Learning to contact a new technology will always encounter various problems, and learning EMC is no exception. EMC (Electromagnetic Compatibility) includes two parts: EMS (Electromagnetic Sensitivity) and EMI (Electromagnetic Interference). Generally speaking, solving EMC problems is actually solving the external radiation interference of electronic equipment, or how to prevent equipment and electronic components from being interfered by external electromagnetic waves. The problem. Learning EMC should pay attention to basic knowledge, such as electromagnetic waves, electromagnetic fields and other introductory theories, and have an urgent desire to learn. In practice, communicate with many others. The effect of learning and communication with a few people is much better than that of learning problems by one person.

The following summarizes the common compatibility problems and specific solutions of EMC engineers for everyone to make study notes.


1. Why do the ground wires and power wires of digital circuits often have large noise voltages? How to reduce these noise voltages?

When the digital circuit is working, it will draw a large current instantaneously. When these transient currents flow through the power line and the ground line, due to the existence of the power line and the ground line inductance, a large kickback voltage will be generated, which is the observed noise Voltage. One way to reduce these noise voltages is to reduce the inductance of the power line and ground line, such as using grid ground, ground plane, power line plane, etc., and the other method is to use appropriate decoupling capacitors (storage Energy capacitor).


2. In practice, it is common to twist multiple wires as high-frequency conductors. It is said that this can reduce the radio frequency impedance of the wires. Why?

This increases the surface area of the wire, thereby reducing the high frequency resistance.


3. Special attention should be paid to the processing of key signals during the electromagnetic compatibility design of circuits or circuit boards. What are the key signals here?

From the perspective of electromagnetic emission, key signal lines refer to periodic signals, such as local oscillator signals, clock signals, address low-order signals, etc.; from the perspective of sensitivity, key signals refer to signals that are sensitive to external electromagnetic interference, such as low power Flat analog signal.


4. How to prevent electrochemical corrosion in the overlapping joints?

Choose a metal with a close electrochemical potential, or seal the contact area to isolate the electrolyte.


5. What is lap joint? Give several methods of lap joint.

The low-impedance (radio frequency) connection between metal components is called lap joint. The lap joint methods include welding, riveting, screw connection, electromagnetic sealing gasket connection, etc.


6. Please list as many methods as possible to reduce the RF impedance of the ground wire.

Try to use a conductor with a large surface area to reduce the resistance of high-frequency current; make the conductor as short as possible to reduce resistance and inductance; plate silver on the surface of the conductor to reduce surface resistance; multiple conductors in parallel to reduce inductance.


7. Why do I see some ground wires grounded through capacitors or inductances in some imported prototypes?

In order to make the ground system present different ground structures for signals of different frequencies.


8. What is the root cause of the ground wire interference problem?

The impedance of the ground wire is the root cause of the ground wire problem. Due to the existence of the ground wire impedance, when a current flows on the ground wire, a voltage will be generated and a potential difference will be formed. When we design the circuit, we assume that each point on the ground wire The potential is the same, and the ground potential is the reference potential for the entire system. The difference between the actual ground potential and the assumed conditions causes various ground problems.


9. When analyzing electromagnetic interference problems, what definition is often used to describe the ground wire?

The ground wire is defined as the return wire of the signal.


10. When there are many wires passing through the panel, filter connectors or filter array boards are often used. What problems should be paid attention to when installing filter connectors or filter array boards?

Install an electromagnetic sealing gasket between the filter connector or filter array board and the chassis panel or stick the gap with conductive tape to prevent electromagnetic leakage at the gap.