How does electronic instrumentation guard electromagnetic interference and suppress it?

The suppression of electromagnetic interference by electronic instruments and meters is an important part of the product's electromagnetic compatibility design. Electromagnetic compatibility here refers to the ability of a device or system to function properly in its electromagnetic environment and does not constitute an unacceptable electromagnetic disturbance to anything in the environment. In recent years, electromagnetic compatibility (EMC) design has gradually become a technical hot spot in international and domestic markets. Electromagnetic compatibility of products has become an important factor in measuring product quality. The State Bureau of Technical Supervision and related ministries and commissions are actively planning to implement electrical appliances in China. Electronic product certification for electromagnetic compatibility. Therefore, it is extremely important to study and discuss the suppression of electromagnetic interference by electronic instruments and meters.

1 The generation of electromagnetic interference

Electromagnetic interference must have three factors at the same time: interference sources, coupling paths, and sensitive receivers.

1.1 Interference sources

In electronic instrumentation and control systems, there is both electromagnetic interference from the instrument or system and external sources of electromagnetic interference.

In electronic instrumentation and control systems, electromagnetic interference occurs due to abrupt changes in voltage and current, component mounting locations, wiring impedances, oscillation circuit interconnections, and coupling of components or circuits.

Outside of electronic instrumentation and control systems, such as lightning, radio radar, navigation systems, broadcast television systems, transient switches, reversing devices, gas spark arrestors, corona discharge, contact potentiometers, sine wave signal sources, electromagnetic pulses And so are all sources of interference.

1.2 Interference Sensitive Receiver

Interference-sensitive receivers refer to the objects of electromagnetic interference hazards. The hazard of electromagnetic interference refers to the influence of electromagnetic interference, which causes slight or serious or devastating damage or damage to the performance and characteristics of the equipment or devices. The danger of electromagnetic interference varies depending on the sensitivity of the instrument. Mild interference causes the performance of the meter to drop, and severe interference can make the meter or control system unable to work or even damage. For example, when people watch TV, the images of the TV screen are shaken, distorted, deformed, and snow-like due to the influence of electromagnetic interference. When people listen to the radio, they are affected by electromagnetic interference and make the radio buzz. Unable to listen normally; The vehicle's receiving and dispatching machine can't receive and send reports during operation because of the interference of the ignition system. In the industrial production, due to the influence of electromagnetic interference, the program of the CNC machine tool may be erroneous and the product cannot be normally processed. In the computer-controlled assembly line production, due to the influence of electromagnetic interference, the computer's production process will be disrupted, and serious will also cause damage to the machine or equipment; in medical treatment, due to the influence of electromagnetic interference, it will cause EEG. , Electrocardiogram, nuclear magnetic resonance instruments and other errors, causing medical accidents. The numerous examples listed above illustrate that with interference, interference-sensitive receivers must also be available to cause interference hazards. Without interference-sensitive receivers, there are no so-called interference effects or interference hazards.

2 electromagnetic interference suppression method

When the electronic instrumentation and control system is designed for electromagnetic compatibility, it is necessary to study and analyze the locations where the equipment may generate interference, the paths that may transmit interference, and the sensitive components that may receive interference, and specifically adopt methods for suppressing electromagnetic interference.

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

2.1 Filtering

Filtering can suppress electromagnetically conductive interference. Sensitive electronic devices conduct electromagnetic interference signals through power lines, telephone lines, control lines, and signal lines. The low-pass filter is often used for conducted interference and can be effectively suppressed. However, in the design of electromagnetic compatibility, the characteristics of the filter must be considered: frequency characteristics, impedance characteristics, rated voltage and voltage loss, rated current, leakage current, insulation resistance, temperature, reliability, size, and so on. Passive lumped parameter element filters and coaxial absorption filters are often used as low-pass filters. Passive lumped parameter filter uses inductors and capacitors to form capacitive, inductive, π-type, T-type, L-type, and C-type filters, which can effectively suppress low-frequency and medium-frequency electromagnetic interference. The frequency of suppression can be reduced. Up to 300MHz. The coaxial absorption filter is filled with an absorbing medium, such as a ferrite material, in a steel pipe passed through the power supply and output lines, or a wear-type ferrite such as a magnetic bead or a magnetic tube is put on the power line to convert the transient energy into Heat energy is consumed so as to suppress interference.

2.2 Shielding

Shielding is a measure used to reduce the penetration of electromagnetic fields outwards or inwards. It is generally used to isolate and attenuate radiated interference. According to its principle, shielding is divided into three types: electrostatic shielding, electromagnetic shielding and magnetic shielding. The function of the electrostatic shield is to eliminate the electromagnetic interference between the two circuits due to the distributed capacitive coupling. The shield body is made of a low-resistance metal material, and the shield body must be grounded. The role of electromagnetic shielding is to prevent the interference of high-frequency electromagnetic fields. The shield body is made of a low-resistance metal material, and the shield metal is used to absorb and reflect the electromagnetic field to achieve the purpose of shielding. The role of magnetic shielding is to prevent the interference of low-frequency magnetic fields. The shield uses high-magnetic, high-saturation magnetic materials to absorb or lose the electromagnetic field to achieve the purpose of shielding.

The influence of electromagnetic interference is closely related to the distance. The closer to the interference source, the greater the interference field strength and the greater the impact. In electronic instruments and meters, the layout of electronic components is often limited by volume. Often, a low-resistance metallic material or a magnetic material is used to form a closed body, and components or locations with insufficient guard spacing are isolated to reduce or prevent static electricity or electromagnetic interference.

2.3 Grounding

In equipment or devices, grounding is used to make the interference current generated by the device or the device itself flow into the earth through the ground wire, and is generally used to suppress conduction interference. The ideal grounding body is a zero-potential, zero-impedance physical entity that serves as a reference point for all signal levels in the relevant circuits. Any unwanted current passing through it does not produce a voltage drop. This ideal grounding entity is actually approximate.

3 Conclusion

For different equipment, the requirements for its electromagnetic compatibility have corresponding standards. When designing electronic instruments and meters, corresponding interference suppression measures should be adopted for different interference sources, so that the performance of the equipment itself can be in its electromagnetic environment. Works normally and does not cause electromagnetic interference to the surrounding things.