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.
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“2-3. Noise generated by digital circuit” - Key points
The current to operate digital circuits contains harmonics, which can be a noise source by itself.
Noise current flows through not only signal lines but also the power supply and ground, causing common mode noise.
Noise can be emitted not only from signal lines but also from various sections such as a print board and cable as an antenna.
Noise emitted by digital circuits is associated with the integral multiplication of the operation frequency. This is called harmonics.