Based on this, the interference formation mechanism in the ground wire can be summarized into the following two points: First, reduce low impedance and power supply feeder impedance. Correctly select the grounding method and isolate ground loops. Grounding methods can be categorized as floating ground, single-point grounding, multi-point grounding, and hybrid grounding. If the interference on the sensitive line mainly comes from the external space or system casing, floating ground can be used to solve this problem. However, floating ground equipment is prone to static electricity accumulation, and when the charge reaches a certain level, electrostatic discharge will occur. Therefore, floating ground is not suitable for general electronic equipment. Single-point grounding is suitable for low-frequency circuits. To prevent power frequency current and other stray currents from generating ground potential differences between points on the signal ground wire, the signal ground wire is isolated from the power supply and safety ground wires, and a single-point connection is made at the point where the power line is connected to the earth. Single-point grounding is mainly suitable for frequencies below 3MHz. Multi-point grounding is the only practical grounding method for high-frequency signals. In radio frequency, it exhibits transmission line characteristics. To ensure the effectiveness of multi-point grounding, when the length of the grounding conductor exceeds 1/8 of the wavelength of the highest frequency, multi-point grounding requires an equipotential grounding plane. Multi-point grounding is suitable for frequencies above 300kHz. Hybrid grounding is suitable for electronic circuits that contain both high-frequency and low-frequency components. Shielding is one of the important measures to improve the electromagnetic compatibility performance of electronic systems and equipment; it can effectively suppress various electromagnetic interferences propagating through space. Shielding can be classified into magnetic field shielding, electric field shielding, and electromagnetic shielding according to its mechanism. For electric field shielding, the following points should be noted: select materials with high conductivity and ensure good grounding. Correctly select the grounding point and a reasonable shape; ideally, the shielding body should be directly grounded.
In today's diverse electronic devices, ranging from consumer electronics like mobile phones and computers to automotive electronic systems and medical equipment, electromagnetic interference (EMC) is generated during operation, requiring the ability to withstand external EMC. EMC testing comprehensively assesses the EMC of electronic devices through a series of specific tests, ensuring stable operation in complex electromagnetic environments. Next, we will delve into the specific tests included in EMC testing and their objectives.
Conducted emissions testing primarily detects how electronic devices transmit electromagnetic interference signals to the outside world through conductors such as power lines, signal lines, and control lines. During the test, an impedance stabilization network (LISN) is used to isolate the device from the power grid and provide a stable impedance environment for the measurement. An EMI receiver is then used to measure the conducted interference signals.
The purpose of testing is to ensure that the intensity of interference signals generated by electronic devices is within the allowable range of standards, thus avoiding interference with other devices on the same power grid. For example, in the conducted emissions test of a desktop computer, if the conducted interference on its power cord exceeds the standard, it may cause noise in audio equipment connected to the same power grid, affecting normal use. Common testing standards such as CISPR 22 (Information Technology Equipment) and CISPR 15 (Lighting Equipment) both have requirements for conducted emissions.
EMC (Electromagnetic Compatibility) testing ensures that electronic equipment or systems do not generate excessive electromagnetic interference (EMI) to other devices during operation, and that the equipment itself can function normally in a given electromagnetic environment without being affected by EMI. The purpose of EMC testing is to ensure that under normal operating conditions, the equipment will not affect other devices due to electromagnetic radiation or interference, and that the equipment's electromagnetic interference to the outside world is kept within a reasonable range, thus complying with national or regional electromagnetic compatibility standards. EMC testing is typically an important step required for CE certification, FCC certification, and International Electrotechnical Commission (IEC) certification.
Signal cables are a significant pathway for conducted interference propagation. Using shielded cables instead of ordinary cables, and ensuring proper grounding of the shielding layer, can effectively suppress conducted interference on the cables. For longer cables, avoid running them parallel to power lines to prevent interference caused by electromagnetic coupling.
Adding a common-mode inductor and filter capacitor at the interface forms a common-mode filter, which can suppress common-mode interference signals. For differential signals, ensure impedance matching of the differential pairs and equal-length traces to reduce signal reflection and common-mode noise generation. Poor grounding will increase conducted interference. Check the equipment's grounding system to ensure that the grounding resistance is low enough and the grounding path is short and straight. Use appropriate grounding methods such as multi-point grounding, single-point grounding, or hybrid grounding, selecting according to the characteristics of the circuit and its frequency range.
For multilayer PCBs, rationally plan ground and power layers to increase ground layer integrity and reduce ground layer segmentation. Simultaneously, ensure that the grounding of different functional modules is independent to avoid mutual interference. For example, analog ground and digital ground should be separated, and connected at appropriate locations using ferrite beads or inductors for single-point connections. ASIM is a comprehensive enterprise specializing in high-performance ESD, TVS diodes, transistors, MOSFETs, and EMI filters, integrating design, R&D, sales, promotion, and brand operation. Equipped with a professional EMC laboratory, it provides one-stop, comprehensive high-quality services for EMC design, testing, and rectification, fully assisting customers in solving electromagnetic compatibility-related problems.
When equipment malfunctions during electrostatic discharge immunity testing, the first step is to check the insulation performance of the equipment's casing and interfaces. Increasing the thickness of the insulation material or using anti-static materials can reduce static electricity buildup and discharge.
Radiated emission testing targets interference signals radiated by electronic devices into the surrounding space in the form of electromagnetic waves. The test is typically conducted in an anechoic chamber, whose inner walls are lined with absorbing materials to absorb reflected electromagnetic waves, simulating a free-space environment and reducing the impact of external electromagnetic interference on the test. Antennas and spectrum analyzers are used to receive and measure the intensity and frequency of the electromagnetic signals radiated by the equipment.
The purpose of this test is to prevent interference signals radiated by electronic devices from affecting other nearby electronic devices. Taking a wireless router as an example, if its radiation emissions exceed the standard, it will not only interfere with the normal operation of other nearby wireless routers, but may also affect Bluetooth devices, wireless mice, and other devices on the same frequency band, leading to unstable or even interrupted signal transmission. The radiation emission testing standards differ for different types of devices; for example, automotive electronic devices must comply with the CISPR 25 standard, while consumer electronics products mostly follow the CISPR 22 standard.
Harmonic current testing primarily focuses on the harmonic currents generated when electronic equipment is connected to the power grid, due to the distortion of the current waveform caused by the presence of nonlinear loads (such as switching power supplies and frequency converters). During testing, the equipment is connected to a testing system, and the content of each harmonic current is analyzed by measuring the current waveform of the equipment under different load conditions.
The purpose of this test is to ensure that harmonic currents generated by electronic equipment do not pollute the power grid, affect the power supply quality, or disrupt the normal operation of other equipment connected to the grid. For example, if a large number of LED lighting devices are connected to the grid and the harmonic current exceeds the limit, it can cause voltage waveform distortion, increase grid losses, and even trigger malfunctions in relay protection devices. The IEC 61000-3-2 standard, developed by the International Electrotechnical Commission, provides detailed regulations on harmonic current limits for equipment of different power levels.
On the circuit board, implement electrostatic discharge (ESD) protection for sensitive circuits and components. ESD protection devices, such as TVS (Transient Voltage Suppressor) diodes and ESD suppressors, can be added to guide the instantaneous high voltage generated by ESD to ground, protecting the circuit from damage. Simultaneously, optimize the circuit layout, keeping sensitive components away from areas prone to ESD, such as interfaces and buttons.
Isolation devices, such as optocouplers and isolation transformers, are used to isolate the interference source from the protected circuit. When designing the circuit, its anti-interference capability is improved, for example, by increasing the signal driving capability and improving the signal-to-noise ratio, so that the circuit can still operate normally when subjected to pulse group interference.
Failure to meet EMC testing standards is not a cause for alarm; the key is to accurately analyze the source of interference and implement effective corrective measures for different test items and interference types. By optimizing circuit layout, strengthening shielding, and improving filtering, EMC issues can be gradually resolved, ensuring the equipment meets electromagnetic compatibility requirements. For electronic engineers, each EMC rectification is a process of technical improvement, laying a solid foundation for creating higher-quality and more reliable electronic equipment.
