A summary of 42 frequently asked questions about electromagnetic compatibility

1. The purpose of electromagnetic compatibility research is to eliminate or reduce natural and perceived electromagnetic interference, reduce its harm, improve the anti-interference ability of equipment or systems, and ensure the electromagnetic compatibility of equipment or systems.

2. Electromagnetic pollution includes hazards to human health such as radio frequency radiation, nuclear electromagnetic pulse, and electrostatic interference, which can damage equipment or systems, as well as their impact on safety and reliability.

3. What is the electromagnetic spectrum management organization? The radio frequency bands are managed by specialized agencies, while in China, the radio frequency bands are allocated and coordinated by the China Radio Regulatory Commission.

4. Basic measures for suppressing electromagnetic interference: shielding, filtering, grounding, and reasonable layout.

5. Characteristics of electromagnetic pulse signal: Electromagnetic pulse is a very serious source of electromagnetic interference, with a wide spectrum coverage, large field strength, and wide range of action. Antennas, transmission lines, cables, and various shielding shells can be induced by it to generate strong pulsed radio frequency currents.

6. Conditions for forming an effective wireless antenna: A. For bipolar/monopole antennas, to form an effective antenna, two electrodes or one electrode and a ground plane are required. When the length of each electrode is 1/4 wavelength, the antenna effect is best. B. For loop antennas, the area product of the loop should be reduced, or the effects of some of its loops should be offset against each other. C. For slot antennas, the maximum size of the slot should be reduced, or a waveguide structure should be used, To reduce radiation below its cutoff frequency.

7. The interference modes of space radiated electromagnetic waves on circuits include antenna coupling, wire inductive coupling, and closed loop coupling.

8. The function of electric field shielding: to eliminate or suppress the electrical coupling between the electric field or alternating electric field and the interfering circuit.

9. The basic conditions for realizing electric field shielding: there is a complete shield and the shield is well grounded.

10. How are the conditions for eliminating or suppressing electric field interference different: A Conditions for eliminating electric field interference: intact and well grounded shielded conductors B Conditions for suppressing electric field: incomplete and well grounded shielded conductors.

11. Common measures to suppress interference from alternating electric fields: Use good grounded conductors to surround the protected circuit as much as possible to achieve electric field shielding.

12. The main measures to improve the magnetic field shielding effect: A material with high magnetic permeability B increase the thickness of the shield C avoid opening holes, and the gap D adopt a double layer shielding structure.

13. The main function of the filter is to limit the frequency band of the receiving device so as to suppress useless signals without affecting useful signals.

14. Common classification methods for filters: A is classified according to the energy loss characteristics of the filter, B is located in the circuit, and C is the frequency characteristic of the active device included in the circuit.

15. Attention should be paid to the installation of power line filters: the installation position of A filter, the isolation of input and output leads, and the grounding of D filter.

16. Draw the actual model of the capacitance:

17. Draw three or more low-pass filters and other basic circuit structures:

18. Grounding: refers to the connection of equipment or systems to the "ground".

19. Purpose of grounding: A Establish a low impedance path connected to the ground, so that lightning current, electrostatic discharge current, etc. directly flow into the ground from the grounding path without affecting the normal operation of the equipment or system and personal safety. B Establish a low impedance path between the equipment shell and nearby metal conductors, so that when there is leakage current in the equipment, it will not endanger personal safety. C Connect all parts of the equipment and the system to a common point or equipotential plane, In order to have a common participation potential and eliminate the possible interference voltage between two suspension circuits, D will ground the shield to make the shield work, E will ground the filter to make the filter work to suppress common mode interference, F will suppress the signal circuit on the circuit pole and connect it to the ground plane to provide a signal return path, G will connect the non important circuit on the vehicle or aircraft to the vehicle body or the metal shell of the aircraft body, To provide a current return path.

20. Methods of interference control grounding: floating grounding, single point grounding, multi-point grounding, and hybrid grounding derived from single point grounding and multi-point grounding.

21. According to the concept in the figure, specify the grounding method: A, single point grounding, B, multi-point node, C, hybrid grounding.

22. According to the diagram concept, grounding function analysis: A When the frequency is below 1MHZ or the length of the ground wire is less than 120, a single point grounding method can be used to prevent radiation and reduce impedance. According to the connection method, it can be divided into parallel single point connection of independent ground wires and series single point grounding of shared ground wires. The latter requires many ground wires, so the connection is complex. B The multi-point grounding wire is relatively short, so it is suitable for high-frequency situations, but loop interference is caused by multi-point grounding, Negative impact on lower frequencies C Hybrid grounding can meet both short circuit and open circuit requirements.

23. What are the grounding methods and locations for the shielding layer of shielded cables? Please draw a schematic diagram:

24. Common methods for reducing low impedance: A. Shorten the length of the ground circuit as much as possible, B. Refine and widen the ground wire, C. Adopt multi-point grounding for the signal grounding of each unit circuit, and ground it nearby.

25. Why do transient disturbances often occur in power grids and circuits: A lightning B switch operation C operation failure

26. What types of conductive disturbances often occur in the power grid: power interruption, frequency offset, voltage sag, surge, voltage noise, harmonics, and transients.

27. Characteristics of surge: For transient voltages with a duration greater than 8.4MS.

28. How transient disturbances are formed during switch opening and closing operations: The contacts of the switch generate arcs during opening and closing, generate electromagnetic disturbances in circuits and spaces, and interfere with other devices in or near the same power grid. There is an inductive load in the control circuit of the A switch. After the switch is disconnected, the inductive load will generate a very high voltage difference between the two ends of the switch. Under certain air humidity conditions, a breakdown discharge phenomenon will occur between the two ends of the switch with a very high voltage difference. As the switch is opened, the distance between the contacts continues to increase, and the required breakdown voltage continues to increase until breakdown is impossible.

29. Suppression method for switching transient disturbance: A reversely parallels the diodes at both ends of the load B reversely parallels the diodes and connects resistors in series.

30. Reasons for electromagnetic interference generated by electromechanical devices: caused by electric arcs generated during motor commutation, or generated by high-frequency/transient voltage.

31. Methods for suppressing electromagnetic interference generated by electromechanical energy: A. When there is poor contact between the brush and the commutator or mechanical vibration is obvious, the arc discharge is severe. To reduce interference, it is necessary to maintain reliable contact between the brush and the commutator, open and close normally, select high-quality brushes, make the commutator surface smooth and clean, maintain appropriate pressure, and in order to suppress arc, shunt capacitors or resistors between the commutator blades of the motor to suppress the path of interference propagation, Provide a low impedance path to bypass the disturbance current and suppress the resulting spike voltage.

Common electrical components for suppressing surge disturbances: spark gaps, metal oxide varistors, and silicon transient absorption diodes.

33. General strategies for controlling electrostatic discharge: A prevents the generation of electrostatic charges B prevents discharge C controls the discharge current path.

34. Basic particularity of electrostatic discharge: The electrostatic discharge current can reach a peak value of tens of amperes in a time range of 0.7~10ns, even exceeding 100A.

35. China Compulsory 3C Certification refers to: A safety certification B, EMC certification C import and export quality certification.

36. In electronic circuit design, in order to minimize EMC interference, what issues should be generally noted in the design of various functional circuits: Selection of components: Consider the structural type and actual characteristics of components, such as capacitance, resistance, inductance, transformer, integrated circuit, etc. Design of B circuits: 1. Power supply, filtering, input/output separation, reduce common impedance 2. Control unit, barrier, and transient disturbance suppression 3. Analog circuit, grounding Stay away from sources of harassment, limit working frequency, and delay rise/fall times.

37. General principles for suppressing the layout of circuit boards: A Separate digital circuits from analog circuits B Separate high-frequency, intermediate frequency, and low-frequency circuits C. Circuits with external signal transmission are well located near the connector end D Connector is placed on one side of the circuit board.

38. Why do power electronic devices generate harmonics: rectifying devices, speed regulating devices, switching power supplies, etc. Due to the nonlinear characteristics during power electronics, harmonic currents can be generated in power electronics systems.

39. The main sources of electromagnetic disturbances in electronic systems include: 1. Operation of high-voltage disconnectors and circuit breakers; 2. Lightning strikes and system short circuits; 3. Partial discharges; 4. Switching operations in secondary systems; 5. Voltage sags, interruptions, imbalances, harmonics, and frequency variations in the power grid during load changes and operational faults; 6. Power frequency and harmonic electric and magnetic fields generated by generators and transformers; 7. Electric and magnetic fields generated by transmission lines around them; 8. Automation equipment, The high-frequency conducted disturbances generated by wireless devices are similar to radiation disturbances, such as lightning strikes, electrostatic discharges, geomagnetic disturbances, and nuclear electromagnetic pulses.

40. The main measures to suppress electromagnetic interference in the power system are as follows: 1. For secondary cables laid near high-voltage conductors, shielding measures should be taken. When the external magnetic field is small, the shielding layer can be grounded at one point, while when the external interference is severe, both ends of the shielding layer should be grounded. 2. For multiple cables, the interference should be suppressed through reasonable cable laying and selecting the correct direction, such as separating low-level signal cables from high-level cables, The secondary cables should be routed as radially as possible to reduce their coupling.

41. What are the general wiring principles for printed circuit boards? The wiring of circuit board A should be short and thick, with uniform lines. Avoid long-distance parallel wiring, and appropriately increase the spacing of parallel lines.

42. Understanding of Some PCB Design Principles 1. Why is the clock line the shortest in PCB design? Because high-speed digital signals or clocks are sources of strong interference, the longer the line, the greater the likelihood of coupling to other parts. 2. Why can't high-frequency signals be routed under printed circuit input/output components? Routing under components can create objective or inductive coupling with them. Why should all connectors be placed on the side of the printed circuit board? Connectors often become effective antenna parts. Placing them on the side of the printed circuit board is beneficial for controlling common mode electricity between connectors