37 Common Electromagnetic Compatibility FAQ

EMC, or Electromagnetic Compatibility, is a critical field in the design of modern electronic devices. It not only determines whether a device can operate normally in complex electromagnetic environments, but also directly impacts the level of interference it generates with its surroundings and other devices.

To help you better address the various challenges in EMC design, we’ve specially curated 37 classic EMC Q&A, which provide you with practical solutions for your projects, helping you easily resolve those seemingly tricky issues. Ready to make new breakthroughs in the field of EMC? Let’s embark on this exciting journey of exploration together!

Question 1: Why Is EMC Design Important for Products?

To ensure the product can operate normally in complex electromagnetic environments without performance degradation or failure due to external interference.
To ensure the product does not generate harmful electromagnetic interference (EMI) that could affect the system or surrounding devices during operation.
To ensure the product complies with relevant standards and regulations, avoiding the risk of product recalls or penalties.
To reduce development and debugging time, and lower product development costs.

Question 2: From Which Aspects Can the EMC Design of a Product Be Approached?

Circuit Design: Carefully select low-noise, high-interference-resistant components, and optimize the circuit layout to minimize sources of interference.
Software Design: Use software algorithms to reduce high-frequency interference, such as optimizing PWM control or implementing anti-interference software mechanisms.
Shielding Structure Design: Use shields or conductive materials to isolate sensitive circuits from external electromagnetic interference.
Signal/Power Line Filtering: Add filtering circuits to suppress high-frequency interference on signal and power lines.
Grounding Design: Select the appropriate grounding method (e.g., single-point grounding or multi-point grounding) based on needs, and optimize the ground loop design to reduce interference.

Question 3: Why Is the Unit "Decibel (dB)" Commonly Used in the Field of EMC?

The decibel scale effectively represents a wide range of amplitudes, and by using a logarithmic scale (dB is a logarithmic unit), it makes the changes in electromagnetic signal strength and noise analysis more intuitive and easier to handle.

Question 4: Why Can't a Spectrum Analyzer Observe Transient Disturbances Like Electrostatic Discharge (ESD)?

Electrostatic discharge is a fast, brief voltage pulse, while spectrum analyzers are primarily used to analyze the frequency spectrum of continuous signals, meaning they cannot capture these short, wide-band transient signals.

To observe electrostatic discharge and other transient disturbances, an oscilloscope should be used, as it can capture the details of voltage changes over time in real-time.

Question 5: How Can You Create a Simple Near-Field Probe Using Coaxial Cable to Diagnose EMI Problems?

Prepare the Coaxial Cable: Choose an appropriate length of coaxial cable (e.g., RG58), ensuring it’s manageable for use.
Strip the Outer Insulation and Shielding: Remove the outer insulation and shielding from one end of the coaxial cable to expose the core conductor.
Make a Coil: Wrap the exposed core conductor into a small loop, approximately 1-2 cm in diameter (around 1-3 turns), forming a closed circular structure.
Solder the Core Conductor: Solder the ends of the loop to the outer shielding of the coaxial cable.

Question 6: How to Choose Shielding Materials When Designing a Shielded Enclosure?

From an electromagnetic shielding perspective, the choice of materials should depend on the type of electromagnetic waves to be shielded. For example:

For electric field waves, plane waves, or high-frequency magnetic field waves, most metal materials (such as aluminum, copper, steel, etc.) are sufficient to meet the shielding requirements.

For low-frequency magnetic field waves, which have stronger penetration ability, materials with high magnetic permeability (such as silicon steel, permalloy, etc.) should be selected for effective shielding.

Question 7: What Are the Key Factors Affecting the Shielding Effectiveness of an Enclosure?

Performance of Shielding Materials: The conductivity and magnetic permeability of the shielding material are fundamental to its shielding effectiveness. For high-frequency interference, good conductivity is more important, while for low-frequency magnetic field interference, materials with higher magnetic permeability are required.
Conductive Discontinuities: Holes, gaps, and interfaces on the enclosure can become primary paths for electromagnetic leakage, directly impacting shielding effectiveness.
Wires Passing Through the Enclosure: Signal cables, power lines, and other wires passing through the enclosure can introduce or leak electromagnetic interference.

Question 8: What Should Be Considered When Shielding Magnetic Field Radiation Sources?

Due to the low wave impedance of magnetic field waves, shielding mainly relies on absorption loss rather than reflection loss. Therefore, the following points should be considered:

Select Materials with High Magnetic Permeability: Materials like permalloy and silicon steel should be chosen to enhance absorption loss of the magnetic field, improving shielding performance.
Design Position of the Shielding Layer: The shielding layer should be as far from the radiation source as possible, as this can help increase the reflection path and improve shielding effectiveness.
Minimize Discontinuities in the Structure: Avoid holes, gaps, and other discontinuities in the shielding layer, as these can become channels for magnetic field leakage, weakening the shielding effect.

Question 9: Why Should Cables Inside the Enclosure Be Kept Away from Gaps and Holes?

Cables typically generate electromagnetic fields, and when placed near gaps and holes, these fields can leak through the discontinuities, thereby reducing the shielding effectiveness.

Question 10: How to Design a Shielded Room for Measuring Human Biometric Magnetic Information and Shielding AC Electromagnetic Fields from Static Magnetic Fields to 1 GHz?

Since low-frequency magnetic fields need to be shielded, materials with high magnetic permeability, such as permalloy, should be chosen. However, it is important to note that the magnetic permeability of permalloy decreases during processing, so heat treatment is necessary to restore its magnetic properties.

To ensure more efficient shielding, the shielded room should be designed in an assembled structure, using pre-processed panels. During the manufacturing process, the panels should be heat-treated according to the design specifications, then carefully transported to the site for installation. During installation, the edges of each panel should overlap to ensure the formation of a continuous magnetic flux path, effectively shielding low-frequency magnetic fields.

However, while this design can effectively shield low-frequency magnetic fields, the presence of gaps and seams may cause high-frequency electromagnetic field leakage. To address this, an additional layer of aluminum shielding can be added outside the permalloy shielding layer to block high-frequency electromagnetic interference (such as high-frequency alternating electromagnetic fields).

Question 11: What Is a Cutoff Waveguide Plate, and How Is It Used?

A cutoff waveguide plate (honeycomb plate) is an array of multiple cutoff waveguides, typically used in situations that require high shielding effectiveness while maintaining a large airflow.

When using it, it’s important to ensure a proper seal between the honeycomb plate and the chassis. This can be done using electromagnetic sealing gaskets or by welding the components together to prevent electromagnetic leakage and ensure effective shielding. Additionally, care should be taken during installation to align the direction of the honeycomb holes, avoiding direct leakage of electromagnetic waves outward.

Question 12: What Are the Common Types of Transparent Shielding Windows?

There are mainly two types: one consists of a glass panel with an embedded metal mesh, and the other has a thin metallic coating on the glass surface.

When using these, it’s crucial to ensure that the metal mesh or conductive coating maintains good electrical contact with the shielding chassis to prevent electromagnetic wave leakage.

Question 13: How Can the Streaking Effect Caused by the Metal Mesh Interlayer Shielding Glass on CRT Displays Be Reduced?

The streaking effect is typically caused by interference between the grid of the metal mesh and the scan lines of the cathode ray tube. To reduce this, you can rotate the mesh so that it forms an angle of 15-20 degrees with the CRT scan lines, which helps minimize interference and the resulting streaks.

Question 14: What Should Be Considered When Using Electromagnetic Sealing Gaskets?

Panel Thickness: Choose an appropriate panel thickness to prevent the rebound force of the gasket from causing deformation. If the panel is thin, the spacing of fastening screws should be increased to ensure sufficient compressive force.
Limit Structure: A limit structure should be set to prevent over-compression of the gasket, which could affect the shielding effectiveness.
Material Selection: Choose suitable metal materials to avoid electrochemical corrosion, which could degrade the performance of the electromagnetic sealing gasket during use.

Question 15: How to Handle Metal Rods Passing Through a Shielded Chassis?

You can use beryllium copper spring clips or other conductive materials to securely connect the metal rod to the shielding chassis, ensuring continuous electromagnetic shielding and preventing signal leakage.

Question 16: What Parameters Should Be Considered When Selecting a Power Line Filter?

The main parameters to consider are the insertion loss of the power line filter (including common-mode and differential-mode loss), rated current and voltage, and effective frequency range. The insertion loss determines the filter's ability to suppress currents and noise; the rated current and voltage ensure that the filter can handle the appropriate load during use; and the frequency range must be selected according to the actual application to ensure the filter can effectively block interference in the relevant frequency bands.

Question 17: Why Is the High-Frequency Filtering Performance of Power Line Filters So Important?

If a filter’s high-frequency characteristics are poor, it may lead to excessive radiation emissions from the equipment, which could negatively affect the equipment’s electromagnetic compatibility. Furthermore, poor high-frequency filtering can make the device more sensitive to pulse interference, reducing its stability and performance.

Question 18: Why Should Power Lines With Filters Be Kept as Far Away from Signal Cables as Possible?

If the power line is too close to signal cables, high-frequency signals from the signal cables may couple onto the power line, especially if the power line has been filtered. This coupling can cause conducted emissions on the power line to exceed limits, thus affecting the device's electromagnetic compatibility, increasing radiation interference, or even turning the power line into a source of interference.

Question 19: Why Should We Not Just Focus on the Compactness of Power Line Filters When Selecting Them?

The size of a filter is primarily determined by the inductance, and smaller filters typically require reducing the size of the inductance, which may result in a lower inductance value and, consequently, a reduction in the filter’s low-frequency filtering performance. Additionally, smaller filters often have more compact internal components, which can reduce their high-frequency performance. Therefore, when selecting power line filters, it’s important to balance size with filtering performance, rather than simply aiming for a more compact design.

Question 20: How Do You Measure the Insertion Loss of a Filter to Get the Most Conservative Result?

To measure insertion loss under the most conservative conditions, it is generally recommended to use a source and load impedance ratio of 0.1:100 (or vice versa) for the test setup. This measurement method ensures that the insertion loss value of the filter is obtained under the most unfavorable conditions, providing the most reliable test results.

Question 21: Why Can AC Line Filters Be Used in DC Applications, but DC Line Filters Cannot Be Used in AC Applications?

In DC filters, the bypass capacitors are typically designed for DC applications and are not suitable for AC conditions. If used in an AC power supply, they may overheat and get damaged, or in some cases, be punctured due to their lower voltage rating. Additionally, the common-mode bypass capacitors in DC filters are usually larger in value, and if used in AC applications, they may cause excessive leakage current, which could violate safety standards.

Question 22: How to Determine the Installation Method for Signal Line Filters?

There are generally two installation methods for signal line filters: PCB-mounted and panel-mounted. In general, PCB-mounted filters are suitable for lower-frequency filtering, while panel-mounted filters are more appropriate for higher-frequency filtering needs.

Question 23: Why Are Three-Terminal Capacitors More Suitable for Interference Filtering?

In traditional capacitors, the lead inductance negatively affects high-frequency filtering performance, but a three-terminal capacitor cleverly combines two lead inductances with the capacitor to form a T-type low-pass filter structure. This design not only effectively reduces the negative impact of lead inductance but also improves the ability to filter high-frequency signals, making it particularly effective for EMI filtering.

Question 24: Why Are Feedthrough Capacitors Ideal for Interference Filtering?

Feedthrough capacitors are a special type of three-terminal capacitor, with the primary advantage of being able to be directly mounted on metal panels. This significantly reduces grounding inductance and nearly eliminates the effects of lead inductance. Additionally, the input and output terminals of feedthrough capacitors are isolated by the metal panel, which effectively eliminates high-frequency coupling, further enhancing their filtering performance.

Question 25: What Is the Difference Between EMI Suppression Cores and Inductor Cores?

Traditional inductor cores typically use low-loss materials to minimize the losses and achieve a high quality factor (Q). On the other hand, EMI suppression cores use materials with higher loss, which effectively absorb electromagnetic energy and reduce the amplitude of interference signals. Therefore, the loss characteristics of EMI suppression cores are more similar to that of resistors.

Question 26: Why Is It Best for Signal Line Filters on External Cables to Have a Rated Voltage Higher Than 200V?

External cables are usually exposed to complex electrical environments and are susceptible to surge, ESD, and other momentary high-voltage interference. These interferences can cause brief high-voltage spikes on the cables, so the filter's capacitors and other components need to have sufficient voltage withstand capability to handle these transient high-voltage surges. Otherwise, the capacitors may break down, or other components could be damaged.

Question 27: How Should a Common-Mode Choke Be Wound?

When winding a common-mode choke, two conductors are typically wound side by side on the magnetic core to ensure that the common-mode currents on both conductors generate opposing magnetic field lines, which cancel each other out. For higher voltages, the "go" and "return" wires should be wound separately to ensure adequate insulation voltage and prevent breakdown. For lower voltages, the wires can be wound together in pairs to reduce space usage and cost.

Question 28: What Should Be Done If Adding a Ferrite Core to the Cable Does Not Significantly Improve the Electromagnetic Radiation Problem?

One reason could be that the cable's original common-mode loop impedance is high, and the impedance added by the ferrite core might not significantly alter the original loop impedance, leading to a minimal suppression effect. To address this, a bypass capacitor can be added at the cable's port to reduce the common-mode loop impedance and further enhance common-mode interference suppression.

Another reason might be that other sources of radiation within the system are contributing to the issue. In this case, simply adding a ferrite core to the cable will not fully solve the problem. It is necessary to check for other potential radiation sources and apply appropriate shielding or isolation measures to comprehensively reduce the device's electromagnetic radiation.

Question 29: What Should Be Considered When Using a Filter Connector or Filter Array Board for Multiple Wires Passing Through a Panel?

It is essential to ensure electromagnetic sealing between the filter and the chassis panel. This can be achieved by installing electromagnetic sealing gaskets or using conductive tape to seal the gaps and prevent electromagnetic leakage.

Question 30: What Is the Root Cause of Grounding Interference Issues?

The root cause of grounding interference is the impedance of the ground line. Since the ground line has impedance, when current flows through it, a voltage drop occurs, leading to a potential difference. In circuit design, we assume that all points on the ground line have the same potential, with the ground line serving as the reference potential for the entire system. However, the actual ground potential differs from this assumption, which leads to various grounding issues.

Question 31: What Are Some Methods for Reducing the RF Impedance of Grounding Lines?

Increase Conductor Surface Area: A larger conductor surface area results in lower resistance when high-frequency current flows through it, thereby reducing signal loss.
Reduce Conductor Length: Minimizing the length of the conductor can significantly reduce resistance and inductance, decreasing high-frequency signal propagation losses on the ground line.
Silver Plating the Conductor Surface: Silver has a higher conductivity than other materials, such as copper. Plating the conductor surface with silver can reduce surface resistance and effectively reduce high-frequency signal loss.
Parallel Multiple Conductors: Connecting multiple conductors in parallel can effectively reduce inductance and impedance, thereby improving the ground line's conductive capacity, especially for high-frequency signals.

Question 32: How to Prevent Electrochemical Corrosion at Tapping Points?

Avoid using metals with significantly different electrochemical potentials, as contact between metals with differing potentials can create a galvanic effect, leading to corrosion. Additionally, sealing coatings, sealants, or other isolation methods can be applied to prevent moisture, air, or other electrolytes from reaching the tapping points, effectively preventing electrochemical reactions.

Question 33: What Are the Key Signals in Circuit or PCB EMC Design?

In EMC design, key signals refer to those that are particularly sensitive to EMI or produce strong electromagnetic radiation. Specifically:

From an electromagnetic emission perspective, key signal lines include periodic signals, such as clock signals, local oscillator signals, and lower address signals. These signals often generate significant electromagnetic radiation in the circuit, making them prone to EMI.

From a sensitivity perspective, key signals are those that are highly susceptible to external electromagnetic interference, such as low-level analog signals. These signals are extremely sensitive to environmental electromagnetic noise, and interference can cause circuit malfunction or errors.

Question 34: Why Do Digital Circuits Often Have High Noise Voltage on Their Ground and Power Lines?

When digital circuits operate, they momentarily draw large amounts of current, especially during switching events. The resulting transient currents flow through the power and ground lines. Due to the inherent inductance of these lines, these transient currents generate significant back EMF (electromotive force), which in turn creates noise voltage.

Question 35: Why Do PCB Designed with Auto-Routing Software Often Have High Radiation?

Auto-routing software used in PCB design often fails to optimize the loop area of periodic signals (such as clock signals or timing signals). A larger loop area increases the electromagnetic radiation of the signal, as it acts as an antenna.

Question 36: Why Is It Not Recommended to Separate Digital and Analog Grounds into Two Layers in Multilayer PCB Design?

In multilayer PCB design, using separate digital and analog grounds can increase parasitic capacitance, especially during high-frequency signal transmission. This can lead to interference and crosstalk between the digital and analog grounds. A better design approach is the "ground splitting" technique, where the digital and analog grounds are kept separate but share a common ground layer.

Question 37: Why Should High-Frequency Circuits Be Placed Away from I/O Cable Connectors in PCB Layout?

The main reason is to prevent high-frequency signals from coupling into the cables via electromagnetic interference, which could generate common-mode voltage (or current). This common-mode voltage can lead to significant radiation interference, negatively affecting EMC performance.

Ending

Through these 37 classic EMC Q&A, we have explored many key electromagnetic compatibility design principles, providing valuable solutions from theory to practical applications. Whether it’s optimizing circuit layouts, selecting appropriate shielding materials, or properly using filters and grounding designs, every detail can have a profound impact on the performance and stability of electronic devices.

As modern electronic devices increasingly rely on operating within complex electromagnetic environments, the importance of EMC design continues to grow. With the advancement of technology, the electromagnetic interference challenges faced by electronic products are becoming more intricate. As designers or engineers, only by continually deepening our understanding and mastery of EMC can we effectively address these challenges.

We hope that these experiences and tips will be helpful for your projects and help elevate your products to higher standards in electromagnetic compatibility. EMC design is not only a technical challenge but also an essential aspect of achieving innovation, ensuring product reliability, and compliance. Let’s continue to explore this field full of both challenges and opportunities, driving the development of more high-performance electronic devices!

You Might Be Interested

Causes of Signal Attenuation in PCB and Optimization Measures

January 20, 2025

Discover the causes of signal attenuation in PCB designs and effective optimization techniques. Learn how to tackle high-frequency signal loss with solutions like repeaters, amplifiers, and low-loss materials to improve system performance.

Ultimate Guide to PCB Potting for Circuit Board Protection

April 5, 2023

PCB potting, an essential in electronics, protects circuits by enveloping them in compounds like epoxy. Vital for high-value applications, it prevents thermal expansion issues, shields against damage, and safeguards from environmental threats. Unlike conformal coating, it covers the entire PCB, ensuring robust protection but at a higher cost. The process involves cleaning, resin preparation, pouring, curing, and hardness verification for effective PCB protection.

Everything You Need to Know About HVAC PCB Control Board

October 26, 2024

Discover everything you need to know about HVAC PCB control boards! This comprehensive guide explains their essential functions, troubleshooting techniques, and types, helping homeowners and technicians ensure optimal performance for heating and cooling systems. Don’t miss out!

Causes and Solutions of ENIG Black Pad Defect

August 20, 2023

In the ENIG finish, the occurrence of black pad defects can render repairs impossible. Our approach should be focused on proactive prevention. In this article, TechSparks presents a comprehensive guide to ENIG black pad issues. Through conceptual introduction, understanding the underlying principles, and outlining preventative measures, we aim to assist you in effectively addressing potential black pad problems that may arise in your projects!

What is Crosstalk in PCB and How to Reduce It

January 12, 2025

Learn how crosstalk affects PCB design and its impact on signal integrity. Explore effective strategies to reduce interference, improve performance, and ensure reliable transmission in your circuits. Click to discover how to optimize your PCB design.

What is Microvia in PCB?

April 8, 2024

Microvias are small holes in PCBs facilitating high-density interconnections. Their reliability is challenged by complex thermo-mechanical environments, requiring careful design and manufacturing. Techniques like mechanical and laser processing are used to create them, advancing PCB technology.

Single-Layer PCB

December 25, 2023

A Single-Layer PCB, or single-sided PCB, with its singular conductive layer, provides a simplified and cost-effective solution for less complex projects. While multi-layer PCBs demand intricate connections and face challenges in alignment and heat dissipation, single-layer PCBs are suitable for low-frequency projects, albeit with limitations in compactness and electromagnetic interference suppression.

High-Speed PCB Signal Integrity Design Tutorial

January 4, 2025

Unlock the secrets of high-speed PCB design! This tutorial covers essential strategies to optimize signal integrity, minimize reflection and crosstalk, and boost system performance in the 5G era.