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Electromagnetic interference is an important part of emcation design

The suppression of electromagnetic interference by electronic instruments is an important component of product electromagnetic compatibility design. Electromagnetic compatibility here refers to the ability of a device or system to work normally in its electromagnetic environment without causing intolerable electromagnetic interference to anything in the environment. In recent years, electromagnetic compatibility (EMC) design has gradually become an international and domestic technology hotspot, and the electromagnetic compatibility of products has become an important factor to measure product quality. The State Bureau of Technical Supervision and relevant ministries are actively planning to implement electromagnetic compatibility certification for electrical and electronic products in China. Therefore, studying and discussing electronic instruments is very important for suppressing electromagnetic interference.


 国产电磁兼容仪器设备


1. Generate electromagnetic interference

Electromagnetic interference must be generated by three factors simultaneously: interference sources, coupling paths, and sensitive receivers.

1.1 Interference sources

In electronic instruments and control systems, there are both electromagnetic interference from within the instrument or system, as well as electromagnetic interference sources from outside.

In electronic instruments and control systems, electromagnetic interference can occur due to sudden changes in voltage and current, installation locations of components, wiring impedances, interconnections of oscillating circuits, and coupling of components or circuits.

Lightning, radio radar, navigation system, radio and television system, transient switch, commutation device, gas spark arrester, corona discharge, contact potentiometer, sine wave signal source, electromagnetic pulse, etc. Outside the electronic instrumentation and control system.

1.2 Interference sensitive receiver

Interference sensitive receivers refer to objects that are harmed by electromagnetic interference. The harm of electromagnetic interference refers to the impact of electromagnetic interference, which causes slight, serious, or destructive damage or damage to the performance and characteristics of equipment or equipment itself. The harm of electromagnetic interference varies depending on the sensitivity of the instrument. Mild interference reduces the performance of the instrument, while severe interference makes the instrument or control system inoperable or even damaged. For example, when people watch TV, due to the influence of electromagnetic interference, the image on the TV screen shakes, twists, deformations, and snowflakes; When people listen to the radio, due to the influence of electromagnetic interference, the radio emits a chirping sound, and the radio cannot receive sound normally; Due to interference from the ignition system, the on-board transceiver cannot receive and receive signals during operation; In industrial production, due to the influence of electromagnetic interference, the program of CNC machine tools may make mistakes, and the products cannot be processed normally; In computer controlled assembly line production, due to the influence of electromagnetic interference, the production process of the computer will be disrupted, and the machine or equipment will be seriously damaged; In medical treatment, due to the influence of electromagnetic interference, electroencephalograms, electrocardiograms, nuclear magnetic resonance instruments, etc. can be mistakenly stored, leading to medical accidents. The numerous examples listed above indicate that in the case of interference, an interference sensitive receiver must be present to cause harm to the interference source. If there is no interference sensitive receiver, there is no interference hazard.


Methods for suppressing electromagnetic interference:

In the electromagnetic compatibility design of electronic instruments and control systems, it is necessary to study and analyze the parts of equipment that may generate interference, the paths that may transmit interference, and the sensitive components that may receive interference, and adopt targeted methods to suppress electromagnetic interference.

There are three main methods for suppressing electromagnetic interference: filtering, shielding, and grounding.


2.1 Filtering

Filters can suppress electromagnetic conducted interference. Sensitive electronic equipment conducts electromagnetic interference signals through power lines, telephone lines, control lines, signal lines, etc. Low-pass filters are commonly used for conducting interference and can be effectively suppressed. However, in the electromagnetic compatibility design, it is necessary to consider the characteristics of the filter: frequency characteristics, impedance characteristics, rated voltage and voltage loss, rated current, leakage current, insulation resistance, temperature, reliability, external dimensions, etc. Low-pass filters usually use passive lumped parameter element filters and coaxial absorption filters. Passive lumped parameter element filters use inductive coils and capacitors to form capacitive, inductive, π, T, L, and C filters, which can effectively suppress low-frequency and intermediate frequency electromagnetic interference, with a suppression frequency of up to 300 MHz. The coaxial absorption filter fills the absorption medium in the worn steel pipes of the power supply inlet and outlet lines, such as ferrite materials, or transmits magnetic beads on the power supply lines, thereby converting instantaneous energy into suppressed heat energy.

2.2 Grounding

In equipment or equipment, grounding is used to allow the interference current generated by the equipment or equipment itself to flow into the earth through the grounding wire, which is usually used to suppress conductive interference. An ideal grounding body is a physical entity with zero potential and zero impedance. As a reference point for all signal levels in each associated circuit, any unnecessary current will not pass through it causing a voltage drop. This ideal grounding entity is actually similar.

 

2.3 Shielding

Shielding is used to reduce external or internal penetration of electromagnetic fields, and is typically used to isolate and attenuate radiated interference. According to its principle, shielding can be divided into electrostatic shielding, electromagnetic shielding, and magnetic shielding. The function of electrostatic shielding is to eliminate electromagnetic interference caused by distributed capacitive coupling between two circuits. The shield is made of low resistance metal material and must be grounded. The function of electromagnetic shielding is to prevent interference from high-frequency electromagnetic fields. The shield body is made of low resistance metal materials, which are used to absorb and reflect electromagnetic fields, thereby achieving the purpose of shielding. The function of a magnetic shield is to prevent interference from low-frequency magnetic fields. The shield body uses magnetic materials with high conductivity and high saturation to absorb or lose electromagnetic fields, thereby achieving the purpose of shielding.

The impact of electromagnetic interference is closely related to distance. The closer to the interference source, the stronger the interference field, and the greater the impact. In electronic instruments, the layout of electronic components is often limited by volume. Low resistance metal materials or magnetic materials are commonly used to manufacture enclosures to isolate components or parts with insufficient protective spacing to reduce or prevent static or electromagnetic interference.

3 Conclusion

For different equipment, there are corresponding standards for their electromagnetic compatibility requirements. When designing electronic instruments, corresponding interference suppression measures should be taken for different interference sources to ensure that the performance of the equipment itself works normally in an electromagnetic environment and does not cause electromagnetic interference to surrounding objects.


keywords: EMC
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