For engineers engaged in the design of microcontroller application systems (both hardware and software), mastering certain EMC testing techniques is essential.
About EMC
EMC:Electromagnetic Compatibility, refers to the ability of a device or system to operate as required in its electromagnetic environment without causing intolerable electromagnetic interference to any devices in that environment.It includes two parts: Electromagnetic Interference (EMI) and Electromagnetic Sensitivity (EMS).Since electrical products can cause electromagnetic interference to other devices during use, or be affected by electromagnetic interference from other devices, it not only relates to the reliability and safety of the product’s operation but can also affect the normal operation of other devices, potentially leading to safety hazards.
Two Main Aspects of EMC Testing
1. Testing the intensity of electromagnetic interference emitted to the outside to confirm compliance with the limit values specified in relevant standards;2. Conducting sensitivity tests under specified electromagnetic interference intensity conditions to confirm compliance with the immunity requirements specified in relevant standards.
EMC Testing for Microcontroller Systems
1. 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 StepsThe following diagram illustrates the steps for analyzing electromagnetic interference emissions and faults for a device or system. Following these steps can improve the efficiency of testing diagnosis.
5. Testing Preparation① Testing site conditions: The EMC testing laboratory should be an anechoic chamber or 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.② Environmental level requirements: The electromagnetic environment levels for conducted and radiated emissions should ideally be far below the limit values specified by standards, generally at least 6dB below the limit values.③ Test bench.④ Isolation of measurement equipment and tested devices.⑤ Sensitivity criteria: Generally provided by the tested party, and monitored and determined through measurement and observation to assess the degree of performance degradation.⑥ Placement of tested devices: To ensure the repeatability of experiments, there are usually specific regulations regarding the placement of tested devices.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 various 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 signals.
Electromagnetic Compatibility Troubleshooting Techniques
1. Solutions for Conducted Problems① Reduce EMI current by connecting a high impedance in series.② Short-circuit EMI current to ground or other circuit conductors by connecting a low impedance in parallel.③ Cut off EMI current using current isolation devices.④ 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 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.a. Dielectric materials and tolerancesMost capacitors used for electromagnetic interference filtering are non-polarized capacitors.b. Differential mode (line-to-line) filtering capacitors.c. Common mode (line-to-ground/case) filtering capacitors.Common mode (CM) decoupling typically uses small capacitors (10 to 100nF). Small capacitors can short-circuit unwanted high-frequency currents before they enter sensitive circuits or when they are far 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 SolutionsFor capacitors, 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:① The working current flowing through should not cause excessive heating or significant drops in inductance;② The current flowing through should not cause magnetic saturation in inductors, especially for high-permeability materials.Possible solutions include:
- Core materials;
- Ferrite and ferrite-loaded cables;
- Inductors, differential mode, and common mode;
- Grounded 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.
Conclusion
In 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 immunity 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 handle shielding, PCB layout, and common mode filtering. Using 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.