26 Apr 2015

Resolving EMI common mode & normal mode noise

Hidetoshi Yamamoto, Product Engineer with EMI division, Murata looks at how to improve common mode and normal mode noise for EMC conformance.

The emission and reception of electromagnetic noise can be a major challenge for electronic engineers.

In this article we look at the differences between common mode and normal mode noise, and then describe the basic configuration of filters suitable for eliminating the noise to improve EMI performance.

What is EMC? Since noise is an electric energy, it is naturally conducted through a conductor if it is connected. However, if the conductor forms just one of a number of conductors inside a cable, the noise conduction is explained in two different ways: common mode and normal mode.

The test in Figure 1 reproduces the situation that is often seen in noise suppression for electronic devices. An electronic device has been connected with an interface cable, and the noise emitted from the cable acts as an antenna. When there is no cable, the level is only low as shown in Figure 1a. However, when a cable is attached, the noise increases in the frequency range from 100MHz to 300MHz as shown in Figure 1b.

EMI noise emitted from a cable of electronic device

Figure 1 Example of noise emitted from a cable of electronic device

However, typically, an interface cable contains a ground line, power line and signal line so which line is conducting the noise?

Common mode EMI noise In fact in Figure 1 the cable might be shielded, in which case the noise might have been conducted through the shield as well. The result is that common noise has been induced to all terminals in the connector that is connected to the cable. In contrast, the ground of an electric circuit is generally considered as a reference point where there is least noise. If noise has been superimposed over this ground as shown in Figure 1, the same noise will be superimposed over the power source and signal. Therefore, the noise that has been superimposed over the ground is sometimes referred to as common mode noise.

Electric circuits are based on an electric current that makes a round trip along the path. If a part of this circuit is extracted as a cable as shown in Figure 2a, the cable is represented by two wires on which an electric current goes out and comes back. These currents are flowing in the opposite direction to each other with the same magnitude. Therefore, the total will be always zero. This way of current flow is called normal mode.

Normal mode EMI noise In contrast, the currents may flow in the same direction on the lines inside a cable as shown in Figure 2b. It is called common mode. The common mode is a component of electric current that flows in the same direction as shown in the figure due to the same voltage applied to each line in some form. As shown in the figure, it is understood that this current is caused by a current that has leaked out via the floating electrostatic capacitance of the load retained against the earth and has then returned to the noise source via the earth. (The current may be caused by a direct connection between the load and noise source without going through the earth)

 Common mode and normal mode for EMI

Figure 2 Common mode and normal mode

Normal mode on two lines as shown in Figure 2a is sometimes called differential mode.

Generally, capacitors (C) and inductors (L) are used to form a low-pass filter in the middle of or at a connection point of cable that works as a noise transmission path in order to block out noise conduction. As shown in Figure 3, inserting a capacitor between the lines and attaching an impedance element in series can form a filter for normal mode.

The current of normal mode noise is in the same direction as the current used for the circuit operations. The values of L and C are adjusted so that the cut-off frequency of the low-pass filter does not go over the components needed for the circuit operations. If all lines are floating with reference to the ground, the circuit is considered as a balanced circuit and an impedance element is used for both lines. In so doing, you need to maintain the balance so that the impedance is the same.

Balanced & unbalanced If one side is grounded such as a case of digital circuit, the circuit is considered as an unbalanced circuit and no impedance element is normally used for the ground. Here, the terms "balanced" and "unbalanced" refer to how the voltage is retained with reference to the earth when conducting normal mode through. If the voltage is symmetrically applied to two lines, it is referred to as balanced, and if it is concentrated on one line, it is referred to as unbalanced. The other line of the unbalanced circuit is the ground to which almost no voltage is applied.

 Filter configuration for normal mode EMI

Figure 3 Example of filter configuration for normal mode

Figure 4 indicates how a filter for common mode can be formed by connecting capacitors to the ground (referred to as Y capacitor). You should try to use a common mode choke coil for the impedance element as much as possible. If there are a number of wires in the cable, it is effective to create a kind of common mode choke coil by turning the cable around a ferrite core or sandwiching the cable with a ferrite core. When common mode noise appears, the noise may show up on the ground to which the Y capacitor is connected. In this case, the effect of Y capacitor is reduced since the Y capacitor has not been connected to an appropriate ground.

In such a case, a ground point to which the Y capacitor is connected needs to be created separately. As shown in the figure, the wiring of this ground is intended to form a returning path of the noise for the noise source.

 Filter for common mode EMI

Figure 4 Basic configuration of filter for common mode

Differential signals In recent years, differential signals have been more commonly used for the high-speed digital transmission such as USB. Differential signals contain common mode noise, which is slightly different from those explained so far.

A differential signal applies a reverse phase signal to each line of a line pair as shown in Figure 5 and receives a signal as a line voltage on the receiver side. If these two currents are symmetrical with each other, the current components are only normal mode, and thus very small noise occurs due to the mechanism shown in Figure 5. Furthermore, in case of receiving a noise induction from the outside, it is less likely to be affected.

 Signal waveform of differential signal

Figure 5 Signal waveform of differential signal

However, if there is a slight imbalance in the signal that has been transmitted by the two lines, the unbalanced components turn into common mode. The factors that are considered to cause imbalance are the rise and fall of time, speed or magnitude deviation, or superimposed common mode noise.

The first three are issues in forming a signal waveform in terms of signal integrity rather than a noise problem Such an imbalance in signal waveform occurs due to a difference in the wire lengths, a kink in the wires, or a difference in the impedances of terminating resistors. The common mode noise occurs due to an imbalance in the signal waveform is observed in the form of harmonics of the signal frequency in the noise spectrum.

Superimposed EMI noise Superimposed noise is often seen when it has been applied from the outside to the power supply or ground of the driver or receiver. Although the noise may seem like signal harmonics, it can occur at a frequency that is not at all related to the signal frequency.

 Factors of causing common mode EMI

Figure 6 Factors of causing common mode EMI

Typically common mode choke coils are used to block out such common mode current and suppress any imbalance in the signal waveforms for differential signals. Usually it is used on the driver side. However, if noise occurs on the receiver side, it is also used there. Parts with a small attenuation for differential mode are chosen for the common mode choke coils used here so that those do not adversely affect the differential signal. In addition to the common mode choke coils, a shielded cable is also used for noise suppression in differential signal.

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About the author

Hidetoshi Yamamoto is a Product Engineer with EMI division of Murata and is based in Japan.

Murata is a leading manufacturer of electronic components, modules, and devices. The complete range of this Technology house includes ceramic capacitors, resistors/thermistors, inductors/chokes, timing devices, buzzers, sensors and EMI suppression filters. Whilst the company is known as a global ceramic capacitor manufacturer, it is also the world leader in Bluetooth & Wi-Fi Modules, the world's no.1 manufacturer of board-mount DC-DC converters and is a key manufacturer of standard and custom AC-DC power supplies.

Established in 1944, Murata is headquartered in Japan and has European offices in Finland, France, Germany, Hungary, Italy, the Netherlands, Spain, Switzerland and the UK.

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