JPH09260885A - Noise reduction mechanism - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable an electrostatic shield structure to display an electrostatic shield effect as much as possible, by a method wherein a means which decreases an object in impedance to grounding is provided to the electrostatic shield structure installed against a noise source. SOLUTION: Provided that noises are transmitted to a conductor 2 apart by a distance d1 from a noise source 1 that produces noises, an electrostatic shield structure 3 is installed at a position apart from the noise source 1 by a distance d2 (d2 <d1 ). An impedance converter 4 as a means which decreases an object in impedance to grounding GND is connected to the electrostatic shield structure 3. By this setup, electrostatic capacitance between the noise source 1 and an equipment influenced by noises transmitted from the noise source 1 is reduced to nearly zero, a noise reduction mechanism of this constitution is capable of displaying an electrostatic shield effect as much as possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EMC designed to reduce the influence on the inside and outside of a device due to noise generated thereby in an environment where an electromagnetic field exists.
(Electro Magnetic Compatible design) A noise reduction mechanism capable of effectively reducing noise generated in a cabinet or the like.

[0002]

2. Description of the Related Art Digital circuits, electrical switching, etc. have inherently very high frequency electrical changes. These electrical changes may propagate in the vicinity as changes in the electric field in the air, for example, and may also propagate as electromagnetic waves in the distance, giving harmful noise to other electronic devices. this house,
In particular, there is an electrostatic shield method as one measure for reducing noise propagation due to a change in electric field. The effect of this electrostatic shield will be described using the model of electrostatic noise coupling in FIG. In the figure, V _S is the noise of the noise source existing at point A, Z _L is the equivalent impedance of the equipment affected by the noise existing at point B, S is the electrostatic shield, Z _C is the electrostatic shield S and the ground. Impedance between them, C _S is the capacitance between A and B when there is no electrostatic shield, C ₁ , C
₂ and C ₃ are between S and A when the electrostatic shield S is installed, S
・ Each capacitance between B and between A and B. in this way,
The capacitance is rearranged by providing the electrostatic shield S, and the magnitude of the noise propagating to the device at the point B is V
_N = V _S · C ₃ / (C _L + C ₂ + C ₃ ). As a result, the noise level V _N is approximately improved to C ₃ / C _S.

By the way, when the noise propagates from the noise source 21 and a fluctuating current flows, the metal piece 22 of λ / 4 size having the noise frequency as shown in FIG. 7 is equivalently equivalent to the antenna as shown in FIG. Since it functions as 22a and amplifies and radiates noise, it adversely affects other electronic devices and the like. Therefore, it is necessary to select the size. Regarding this size, the same applies to a metal housing 23 having a built-in device serving as a noise source 21 as shown in FIG. 8A, and the metal housing 23 itself has a noise frequency of λ / 4. In the case of the size, it may function as the antenna 23a. In FIG. 8B, the inverter unit 24,
EMC with built-in X-ray tube controller such as high-speed starter 25
A cabinet (housing) 26 is shown. Also in the EMC cabinet 26 having such a shield box structure, when the vertical dimension is, for example, about 1.8 m, λ / of 30 MHz is obtained.
4 (Consideration of reduction ratio) Functions as an antenna and generates large radiation noise.

FIG. 9 shows an example of the structure of a detection unit 28 isolated from a main body 27 in a medical device for detecting a biosignal such as an electrocardiogram. Body 2
The high-speed logic circuit for data processing and displaying the biomedical signal is built in 7 and the detection unit 28 has a cable 30 having an electrode 29 for contacting the living body at the tip thereof.
Is connected. The leak current of the detection unit 28 is strictly regulated from the viewpoint of biological safety, and thus is isolated from the main body 27 in this manner, and the power source and the like are sent from the main body 27 via a transformer or the like. . In such a device, a high-frequency logic circuit in the main body 27 isolates a high-frequency noise from the circuit, particularly jumps into the cable 30 up to the electrode 29, and the radiation noise is secondarily radiated from the cable 30 to cause another electron to be emitted. May adversely affect equipment.

On the other hand, an active ground circuit for obtaining a low impedance and constant potential point is disclosed in Japanese Patent Laid-Open No. 6-188842. As shown in FIG. 10, this active ground circuit applies a reference potential to the impedance converter 31 having a high input impedance, connects the input to the ground potential, and obtains a low impedance at the output.

[0006]

However, in the electrostatic noise coupling model shown in FIG. 6, the magnitude V _N of the noise propagating to the device is approximately C ₃ by providing the electrostatic shield S. / C _S , but in an actual device, the distance between the electrostatic shield S and the ground is large, so the impedance Z _C between them is large, and therefore C ₃ is not small and the effect of the electrostatic shield is sufficient. There was a problem that I could not get it.

As shown in FIGS. 7 and 8, when the metal piece or the metal case at the position where the noise propagates has a λ / 4 size of the noise frequency, the metal piece or the metal case is used. Equivalently functions as an antenna and amplifies and radiates noise, which adversely affects other electronic devices and the like.

As shown in FIG. 9, in the structure of the detection unit isolated from the main body, the high-frequency noise from the high-speed logic circuit etc. in the main body jumps into the isolated circuit, especially the cable to the electrode,
There is a problem that radiation noise may be secondarily radiated from the cable and adversely affect other electronic devices and the like.

Further, the active ground circuit shown in FIG. 10 shows only a circuit capable of obtaining a low-impedance constant potential point that can be used as a ground point.

The present invention has been made in view of the above,
First, the effect of the electrostatic shield can be maximized, and even when the second metal casing has a size of λ / 4 of the frequency of noise, it does not become an antenna, and
Another object of the present invention is to provide a noise reduction mechanism that prevents secondary radiation of radiation noise from a shielded cable connected to a circuit or the like isolated from the main body.

[0011]

In order to solve the above-mentioned problems, the invention according to claim 1 provides an electrostatic shield structure provided for a noise source with a low impedance for reducing the impedance to ground. The point is to connect the means. With this configuration, the capacitance between the noise source and the equipment that is affected by noise will be close to zero, and the effect of the electrostatic shield will be maximized, and the amount of noise that propagates to the equipment, etc. Can be made sufficiently small.

A second aspect of the present invention is characterized in that, in the noise reduction mechanism according to the first aspect, the electrostatic shield structure is made of a good conductive metal containing aluminum and copper. With this configuration, the entire area of the electrostatic shield structure has a low impedance with respect to the ground, and the effect of the electrostatic shield can be maximized.

A third aspect of the present invention is characterized in that a metal casing for preventing noise transmission to the inside or the outside is connected to an impedance lowering means for reducing impedance to ground. With this configuration, even when the metal case has a size of λ / 4 of the noise frequency, the high-frequency noise induced in the metal case flows into the low impedance point and does not become an antenna, preventing the generation of electromagnetic waves. To be done.

A fourth aspect of the present invention is characterized in that a shield body in a shielded cable for transmitting a desired signal is connected to a means for lowering impedance to reduce impedance to ground. With this configuration,
The shield body functions as an electrostatic shield, and secondary radiation of radiation noise from the shielded cable is prevented.

A fifth aspect of the invention is characterized in that, in the noise reduction mechanism according to the fourth aspect, the other end of the impedance lowering means is isolated from a ground potential point. With this configuration, in a medical device or the like, when the shielded cable is connected to a detection unit or the like isolated from the main body and transmits a biological signal or the like, the other end of the impedance lowering means is connected to the main body and the detection unit. By using an isolated connection structure in which the detection unit is connected to the ground via a floating capacitance or the like, the detection unit is reliably isolated from the main body, and safety as a medical device or the like is maintained.

According to a sixth aspect of the present invention, in the noise reduction mechanism according to any one of the first to fifth aspects, the impedance lowering means is provided with a reference potential and has a high impedance on the input side and a low impedance on the output side. The gist is that it is an impedance converter that obtains impedance.
With this configuration, even if the ground side impedance is high, it is possible to reduce the impedance of the connection side with the electrostatic shield structure, etc., and it is possible to connect the ground side via stray capacitance etc. as described above. Become.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1 and 2 are views showing a first embodiment of the present invention. Now, for example, it is assumed that the harmonics of the clock from the clock oscillator on the digital PC board become noise. In FIG. 1, 1 is a noise source that generates such noise, and a distance d ₁ from the noise source ₁
If noise is propagated to the conductor 2 at a distance, the electrostatic shield structure 3 is separated from the noise source 1 by a distance d ₂
It is installed at the position of (d ₂ <d ₁ ). The electrostatic shield structure 3 is made of a highly conductive metal material such as aluminum or copper. Then, the electrostatic shield structure 3 is connected to an impedance converter 4 as a means for reducing the impedance to the ground GND. In FIG. 1, the impedance converter 4 is connected to the point A of the shield structure 3, but may be further connected to a plurality of points such as the point B.

FIG. 2 shows the configuration of the impedance converter 4. The impedance converter 4 is composed of a voltage follower using an operational amplifier, and the input side (non-inverting input terminal (+)) has a high impedance and the output side has a low impedance. As the operational amplifier, one having better frequency characteristics than the frequency of target noise is used. In particular, since the phase characteristic drops from a low frequency, the phase compensation capacitor 6 may be connected between the inverting input terminal (-) and the output terminal. A filter function circuit 7 may be connected to the high impedance input of the operational amplifier to stabilize the potential. As the filter function circuit 7, a high pass filter by CR, a common mode canceling function of an operational amplifier, a noise cut transformer, and other ready-made signal filters are used. Reference numeral 8 is a reference potential addition point.

In FIG. 1, since the potential of the ground GND is stably guided to the high impedance input point (non-inverting input terminal (+)) of the impedance converter 4 connected to the point A, its lead is the shielded cable 5. Good. Further, in order to further stabilize the potential of the shield braid (shield body) in the shielded cable 5, even if the same impedance converter 4 as above is connected to the point C which is as close as possible to the ground potential in the shield braid. Good.

As described above, in the noise reduction mechanism of this embodiment, the impedance converter 4 is connected to the electrostatic shield structure 3 to lower the impedance to the ground GND, so that the noise source 1 and the conductor 2 are connected. The electrostatic capacitance (C ₃ described above) between and becomes small near zero, the effect of the electrostatic shield is maximized, and the magnitude of noise propagating to the conductor 2 can be made sufficiently small. . Therefore, the radiation noise is not secondarily radiated from the conductor 2.

The second embodiment of the present invention is shown in FIGS.
An embodiment will be described. In the present embodiment, a noise reduction mechanism is provided in a metal housing such as an EMC cabinet to form a low noise cabinet. Now the noise source 11
As noise from the motor drive inverter harmonics 4
It is assumed that 2 MHz is generated. As shown in FIG. 3A, when the above noise source 11 is housed in the metal housing 9 expected to have an electrostatic shield function with a height of 1.8, noise is first stored in the metal housing 9. Propagate. Since the height (frame length) of the metal casing 9 is λ / 4 size of the harmonic of 42 MHz, the high frequency noise current flowing in the metal casing 9 causes the metal casing 9 to function as an antenna and generate a strong electromagnetic wave of 42 MHz. Will be done. The metal casing 9 has a connection mechanism with the ground GND in order to function as an electrostatic shield, but the high-frequency noise current induced in the metal casing 9 can be sufficiently reduced due to the high impedance between the metal casing 9 and the ground GND. Not not. On the other hand, in the present embodiment, a part of the inside of the metal housing 9, for example, D in FIG.
As shown in (b) and (c) of FIG. 3, the impedance converter 4 mounted in the shield 10 is installed at the position indicated by, so that the low impedance output side thereof is a metal casing. The body 9 is brought into surface conduction E, and the high impedance input side is led to the ground GND through the shielded cable 5 as in the first embodiment. FIG.
As shown in (d), the impedance converter 4 mounted in the shield 10 may be installed outside the metal casing 9 corresponding to the point D.

As described above, in the noise reduction mechanism of this embodiment, the impedance converter 4 is connected to the metal casing 9 to lower the impedance to the ground GND, so that the metal casing 9 can reduce the frequency of noise. Even in the case of the λ / 4 size, the high frequency noise current induced in the metal casing 9 flows into the low impedance point, and it is possible to prevent the generation of electromagnetic waves without forming an antenna.

FIGS. 4A and 4B show the third embodiment of the present invention.
An embodiment will be described. The present embodiment, in a medical device for detecting a biological signal such as an electrocardiogram, a detection unit isolated from the main body, in particular, a shield cable connected to the detection unit is provided with a noise reduction mechanism,
This is intended to prevent secondary noise from being radiated from the shielded cable due to high frequency noise from the high speed logic circuit in the main body. 4 (a) and (b)
9, the same or equivalent devices as those in FIG. 9 are designated by the same reference numerals as those used above, and the duplicated description will be omitted. In the present embodiment, the cable connected to the detection unit 28 is the shield cable 12, and the low impedance output point of the impedance converter 4 is connected to the shield braid (shield body) 12a. However, the detection unit 2 to which the shielded cable 12 is connected
Since 8 is isolated from body 27, the high impedance input point of impedance converter 4 cannot be directly connected to ground GND and must be isolated. Therefore, the high-impedance input point of the impedance converter 4 and the ground GND are connected by the high voltage capacitor 13 (FIG. 4A), or the high-impedance input point of the impedance converter 4 and the ground GND are connected to the main body 27. The means for connecting the detection unit 28 with the stray capacitance 14 (FIG. 4B) is used.

As described above, the noise reduction mechanism of this embodiment is provided with the shield braid 12a of the shielded cable 12.
Connect the impedance converter 4 to the ground GND
Since the impedance to the
a functions as an electrostatic shield, and the shielded cable 12
Radiation noise is never emitted from the secondary radiation.

Next, examples of experimental results for further confirming the effects of the above-described embodiments will be described with reference to FIGS.
This will be described using (d).

Method: Read the rectangular wave output (20 MHz) emitted from the oscillator (HP81116A: waveform generator) 1
5, the metal piece (copper tape) 16 receives the high-frequency electrostatic induction from the lead 15. And
The potential difference between the metal piece 16 and the ground GND at that time is used as an electrostatic induction voltage in an oscilloscope (Tektro TDS5
40A), and the effect of reducing electrostatic induction was observed by comparing PP values obtained from the observed waveform. For comparison,
The experimental apparatus prepared the following three types in which the space between the lead 15 and the metal piece 16 was different. The structure in which there is nothing in the space from the lead 15 to the metal piece 16 (FIG. 5A). Note that 17 in FIG. 5A is an oscilloscope probe.
An electrostatic shield (copper tape) 18 of the same size as the metal piece 16 is provided in the space from the lead 15 to the metal piece 16, and the electrostatic shield 18 is connected to the ground GND by a capacitor 19 (FIG. 5B). An electrostatic shield 18 having the same size as the metal piece 16 is provided in the space from the lead 15 to the metal piece 16, and the electrostatic shield 18 is connected to the ground GND via the impedance converter 4 shown in FIG. FIG. 5 (c)).

Results: The PP values obtained from the oscilloscope observation waveforms are as follows. The structure has a PP value of 5 mV. The structure has a P-P value of 4.5 mV, which is a 10% reduction. Has a PP value of 4 mV,
20% reduction.

Conclusion: By using the impedance converter 4, the high frequency electrostatic induction can be further suppressed as compared with the case where only the electrostatic shield 18 is used.

[0030]

As described above, according to the invention of claim 1, the electrostatic shield structure provided for the noise source is connected to the impedance lowering means for reducing the impedance to the ground. , The capacitance between the noise source and the equipment affected by the noise becomes close to zero, and the effect of the electrostatic shield is maximized to ensure that the noise propagated to the equipment is sufficiently large. Can be made smaller.

According to the second aspect of the present invention, since the electrostatic shield structure is made of a good conductive metal including aluminum and copper, the entire area of the electrostatic shield structure has a low impedance with respect to the ground, and is therefore static. It is possible to maximize the effect of the electric shield.

According to the third aspect of the present invention, the metal casing which should prevent the transmission of noise to the inside or the outside is connected to the impedance lowering means for reducing the impedance to the ground. Frequency of λ /
Even in the case of four sizes, high-frequency noise induced in the metal housing can flow into the low impedance point, and the metal housing can be prevented from becoming an antenna.

According to the fourth aspect of the invention, since the shield body in the shielded cable for transmitting the desired signal is connected to the impedance lowering means for reducing the impedance to the ground, the shield body functions as an electrostatic shield. It is possible to prevent secondary radiation of radiation noise from the shielded cable.

According to the fifth aspect of the present invention, the other end of the impedance lowering means is isolated from the ground potential point. Therefore, in a medical device or the like, the shield cable is isolated from the main body of the detection unit or the like. In the case of transmitting a biological signal or the like, the other end of the impedance lowering means is connected to the ground through the stray capacitance between the main body and the detection unit, etc. The detection unit can be reliably isolated from the main body, and the safety as a medical device or the like can be maintained.

According to the invention of claim 6, since the impedance lowering means is an impedance converter to which a reference potential is applied and which has a high impedance on the input side and a low impedance on the output side, the impedance on the ground side is high. Also, the impedance of the connection side with the electrostatic shield structure or the like can be reduced, and the ground side can be connected via the floating capacitance or the like as described above.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a noise reduction mechanism according to the present invention.

FIG. 2 is a block circuit diagram of an impedance converter applied to the first embodiment.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing a third embodiment of the present invention.

FIG. 5 is a diagram for explaining an experimental example for confirming the effect of each of the above-described embodiments.

FIG. 6 is a diagram showing a model of electrostatic noise coupling for explaining the effect of an electrostatic shield.

FIG. 7 is a diagram for explaining the behavior of a metal piece that has undergone noise propagation.

FIG. 8 is a diagram for explaining the behavior of a metal housing that has undergone noise propagation.

FIG. 9 is a diagram for explaining noise generated from a connection cable to the isolation unit.

FIG. 10 is a diagram showing a conventional active ground circuit.

[Explanation of symbols]

 1, 11 Noise source 3 Electrostatic shield structure 4 Impedance converter (impedance lowering means) 8 Reference potential addition point 9 Metal housing 12 Shield cable 12a Shield braid (shield body) 27 Main body 28 Detection isolated from the main body unit

Claims (6)

[Claims] 1. A noise reduction mechanism comprising an electrostatic shield structure provided for a noise source, and an impedance lowering means for reducing impedance to ground connected to the electrostatic shield structure. 2. The noise reduction mechanism according to claim 1, wherein the electrostatic shield structure is made of a good conductive metal including aluminum and copper. 3. A noise reduction mechanism characterized in that a low-impedance reducing means for reducing the impedance to the ground is connected to a metal casing for preventing noise transmission to the inside or the outside. 4. A noise reduction mechanism comprising a shield body in a shielded cable for transmitting a desired signal, and a low-impedance reducing means for reducing impedance to ground connected thereto. 5. The noise reduction mechanism according to claim 4, wherein the other end of the impedance lowering means is isolated from a ground potential point. 6. The impedance lowering means is an impedance converter to which a reference potential is applied and which has a high impedance on the input side and a low impedance on the output side. Noise reduction mechanism.