Although the chapters so far have mainly described the occurrence and conduction of noise, many of the electromagnetic noise interferences are actually conducted through space via radio waves. This chapter describes the spatial conduction of noise.
The spatial conduction of noise can be classified into two types of problems: a problem that happens in a relatively short distance (when the circuits inside the same electronic device are interfering with each other) or a problem that happens in a relatively long distance (when noise is emitted as radio waves and interferes with an electronic device in a neighbor's house). These two problems are different in terms of the level of reduction in interference depending on the distance, and the latter problem has an influence in a longer distance. Although the latter problem is often the one that has regulations for unwanted emissions based on noise regulations, the former is also important for designing electronic devices.
In this chapter, we will first discuss the interference between circuits (problem of short distance) and then describe the antenna theory (problem of long distance) and shielding to block this problem. For the sake of simplifying the explanation, some phenomena may be extremely simplified in the explanation through our unique interpretation. For detailed and precise theory, please refer to technical books. [Reference 1, 2, 3, 4]
The contents of this chapter cover the section from the transmission path to the antenna as shown in Fig.1. Just like the previous chapter, technical terms and concepts will be gradually introduced along with the explanation.
4-2. Spatial noise conduction and its countermeasures
As described in Chapter 1, noise conduction occurs through conductor conduction and spatial conduction. Although the conductor conduction has been mainly described so far, this chapter will describe the spatial conduction as well as noise suppression to block it out.
4-2-1. Spatial noise conduction model and shield
(1) Spatial noise conduction
As shown in Fig. 4-2-1, the main mechanisms of the spatial noise conduction are considered to be as follows:
Emission and reception of radio waves.
As an example, Fig. 4-2-1 shows how the spatial noise conduction occurs inside an electronic device and how the noise is eventually emitted from the cable. These three mechanisms of spatial conduction are also applied to the case of noise conduction to the outside of the electronic device as well as to the case of noise reception.
Fig. 4-2-1 Model of spatial noise conduction
To block out the spatial noise conduction in the air, the target object should be shielded as shown in Fig. 4-2-2. Shield means that the target object is covered by a good conductor (or magnetic body) such as a metal etc. Shielding can be applied to both noise source side and receiver side. Although the target circuits are individually shielded in Fig 4-2-2, the entire electric device or the entire room (called shield room) may be covered.
Although the mind-set for shielding is slightly different depending on the noise induction model, the embodiment is almost the same. The reason is that even a thin metal foil can provide a sufficiently good effect in the frequency range of above several MHz unless it is under extreme conditions. In many cases, it requires ground connection and its effectiveness significantly varies depending on how good the ground is.
4-2-2. Electrostatic induction
(1) Electric field transmits noise
Generally, an object that has a voltage creates an electric field around it. A phenomenon in which this electric field affects the surrounding circuits as shown in Fig. 4-2-3 is called electrostatic induction. A circuit diagram that represents this phenomenon is shown in Fig. 4-2-3(b), wherein a floating electrostatic capacitance
has been created from the noise source to the victim and thereby a current path has been created.
The noise voltage
caused by the electrostatic induction increases when the the noise source voltage
becomes greater and the floating electrostatic capacitance
becomes greater. The floating electrostatic capacitance
increases when the distance between the noise source and victim is shorter and the noise source and victim are bigger in size.
Fig. 4-2-3 Electrostatic induction
(2) High impedance circuit is susceptible to noise
Often the floating electrostatic capacitance
is a very small amount of about several pF or less. For example, the floating electrostatic capacitance between the lines shown in Fig. 4-2-3(a) is approx 1pF, given that the gap is 10mm, the parallel length is 100mm, and the diameter of the line is as thin as 1mm (when the dielectric constant of the substrate is ignored).
Therefore, the ratio of the impedance of
to the entire circuit in Fig. 4-2-3(b) is relatively large. If the impedance
of the circuit that becomes a victim of noise can be made smaller than this, the induction voltage
can be reduced by voltage dividing. Generally speaking, this is one of the reasons why high impedance circuits are more likely to pick up noise (low impedance circuits are less likely).
Generally, electrostatic induction refers to general noise induction caused by an electric field. In order to simplify the circuit model, we only focus attention to the floating electrostatic capacitance between lines as shown in Fig. 4-2-3.
(3) How to reduce electrostatic induction
In order to reduce electrostatic induction, the following measures (among others) are generally taken:
Increase the distance (the floating electrostatic capacitance is reduced).
Reduce the size of wiring etc.
Reduce the length of the parallel wiring section (reduce the floating electrostatic capacitance).
Provide an electrostatic shield (cover either the noise source or victim with a metal plate, which is then connected to the ground).
Reduce the voltage of noise source (use an EMI suppression filter).
Reduce the impedance or sensitivity of the receiver (use an EMI suppression filter).
Among those listed above, the following section will describe the electrostatic shield.
4-2-3. Electrostatic shield
Fig. 4-2-4 shows an example of electrostatic shield. A metal plate that is connected to the ground is placed between the noise source and victim so as to block out the effect of electric field.
Fig. 4-2-4 Electrostatic shield
As shown in Fig. 4-2-4(b), electrostatic shields reduce the effect on the noise victim by bypassing the noise current to the ground. Therefore, grounding (making a connection to the ground) is required. In case of shielding high frequency noise, it is not necessary to make a connection to the earth. It is good enough to make a connection to the ground of the enclosure or circuit. However, the impedance of the ground should be as low as possible to let the noise current flow smoothly.
Generally speaking, electrostatic shield refers to a shield for electrostatic field. In case of blocking high frequency noise in proximity to the wiring as shown in Fig. 4-2-4, the effect of electromagnetic shield (described later) has been involved.
Shielding can be applied to both noise source side and victim side. In case of shielding on the victim side, it is connected to the ground of the affected circuit.
4-2-4. Electromagnetic induction
(1) Magnetic field transmits noise
Generally, an electric current flowing through a wire creates a magnetic field around it. A phenomenon in which this magnetic field affects the surrounding circuits as shown in Fig. 4-2-5 is called electromagnetic induction. From the viewpoint of circuit, you can think that an induction voltage occurs in the affected circuit due to the mutual inductance
between the two circuits as shown in Fig. 4-2-5(b). The inductors (coils) that are connected with
in the figure refer to the inductance of the current loop created by the circuit wiring etc. and does not represent particular components.
Just like a case of electrostatic induction, the noise voltage
caused by the electromagnetic induction increases when the noise source current
becomes greater and the mutual inductance
becomes greater. Furthermore, the mutual inductance
increases when the distance between the noise source and victim becomes shorter and the parallel section of current becomes larger.
(2) Current loop causes problems
The magnitude of the mutual inductance
needs to consider the entire current loop. For example, in case of the thin line (gap 10mm, parallel length 100mm, diameter 1mm) that was used in the above-mentioned example of floating electrostatic capacitance, the mutual inductance of only the relevant section is approx. 40nH.
However, the current always requires a return route (ground etc.). As an example of this return route taking a slightly longer way, if there is a ground 100mm below both lines, the mutual inductance increases to about 100nH. (Since this estimation does not include the circuits on both ends of the lines, it could be even more if these circuits are considered)
In contrast, as an example of the return route taking the shortest way, if there is a ground plane 1mm below the lines, the mutual inductance is reduced to about 0.5nH.
As above, the value of the mutual inductance varies depending on how the return route of the current takes. To reduce the mutual inductance, you need to reduce the total area of the current loop created by the circuits on both ends of the lines, and the ground.
Fig. 4-2-5 Electromagnetic induction
(3) How to reduce electromagnetic induction
In order to reduce electromagnetic induction, the following measures (among others) are generally taken:
Increase the distance (the mutual inductance is reduced).
Reduce the current loop area of wiring etc.
Current loops should be perpendicular to each other (the mutual inductance is reduced)
Provide an electromagnetic shield (cover either the noise source or victim with a metal plate)
Reduce the current of noise source.
Attach an EMI suppression filter to the receiver (bypass capacitor, ferrite bead etc.)
Among those listed above, the following section will describe the electromagnetic shield.
4-2-5. Electromagnetic shield
(1) Magnetic fields can be stopped without using a magnetic body
An example of electromagnetic shield is shown in Fig. 4-2-6. A metal plate is placed between the noise source and victim so as to block out the magnetic flux that goes through the metal plate. Since this effect of blocking out the magnetic flux is mainly caused by the eddy current that flows through the metal plate, the metal plate does not need to be a magnetic body. However, an electric current needs to be allowed to flow through it. That is to say, if there is a gap in the metal plate, the shielding effect would be significantly impaired.
Please also be aware that the magnetic field caused by a direct current or low frequency waves cannot be blocked by an electromagnetic shield. In such a case, you need to use a magnetic shield, which will be described later.
(2) Connected to the ground in most cases
In principle, electromagnetic shield does not require grounding as indicated by the circuit in Fig. 4-2-6(b). However, if you are shielding a cable, both ends should be connected the ground. This is because the effect of minimizing the current loop can be gained by using the shield inner surface as the return route of the current. For example, one type of the ideal shielded cables is coaxial cable, which uses the outer conductor as the return route of signal current. This allows keeping the area of the current loop against the external magnetic field to almost zero.
In many cases of noise suppression, both electromagnetic induction and electrostatic induction are involved. So, if the metal plate for electromagnetic shield is connected to the ground, it can also work as an electrostatic shield. Therefore, electromagnetic shields are also connected to the ground in many cases.
Fig. 4-2-6 Electromagnetic shield
4-2-6. Emission and reception of radio waves
(1) Radio waves transmit noise if the distance increases
In addition to the electrostatic induction and electromagnetic induction, the spatial noise conduction can be caused by being once turned into radio wave, which can fly into the air and interfere with other circuits as shown in Fig. 4-2-7.
The electrostatic induction and electromagnetic induction are phenomena that occur in a relatively short distance and those effects are reduced inversely proportional to the square or cubic of the distance. Therefore, it is effective to separate the circuits from each other. Although the interference via radio wave is reduced in accordance with the distance, the degree of reduction is not so high and thus noise can travel over a relatively long distance.
Therefore, you can say that the spatial conduction is mainly caused by induction via an electric field or magnetic field in a short distance while it is mainly caused by induction via radio wave in a long distance.
(2) Near field and far field
These phenomena are caused by the electromagnetic field construction around the antenna that emits noise. The area relatively near the antenna is called near field, while the area relatively far away is called far field. As a rough guide, the transition is about a distance of
from the noise source as shown in Fig. 4-2-7.
The transition distance is inversely proportional to the frequency. Although the distance is as far as 5m for 10HMz, it will be around 50cm for 100MHz and 5cm for 1GHz. In case of inside a general electronic device, you need to consider the induction caused by radio wave in the frequency range of over 1GHz (the frequency range that is used by mobile phone and wireless LAN etc.).
(3) Wave impedance
One of the characteristics of the noise that is transmitted in the air as a radio wave is that the ratio of the electric field to the magnetic field is constant (377 ohms). This ratio of the electric field to the magnetic field is called wave impedance. In the case of near field, either the electric field or the magnetic field may be more than the other, which may create spots where the wave impedance is extremely large or small. The effectiveness of shielding is affected by that. Since the wave impedance is constant in the far field, the shielding effect is stable.
A circuit that sends and receives radio waves is called antenna. In terms of noise suppression, you need to create a circuit that emits and receives least possible noise. That means that the circuit should be designed by trying not to create an effective antenna. The near field, far field and antenna will be further described in a separate section.
(5) Use electromagnetic shields for shielding
Shielding radio waves is performed by the above-mentioned electromagnetic shield. That means that the electromagnetic shield blocks out both high frequency magnetic field and electric field. The effect of the electromagnetic shield will be described in a separate section.
Fig. 4-2-7 Transition between near field and far field
4-2-7. Magnetic shield
Electromagnetic shield does not have any effect on the very low frequency magnetic field including a direct current magnetic field AC power supply etc. In such a case, magnetic shield is effective. The magnetic shield covers up the target object with a magnetic body as shown in Fig. 4-2-8 and reduces the magnetic field around the target object by inducting and bypassing the magnetic field lines into the magnetic body. In order to improve the bypass effect, a thicker material with a large magnetic permeability needs to be used.
Fig. 4-2-8 Magnetic shield (conceptual diagram)
4-2-8. How to make the shield lighter
(1) Difficult to make a perfect shield
In order to completely block out the spatial conduction (40 dB or more as a guideline), the entire periphery of the target object needs to be covered with a shield material as shown in Fig. 4-2-9. However, shields are large parts, and the weight and costs are the issues.
Fig. 4-2-9 Shield configuration
As shown in Fig. 4-2-3 and Fig. 4-2-5, shielding can be effective to some extent even though it is merely a shield plate placed in the middle or in an extreme case, ground lines laid on both sides of the wiring in question (called guard trace). However, you can only expect an effect of up to about 10dB from such an incomplete shield.
(2) Eliminate noise in the area of conductor conduction
A circuit needs an antenna to emit and receive noise. If you can eliminate noise by inserting an EMI suppression filter between this antenna and the circuit, the noise can be eliminated in the area of noise conductor conduction and thus shielding is not required.
Fig. 4-2-10 Suppression of spatial conduction using EMI suppression filters
Although low-pass filters that use capacitors and inductors are generally used as EMI suppression filters, the components that are advantageous for noise suppression vary depending on the noise induction mechanism.
(3) Filter for electrostatic induction
For example, in case of the electrostatic induction shown in Fig. 4-2-11, it is assumed that the circuit impedance is extremely high due to the small electrostatic capacity of the floating electrostatic capacitance
that mediates noise. In such a case, bypass capacitors are considered to be more advantageous than impedance components such as a ferrite bead.
Fig. 4-2-11 Example of filter configuration effective for electrostatic induction
(4) Filter for electromagnetic induction
In case of the electromagnetic induction shown in Fig. 4-2-12, it is important to reduce the current on the noise source side and to reduce the voltage on the noise receiver side. Impedance components are considered to be advantageous for reducing current, while bypass capacitors are considered to be advantageous for reducing voltage.
The above explanations are merely qualitative and the level of impedance varies depending on the frequency. However, the noise suppression can be effectively carried out by choosing a circuit in consideration of the noise induction mechanism.
Fig. 4-2-12 Example of circuit configuration effective for electromagnetic induction
“4-2 Spatial noise conduction and its countermeasures” - Key points
- Electrostatic induction is caused by voltage
- Electromagnetic induction is caused by current
Induction occurs via radio wave in a relatively long distance
- Shield is used for blocking out the above induction
EMI suppression filters are used in the area of conductor conduction for blocking out induction without any shield