What is Conducted Noise? Its Types and Countermeasures

Are you familiar with the concept of noise? Usually, when you are listening to music or talking on the phone, a coarse sound may appear, and you might describe it as “noisy.” The concept of noise is also used in the context of electrical circuits, where it often is the reason for vexing problems. Understanding noise is essential to achieve stable operation of electronic circuits.

This article focuses on conducted noise among the various types of noise, and introduces its types and countermeasures.

What is noise?

In electronic circuits, noise is any electrical signal other than one generated to achieve a purpose. In everyday life, you may have experienced choppy phone calls or disrupted streaming video. All of these phenomena are treated as noise.

Have you ever noticed that as soon as you start using a microwave oven, the Wi-Fi in your home becomes unstable? This is because the radio frequency emitted by the microwave oven is the same as the frequency used by Wi-Fi, causing interference.

Noise in electronic circuits can be caused by internal or external factors. Internal factors include high-frequency ringing and spikes that are caused by a printed circuit board itself or components on the circuit. Ringing is a temporary fluctuation that occurs when the input signal changes, and spikes are momentary increases in voltage. External factors are events that occur outside the circuit, such as electrostatic discharges, lightning surges, radio waves, and electromagnetic waves.

Noise is also called electromagnetic interference (EMI). Noise immunity is related to electromagnetic susceptibility (EMS), and compatibility between EMI and EMS is called electromagnetic compatibility (EMC).

What is conducted noise?

Noise is classified into radiated and conducted noise according to the path it travels. Radiated noise is transmitted through space, while conducted noise is physically transmitted through electronic circuits and cables. This article describes conducted noise.

Consider a simple circuit with a DC power supply and a substrate board. Current flows from the positive to negative side of the power supply through the board. If 10 watts of power is required to operate the board, ideally,10 volts of voltage would result in 1 ampere of current. In reality, however, because of added noise, unwanted current may flow or some current may be lost.

In other words, in order for current to flow under ideal conditions and to ensure utmost performance and stable operation, the circuit must be designed to provide the required power with noise taken into account. To do this, it is necessary to know what kind of noise is generated and where it occurs.

Types of conducted noise

There are two types of conducted noise, differential mode noise and common mode noise, depending on the direction of noise propagation.

・Differential Mode Noise
This noise, also called normal mode noise, occurs in power supply lines. While differential mode noise flows in the same direction as the current on the circuit, the direction of flow is opposite between the positive and negative sides of the input power supply. This is why this noise current is called differential mode.

・Common Mode Noise
This noise occurs between the power supply and ground. The noise current flows in the same direction as that on the positive and negative sides of the power supply. This is why this noise current is called common mode.

These two types of noise can occur simultaneously on a single circuit. Therefore, countermeasures must be taken for each type of noise.

Countermeasures for conducted noise

With an understanding of the types and characteristics of noise discussed thus far, noise suppression measures can be implemented.

The key factors in implementing noise suppression are the intensity of the countermeasure used, and the cost incurred. There is a trade-off between the degree/intensity of the measure and the cost. The stronger the measure, the more noise can be suppressed, but the higher the cost. Accordingly, it is important to strike a balance between the two.

There are a number of methods of noise suppression. The level of noise reduction can be adjusted for each method. Consider what level of noise is acceptable based on case studies and other factors to avoid excessive countermeasures.

Effective measures to deal with conducted noise include designing circuits so that noise escapes to ground, and using components that are not susceptible to noise generation. This minimizes the generation of noise that would require subsequent countermeasures. An effective grounding design not only provides noise suppression, but also helps to improve the performance and stability of the circuit.

However, the amount of noise that can be prevented by circuit design alone is limited due to physical constraints and component limitations, and other preventive measures may be required. For example, effective noise suppression components such as noise-reducing filters and bypass capacitors should be added to the circuit. It is necessary to determine at which points in the circuit noise is generated, and to introduce measures at all points where noise would be generated.

Differential mode noise countermeasures include the use of reduction filters, decoupling, and loop area reduction. Decoupling is the practice of blocking DC currents and allowing only AC currents to flow, and is implemented using decoupling capacitors or decoupling coils.

Common-mode noise countermeasures include shortening wiring, reducing crosstalk, and increasing impedances to cut off common-mode paths. Crosstalk is a phenomenon in which signals from one circuit interfere with adjacent circuits; it can be reduced by shortening the wiring. Impedance is resistance in an AC circuit. After implementing countermeasures, be sure to test them to see if they are working effectively.

Noise can be prevented if we know what causes it!

Noise can occur in any circuit. To prevent noise, it is advisable to start with the ground design. First, locate the noise source and determine what type of noise is generated, in what direction, and in what amount. Based on this information, countermeasure components should be mounted in the necessary areas and the stable operation of the circuit should be confirmed. During the design phase, testing and countermeasures are repeated until satisfactory circuits are obtained.

Finally, let us review the terms described in this article.

• ・Noise: Electrical signals other than those generated to achieve a purpose.
• ・Radiated noise: Noise that propagates through space
• ・Conducted noise: Noise that propagates through components and cables on a circuit
• ・Differential mode noise: Conducted noise that transmits from the positive to the negative of a power supply. Countermeasures include implementing reduction filters, decoupling, and loop area reduction
• ・Common mode noise: Conducted noise that occurs between the power supply and ground. Countermeasures include shortening wiring, reducing crosstalk, and increasing impedance.

Other types of noise exist, and their causes and countermeasures vary. Please also refer to the following information on noise: