2-1. Introduction
Chapter 1 described the mechanism of causing interference by
electromagnetic noise and the overview of noise suppression. Noise
suppression is mainly implemented in the noise transmission path
with use of shields and filters as the typical means. In order to
use these means efficiently, it is important to have a deep
understanding of the mechanism of the occurrence and propagation of
electromagnetic noise.
With close reference to the principle of noise occurrence described
in Chapter 1 in terms of noise source, there are three factors:
noise source origin, transmission path and antenna (assuming that
the noise interference is eventually propagated in the form of
electromagnetic waves, antenna is included here)
[Reference 1]
as shown in Fig. 2-1-1(a). In case of becoming a noise victim,
exactly the same schematic diagram can be used as shown in Fig.
2-1-1(b) by flipping the drawing horizontally and replacing the
noise source origin with a noise receiver. That means the mechanism
can be considered as the same in both cases of generating and
receiving noise.
Therefore, in order to have a deep understanding of the mechanism of
noise interference, Chapters 2 to 5 focus on the issues regarding
the noise source side and describe the basic theory of the
mechanisms of causing, transmitting and emitting noise. In these
chapters, shields for shutting out noise and ground connections are
also briefly described. Another important factor “filter” will be
described in detail in a later chapter.
First of all, Chapter 2 will describe the mechanism of causing
noise.
2-2. Noise source origin
There are various situations to cause an electric current that can
be a noise source. For example, a certain signal component is needed
for the operation of one circuit while causing a problem for other
circuits. In another case, noise may be inevitably generated despite
that none of the circuits want it. Sometime the noise may be caused
rather as a result of carelessness. Of course, the mind-set towards
the noise suppression varies depending on each situation. However,
it is easier to handle the situation if you can understand how the
particular noise has been caused.
In this chapter, we will take the following three typical cases of
noise sources to understand the mechanism of causing noise and the
general coping strategies.
-
(i)
Signal
-
(ii)
Power supply
-
(iii)
Surge
2-2-1. In case that signal becomes a noise source or victim
In this section we call the lines that primarily transmit
information as signal lines. Generally in order to transmit
information through an electric circuit, some amount of electric
current is required even if it is very small. Then, the current
creates an electromagnetic field around it. When the current changes
in accordance with the information, it emits radio waves to the
surrounding, which in turn is causing noise.
As the amount of information increases, the frequency of the
electric current that goes through the signal line increases, or
more lines may be required. Generally the higher the current
becomes, or the higher the number of lines becomes, the stronger the
radio wave is likely to be emitted. Therefore, the higher the
performance of electronic devices becomes, and the more information
is handled, the more likely that the signal lines used in the
electronic devices can easily cause noise interference.
Electrical circuits that transmit information can be broadly
classified into analog circuit and digital circuit, wherein analog
signal and digital signal are respectively used. The general
characteristics of those are described below from the viewpoint of
circuit noise.
(1) Analog circuit
From the viewpoint of noise source origin, analog circuits tend to
generate relatively less noise since those only use limited
frequencies and are designed to control the flow of electric
current.
But still, if some of the energy leaks to the outside, it can be a
cause of noise interference. For example, receivers for TV or radio
use a signal with a constant frequency called local oscillator
frequency in order to selectively amplify the targeted frequency
from the radio waves that have been received by the antenna. If this
leaks to the outside, it can cause interference to other devices. In
order to prevent this from happening, the tuner section is shielded
or EMI suppression filter are used for the wiring.
In contrast, from the viewpoint of noise victim, since analog
circuits often deal with faint signals and the information is
affected by even small fluctuations, the circuit tends to easily
become a noise victim. For example, if noise enters to the first
stage of a voice amplifier, the speaker will make a loud noise sound
by detecting and amplifying it. In order to prevent this from
happening, highly sensitive voice amplifiers are shielded or EMI
suppression filters are used for the wiring.
(2) Digital circuit
From the viewpoint of noise source origin, digital circuits are
likely to become noise sources as the transition between the signal
levels of 0 and 1 happens in a very short period of time, which
contains an extremely wide range of frequency components. In order
to prevent the noise emission, shields and EMI suppression filters
are used for digital signals. Noise generated by digital circuits is
an important topic and will be described in detail in Section 2-3,
since it is not only related to signals but also related to power
supply.
However, from the viewpoint of noise victim, the signals are
expressed with only two status of 0 and 1 (nothing in between) and
have a relatively large amplitude. In addition, the information
would not be affected by small induction. So it is unlikely to
become a noise victim. However, if it gets high level noise even if
it goes for a split second, the data will be completely altered. So
it has a vulnerability to pulse noise such as an electrostatic
discharge. (Electro-Static Discharge is also called ESD for short)
2-2-2. In case that power supply becomes a noise source
Since power supply is essentially a circuit that provides only
direct current or commercial frequency, it should be unlikely to
become a cause or pathway of electromagnetic noise. However, in many
cases, it actually becomes a cause or pathway of noise. This is
considered to be due to reasons as follows:
-
(i)
even though the voltage seems stable, its electric current may
contain a large amount of high-frequency current flowing to
operate the electric circuit
-
(ii)
since the power line is a shared wire in the circuit, noise is
circulated and affects the entire circuit
-
(iii)
since the ground in particular is often shared throughout the
equipment and provides a common voltage, it is hard to separate
it
-
(iv)
since it is the energy source for the equipment, the noise
energy also becomes large
Typical examples where the power supply causes noise are contact
noise and switching power supply.
The contact noise is a type of noise that occurs at a point of
contact when the source current is turned on/off with a switch (it
is particularly strong when turned off), which has the same meaning
as the switching surge described in (2) of Section 2-2-3. See
Section 2-2-3 for details. Since a very high voltage occurs and the
flow of transient but high-frequency current spreads radio waves, it
can cause a circuit failure or can lead malfunction of the
surrounding electronic devices.
Switching power supply is a circuit that changes voltage and
frequency by intermittently flowing the electric current with use of
semiconductor. Since the section of intermitting the electric
current generates a high-frequency energy, it causes noise
interference when it leaks out to the outside. For example, the
chopper-type DC-DC converter shown in Fig. 2-2-7 generates the
output voltage by intermittently flowing the direct current with use
of transistor. This intermittent current contains a high-frequency
energy. Although most of this energy is usually absorbed by input
capacitors and/or output smoothing circuit, even a small amount of
leakage can be a noise source for the surrounding circuits. In order
to eliminate noise in the switching power supply, a low-pass filter
that uses L and C is used in addition to the input capacitors and/or
output smoothing circuit (noise can also be suppressed by improving
the performance of input capacitors and output smoothing circuit).
Apart from DC-DC converter, an inverter that drives a motor is also
a type of switching power supply that can generate noise.
In contrast, from the viewpoint of noise victim, power supply is a
circuit that is relatively less likely to get affected. Since the
amount of energy used internally is large, it will not be easily
affected by interference.
However, power supply can be a conduction path for noise. As shown
in Fig. 2-2-8, power line is a conductor to directly join electronic
devices to each other and is one of the important conduction paths
for noise. For example, when an electronic device is affected by
noise, or when an electronic device emits noise, the AC power cable
becomes a doorway for noise. Therefore, many electronic devices use
EMI suppression filters in the power line. Fig. 2-2-9 shows an
example of the configuration of EMI suppression filter for AC power
supply.
Since the EMI suppression filters used for power supply generally
draw an electric current significantly larger than that of signals,
parts that are capable of drawing a large current are required.
2-2-3. Noise occurrence due to surge
An unintended excessive voltage or current due to electrostatic
discharge or on-off switching is called surge. Since the level of
voltage and/or current is significantly larger than that of normal
circuit operation, it can cause malfunction or damage to the
circuit. In order to prevent this from happening, surge absorbers
are used for the wiring, which surge can enter.
Typical surges are electrostatic surge, switching surge and
lightning surge etc. Surge is one of the major categories of EMC
measures. It is summarized as follows:
(1) Electrostatic surge
As shown in Fig. 2-2-11, electrostatic surge is a transient noise
that happens when an electric charge accumulated in a very small
floating electrostatic capacity of about several 100pF sustained in
a human body or equipment is discharged to an electronic device or
surrounding object. Although its energy is only small, its voltage
is as high as several kV or more and a larger current flows
instantaneously. Therefore, if it is directly applied to a circuit,
it can damage the circuit. Even if it is not directly applied, the
circuit may malfunction when the signal line suffers from
electromagnetic induction, or when the potential of power supply or
ground fluctuates.
One of the tests to simulate electrostatic surge is for example,
IEC61000-4-2. Please refer to the test for details.
As shown in Fig. 2-2-12, in order to reduce the interference by
static charge,
-
(i)
cover up with insulator so as to block discharge, or otherwise,
cover up with metal so as to divert it.
-
(ii)
release the discharge current through a pathway that does not
affect the circuit (release it to the solid earth to avoid
flowing into the signal ground: SG).
-
(iii)
use an appropriate surge absorber.
(2) Switching surge
When the electric current changes suddenly due to relaying or
switching on-off (especially when the circuit is turned off), a
transient high voltage is induced at the contact point due to the
inherent inductance of the circuit. The phenomenon is called
switching surge. The contact noise mentioned in Section 2-2-2 is the
noise caused by a switching surge.
Since an extremely high voltage occurs, it can show a spark as shown
in Figs. 2-2-13 and 2-2-14 or can spread radio waves due to the
strong damped oscillating current by the floating electrostatic
capacitance at the contact point resonating with inductance.
Therefore, it can damage other electronic devices that are sharing a
circuit or cause their malfunction. Since this damped oscillating
current contains high-frequency components, it can cause reception
interference to radios and TVs.
Since resonance that causes a damped oscillating current is an
important topic in noise suppression, it will be further described
in a different section.
Other than relays and switches, noise generated by DC motor, for
example is often caused by the commutator switching the current.
Therefore this can be considered as a type of switching surge.
As shown in Fig. 2-2-15, in order to reduce the interference by
switching surge,
-
(i)
use a surge absorber such as a capacitor, varistor and snubber
at contact point.
-
(ii)
provide a shield to shut out any electromagnetic effect.
-
(iii)
use EMI suppression filters for the noise-transmitting wires and
victimized circuits.
In order to achieve some improvement just by shields and filters, it
is important to know where the sections that provide the pathway and
antenna for noise are. For example, in Fig. 2-2-15, only shielding
the switch section does not achieve any improvement at all in most
cases (since the wiring outside the shield works as an antenna and
emits most of radio waves).
(3) Lightning surge
Since lightning is a natural phenomenon and has an extremely large
energy, it is very difficult to provide protection against a direct
hit. In many cases, instead of providing protection against a direct
hit, electronic devices are provided with protection against induced
lightning.
Induced lightning is a high voltage induced to a relatively long
wiring such as a power line or communication line when a lightning
stoke occurs near an electronic device. Possible mechanisms of
causing an induced lightning are: electric charge is induced to an
electric line due to the electric field caused by thunder cloud and
then the electric charge is released by a lightning stoke; or a
magnetic field due to the lightning current causes an induced
electromotive force in an electric line. Not as hard as direct
lightning stroke, but induced lightning has a large energy that is
enough to damage electric circuits. So protection is needed.
In order to provide protection against induced lightning, a surge
absorber such as a varistor is used at the section where power lines
and communication lines go in and out the electronic device.
“2-2. Noise source origin” - Key points
The three points as listed below were described as examples of
noise sources and victims. Now we understand that noise
suppression can be all different depending on the mechanism of
causing noise and what kind of noise you are dealing with. In
order to efficiently implement noise suppression, you need to
investigate the noise that is causing interference and choose
appropriate means in accordance with the cause. Since digital
circuit and resonance phenomenon are important topics regarding
noise sources, these will be further described in different
sections.
- Signal
- Power supply
- Surge