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01、Introduction
For engineering technicians engaged in the design of microcontroller application systems (both hardware and software), mastering certain EMC testing techniques is essential.
02、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 may also affect the normal operation of other devices, potentially leading to safety hazards.
03、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 disturbance immunity requirements specified in relevant standards.
04、EMC Testing for Microcontroller Systems
4.1 Testing Environment
To 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.
4.2 Testing Equipment
Electromagnetic 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 disturbance measurement.
4.3 Measurement Methods
There 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 sensitivity (immunity) testing, and radiated sensitivity (immunity) testing.
4.4 Testing Diagnosis Steps
The following diagram outlines the steps for analyzing electromagnetic interference emissions and faults of a device or system. Following these steps can improve the efficiency of testing diagnosis.
4.5 Testing Preparation
① Testing site conditions: The EMC testing laboratory should be 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.
② Environmental level requirements: The electromagnetic environment levels for conducted and radiated emissions should ideally be well below the limit values specified by standards, generally keeping the environmental level at least 6dB below the limit.
③ Test bench.
④ Isolation of measuring equipment and the device under test.
⑤ Sensitivity criteria: Generally provided by the tested party, and monitored and judged in real-time to determine the extent of performance degradation through measurement and observation.
⑥ 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.
4.6 Types of Tests
Conducted emission testing, radiated emission testing, conducted immunity testing, radiated immunity testing.
4.7 Common Measurement Instruments
Electromagnetic 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 of voltage amplitude as it varies with frequency, and the waveform it displays is called the spectrum. The spectrum analyzer overcomes the shortcomings 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.
05、Troubleshooting Techniques for Electromagnetic Compatibility
5.1 Solutions for Conducted Problems
① 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.
5.2 Capacitive Solutions for Electromagnetic Compatibility
A common phenomenon is to not view one side of the filter capacitor as directly connected to a separate impedance, but rather as 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/f
Where: 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 tolerances
Most 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 (10-100nF). Small capacitors can short unwanted high-frequency currents to the case 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 to use leadless components.
5.3 Inductive, Series Loss Electromagnetic Compatibility Solutions
In terms of capacitance, if Zs and Z1 are not purely resistive, their actual values must be used when calculating frequency. When capacitors are in series with 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 the inductance, especially for high permeability materials.
Possible solutions include:
Magnetic core materials;
Ferrite and ferrite-loaded cables;
Inductors, differential mode, and common mode;
Grounded chokes;
Combined inductive-capacitive components.
5.4 Solutions for Radiated Problems
In 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 similarly to 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.
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