JP2010283415A - Noise-countermeasure circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a noise-countermeasure circuit which is configured to suppress mode conversion from a normal mode to a common mode and capable of suppressing the generation of common mode noise. <P>SOLUTION: The noise-countermeasure circuit 4, on the side of a transmission side circuit 1, includes a common-mode choke coil 12, parasitic capacitances 10A and 10B inside a transceiver 10, and a capacitor 13. The lines 31 and 32 of a differential transmission line 3 are connected to the transceiver 10, and the capacitor 13 is connected to the line 31, between the transceiver 10 and the common mode choke coil 12. AS a result, the capacitor 13 is connected, in parallel with the parasitic capacitance 10A on the line 31. The capacitor 13 is an element for balancing the capacitance on the side of the line 31 and the capacitance on the side of the line 32, and the capacitance value C3 is set so that (the capacitance value C1)+(the capacitance value C3)=the capacitance value C2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a noise countermeasure circuit for suppressing noise generated on a differential transmission line used in an in-vehicle LAN (Local Area Network) or the like.

In general, in a system that performs high-speed communication using a differential transmission path, such as CAN (Controller Area Network) for in-vehicle LAN, a common mode choke coil is disposed in front of the communication IC (Integrated Circuit). Suppresses the occurrence of common mode noise.
Conventionally, as this type of common mode choke coil, for example, as disclosed in Patent Document 1, there is a structure in which two conductive wires are wound around a toroidal ferrite core.
FIG. 10 is a circuit diagram showing a conventional noise countermeasure circuit using this common mode choke coil.
In CAN or the like, as shown in FIG. 10, the lines 111 and 112 constituting the differential transmission path are drawn from the output end of the transceiver 110 which is a communication IC, and the termination resistor 113 is connected between the lines 111 and 112 in the vicinity of the transceiver 110. By connecting to the terminal circuit, a termination circuit is formed. The common mode choke coil 100 is interposed in the lines 111 and 112 between the transceiver 110 and the terminating resistor 113 to constitute a noise countermeasure circuit.
With this configuration, common mode noise that has entered the lines 111 and 112 is suppressed by the common mode choke coil 100.

Japanese Utility Model Publication No. 59-70323

However, the conventional noise countermeasure circuit described above has the following problems.
In the noise countermeasure circuit shown in FIG. 10, since the transceiver 110, the common mode choke coil 100, and the termination resistor 113 are arranged, the parasitic capacitances 121 and 122 generated in the transceiver 110 and the common mode choke coil 100 are formed. A series resonance circuit is formed by the normal mode inductance. For this reason, ringing having the resonance frequency of the series resonance circuit occurs at the rise and fall of the differential signal in the normal mode.
However, in the common mode choke coil 100 having the structure as described in Patent Document 1, magnetic flux generated by the normal mode differential signal when the normal mode differential signal flows through the lines 111 and 112 of the differential transmission path. The cancellation is incomplete. For this reason, the inductance in the normal mode becomes extremely high and ringing is likely to occur.
In such a state, when a difference occurs in the capacitance values of the parasitic capacitors 121 and 122 generated in the transceiver 110, a phase difference and amplitude disturbance occur in the normal mode differential signal flowing through the lines 111 and 112. As a result, a common mode component is generated, and so-called mode conversion from the normal mode to the common mode occurs. This common mode component occurs at the rising and falling portions of the signal and generates common mode noise. In addition, as described above, if ringing with a large amplitude is present at the rising or falling portion, the common mode component becomes large, and there is a possibility that common mode noise with an extremely large amplitude may be radiated.

  The present invention has been made to solve the above-described problems, and provides a noise countermeasure circuit that can suppress the occurrence of common mode noise by suppressing the mode conversion from the normal mode to the common mode. With the goal.

In order to solve the above-mentioned problems, the invention of claim 1 is directed to a communication IC for transmitting or receiving a differential signal, a differential transmission line connected to the communication IC, and an interposition in the differential transmission line. A noise countermeasure circuit comprising a noise countermeasure component having a pair of coils connected to each line of a differential transmission path, wherein the communication IC is in a state of being electrically connected to one line of the differential transmission path. So that the sum of the capacitance values of the first parasitic capacitance and the capacitor generated in the communication IC is substantially equal to the capacitance value of the second parasitic capacitance generated in the communication IC in a state of being electrically connected to the other line of the differential transmission path. The capacitor is configured to be connected in parallel with the first parasitic capacitance to one line that is between the communication IC and the noise countermeasure component.
With this configuration, when a differential signal is transmitted / received by the communication IC, the differential signal propagates on the differential transmission path. However, in the normal mode state, the pair of signals constituting the differential signal have opposite phases Flows on the differential transmission line. In the common mode state, a common mode signal that causes radiation noise flows on the differential transmission path, but this signal is attenuated by a noise countermeasure component interposed in the differential transmission path.
By the way, since the noise countermeasure component interposed in the differential transmission path has a pair of coils, if a pair of parasitic capacitances respectively conducted to the differential transmission path are generated in the communication IC, it is normal. During the mode, a series resonance circuit is formed by the coil inductance and the parasitic capacitance. Then, when the differential signal rises or falls, ringing having the resonance frequency of the series resonance circuit is generated and superimposed on the differential signal.
In such a state, when there is a capacitance difference between a pair of parasitic capacitances, a phase difference or amplitude disturbance occurs in the differential signal, and the differential signal where the ringing is superimposed has a rising or falling portion. A large common mode component is generated, and very large amplitude common mode noise is radiated.
However, in the present invention, the capacitance difference is eliminated by adding a capacitor to the first and second parasitic capacitances having a capacitance difference. That is, the capacitor is connected in parallel with the first parasitic capacitance, and the capacitance value sum of the first parasitic capacitance and the capacitor is set to be approximately equal to the capacitance value of the second parasitic capacitance. The capacitance on one line side of the differential transmission path and the capacity on the other line side are balanced, and the phase difference between the differential signals is extremely reduced. As a result, mode conversion from the normal mode to the common mode is suppressed.

  According to a second aspect of the present invention, in the noise countermeasure circuit according to the first aspect, the noise countermeasure component is a common mode choke coil.

  As described above in detail, according to the noise countermeasure circuit of the present invention, the phase difference between the differential signals is obtained by balancing the capacitance on one line side of the differential transmission line with the capacitance on the other line side. By reducing the number, the mode conversion from the normal mode to the common mode is suppressed, so that there is an excellent effect that radiation of common mode noise can be effectively prevented.

1 is a system diagram of a differential transmission line to which a noise countermeasure circuit according to a first embodiment of the present invention is applied. It is a circuit diagram which shows the noise countermeasure circuit provided in the transmission side circuit. It is a circuit diagram for demonstrating the effect | action and effect of a noise countermeasure circuit. It is a wave form diagram which shows the differential signal at the time of normal mode. It is a wave form diagram showing a differential signal in the state where ringing was superimposed. It is a wave form diagram which shows the state which the phase difference produced in the differential signal which has ringing. It is a circuit block diagram for an evaluation test. It is a diagram which shows ringing. It is a diagram which shows the noise level in each frequency. It is a circuit diagram which shows the conventional noise countermeasure circuit.

  The best mode of the present invention will be described below with reference to the drawings.

  FIG. 1 is a system diagram of a differential transmission line to which a noise countermeasure circuit according to a first embodiment of the present invention is applied. FIG. 2 is a circuit diagram showing a noise countermeasure circuit provided in a transmission side circuit. .

  The differential transmission path system shown in FIG. 1 is, for example, an in-vehicle LAN, and includes a transmission side circuit 1, a reception side circuit 2, and a differential transmission path 3. The noise countermeasure circuits 4 and 5 of this embodiment are provided in the transmission side circuit 1 and the reception side circuit 2, respectively.

  The transmission side circuit 1 includes a transceiver 10 that is a communication IC connected to the differential transmission path 3, a termination resistor 11, a common mode choke coil 12 that is a noise countermeasure component, and a capacitor 13.

The transceiver 10 is connected to one line 31 and the other line 32 of the differential transmission path 3, and has a function of transmitting differential signals S + and S− to the lines 31 and 32.
In the transceiver 10, a parasitic capacitance 10 </ b> A is generated as a first parasitic capacitance inside the line 31 of the differential transmission path 3, and a parasitic capacitance 10 </ b> B as a second parasitic capacitance is generated inside the line 32. Arise.
In this embodiment, there are differences in the capacitance values C1 and C2 of the parasitic capacitances 10A and 10B generated in the transceiver 10, and the capacitance value C1 of the parasitic capacitance 10A is smaller than the capacitance value C2 of the parasitic capacitance 10B. Will be described.
The termination resistor 11 is an element for preventing reflection of the differential signal, and is connected across the lines 31 and 32 of the differential transmission path 3.

  As shown in FIG. 2, the noise countermeasure circuit 4 on the transmission side circuit 1 side includes a common mode choke coil 12, parasitic capacitors 10 </ b> A and 10 </ b> B in the transceiver 10, and a capacitor 13.

  The common mode choke coil 12 is an element for removing common mode noise on the differential transmission path 3 and is interposed on the differential transmission path 3. Specifically, the common mode choke coil 12 includes a pair of coils 12A and 12B connected to the lines 31 and 32 of the differential transmission path 3, respectively.

The capacitor 13 is an element for balancing the capacitance on the line 31 side of the differential transmission path 3 and the capacitance on the line 32 side, and has a capacitance value C3.
Specifically, the capacitor 13 is connected to the line 31 between the transceiver 10 and the common mode choke coil 12 with one end 13a connected to the line 31 and the other end 13b grounded. Thereby, the capacitor 13 is in a state of being connected in parallel with the parasitic capacitance 10 </ b> A on the line 31.
The capacitance value C3 of the capacitor 13 is set to a numerical value that balances the capacitance on the line 31 side and the capacitance on the line 32 side, and the capacitance value C1 + the capacitance value C3 = the capacitance value C2.

  As shown in FIG. 1, the configuration of the reception side circuit 2 is substantially the same as that of the transmission side circuit 1, and includes a receiver 20 that is a communication IC, a termination resistor 21, a common mode choke coil 22, and a capacitor 23. ing.

That is, the receiver 20 is connected to the lines 31 and 32 of the differential transmission path 3 and has a function of receiving the differential signals S + and S− from the lines 31 and 32.
Inside the receiver 20, a parasitic capacitance 20 </ b> A that is conductive with the line 31 and a parasitic capacitance 20 </ b> B that is conductive with the line 32 are generated.
The termination resistor 21 is an element for preventing reflection of a differential signal, similarly to the termination resistor 11, and is connected between the lines 31 and 32.
The common mode choke coil 22 is also an element for removing common mode noise on the differential transmission path 3 and has a pair of coils 22A and 22B connected to the lines 31 and 32 of the differential transmission path 3, respectively. is doing.

The noise countermeasure circuit 4 on the reception side circuit 2 side is composed of the common mode choke coil 22, the parasitic capacitances 20 </ b> A and 10 </ b> B in the receiver 20, and the capacitor 23 as in the transmission side. Since the capacitance value C5 of 20B is smaller than the capacitance value C4 of the parasitic capacitance 20A, the connection part of the capacitor 23 is different.
That is, the capacitor 23 has a capacitance value C 6 and is connected to a line 32 between the receiver 20 and the common mode choke coil 22. The capacitance value C6 of the capacitor 23 is set such that the capacitance value C5 + the capacitance value C6 = the capacitance value C4.

Next, the operation and effect of the noise countermeasure circuit of this embodiment will be described.
FIG. 3 is a circuit diagram for explaining the operation and effect of the noise countermeasure circuit.
Since the noise countermeasure circuit 4 of the transmission side circuit 1 and the noise countermeasure circuit 5 of the reception side circuit 2 have almost the same operations and effects, the noise countermeasure circuit provided in the transmission side circuit 1 is used here to avoid duplication. Only the operation and effect of 4 will be described.

As shown in FIG. 3, when the differential signals S + and S− are transmitted from the transceiver 10 onto the differential transmission line 3, in the normal mode state, the differential signals S + and S− 32.
When a common mode signal (not shown) in the same direction that causes radiation noise flows in the same direction on the lines 31 and 32 of the differential transmission path 3, the common mode choke coil 12 causes these noise signals to be transmitted. Is attenuated.

4 is a waveform diagram showing the differential signals S + and S− in the normal mode, FIG. 5 is a waveform diagram showing the differential signals S + and S− in a state where ringing is superimposed, and FIG. It is a wave form diagram which shows the state which the phase difference produced in differential signal S + and S- which have ringing.
As shown in FIG. 4, in the normal mode state, when there is no difference in conditions between the line 31 side and the line 32 side of the differential transmission path 3, no phase difference occurs between the differential signals S + and S-. .

However, as shown in FIG. 2, the coils 12A and 12B of the common mode choke coil 12 are present on the differential transmission path 3, and the parasitic capacitances 10A and 10B in the transceiver 10 are present. Sometimes, the inductance of the common mode choke coil 12 and the capacitance such as parasitic capacitance form a series resonance circuit of inductance and capacitance.
Therefore, as shown in FIG. 5, ringing R having the resonance frequency of the series resonance circuit is generated and superimposed on the differential signals S + and S−.

In this state, assuming that the capacitances connected to the lines 31 and 32 are only the parasitic capacitances 10A and 10B, as described above, the capacitance value C1 of the parasitic capacitance 10A is more than the capacitance value C2 of the parasitic capacitance 10B. Therefore, there is a phase difference between the differential signals S + and S−.
As a result, as shown in FIG. 6, a large common mode component CM is generated at the rising and falling portions of the differential signals S + and S− on which the ringing R is superimposed, as shown by the two-dot chain line in FIG. The common mode component CM may radiate large-amplitude common mode noise.
However, in this embodiment, as shown in FIG. 2, the capacitor 13 is connected to the line 31 in parallel with the parasitic capacitance 10A, and the capacitance value sum C1 + C2 of the parasitic capacitance 10A and the capacitor 13 is the capacitance of the parasitic capacitance 10B. It is set to be equal to the value C2.
Accordingly, since the capacitance on the line 31 side and the capacitance on the line 32 side of the differential transmission path 3 are balanced, the phase difference between the differential signals S + and S− as shown in FIG. 6 hardly occurs. . As a result, there is no mode conversion from the normal mode to the common mode, and no common mode component CM is generated, thereby preventing radiation of common mode noise.

The inventors conducted the following evaluation test in order to confirm this effect.
FIG. 7 is a circuit configuration diagram for the evaluation test.
This evaluation test was performed using a CANBUS conductive noise evaluation circuit, which is one of the in-vehicle LAN interface standards.
Specifically, as shown in FIG. 7, an impedance matching circuit including a 120Ω resistor 211 and a 4.7 nF capacitor 212 is configured in the transmission circuit 1 on the lines 31 and 32 of the differential transmission path 3. . These lines 31 and 32 intersect at the tip to form one transmission path 35, and this transmission path 35 is connected to the spectrum analyzer 220. Reference numeral 213 denotes a 50Ω resistor.
At this time, the termination resistor 11 was set to 60Ω, and the common mode choke coil 12 having a common mode inductance of about 50 μH and a normal mode inductance of about 3 μH was used.
Further, in the transceiver 10, when the capacitance values C1 and C2 of the parasitic capacitors 10A and 10B at 1 MHz were measured, they were 8.2 pF and 6.9 pF, respectively. That is, in this evaluation test, the transceiver 10 that generates the parasitic capacitance 10A having a capacitance value 1.3 pF larger than the parasitic capacitance 10B is used.

  In the evaluation test, in this circuit configuration, differential signals S + and S− that are rectangular waves of 250 kHz are output from the transceiver 10 to the differential transmission path 3. The differential signals S + and S− are sent to the transmission line 35 through an impedance matching circuit including the common mode choke coil 12, the terminating resistor 11, the resistor 211, and the capacitor 212. As a result, the differential signals S + and S− are added in the transmission path 35 to become the common mode component CM. Therefore, the common mode component CM is measured by the spectrum analyzer 220.

FIG. 8 is a diagram showing ringing, and FIG. 9 is a diagram showing a noise level at each frequency.
First, in the evaluation circuit shown in FIG. 7, the waveforms of the differential signals S + and S− having no phase difference in the normal mode were measured. As shown in FIG. It occurred around 2 μs. The ringing frequency in FIG. 8 is about 38 MHz, which also matches the resonance frequency of the series resonance circuit of the inductance of the common mode choke coil 12 and the parasitic capacitors 10A and 10B.

Next, the noise level was measured when the 1 pF capacitor 13 was inserted into the line 31, when it was inserted into the line 32, and when it was not inserted into any of the lines 31 and 32.
At this time, the measurement frequency was 0.15 MHz to 1000 MHz, and the bandwidth of the spectrum analyzer 220 was 10 kHz.
In actual measurement, it was also possible to observe a noise level of a harmonic of 250 kHz, which is the fundamental frequency of the differential signals S + and S−. However, in FIG. 9, for easy understanding, an envelope curve passing through the apex of the fundamental and harmonic noise levels is shown.

As shown by a box A in FIG. 9, it can be confirmed that the noise level varies depending on whether the capacitor 13 is connected or not in the frequency range of 20 MHz to 100 MHz. This difference is particularly noticeable at the ringing resonance frequency of 38 MHz.
That is, as indicated by the solid noise level curve N1, when the 1 pF capacitor 13 was inserted into the line 31, the noise level was the highest. When the capacitor 13 is inserted into the line 32 as shown by the very solid noise level curve N2, the noise level is the lowest, and as shown by the broken line noise level curve N3, both of the lines 31 and 32 are used. When not inserted, the noise level was between these levels.

  From this evaluation test, since the parasitic capacitance 10A is higher by 1.3 pF than the parasitic capacitance 10B, adding a 1 pF capacitor 13 to the line 31 further increases the balance of the capacitance balance between the lines 31 and 32. It was confirmed that the noise was increased and that the capacitor 13 was added to the line 32 to balance the capacitance between the lines 31 and 32 and to reduce the common mode noise.

In addition, this invention is not limited to the said Example, A various deformation | transformation and change are possible within the range of the summary of invention.
For example, in the above embodiment, the common mode choke coil is applied as the noise countermeasure component, but the present invention is not limited to this. As a noise countermeasure component, it has a pair of coils connected to each line of the differential transmission line, and when a common mode signal flows, it generates mutual inductance between the coils and increases the inductance of the entire component Any structure can be used.

  DESCRIPTION OF SYMBOLS 1 ... Transmission side circuit, 2 ... Reception side circuit, 3 ... Differential transmission path, 4, 5 ... Noise suppression circuit, 10 ... Transceiver, 20 ... Receiver, 10A, 10B, 20A, 20B ... Parasitic capacitance, 11, 21 ... Termination resistor, 12, 22 ... Common mode choke coil, 12A, 12B, 22A, 22B ... Coil, 13, 23 ... Capacitor, 31, 32 ... Line, A ... Box, C1-C6 ... Capacitance value, CM ... Common mode component , S +, S−... Differential signal.

Claims (2)

A communication IC for transmitting or receiving a differential signal, a differential transmission path connected to the communication IC, and a pair connected to each line of the differential transmission path interposed in the differential transmission path A noise countermeasure circuit comprising a noise countermeasure component comprising a coil of
The capacitance value sum of the first parasitic capacitance and the capacitor generated in the communication IC in a state of being conductive with one line of the differential transmission path is in the communication IC in a state of being conductive with the other line of the differential transmission path. The capacitor is connected in parallel with the first parasitic capacitance to the one of the lines and between the communication IC and the noise countermeasure component so as to be approximately equal to the capacitance value of the generated second parasitic capacitance. did,
Noise suppression circuit characterized by that. The noise suppression circuit according to claim 1,
The noise suppression component is a common mode choke coil.
Noise suppression circuit characterized by that.

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