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From the perspective of electromagnetic induction, community teacher Tao Xianfang believes that a good electronic product must not only meet functional requirements but also excel in circuit design (ECD) and electromagnetic compatibility design (EMCD). The quality and technical performance of the product are heavily influenced by these aspects. While many engineers begin their careers with an understanding of electronic components, they often struggle when transitioning to electromagnetic compatibility design. The foundation of EMC design lies in electromagnetic field theory, specifically electromagnetic induction knowledge. Consider a scenario where multiple electronic devices operate in the same space, generating electromagnetic fields around them. These fields can interfere with each other through conduction or radiation, potentially destabilizing the system or causing crashes—the root cause being electromagnetic interference (EMI). EMI is prevalent in electronic products, arising not only from interactions between devices but also between components within a system. The two primary modes of interference are conducted and radiated, with conducted interference further subdividing into common mode and differential mode interference. The causes of interference are complex, with electrostatic discharge interference being central.
Ensuring system stability in the absence of external influences requires adherence to a three-step rule for improving design efficiency and solving electrostatic discharge interference comprehensively. To mitigate radiation interference, three key strategies are:
1. Shielding;
2. Minimizing the area of current loops (to reduce magnetic field interference) and the length of live conductors (to reduce electric field interference);
3. Avoiding resonance conditions where the length of the conductor matches an integer multiple of the quarter-wavelength of the interference signal, as this maximizes radiation interference.
Common EMI suppression methods include shielding, spread spectrum techniques, and filtering. Most electromagnetic shielding methods target noise above 300 MHz. Additionally, using composite materials such as vacuum-plated plastics with nickel coatings is a common approach to isolate electromagnetic waves. The spread spectrum technique involves spreading the clock signal to reduce peak signal amplitudes and levels. Some BIOS systems now offer built-in spread spectrum functionality, allowing users to adjust settings themselves. However, achieving the right balance between signal distortion and EMI attenuation is crucial, typically ranging from 1% to 1.5%. Exceeding 3% can distort signals excessively, rendering the approach impractical.
Filtering is the most commonly used method among design engineers due to its low cost and ease of implementation via SMD processes. Filters are tailored based on specific control requirements. For instance, a large current bead can be employed on power traces, while general beads suppress specific frequency noise signals. CMFs are used to manage differential mode noise on lines like USB, 1394, and LVDS. The effectiveness of any solution depends on the context; if it works, it’s a good method.
Addressing radiation-conducted EMI presents a significant challenge, with several solutions:
1. Adding an LC filter circuit at the interference source;
2. Introducing noise to ground via bypass capacitors on the I/O side;
3. Shielding electromagnetic waves with enclosures;
4. Expanding the PCB area;
5. Using flat cables or solid wires sparingly within products;
6. Twisting internal physical wires to suppress noise radiation, adding bypass capacitors at the I/O ends of flat cables;
7. Adding a Common Mode Filter at the beginning or end of differential mode signal lines;
8. Adhering to analog and digital wiring principles.
EMI formation can be categorized into common mode and differential mode radiation. Common mode radiation includes common ground impedance interference and electromagnetic field interference to wires. The former arises from shared ground resistance between noise sources and victim circuits, solved by avoiding common interference. The latter results from high electromagnetic energy fields affecting inter-device wiring, addressed through shielding. Differential mode radiation refers to interference between wires, where noise from one wire infects others, creating near-field interference. This can be mitigated by increasing the distance between wires.
In the "EMC Solution for Improving Handset Sensitivity," Murata experts highlighted how modern electronic products are evolving with higher performance, leading to complex changes in electronic devices. Mobile phones, for instance, feature lower IC voltages, reduced energy, and faster interface communication speeds. Additionally, the growing number of electronic controls in automotive applications has increased system complexity. These factors make addressing EMC challenges increasingly important. As the market evolves, so do safety and testing trends.
IEC categorizes electromagnetic compatibility standards into three types: basic standards, general standards, and product standards. Basic standards divide into emission and immunity standards. General standards classify environments into classes A and B, with A representing industrial zones and B civilian. Knowing which standard applies is critical. If a product lacks a specific EMC standard or applicable product class standard, the general EMC standard should be used. A product standard is preferable, followed by selecting the most relevant standard.
Shou Jianxia, Secretary General of the National Radio Interference Standardization Technical Committee, discussed the latest international and domestic EMC standards, recent developments, and testing industries at the Circuit Protection and Electromagnetic Compatibility symposium in Shanghai.
In the Chengdu station of the event, the Electronic Component Technology Network invited European EMC technology and standards expert Dr. Gerd Jeromin to focus on European radio and communication equipment EMC challenges, system solutions, and methods, as well as European standards and testing. Click to watch: EMC Standards and Wireless Communication Device Planning and Application Trends.