2-3. Noise generated by digital circuit
Since digital circuit makes it easy to design electronic devices as well as
significantly improving the performance, it has been widely used in electronic
devices. On the other hand, it can relatively generate noise more easily, and it
is a typical circuit that requires measures for unwanted emissions in accordance
with the noise regulations.
Fig. 2-3-1 shows the types of noise that can be emitted by electronic devices
that use digital circuits. Typically it is generated over a wide frequency
range, which causes reception interference if it goes over the frequency range
for TV and/or radio etc. This section will describe the mechanism of generating
such noise from digital circuits.
2-3-1. Relationship between signal frequency and noise
As shown in Fig 2-3-2, digital circuits transmit the information by switching
the signal level between High and Low to operate the circuits. A high-frequency
current flows in the signal line at the moment of switching this signal level.
The current flows not only in the signal line, but also in the power supply and
ground. These high-frequency currents used in digital circuits are considered to
be causes of noise. These currents will be further described in Section 2-3-2
onwards.
Figs. 2-3-3 and 2-3-4 show an example of measurements with varied signal
frequencies. The figures take a clock generator as an example of a digital
circuit and measure the noise generated by the generator with an antenna placed
3 m away inside a measurement field called radio wave dark room. You can see
that the interval and level of the frequency at which the noise is observed
change, as the signal frequency of the clock generator changes from 4MHz to
20MHz and then to 66MHz. In this way, the noise is observed at discrete
frequencies in a clock signal, and these components are called harmonics of the
signal. Harmonics will be further described in a later section.
In the measurement result of noise in Fig. 2-3-4, the line represented by H
shows the measurement result for the radio wave of horizontal polarization while
the line represented by V shows the measurement result for the radio wave of
vertical polarization. In this course, the same will be applied to the following
figures unless otherwise noted.
2-3-2. Why digital circuits generate noise
In order to explain noise generated by digital circuits, we look at a simplified
circuit as an example that is composed of one signal line between two ICs.
As shown in Fig. 2-3-5, we consider a case wherein the information is
transferred by a single signal line that connects two digital ICs with each
other. The current flowing between two ICs can be simplified as shown in Fig.
2-3-6. [Reference 4]
In Figs. 2-3-5 and 2-3-6, the signal is transmitted by a single line from the
left driver to the right driver. The change to the signal voltage can be
considered to be made by connecting the switch (composed of a transistor) that
is attached to the signal line inside the driver to the power supply side or the
ground side. When the switch is turned on the driver side, the gate capacity of
the input terminal (very small electrostatic capacity of several pF) is charged
or discharged on the receiver side. It is considered that the information is
transmitted from the driver to the receiver when the signal voltage of the
driver output changes in accordance with the charging and discharging of this
capacity.
Fig. 2-3-7 shows the schematic diagram of the current flow and voltage shift at
the switching moment. Fig. 2-3-7 shows a modeling with the output resistance (R)
of the driver IC. The speed at which the signal level switches varies depending
on this output resistance and gate capacity. Please note that this model has
been simplified so much just to show the operation of the circuit and is not
sufficient to explain noise. A more realistic model will be described later.
In this case, the current flowing between two ICs goes through the orange path
on the charging side of the gate capacity in Fig. 2-3-6, while it goes through
the blue path in the figure on the discharging side. You can consider that this
current is causing the noise generated from the digital circuit.
Since this current is made by charging and discharging the gate capacity
(capacitor), it flows like a spike at the moment of signal switching as shown in
Fig. 2-3-8(b). Since this waveform contains various frequencies, it is emitted
through the wiring as an antenna causing noise interference. Such a sudden
change in the current causes an induction voltage in accordance with the
parasitic inductance of the circuit. This voltage also becomes a cause of noise.
Since the origin of the noise is on-off switching inside the driver, you can say
that the noise source is inside the driver in the model of Fig. 2-3-5.
2-3-3. Short-circuit current
Fig. 2-3-6 indicates another green current. This current is called short-circuit
current, which also becomes a cause of noise.
Since C-MOS digital IC has a moment at which the power supply and ground are
connected with each other when the switch inside the driver switches, a
spike-like current may occur as shown in (3) of Fig. 2-3-8(b). This current is
called short-circuit current. This current does not flow into the signal line.
But it flows into the power supply and ground as a sharply changing current.
Therefore, it can be a cause of noise in the power supply and ground. Fig. 2-3-8
indicates that this current flows through up and down the switch inside the
driver.
Unlike signal current, this short-circuit current occurs in the same direction
at both rise and fall of signal. Therefore, from the viewpoint of frequency, it
is considered to have a frequency that is a double of the signal cyclic
frequency. Sometimes remembering this nature comes in useful when separating the
noise source or pathway from the generated noise frequency.
The components called harmonics that can cause noise occur at frequencies of the
integral multiplication of cyclic frequency. This will be further described
later. The noise generated by short-circuit current tends to appear at
frequencies that overlap with the even harmonics of the signal (integral
multiplication of double signal frequency). Therefore, if the even harmonics
cause a problem, the power supply is possibly a cause of the problem not just
the signal.
In order to simplify the model, Fig. 2-3-6 indicates that the gate capacity is
between the signal line and the ground. However, realistically the gate capacity
also exits between the signal line and the power supply. So there are current
pathways to both power supply and ground.
2-3-4. Decoupling capacitor
The current pathways shown in Fig. 2-3-6 not only include the signal line, but
also the power supply and ground. That means connecting a signal line is not
enough to transmit the signal, and you always need to connect it to the power
supply and ground.
Fig. 2-3-6 also indicates “decoupling capacitor” on the left hand side. This is
a type of bypass capacitor for connection between the power supply and the
ground. Although this capacitor is used to stabilize IC power supply voltage or
to instantaneously supply the source current, it is also playing a role of
current pathway to transmit the signal in the case of Fig. 2-3-6. The operation
of decoupling capacitor will be further described in Section 3-1.
Let's think about the pathway of the current if this capacitor is missing. As
shown in Fig. 2-3-10, the electric current that flows through the power supply
and ground would flow via the power supply that is far away from the IC and thus
have a large inductance, being unable to flow normally (therefore, the signal
pulse waveform gets deformed, or the IC operation speed slows down). In
addition, since the current that causes noise flows through circuits in a wide
area, noise will be generated more.
Therefore, decoupling capacitors are very important parts for digital IC not
only for stabilizing the power voltage (called “PI” - Power Integrity), but also
for transmitting signals correctly (called “SI” - Signal Integrity) as well as
suppressing electromagnetic noise (EMI). From the viewpoint of EMI suppression,
the operation of decoupling capacitor is represented by confining the
high-frequency current that contains noise flowing into the power supply and
ground inside the vicinity of IC as shown in Fig. 2-3-10.
The smaller the loop of current pathway via decoupling capacitor becomes, the
smaller the amount of noise generated. The signal quality will also be improved.
Therefore, the decoupling capacitor should be placed as close as possible to the
IC. Section 3-1 will explain how to use decoupling capacitor in detail.
2-3-5. Induction of common mode noise
The signal current shown in Fig. 2-3-6 makes a current loop by itself and thus
emits radio waves by using this loop as an antenna as shown in Fig. 2-3-11.
Here, let’s call this as a noise emission by normal mode current. (In order to
simplify the mechanism of noise emission, this example is modeled by a loop
antenna. Since real-world electronic devices have more complicated shapes, which
cannot be represented only by a loop antenna.)
Real-world electronic devices also emit noise other than normal mode shown in
Fig. 2-3-11. As shown in Fig. 2-3-6, the current flows not only into the signal
line but also into the ground and power line. This current may result in
generating more influential noise called common mode noise as shown in Fig.
2-3-12. The mechanism of generating common mode noise will be further described
in Section 5-3.
The common mode noise will also come up not only to the ground but also to the
power supply and signal line. Since the ground stretches to all around the print
board, if common mode noise is generated, it can be emitted from the print board
itself as an antenna or can be emitted from various cables connected to the
print board as antennas. Since the size of conductor that works as an antenna is
significantly larger than the signal line, it emits strong noise even though the
voltage is only small.
Fig. 2-3-13 shows a conceptual diagram of emission from an electronic device
including common mode noise. The portion of the emission due to signal current
is emitted by (1) normal mode. Since the antenna is small, the noise emission
travels to a relatively small area. However, if common mode noise is induced by
this current, the entire print board (2) can becomes an antenna, or the cable
(3) can become an antenna, resulting in stronger noise emission.
Since the common mode noise is not only emitted easily but also conducted
through the ground and power supply, it is hard to stop its propagation when it
is once generated. For example, the cables in Fig. 2-3-13 are connected to an
interface IC. The common mode noise then conducts through the cables via the
power supply and ground of this IC.
In order to efficiently implement noise suppression, it is important to prevent
the generation of common mode noise. For this purpose, the impedance of the
ground is lowered so as to suppress the occurrence of common mode noise (called
ground enforcement), or the causal current is blocked with use of EMI
suppression filters in the signal line.
2-3-6. Harmonics in signal
As described above, the signal-transmitting electric current itself can be a
cause of noise in digital circuits. Fig. 2-3-14 shows an example of measurement
showing the process of 20 MHz clock signal changing into noise.
Although the voltage waveform of the digital signal is a simple rectangular wave
as shown in Fig. 2-3-14(a), it can be disassembled into a spectrum discretely
distributed over a wide frequency range as shown in Fig. 2-3-14(b). These
components are called harmonics. When some part of the energy of these harmonics
is released, it is observed as noise as shown in Fig. 2-3-14(c), causing noise
interference.
As described in Section 2.1, noise needs a transmission path and antenna to
emit. In electronic devices that use digital circuits, the wiring that connects
ICs with each other, print board, cable and metal casing etc. can work as the
transmission path and antenna. In general, the higher the frequency becomes, the
more easily the frequency is emitted as radio waves. Therefore harmonic noise
(several 100MHz or higher) tends to seem more prominent in Fig. 2-3-14(c) which
measures the emitted noise than Fig. 2-3-14(b) which directly measures the
signal.
In order to efficiently suppress noise, it is important to understand the nature
of the harmonics (indicated in Fig. 2-3-14(b)) included in the original signal.
In the next section, the nature of the harmonics will be described.
“2-3. Noise generated by digital circuit” - Key points
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The current to operate digital circuits contains harmonics, which can be
a noise source by itself.
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Noise current flows through not only signal lines but also the power
supply and ground, causing common mode noise.
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Noise can be emitted not only from signal lines but also from various
sections such as a print board and cable as an antenna.
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Noise emitted by digital circuits is associated with the integral
multiplication of the operation frequency. This is called harmonics.