Signal transmission circuit and electronic apparatus

Abstract

To increase intensity of a signal to transmit.SOLUTION: In an electronic apparatus 1000, a signal transmission circuit 1 includes: a common mode filter 12 including first and second inductors 121, 122; second circuits 18, 19 including first and second capacitors 181, 191; a third circuit 20 having an input end 201 and an output end 202, for reflecting a predetermined amount of signals in a predetermined frequency band which includes a frequency of a transmission signal in an input signal, and for passing other components; a first transmission line 13 connected to an end of the first inductor, from a communication circuit 11 which outputs a differential signal, through the first circuit; a second transmission line 14 having characteristic impedance different from the impedance of the third circuit, connected to an end of the second inductor from the communication circuit through the second circuit; a third transmission line 15 connected to the other end of the first inductor and an output terminal; a fourth transmission line 16 connected to the other end of the second inductor and the input end; and a fifth transmission line 17 connected to the output end and the ground.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a signal transmission circuit and an electronic device.

Background Art

[0002] Patent Document 1 discloses a transmission circuit that includes a common-mode filter (common-mode choke) having two input connection portions and two output connection portions and transmits a signal via a coaxial cable. And it is disclosed that one of the two output connection portions is connected to the coaxial cable and the other is connected to the chassis via a series connection of a resistor and a capacitor.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] It is required to improve the strength of the signal to be transmitted.

Means for Solving the Problems

[0005] The signal transmission circuit according to the embodiment includes a common mode filter including a first inductor and a second inductor, a first circuit including a first capacitor, a second circuit including a second capacitor and having the same circuit configuration as the first circuit, a third circuit having a circuit configuration different from that of the second circuit, reflecting a predetermined amount of signals in a predetermined frequency band including the frequency of the transmission signal among the input signals and passing other components, and having an input end and an output end, a first transmission line connected to one end of the first inductor via the first circuit from a communication circuit that outputs a differential signal, a second transmission line connected to one end of the second inductor via the second circuit from the communication circuit and having a characteristic impedance different from the impedance of the third circuit, a third transmission line connected to the other end of the first inductor and the output terminal, a fourth transmission line connected to the other end of the second inductor and the input end, and a fifth transmission line connected to the output end and the ground.

Advantages of the Invention

[0006] According to the present invention, the strength of the signal to be transmitted can be improved.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0008] The signal transmission circuit according to the present disclosure is configured as follows. Hereinafter, the same or equivalent components, members, or steps shown in each drawing are denoted by the same reference numerals, and duplicate explanations are omitted as appropriate. Also, the dimensions of each member in each drawing are appropriately enlarged or reduced for ease of understanding.

[0009] In addition, in this specification, the expression "equal" includes cases where two elements are not completely equal but can be regarded as substantially equal. That is, the expression "equal" includes cases where two elements can be regarded as substantially equal, for example, when they differ by about several percent.

[0010] (First Embodiment) FIG. 1 is a schematic diagram of the configuration of an electronic device according to the first embodiment. In FIG. 1, for ease of understanding, in addition to the electronic device 1000, the electronic device 2000 whose signal is to be transmitted and the coaxial cable 3000 are also shown.

[0011] As shown in FIG. 1, the electronic device 1000 is connected to the electronic device 2000 via the coaxial cable 3000. The electronic device 1000 and the electronic device 2000 are, for example, in-vehicle devices. The electronic device 1000 as an in-vehicle device is, for example, an in-vehicle camera that images the periphery of a vehicle.

[0012] Also, the electronic device 2000 as an in-vehicle device is, for example, a display device. The electronic device 2000 is, for example, a display device mounted on a vehicle such as a car navigation, a display audio, or a monitor. The electronic device 2000 may be a display device that displays an image captured by the in-vehicle camera. The electronic device 1000 includes a signal transmission circuit 1. The signal transmission circuit 1 transmits the signal generated by the electronic device 1000 to the electronic device 2000 via the coaxial cable 3000. For example, when the electronic device 1000 is an in-vehicle camera, the signal transmission circuit 1 transmits the signal of the image captured by the electronic device 1000 to the electronic device 2000 via the coaxial cable 3000.

[0013] The signal transmission circuit 1 includes a common mode filter 12 including a first inductor 121 and a second inductor 122, a first transmission line 13, a second transmission line 14, a third transmission line 15, a fourth transmission line 16, a fifth transmission line 17, a first circuit 18, a second circuit 19, and a third circuit 20.

[0014] Also, the signal transmission circuit 1 may include a communication circuit 11 and an output terminal 21. By providing the communication circuit 11 in the signal transmission circuit 1, the lengths of the first transmission line 13 and the second transmission line 14 can be shortened compared to the case where the communication circuit 11 is provided outside the signal transmission circuit 1. Therefore, the attenuation amount of the energy of the signal flowing through the transmission line can be reduced. Also, the possibility of noise generation in the signals flowing through the first transmission line 13 and the second transmission line 14 can be reduced.

[0015] Furthermore, when the electronic device 1000 is an in-vehicle device, the electronic device 1000 may include a housing, and the signal transmission circuit 1 may include a body ground 22 and a housing ground 23. The body ground 22 is a connection point between the vehicle body and the signal transmission circuit 1. Also, the housing ground 23 is a connection point between the housing of the electronic device 1000 and the signal transmission circuit 1. Note that since the electronic device 1000 is stored in a non-conductive case, for example, a resin case, there may be a case where the signal transmission circuit 1 cannot be connected to the vehicle body. In that case, the signal transmission circuit 1 does not have to include the body ground 22.

[0016] In the following description, the case where the signal transmission circuit 1 includes the communication circuit 11, the output terminal 21, the body ground 22, and the housing ground 23 will be described as an example.

[0017] The communication circuit 11 has a first output node 111 and a second output node 112, and outputs a differential signal via the first output node 111 and the second output node 112. One end of the first transmission line 13 is connected to the first output node 111 of the communication circuit 11. Thereby, the communication circuit 11 outputs a first signal as a differential signal from the output node 111 to the first transmission line 13.

[0018] Also, one end of the third transmission line 15 is connected to the second output node 112 of the communication circuit 11. Thereby, the communication circuit 11 outputs a second signal as a differential signal from the second output node 112 to the third transmission line 15. Here, the first signal and the second signal are high-frequency signals with opposite phases to each other. The signal transmission circuit 1 of the present embodiment transmits a high-frequency signal of 100 MHz or more as an example.

[0019] The other end of the first transmission line 13 is connected to one end of the first inductor 121 via the first circuit 18. The first signal output from the first output node 111 is input to the first inductor 121 via the first transmission line 13. The first circuit 18 includes a first capacitor 181. The first circuit 18 may include only the first capacitor 181. The first capacitor 181 blocks the DC component among the DC component and the AC component included in the first signal. As an example, in the present embodiment, the capacitance of the first capacitor 181 is 100 nF.

[0020] The other end of the second transmission line 14 is connected to one end of the second inductor 122 via the second circuit 19. The second signal output from the second output node 112 is input to the second inductor 122 via the third transmission line 15. The first circuit 18 and the second circuit 19 have the same circuit configuration. The second circuit 19 includes a second capacitor 191. The second circuit 19 may include only the second capacitor 191. The second capacitor 191 blocks the DC component among the DC component and the AC component included in the second signal. As an example, in the present embodiment, the capacitance of the second capacitor 191 is 100 nF.

[0021] In this embodiment, the first transmission line 13 and the second transmission line 14 are formed of, for example, copper wires. The characteristic impedance of the first transmission line 13 and the second transmission line 14 is determined by, for example, the thickness of the copper wires.

[0022] By the way, when the characteristic impedance of the first transmission line 13 is different from the characteristic impedance of the second transmission line 14, the waveforms of the first signal and the second signal change in the process of passing through the first transmission line 13 and the second transmission line 14, and there is a possibility that the first signal and the second signal are not in opposite phases to each other. As a result, common-mode noise may occur in the first signal and the second signal. However, in this embodiment, the characteristic impedances of the first transmission line 13 and the second transmission line 14 are both, for example, 50 Ω. That is, the characteristic impedance of the first transmission line 13 is equal to the characteristic impedance of the second transmission line 14. Thereby, the signal transmission circuit 1 in this embodiment can reduce the possibility of common-mode noise occurring in the first signal and the second signal.

[0023] The common-mode filter 12 includes a first inductor 121 and a second inductor 122. One end of the first inductor 121 is connected to the first transmission line 13, and the other end is connected to the third transmission line 15. The first signal is input to the first inductor 121 from the first transmission line 13. Further, the first inductor 121 outputs a third signal, which is a signal based on the first signal, to the third transmission line 15.

[0024] One end of the second inductor 122 is connected to the second transmission line 14, and the other end is connected to the fourth transmission line 16. The second signal is input to the second inductor 122 from the second transmission line 14. Further, the second inductor 122 outputs a fourth signal, which is a signal based on the second signal, to the fourth transmission line 16.

[0025] The common mode filter 12 is a filter that attenuates the common mode noise contained in the input first signal and second signal and allows components that are not common mode noise to pass through. That is, by including the common mode filter 12, the signal transmission circuit 1 can reduce the common mode noise when common mode noise is present in the first signal and the second signal. When common mode noise is present in the first signal and the second signal, the third signal is the signal obtained by attenuating the common mode noise contained in the first signal, and the fourth signal is the signal obtained by attenuating the common mode noise contained in the second signal.

[0026] Therefore, when no common mode noise is present in the first signal and the second signal, the first signal and the third signal have the same waveform, and the second signal and the fourth signal have the same waveform.

[0027] The third transmission line 15 is connected to the other end of the first inductor 121 and the output terminal 21. Also, a coaxial cable 3000 is connected to the output terminal 21. The third signal input from the first inductor 121 to the third transmission line 15 is output to the coaxial cable 3000 via the output terminal 21.

[0028] The third circuit 20 has an input end 201 and an output end 202. The third circuit 20 has a circuit configuration different from that of the first circuit 18 and the second circuit 19. Specifically, the third circuit 20 may include a first resistor 203 and a third capacitor 204 connected in parallel to the first resistor 203. As an example, in the present embodiment, the resistance value of the first resistor 203 is 50 Ω, and the capacitance of the third capacitor 204 is 10 pF. In the present embodiment, the characteristic impedance of the second transmission line 14 and the impedance of the third circuit 20 are different. The impedance of the third circuit 20 may be smaller or larger than the characteristic impedance of the second transmission line 14. In the present embodiment, the impedance of the third circuit 20 is smaller than the characteristic impedance of the second transmission line 14. Details of the impedance of the third circuit 20 will be described later.

[0029] The fourth transmission line 16 is connected to the other end of the second inductor 122 and the input terminal 201. The fourth transmission line 16 is preferably as short as possible. By shortening the fourth transmission line 16, the phase shift between the signal flowing from the other end of the second inductor 122 to the input terminal 201 and the signal reflected in the third circuit 20 described later can be reduced. Also, when the second inductor 122 and the third circuit 20 can be directly connected, the fourth transmission line 16 can be omitted. The case where the second inductor 122 and the third circuit 20 can be directly connected is, for example, the case where the surface mounting pads of the second inductor 122 and the third circuit 20 can be directly connected.

[0030] The fifth transmission line 17 is connected to the output terminal 202 and ground or earth. In the present embodiment, the fifth transmission line 17 is connected to the output terminal 202 and ground. Here, the ground is the body ground 22 and the chassis ground 23. As described above, the signal transmission circuit 1 may not include the body ground 22. In that case, the ground is the chassis ground 23.

[0031] Generally, when transmitting a signal, attenuation of the signal may occur because a part of the energy of the signal is converted into other energy such as heat. Also, the amount of attenuation increases as the frequency of the signal is higher. Therefore, for example, when transmitting a high-frequency signal from the electronic device 1000 to the electronic device 2000 via the coaxial cable 3000, the energy of the third signal output from the electronic device 1000 may be attenuated before it is input to the electronic device 2000. Depending on the amount of attenuation of the energy of the third signal, the intensity of the third signal input to the electronic device 2000 may be lower than the threshold value of the intensity of the signal that can be detected by the electronic device 2000. As a result, there is a possibility that the signal cannot be detected in the electronic device 2000.

[0032] In the signal transmission circuit 1 of this embodiment, the third circuit 20 reflects a part of the input signal. Then, the signal reflected by the third circuit 20 is output from the electronic device 1000 via the third transmission line 15. As a result, the energy of the signal reflected by the third circuit 20 can be added to the third signal. Consequently, it is possible to improve the intensity of the signal output from the electronic device 1000. Hereinafter, the principle by which the third circuit 20 reflects a part of the input signal will be described.

[0033] First, the impedance of the third circuit 20 will be described. The impedance of the third circuit 20 is determined by the resistance value of the first resistor 203, the capacitance of the third capacitor 204, and the frequency of the signal input to the third circuit 20. Here, the signal input to the third circuit 20 is the fourth signal.

[0034] FIG. 2 shows the simulation results of the relationship between the frequency of the signal input to the third circuit 20 of this embodiment and the impedance of the third circuit 20. In FIG. 2, the horizontal axis represents the frequency of the signal input to the third circuit 20, and the vertical axis represents the impedance of the third circuit 20.

[0035] According to FIG. 2, it can be seen that the higher the frequency of the signal input to the third circuit 20, the lower the impedance of the third circuit 20. Also, according to FIG. 2, it can be seen that the decrease in the impedance of the third circuit 20 occurs when the frequency of the signal input to the third circuit 20 becomes higher than about several tens of MHz.

[0036] Here, the frequency of the signal input to the third circuit 20 when the decrease in the impedance of the third circuit 20 starts to occur is called the start frequency of decrease. Further, when a signal having a frequency lower than the start frequency of decrease is input, the impedance of the third circuit 20 coincides with the resistance value of the first resistor 203. Therefore, as also shown in FIG. 2, the impedance of the third circuit 20 when a signal having a frequency lower than the start frequency of decrease is input is 50Ω, which coincides with the characteristic impedance of the second transmission line 14.

[0037] Further, the start frequency of the decrease is determined by the resistance value of the first resistor 203 and the capacitance of the third capacitor 204. Here, in the present embodiment, in order to match the impedance of the third circuit 20 when a signal having a frequency lower than the start frequency of the decrease is input and the characteristic impedance of the second transmission line 14, the resistance value of the first resistor 203 is fixed at 50 Ω. Therefore, when changing the start frequency of the decrease, the capacitance of the third capacitor 204 is changed. That is, in the present embodiment, the larger the capacitance of the third capacitor 204 is made than 10 pF, the smaller the start frequency of the decrease becomes. Also, the smaller the capacitance of the third capacitor 204 is made than 10 pF, the larger the start frequency of the decrease becomes.

[0038] As described above, in the present embodiment in which a high-frequency signal of 100 MHz or higher is transmitted, the characteristic impedance of the second transmission line 14 and the impedance of the third circuit 20 are different. As a result, signal reflection occurs in the third circuit 20. Specifically, the third circuit 20 reflects a part of the input signal and passes the rest. Here, the signal input to the third circuit 20 is the fourth signal. That is, the third circuit 20 reflects a fifth signal, which is a part of the fourth signal, toward the second inductor 122 and passes a sixth signal, which is a signal other than the fifth signal in the fourth signal. The sixth signal that has passed through the third circuit 20 flows to the body ground 22 and the housing ground 23.

[0039] The fifth signal reflected by the third circuit 20 is input to the second inductor 122. When the fifth signal is input to the second inductor 122, due to the mutual induction between the first inductor 121 and the second inductor 122, the energy of the fifth signal is transmitted from the second inductor 122 to the first inductor 121, and a seventh signal is generated in the first inductor 121. The seventh signal is output from the first inductor 121 to the coaxial cable 3000 via the third transmission line 15 and the output terminal 21.

[0040] From the above, in addition to the third signal, the seventh signal is also input to the third transmission line 15 and the coaxial cable 3000. As a result, it is possible to add the energy of the seventh signal to the third signal, which is a high-frequency signal and whose energy attenuates during transmission. Therefore, the intensity of the signal output from the electronic device 1000 can be improved.

[0041] Further, the reflection coefficient Γ representing the ratio of the signal reflected by the third circuit 20 is expressed by the following equation using Z1, which is the characteristic impedance of the second transmission line 14, and Z2, which is the impedance of the third circuit 20. Γ=(Z1-Z2) / (Z1+Z2)

[0042] Therefore, in the present embodiment, the larger the difference between the characteristic impedance of the second transmission line 14 and the impedance of the third circuit 20, the larger the reflection coefficient, and thus the larger the amount of the signal reflected by the third circuit 20. Also, as shown in FIG. 2, as the frequency of the signal input to the third circuit 20 increases, the impedance of the third circuit 20 decreases significantly, so the difference from the characteristic impedance of the second transmission line 14 also increases. From the above, in the present embodiment, the higher the frequency of the signal to be transmitted, the more signals the third circuit 20 can reflect. Therefore, the higher the frequency of the signal to be transmitted, the more energy (the energy of the seventh signal) can be added to the third signal.

[0043] FIG. 3 is a schematic diagram of a reference example of an electronic device 1100 in which the invention in the present disclosure is not implemented. The signal transmission circuit 2 of the reference example in FIG. 3 includes a resistor 100 in place of the third circuit 20 in the signal transmission circuit 1 of FIG. 1. Regarding other configurations, since they are the same as those of the electronic device 1000 in the first embodiment, the same components as those of the electronic device 1000 in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. The resistance value of the resistor 100 is 50 Ω. That is, in the reference example, the characteristic impedance of the second transmission line 14 is equal to the resistance value of the resistor 100. As a result, in the reference example, no signal reflection occurs in the resistor 100, and all the signals input to the fourth transmission line 16 flow to the body ground 22 and the chassis ground 23.

[0044] FIG. 4 is a simulation result of an eye pattern of a signal passing through the coaxial cable 3000 of the reference example shown in FIG. 3. FIG. 5 is a simulation result of an eye pattern of a signal passing through the coaxial cable 3000 of the present embodiment shown in FIG. 1.

[0045] In FIGS. 4 and 5, the horizontal axis represents time, and the vertical axis represents amplitude (relative value). The amplitude A in FIG. 4 indicates the magnitude of the amplitude of the eye pattern at 200 psec. The amplitude B in FIG. 5 also indicates the magnitude of the amplitude of the eye pattern at 200 psec.

[0046] In the eye pattern, the larger the amplitude, the higher the signal strength. Comparing the amplitude A and the amplitude B, it can be seen that the present embodiment in which signal reflection occurs in the third circuit 20 has a larger amplitude and higher signal strength than the reference example in which no signal reflection occurs in the resistor 100. Therefore, compared with the reference example, in the present embodiment, the signal passing through the coaxial cable 3000, that is, the signal output from the electronic device 1000, has a high strength, so the possibility that the signal cannot be detected in the electronic device 2000 can be reduced.

[0047] As described above, the signal transmission circuit 1 of the present embodiment includes a common mode filter 12 including a first inductor 121 and a second inductor 122, a first circuit 18 including a first capacitor 181, a second circuit 19 including a second capacitor 191 and having the same circuit configuration as that of the first circuit 18, a third circuit 20 having a circuit configuration different from that of the second circuit 19, reflecting a predetermined amount of signals in a predetermined frequency band including the frequency of the transmission signal among the input signals and passing other components, and having an input terminal 201 and an output terminal 202, a first transmission line 13 connected to one end of the first inductor 121 from the communication circuit 11 via the first circuit 18, a second transmission line 14 connected to one end of the second inductor 122 from the communication circuit 11 via the second circuit 19 and having a characteristic impedance different from the impedance of the third circuit 20, a third transmission line 15 connected to the other end of the first inductor 121 and an output terminal 21, a fourth transmission line 16 connected to the other end of the second inductor 122 and the input terminal 201, and a fifth transmission line 17 connected to the output terminal and the ground. By configuring in this way, the signal transmission circuit 1 of the present embodiment can improve the strength of the signal to be transmitted.

[0048] (Second Embodiment) Hereinafter, a second embodiment of the electronic device according to the present disclosure will be described with reference to the drawings. The electronic device 1200 of the present embodiment includes a fourth circuit 24 between the fourth transmission line 16 and the fifth transmission line 17 in addition to the third circuit 20 in the first embodiment. In the present embodiment, the resistance value of the first resistor 203 and the capacitance of the third capacitor 204 are different from those in the first embodiment. Since other configurations are the same as those of the electronic device 1000 in the first embodiment, the same components as those of the electronic device 1000 in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

[0049] FIG. 6 is a schematic diagram of the configuration of the electronic device 1200. As shown in FIG. 5, the electronic device 1200 includes a signal transmission circuit 3. The signal transmission circuit 3 includes a fourth circuit 24 connected in series between the output terminal 202 and the fifth transmission line 17. The third circuit 20 includes a first resistor 203 and a third capacitor 204 connected in parallel with the first resistor 203.

[0050] The fourth circuit 24 includes a second resistor 241 and a fourth capacitor 242 connected in parallel with the second resistor 241. The fourth circuit 24 further includes a third resistor 243, a fifth capacitor 244 connected in parallel with the third resistor 243, a fourth resistor 245, and a sixth capacitor 246 connected in parallel with the fourth resistor 245. The parallel connection of the second resistor 241 and the fourth capacitor 242, the parallel connection of the third resistor 243 and the fifth capacitor 244, and the parallel connection of the fourth resistor 245 and the sixth capacitor 246 may be connected in series in this order. An example of the configuration will be described below.

[0051] In the present embodiment, the capacitances of the third capacitor 204, the fourth capacitor 242, the fifth capacitor 244, and the sixth capacitor 246 are different from each other. As an example, in the present embodiment, the capacitance of the third capacitor 204 is 100 pF, the capacitance of the fourth capacitor 242 is 10 pF, the capacitance of the fifth capacitor 244 is 1 pF, and the capacitance of the sixth capacitor 246 is 0.1 pF. Also, as an example, in the present embodiment, the resistance values of the first resistor 203, the second resistor 241, the third resistor 243, and the fourth resistor 245 are all 12.5 Ω.

[0052] Hereinafter, the third circuit 20 and the fourth circuit 24 are regarded as one circuit and referred to as the fifth circuit. In the present embodiment, the impedance of the fifth circuit is determined by the resistance values of the first resistor 203, the second resistor 241, the third resistor 243, and the fourth resistor 245, the capacitances of the third capacitor 204, the fourth capacitor 242, the fifth capacitor 244, and the sixth capacitor 246, and the frequency of the signal input to the fifth circuit.

[0053] FIG. 7 is a simulation result showing the relationship between the frequency of the signal input to the fifth circuit of the present embodiment and the impedance of the fifth circuit. In FIG. 7, the horizontal axis represents the frequency of the signal input to the fifth circuit, and the vertical axis represents the impedance of the fifth circuit.

[0054] When a signal having a frequency lower than the start frequency of decrease is input, the impedance of the fifth circuit coincides with the sum of the resistance values of the first resistor 203, the second resistor 241, the third resistor 243, and the fourth resistor 245. Therefore, as also shown in FIG. 7, the impedance of the fifth circuit when a signal having a frequency lower than the start frequency of decrease is input is 50 Ω. The resistance values of the respective resistors may be appropriately set so that the characteristic impedance of the second transmission line 14 and the impedance of the fifth circuit when a signal having a frequency lower than the start frequency of decrease is input are equal.

[0055] Also, according to FIG. 7, it can be seen that the higher the frequency of the signal input to the fifth circuit, the lower the impedance of the fifth circuit. In the present embodiment, the start frequency of decrease is determined by the capacitances of the first resistor 203, the second resistor 241, the third resistor 243, the fourth resistor 245, the third capacitor 204, the fourth capacitor 242, the fifth capacitor 244, and the sixth capacitor 246. Here, in the present embodiment, in order to match the impedance of the fifth circuit when a signal having a frequency lower than the start frequency of decrease is input and the characteristic impedance of the second transmission line 14, the sum of the resistance values of the respective resistors is fixed at 50 Ω. Therefore, when changing the start frequency of decrease, at least one of the capacitances of the respective capacitors is changed. When at least one of the capacitances of the respective capacitors is increased, the start frequency of decrease becomes smaller. Also, when at least one of the capacitances of the respective capacitors is decreased, the start frequency of decrease becomes larger. As an example, by increasing the capacitance of each capacitor at the same ratio, the start frequency of the drop can be decreased without changing the amount of change in the impedance of the fifth circuit with respect to the amount of change in the frequency of the signal input to the fifth circuit when a signal having a frequency higher than the start frequency of the drop is input. Similarly, by decreasing the capacitance of each capacitor at the same ratio, the start frequency of the drop can be increased without changing the amount of change in the impedance of the fifth circuit with respect to the amount of change in the frequency of the signal input to the fifth circuit when a signal having a frequency higher than the start frequency of the drop is input. The capacitance of each capacitor may be appropriately set so as to obtain a desired start frequency of the drop.

[0056] From the above, also in this embodiment that performs transmission of a high-frequency signal of 100 MHz or more as in the first embodiment, the characteristic impedance of the second transmission line 14 is different from the impedance of the fifth circuit. Thereby, the fifth circuit can reflect a fifth signal which is a part of the fourth signal. As a result, a seventh signal is generated in the second inductor 122, and the energy of the seventh signal can be added to the third signal. Therefore, the intensity of the signal output from the electronic device 1000 can be improved.

[0057] Also, comparing FIG. 2 and FIG. 7, it can be seen that in FIG. 7, the amount of change in the impedance of the fifth circuit with respect to the amount of change in the frequency of the signal input to the fifth circuit is small. Depending on the performance of the coaxial cable 3000, since the attenuation amount of the energy of the third signal is small, the amount of the signal reflected by the third circuit 20 or the fourth circuit 24 may be small. In such a case, as in the present embodiment, the number of parallel connections of the resistor and the capacitor provided between the fourth transmission line 16 and the fifth transmission line 17 is plural, and the capacitances of the respective capacitors are made different from each other, so that even when a high-frequency signal is input, it is possible to prevent the impedance from decreasing significantly. Thereby, the amount of the signal reflected by the fifth circuit can be reduced. Note that, the more the number of parallel connections of the resistor and the capacitor provided between the fourth transmission line 16 and the fifth transmission line 17 is increased, the smaller the amount of change in the impedance of the fifth circuit with respect to the amount of change in the frequency of the signal input to the fifth circuit can be made. Therefore, depending on the performance of the coaxial cable 3000, the number of parallel connections of the resistor and the capacitor may be determined. In the present embodiment, the number of parallel connections of the resistor and the capacitor is four, but depending on the performance of the coaxial cable 3000, the number of parallel connections of the resistor and the capacitor may be two or three, or may be five or more.

[0058] FIG. 8 is a simulation result of an eye pattern of a signal passing through the coaxial cable 3000 of FIG. 1 showing the schematic configuration of the present embodiment. The horizontal axis of FIG. 8 represents time, and the vertical axis represents amplitude (relative value). The amplitude C in FIG. 8 indicates the magnitude of the amplitude of the eye pattern at 200 psec. Comparing the amplitude C with the amplitude A in FIG. 4 showing the simulation result of the eye pattern in the reference example, it can be seen that in the present embodiment where signal reflection occurs in the fifth circuit, the amplitude of the eye pattern is larger and the signal intensity is higher than in the reference example where no signal reflection occurs at the resistor 100. Therefore, compared with the reference example, in the present embodiment, the intensity of the signal passing through the coaxial cable 3000, that is, output from the electronic device 1000, is high, so the possibility that the signal cannot be detected in the electronic device 2000 can be reduced.

[0059] Also, when comparing FIG. 8 with FIG. 5 showing the simulation results of the eye pattern in the first embodiment, in FIG. 8, since the amount of signal reflecting frequencies other than the desired frequency is small, the upper end of the amplitude of the eye pattern at 200 psec is closer to 1.0 and the lower end is closer to 0.0. That is, in this embodiment, compared with the first embodiment, the EMC (Electro Magnetic Compatibility) performance is good, and it is possible to reduce the possibility of emitting noise to the outside.

[0060] As described above, each embodiment of the present disclosure has been described. However, the above-described embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These novel embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope. Furthermore, components across different embodiments and modifications may be appropriately combined. Also, the effects in each embodiment described in this specification are merely examples and are not limited, and there may be other effects.

[0061] (Supplementary Note) One aspect of the present disclosure is as follows. [First Aspect] A common mode filter including a first inductor and a second inductor, A first circuit including a first capacitor, A second circuit including a second capacitor and having the same circuit configuration as the first circuit, A third circuit having a circuit configuration different from that of the second circuit, reflecting a predetermined amount of signals in a predetermined frequency band including the frequency of the transmission signal among the input signals, passing other components, and having an input end and an output end, A first transmission line connected to one end of the first inductor via the first circuit from a communication circuit that outputs a differential signal, A second transmission line that is connected to one end of the second inductor via the second circuit from the communication circuit and has a characteristic impedance different from the impedance of the third circuit, A third transmission line connected to the other end of the first inductor and the output terminal, A fourth transmission line connected to the other end of the second inductor and the input end, A fifth transmission line connected to the output end and ground or earth, A signal transmission circuit comprising the same. According to this aspect, the energy of the signal reflected by the third circuit can be added to the signal with attenuated energy flowing through the coaxial cable via the third transmission line. Thereby, the intensity of the signal to be transmitted can be improved. [Second Aspect] The impedance of the third circuit is smaller than the characteristic impedance of the second transmission line, The signal transmission circuit according to the first aspect. According to this aspect, a part of the signal input to the third circuit can be reflected to the second inductor. [Third Aspect] The third circuit includes a first resistor and a third capacitor connected in parallel with the first resistor, The signal transmission circuit according to the first aspect or the second aspect. According to this aspect, a part of the signal input to the third circuit can be reflected to the second inductor. [Fourth Aspect] The signal transmission circuit further comprises a fourth circuit connected in series between the output end and the fifth transmission line, The fourth circuit includes a second resistor and a fourth capacitor connected in parallel with the second resistor, The signal transmission circuit according to the third aspect. According to this aspect, the change amount of the impedance of the third circuit and the fourth circuit with respect to the change amount of the frequency of the signal input to the third circuit and the fourth circuit can be reduced. Thereby, the possibility of reflecting a signal larger than necessary can be reduced. [Fifth Aspect] The capacitance of the third capacitor and the capacitance of the fourth capacitor are different. The signal transmission circuit according to the fourth aspect. According to this aspect, it is possible to reduce the amount of change in the impedance of the third circuit and the fourth circuit with respect to the amount of change in the frequency of the signal input to the third circuit and the fourth circuit. As a result, the possibility of reflecting a signal larger than necessary can be reduced. [Sixth Aspect] The characteristic impedance of the first transmission line and the characteristic impedance of the second transmission line are equal. The signal transmission circuit according to any one of the first aspect to the fifth aspect. According to this aspect, common mode noise is unlikely to occur in each signal passing through the first transmission line and the second transmission line. [Seventh Aspect] The first circuit includes only the first capacitor. The second circuit includes only the second capacitor. The capacitance of the first capacitor and the capacitance of the second capacitor are equal. The signal transmission circuit according to any one of the first aspect to the sixth aspect. According to this aspect, common mode noise is unlikely to occur in each signal passing through the first transmission line and the second transmission line. [Eighth Aspect] Comprising the communication circuit. The signal transmission circuit according to any one of the first aspect to the seventh aspect. According to this aspect, compared with the case where the communication circuit is provided outside the signal transmission circuit, the length of the transmission line can be shortened, so that the amount of attenuation of the energy of the signal flowing through the transmission line can be reduced. In addition, the possibility of noise occurring in the signal flowing through the transmission line can be reduced. [Ninth Aspect] An electronic device including the signal transmission circuit according to any one of the first aspect to the eighth aspect. According to this aspect, the energy of the signal reflected by the third circuit can be added to the signal with attenuated energy flowing through the coaxial cable via the third transmission line. Thereby, the intensity of the signal to be transmitted can be improved.

Explanation of Signs

[0062] 1000, 1100, 1200 Electronic devices 1, 2, 3 Signal transmission circuits 11 Communication circuit 12 Common mode filter 121 First inductor 122 Second inductor 13 First transmission line 14 Second transmission line 15 Third transmission line 16 Fourth transmission line 17 Fifth transmission line 18 First circuit 181 First capacitor 19 Second circuit 191 Second capacitor 20 Third circuit 201 Input terminal 202 Output terminal 203 First resistor 204 Third capacitor 24 Fourth circuit 241 Second resistor 242 Fourth capacitor

Claims

1. A common mode filter including a first inductor and a second inductor; A first circuit including a first capacitor; A second circuit including a second capacitor and having the same circuit configuration as the first circuit; A third circuit having a circuit configuration different from that of the second circuit, reflecting a predetermined amount of signals in a predetermined frequency band including the frequency of the transmission signal among the input signals, passing other components therethrough, and having an input end and an output end; A first transmission line connected to one end of the first inductor via the first circuit from a communication circuit that outputs a differential signal; A second transmission line connected to one end of the second inductor via the second circuit from the communication circuit and having a characteristic impedance different from that of the third circuit; A third transmission line connected to the other end of the first inductor and an output terminal; A fourth transmission line connected to the other end of the second inductor and the input end; A fifth transmission line connected to the output end and ground or earth; A signal transmission circuit comprising the same.

2. The impedance of the third circuit is smaller than the characteristic impedance of the second transmission line. The signal transmission circuit according to Claim 1.

3. The third circuit includes a first resistor and a third capacitor connected in parallel with the first resistor. The signal transmission circuit according to Claim 1 or 2.

4. The signal transmission circuit further comprises a fourth circuit connected in series between the output end and the ground. The fourth circuit includes a second resistor and a fourth capacitor connected in parallel with the second resistor. The signal transmission circuit according to Claim 3.

5. The capacitance of the third capacitor and the capacitance of the fourth capacitor are different. The signal transmission circuit according to Claim 4.

6. The characteristic impedance of the first transmission line and the characteristic impedance of the second transmission line are equal. The signal transmission circuit according to Claim 1.

7. The first circuit includes only the first capacitor. The second circuit includes only the second capacitor. The capacitance of the first capacitor and the capacitance of the second capacitor are equal. The signal transmission circuit according to Claim 1.

8. Comprising the communication circuit. The signal transmission circuit according to Claim 1.

9. An electronic device comprising the signal transmission circuit according to Claim 1. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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