first transmission line; the second branch transmission line is connected to the second transmission line; 2. The differential transmission circuit according to claim 1, wherein an adjacent line length, which is the length from a branch point closest to the first resistor to a branch point adjacent to the branch point, in the first transmission line is equal to or less than a termination line length, which is the length from the second resistor to the branch point closest to the second resistor.

3. 3. The differential transmission circuit according to claim 2, wherein an adjacent line length, which is the length from a branch point closest to the first resistor to a branch point adjacent to the branch point, in the second transmission line is equal to or less than a termination line length, which is the length from the second resistor to the branch point closest to the second resistor.

4. 3. The differential transmission circuit according to claim 2, wherein an adjacent line length, which is the length from a branch point closest to the second resistor to a branch point adjacent to the branch point, in the first transmission line is equal to or less than a termination line length, which is the length from the first resistor to the branch point closest to the first resistor.

5. 5. The differential transmission circuit according to claim 4, wherein an adjacent line length, which is the length from a branch point closest to the second resistor to a branch point adjacent to the branch point, in the second transmission line is equal to or less than a termination line length, which is the length from the first resistor to the branch point closest to the first resistor.

6. 6. The differential transmission circuit according to claim 1, wherein the lengths between the branch points of the first transmission line and the second transmission line are substantially equal.

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