JP2010135607A - Printed circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve noise resistance to common mode noise in a printed circuit board. <P>SOLUTION: First and second circuit elements 15 and 16 are mounted on a signal line 2 and a signal ground line 3 of a receiving circuit 4 which receives a signal transmitted from a signal source 1, respectively, and these circuit elements are connected to an input end 5 and a reference end 6 of the receiving circuit 4 via grounded third and fourth circuit elements 17 and 18. A bridge circuit is constituted at a prestage of the receiving circuit 4, and impedance of the circuit elements 15-18 is so adjusted as to approach the balancing condition of the bridge circuit, thereby the noise voltage applied to the receiving circuit 4 is reduced. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a printed circuit board having improved noise resistance against common mode noise.

  As electronic devices have become more advanced in recent years, various noise problems that affect circuit operation have become more complex due to higher speed of operation and higher wiring density.

  Increasing the density of the printed circuit board leads to a reduction in the ground area, and common mode noise that enters the printed circuit board from the casing or cable of the electronic device greatly affects the operation of the circuit.

  Especially at high frequencies, some common mode noise changes its form and adversely affects the circuit due to the imbalance in signal transmission, the stray capacitance with the housing and the ground, and the stray inductance. Also become.

  Conventionally, as a countermeasure against common mode noise, as shown in FIG. 8, a method of suppressing common mode noise by attaching a component 123 having mutual inductance to both the signal line 102 and the signal ground line 103 of the signal source 101 is known. (See Non-Patent Document 1).

  Further, as shown in FIG. 9, a method of bypassing the signal line 102 and the signal ground line 103 to the housing by capacitors 124 and 125, respectively, is known.

Murata's Web Noise Countermeasures Collection 20-30

  In the conventional technique, as shown in FIG. 8, for example, a common choke coil is used as the component 123 having mutual inductance.

  The common choke coil works so that the magnetic flux cancels out when the current in the normal mode flows, whereas the coil strengthens the magnetic flux when the current in the common mode flows. It is possible to suppress efficiently.

  However, the common choke coil is more expensive than circuit elements such as resistors, capacitors, and inductors, and has a problem of increasing the cost required for noise countermeasures.

  Further, as a technique that can be expected to have the same effect as a common choke coil, there is a technique in which a magnetic core is provided around a cable.

  Since the magnetic core is less expensive than the common choke coil, it can be used for noise countermeasures at a relatively low cost. However, in order to provide the magnetic core, a certain amount of space is required. There was a problem that it would be an adverse effect of miniaturization.

  Further, when the magnetic core is circulated around the cable, there is a problem that stress is applied to the cable and reliability of connection of the cable and the connector is lowered.

  Furthermore, as a common problem in the technology of providing countermeasure parts having these mutual impedances, there is a problem that a specific narrow high-frequency noise component can be attenuated, but sufficient countermeasures cannot be taken against noise over a wide band such as static electricity. .

  On the other hand, as shown in FIG. 9, in the configuration in which the signal line and the signal ground line are respectively connected to the housing via a capacitor, the noise can be reduced by adding a capacitor to the common mode noise generated in the signal source. Can be bypassed to the housing. Therefore, it is possible to take an inexpensive and space-saving measure against common mode noise.

  For common mode noise entering through the casing, the noise voltage applied to the receiving circuit is determined by the ratio of the capacitance of the capacitor bypassed to the casing and the stray capacitance, so that the bypass capacitor preferably has a larger capacity.

  However, since the bypass capacitor has a great influence on the leakage current, the capacity cannot be made too large. For this reason, there has been a problem that a sufficient attenuation effect cannot be obtained for noise superimposed on the signal line or the signal ground line via the casing due to capacitive coupling.

  In addition, even if a bypass capacitor is added to the signal line and signal ground line, the deviation of the balance between the signal line and signal ground line cannot be improved. There was a problem that I could not.

  An object of the present invention is to provide a printed circuit board capable of sufficiently increasing noise resistance against common mode noise using an inexpensive circuit element mounted on the printed circuit board.

  In order to achieve the above object, a printed circuit board according to the present invention is a printed circuit board including a receiving circuit for a signal transmitted from a signal source, connected to a signal line from the signal source, and an input terminal of the receiving circuit. A first wiring pattern connected to the signal line, a second wiring pattern connected to the reference terminal of the receiving circuit, and a first wiring pattern connected to the grounded housing. A GND pattern; a second GND pattern connected to a reference end of the receiving circuit; a first circuit element mounted on the first wiring pattern; and a second circuit mounted on the second wiring pattern. A circuit element, a third circuit element having one end mounted on the input end side with respect to the first circuit element in the first wiring pattern, and the other end mounted on the first GND pattern, and one end The second wiring path Than the in over down the second circuit element is mounted on the reference end and the other end and having a fourth circuit element mounted on the first GND pattern.

  According to the above configuration, the bridge circuit having the input terminal and the reference terminal of the receiving circuit as an intermediate point with respect to the noise voltage entering from the housing can be configured with only inexpensive circuit elements.

  As a result, the same attenuation effect can be obtained not only for the common mode of the signal source but also for noise superimposed on the signal line and the GND line of the signal source.

  By adjusting the impedance of the circuit element so as to be close to the equilibrium condition of the bridge circuit, it is possible to sufficiently reduce the noise voltage applied to the receiving circuit.

  The best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 shows a first embodiment of the present invention, in which a printed circuit board 7 is attached to a housing 8 via screws in screw holes 9. A reception circuit 4 is mounted on the printed circuit board 7, and the signal source 1 and the reception circuit 4 provided outside the printed circuit board 7 are electrically connected, and a signal is transmitted from the signal source 1 to the reception circuit 4. Is done. The receiving circuit 4 has terminals such as an input end 5 and a reference end 6.

  The signal line 2 connected to the signal source 1 is connected to a first pattern (first wiring pattern) 10 on the printed circuit board 7 connected to the input terminal 5 of the receiving circuit 4. A signal ground line 3, which is a GND line connected to the signal source 1, is connected to the second pattern 11 (second wiring pattern) on the printed circuit board 7 connected to the reference end 6 of the receiving circuit 4. It is connected.

  The printed circuit board 7 is separated from the first GND pattern 13 connected to the grounded housing 8 and the first GND pattern 13 and is connected to the reference end 6 of the receiving circuit 4. The GND pattern 14.

  A first circuit element 15 is mounted in the middle of the first pattern 10, and a second circuit element 16 is mounted in the middle of the second pattern 11.

  Further, one end of the third circuit element 17 is mounted on the input end side of the first pattern 10 with respect to the first circuit element 15, and the other end of the third circuit element 17 is connected to the first GND pattern 13. Has been implemented. One end of the fourth circuit element 18 is mounted on the reference end side of the second pattern 12 with respect to the second circuit element 16, and the other end of the fourth circuit element 18 is connected to the first GND pattern 13. Has been implemented.

  FIG. 2A is a circuit diagram showing the connection structure of the printed circuit board 7. For a voltage source based on the first GND pattern 13, a bridge circuit as shown in FIG. It is composed. When calculating the attenuating action of the common mode noise 20, the signal source 1 can be short-circuited as shown in FIG.

  Here, the impedance of the first circuit element 15 is Z1, the impedance of the second circuit element 16 is Z2, the impedance of the third circuit element 17 is Z3, and the impedance of the fourth circuit element 18 is Z4.

  At this time, when the common mode noise 20 indicated by Vcom is added, a noise voltage appearing between the input terminal 5 and the reference terminal 6 of the receiving circuit 4 indicated by Vin is expressed by the following equation.

  Vin = Vcom * | Z3 / (Z1 + Z3) -Z2 / (Z2 + Z4) |

  Here, the equilibrium condition of the bridge circuit in FIG. 2B is expressed as Z1 * Z4 = Z2 * Z3, and Vin = 0 is obtained by determining the impedance of each circuit element so as to satisfy the equilibrium condition.

  Further, by determining the impedance of each circuit element so as to be approximately Z1 * Z4 = Z2 * Z3, Vin can be made sufficiently small with respect to Vcom, and the input terminal 5 and the reference terminal 6 of the receiving circuit 4 can be reduced. The noise voltage appearing during the period can be sufficiently reduced.

  CMRR (common-mode noise removal ratio) indicating the ratio at which the common mode noise 20 indicated by Vcom is converted into normal mode noise appearing between the input terminal 5 and the reference terminal 6 of the receiving circuit indicated by Vin is expressed by the following equation. The

  CMRR = 20 * log (Vcom / Vin) = 20 * log | {(Z1 + Z3) * (Z2 + Z4)} / (Z3 * Z4-Z1 * Z2) |

  At this time, the larger the CMRR value, the less affected by the common mode noise 20, and the relationship between Z1, Z2, Z3, and Z4 is preferably as large as possible, for example, 60 dB or more.

  From the above, by providing a bridge circuit in front of the receiving circuit 4, not only the common mode noise of the signal source 1, but also the signal line 2 and the signal ground line 3 via the housing 8 by capacitive coupling or inductive coupling. A sufficient attenuation effect can be obtained even with superimposed noise.

  Further, by using the third circuit element 17 and the fourth circuit element 18 as capacitors, the signal line 2 and the signal ground line 3 also function as a filter circuit, and in addition to the effects of the bridge circuit, noise resistance. Can be further increased.

  FIG. 3 shows the second embodiment. Only the first GND pattern 13 and the second GND pattern 14 are connected by a high-resistance circuit element 21 of 1 kΩ or more which is a resistance element. Different. Since the first GND pattern 13 and the second GND pattern 14 can be sufficiently separated, attenuation of the signal waveform transmitted from the signal source 1 can be suppressed.

  FIG. 4 shows the third embodiment. The only difference between the first GND pattern 13 and the second GND pattern 14 is that the first GND pattern 13 and the second GND pattern 14 are connected between the grounds with the connection portion 22 between the grounds having a sufficiently small pattern width. Different. Impedance between the first GND pattern 13 and the second GND pattern 14 can be reduced, and noise radiation on the printed circuit board can be suppressed.

  At this time, it is preferable that the bridge circuit is connected at a position as far as possible so as not to affect the impedance balance condition of the bridge circuit as much as possible.

  FIG. 5 shows a fourth embodiment, which is different from the first embodiment only in that the first pattern 10 and the second pattern 11 are connected by a capacitor 23. It is possible to increase the balance between the first pattern 10 connected to the signal line 2 and the second pattern 11 connected to the signal ground line 3 in the previous stage of the bridge circuit.

  At this time, it is possible to take measures against common mode noise in two stages of the capacitor 23 and the bridge circuit, and noise resistance against the common mode noise can be further improved.

  FIG. 6 shows a fifth embodiment, in which the signal source 1 and the printed circuit board 7 are connected by a cable having the signal line 2 and the signal ground line 3, and the magnetic core 24 is wound around the cable. . According to this configuration, it is possible to increase the balance between the signal line 2 and the signal ground line 3 in the cable.

  At this time, it is possible to take measures against common mode noise in two stages of the magnetic core 24 and the bridge circuit, and noise resistance against the common mode noise can be further improved.

  FIG. 7 shows a sixth embodiment, which differs from the first embodiment only in that the signal source 1 and the receiving circuit 4 are arranged on the same printed circuit board. By providing the circuit element constituting the bridge circuit in the previous stage of the receiving circuit 4, the noise voltage applied to the receiving circuit 4 can be reduced.

1 is a plan view showing a printed circuit board according to Embodiment 1. FIG. FIG. 2 shows the connection structure of FIG. 1, wherein (a) is a circuit diagram of the apparatus of FIG. 1, and (b) is an equivalent circuit diagram supplementing (a). 6 is a plan view showing a printed circuit board according to Embodiment 2. FIG. 6 is a plan view showing a printed circuit board according to Embodiment 3. FIG. 6 is a plan view showing a printed circuit board according to Embodiment 4. FIG. 10 is a plan view showing a printed circuit board according to Embodiment 5. FIG. 10 is a plan view showing a printed circuit board according to Embodiment 6. FIG. It is a circuit diagram which shows the common mode countermeasure of the printed circuit board by one prior art example. It is a circuit diagram which shows the common mode countermeasure of the printed circuit board by another prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Signal source 2 Signal line 3 Signal ground line 4 Reception circuit 5 Input end 6 Reference end 7 Printed circuit board 8 Case 9 Screw hole 10 1st pattern 11 2nd pattern 13 1st GND pattern 14 2nd GND Pattern 15 First circuit element 16 Second circuit element 17 Third circuit element 18 Fourth circuit element 19 High resistance circuit element 23 Capacitor 24 Magnetic core

Claims (6)

In a printed circuit board including a receiving circuit for a signal transmitted from a signal source,
A first wiring pattern connected to a signal line from the signal source and connected to an input end of the receiving circuit;
A second wiring pattern connected to a GND line from the signal source and connected to a reference end of the receiving circuit;
A first GND pattern connected to a grounded housing;
A second GND pattern connected to a reference end of the receiving circuit;
A first circuit element mounted on the first wiring pattern;
A second circuit element mounted on the second wiring pattern;
A third circuit element having one end mounted on the input end side of the first circuit pattern in the first wiring pattern and the other end mounted on the first GND pattern;
A fourth circuit element having one end mounted on a reference end side with respect to the second circuit element in the second wiring pattern and the other end mounted on the first GND pattern;
A printed circuit board comprising:   When the impedance of the first circuit element is Z1, the impedance of the second circuit element is Z2, the impedance of the third circuit element is Z3, and the impedance of the fourth circuit element is Z4, Z1 * Z4 The printed circuit board according to claim 1, wherein: Z2 * Z3.   The printed circuit board according to claim 1, wherein each of the third circuit element and the fourth circuit element is a capacitor.   3. The printed circuit board according to claim 1, wherein the first GND pattern and the second GND pattern are connected via a resistance element. 4.   The printed circuit board according to claim 1, wherein the first wiring pattern and the second wiring pattern are connected via a capacitor.   The printed circuit board according to claim 1, wherein a cable having the signal line and the GND line is provided, and a magnetic core is wound around the cable.

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