How to Easily Handle EMC Testing for Microcontroller Systems?

▌AbstractThis article discusses the definition of EMC, testing methods for EMC in microcontroller application systems, the application of new devices and materials for EMC, and troubleshooting techniques. Anyone involved in the research, development, production, or supply of electronic products must conduct EMC electromagnetic compatibility testing.▌IntroductionElectromagnetic Compatibility (EMC) refers to the ability of a device or system to operate normally in its electromagnetic environment without causing unacceptable electromagnetic interference to anything in that environment.EMC testing includes two main aspects: testing the intensity of electromagnetic interference emitted to the outside to confirm compliance with relevant standard limits; and conducting sensitivity tests under specified electromagnetic interference conditions to confirm compliance with relevant standard immunity requirements.For engineers involved in the design of microcontroller application systems, mastering certain EMC testing techniques is essential. EMC stands for Electro-Magnetic Compatibility, which includes both Electromagnetic Interference (EMI) and Electromagnetic Sensitivity (EMS). Since electrical products can interfere with other devices or be affected by interference from other devices, it not only relates to the reliability and safety of the product but can also impact the normal operation of other devices, potentially leading to safety hazards.▌EMC Testing for Microcontroller Systems1. Testing EnvironmentTo ensure the accuracy and reliability of test results, electromagnetic compatibility measurements have high requirements for the testing environment, which can include outdoor open areas, shielded rooms, or anechoic chambers.2. Testing EquipmentElectromagnetic compatibility measurement equipment is divided into two categories: one is electromagnetic interference measurement equipment, which can measure electromagnetic interference when connected to appropriate sensors; the other is for electromagnetic sensitivity measurement, which simulates different interference sources and applies them to various tested devices through appropriate coupling/decoupling networks, sensors, or antennas for sensitivity or immunity measurement.3. Measurement MethodsThere are many measurement methods for electromagnetic compatibility testing based on different standards, but they can be summarized into four categories: conducted emission testing, radiated emission testing, conducted immunity testing, and radiated immunity testing.4. Testing Diagnosis StepsFigure 1 shows the steps for analyzing electromagnetic interference emissions and faults of a device or system. Following these steps can improve the efficiency of testing diagnosis.5. Testing Preparation

• Testing site conditions: The EMC testing laboratory consists of an anechoic chamber and a shielded room. The former is used for radiated emission and radiated sensitivity testing, while the latter is used for conducted emission and conducted sensitivity testing.

• Environmental level requirements: The electromagnetic environment levels for conducted and radiated emissions should ideally be well below the standard limit values, generally at least 6dB lower than the limit.

• Testing table.

• Isolation of measurement equipment and the device under test.

• Sensitivity criteria: Generally provided by the tested party, monitored and determined through measurement and observation to assess the degree of performance degradation.

• Placement of the device under test: To ensure the repeatability of the experiment, there are usually specific regulations regarding the placement of the device under test.

6. Types of TestsConducted emission testing, radiated emission testing, conducted immunity testing, radiated immunity testing.7. Common Measurement InstrumentsElectromagnetic interference (EMI) and electromagnetic sensitivity (EMS) testing require many electronic instruments, such as spectrum analyzers, electromagnetic field interference measurement instruments, signal sources, amplifiers, oscilloscopes, etc. Due to the wide frequency range of EMC testing (20Hz to 40GHz), large amplitude (from μV to kW), and various modes (FM, AM, etc.), as well as different orientations (horizontal, tilted, etc.), it is crucial to use electronic instruments correctly.The appropriate instrument for measuring electromagnetic interference is the spectrum analyzer. A spectrum analyzer is an instrument that displays the relationship between voltage amplitude and frequency, showing a waveform called a spectrum. The spectrum analyzer overcomes the limitations of oscilloscopes in measuring electromagnetic interference, allowing for precise measurement of interference intensity at various frequencies, and can directly display the spectral components of the signal.When addressing electromagnetic interference issues, the most critical task is to identify the source of the interference. Only by accurately locating the source can effective measures be proposed to mitigate the interference. Determining the source of interference based on the frequency of the signal is the simplest method, as frequency characteristics are the most stable among all signal features, and circuit designers are often well aware of the signal frequencies at various points in the circuit. Therefore, knowing the frequency of the interference signal allows for inferring which part of the circuit is generating the interference.For electromagnetic interference signals, since their amplitude is often much smaller than that of normal operating signals, measuring them with a spectrum analyzer is quite straightforward. The narrow intermediate frequency bandwidth of the spectrum analyzer allows it to filter out signals that differ from the interference signal frequency, enabling precise measurement of the interference signal frequency and thus identifying the circuit that generates the interference signal.▌Electromagnetic Compatibility Troubleshooting Techniques1. Solutions for Conducted Problems

• Reduce EMI current by connecting a high impedance in series;

• Short-circuit EMI current to ground or other circuit conductors by connecting a low impedance in parallel;

• Cut off EMI current using current isolation devices;

• Suppress EMI current through its own action;

2. Capacitive Solutions for Electromagnetic CompatibilityA common phenomenon is to view one side of the filter capacitor as directly connected to a separate impedance rather than connected to a transmission line. A typical case is when the length of an input/output line reaches or exceeds 1/4 wavelength, the transmission line becomes “long.” This change can be approximately represented by the formula: l≥55/fWhere: l is in meters, and f is in MHz. This formula considers the average propagation speed, which is 0.75 times that of free space theory.

• Dielectric materials and tolerances: Most capacitors used for electromagnetic interference filtering are non-polarized capacitors.

• Differential mode (line-to-line) filtering capacitors.

• Common mode (line-to-ground/case) filtering capacitors.

Common mode (CM) decoupling typically uses small capacitors (10–100nF). Small capacitors can short-circuit unwanted high-frequency currents to the case before they enter sensitive circuits or when they are further away from noisy circuits. To achieve good high-frequency attenuation, minimizing or eliminating parasitic inductance is key. Therefore, it is necessary to use ultra-short leads, especially preferring leadless components.3. Inductive, Series Loss Electromagnetic Compatibility SolutionsIn terms of capacitance, if Zs and Z1 are not purely resistive, their actual values must be used when calculating frequency. When capacitors are connected in series in power or signal circuits, the following must be satisfied:

• The working current flowing through should not cause excessive heating of the inductance or excessive drop;

• The current flowing through should not cause magnetic saturation of the inductance, especially for high-permeability materials.

Solutions include: magnetic core materials; ferrite and ferrite-loaded cables; inductors, differential mode, and common mode; grounding chokes; combined inductive-capacitive components.4. Solutions for Radiated ProblemsIn many cases, radiated electromagnetic interference issues may arise during the conducted phase and can be eliminated. Some solutions can suppress interference devices in the radiated transmission path, functioning like field shielding. According to shielding theory, the effectiveness of such shielding mainly depends on the frequency of the electromagnetic interference source, the distance to the shielding device, and the characteristics of the electromagnetic interference field—electric field, magnetic field, or plane wave.

• Conductor strips

Using copper or aluminum strips can quickly and easily establish a direct shielding and low-resistance connection or bus. They are convenient for both temporary and relatively permanent solutions. Thickness ranges from 0.035 to 0.1mm, and the back is coated with a conductive adhesive for installation. If using copper conductive tape, its resistance is approximately 20mΩ/cm².Applications: electrical shielding enclosures; locating leakage points during failures; as an emergency solution, converting plastic connectors to metal, shielding ordinary flat cables, etc.

• Mesh shielding strips and zippered enclosures

Tinned steel mesh strips: primarily used for installation on an already assembled electrical enclosure as an easily installed band-type shielding cover. To reduce magnetic field radiation or sensitivity issues, steel mesh strips are an effective solution.Zippered shielding enclosures: used when there are clear indications that electrical fields are the main cause of EMI coupling.

• EMI sealing gaskets

Applications: EMI sealing gaskets are the most commonly used method to address radiation issues, sensitivity issues, ESD, electromagnetic pulses, and TEMPEST issues when the following conditions exist:* The chassis leakage has been identified as the main radiation path.* The mating surfaces are not smooth, flat, or hard enough to provide good contact.

• EMI shielding for windows and ventilation panels

Suitable for shielding apertures.The approximate model for plane wave is: SE≈104(-20-lgl)-20lgfWhere SE is in dB; l is the size of the mesh or aperture in mm; f is in MHz. Of course, as frequency decreases, the upper limit of the shielding efficiency SE of the mesh is limited by the metal itself. In the near field, for H field shielding, the shielding power SHE is not affected by frequency and can be approximated by the formula: SEH≈10lg(πr/l)Where r is the distance between the source and the shielding cover, and l is the mesh size, both in mm.

• Conductive coatings

Used to establish EMI shielding on the plastic casing of a system, enhance the shielding effectiveness SE of existing ordinary or deteriorated conductive surfaces, prevent ESD or static accumulation, and increase the contact area of mating surfaces or gaskets.

• Conductive foils

Aluminum is a good conductor, with no absorption loss below 10MHz, but it has good reflection loss for electric fields at any frequency. For applications, please refer to relevant materials.

• Conductive fabrics

Can be applied in any shielding situation requiring 30–30dB attenuation in the frequency range from 100kHz to GHz.▌ConclusionIn practical EMC testing applications, in addition to certification testing by standard qualification laboratories, two other feasible methods are also recognized in the industry: TCF (Technical Construction File) and Self Certification. Testing for anti-interference capability is a very practical testing item. The best way to achieve electromagnetic compatibility is to treat all digital and analog circuits as circuits responding to high-frequency signals, using high-frequency design methods to address electrical shielding, PCB layout, and common mode filtering. Adopting a solid ground plane and power plane is also important, even for analog circuits, as this helps limit high-frequency common mode loops. Most transient interferences are high-frequency and generate strong radiated energy.

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