What is EMC?

EMC stands for Electromagnetic Compatibility, which is defined by the International Electrotechnical Commission (IEC) standards as the ability of equipment or systems to operate in an electromagnetic environment without causing unbearable electromagnetic interference to any equipment in their environment. Among them, EMC includes two parts: EMI (electromagnetic interference) and EMS (electromagnetic anti-interference). EMI refers to the electromagnetic interference generated by equipment in the environment during normal operation, which cannot exceed a certain limit; EMS refers to a device that has a certain degree of immunity to electromagnetic interference present in its environment. This introduction covers conducted emission, radiated emission, ESD electrostatic discharge immunity testing, and surge (impact) immunity testing.

Introduction to EMI Testing Projects

        Radiation emission (EUT test equipment, AE auxiliary equipment)

        The purpose of radiation emission measurement is to determine whether the electromagnetic interference energy emitted by EUT through space propagation meets the standards.

        The commonly used anechoic chambers for radiation emission testing are the 3-meter method and the 10 meter method. Today, we will introduce the 3-meter method 966 anechoic chamber.

        Measurement frequency range: low frequency 30MHz~1GHz, high frequency 1GHz~18GHz, the measurement frequency is determined by the highest operating frequency inside the EUT.

Arrangement of EUT: The EUT is required to be arranged according to its actual working state (all interconnecting cables and connected external devices are connected according to their actual state). If other equipment is needed to assist in testing, the EUT should be arranged accordingly. So the distance between the EUT and the auxiliary testing equipment should be ≥ 10cm. The EUT should be placed on a wooden testing platform 3 meters away from the center of the antenna, and the height of the wooden table should be 0.8m. (Referring to GB/T 9254.1-2021 regulatory specifications)

Conductive emission

        Any non portable device has a cable interconnection relationship with other devices, whether through power cables or signal cables. As long as this interconnection relationship exists, the device has a way to conduct its common mode current to the interconnected devices. This phenomenon is called conducted interference, also known as conducted emission.

        Conducted emission testing is to test whether the interference generated by the tested equipment on the public power grid through the power line during operation meets the standards. Conductive emission consists of two parts: the power end CE and the communication end CE

        Power end CE: Test the interference transmitted by the testing equipment to the AC power grid or DC distribution network through its own power port, with a common testing frequency range of 150kHz to 30MHz.

        Communication end CE: The testing frequency band is the same as above. The communication end described here refers to ports connected to the public network, such as network ports, ISDN ports, etc., which have CE testing requirements, while there are no CE requirements for audio and video ports.

        Arrangement of EUT: The EUT is required to be arranged according to its actual working state (all interconnecting cables and connected external devices are connected according to their actual state). If other equipment is needed to assist in testing, the EUT should be arranged accordingly. So the distance between EUT and auxiliary testing equipment should be ≥ 10cm, and EUT should be placed on a wooden testing table with a height of 0.8m. (Referring to GB/T 9254.1-2021 regulatory specifications)

Introduction to EMS Testing Projects

        ESD electrostatic discharge immunity test

        The purpose is to investigate the anti-interference ability of the tested equipment when receiving direct discharge or electrostatic field interference generated by external static power sources (such as charged human bodies, charged equipment, etc.).

        EUT layout: The EUT should be placed on a non-conductive table (0.8 ± 0.08) meters above the grounding reference plane. The test should be conducted within the range of ambient temperature: 15 ℃ -35 ℃, relative humidity: 30% -60%, atmospheric pressure: 86kpa-106kpa.

        (Referring to GB/T 17626.2-2018 regulatory specifications)

        Electrostatic discharge requirements: The testing level for electrostatic discharge test is 2kV, 4kV, 6kV, 8kV for contact discharge; Air discharge: 2kV, 4kV, 8kV, 15kV. General consumer electronic devices, IoT devices, and other products generally pass the B level for contact discharge testing at ± 4KV, and the B level for air discharge at 8KV.

        Evaluation of Test Result Level:

        A performs normally within the limits specified by the manufacturer, client, or purchaser; The B function or performance is temporarily lost or reduced, but can be restored on its own after the disturbance stops, without the need for operator intervention; C temporarily loses or reduces functionality or performance, but requires operator intervention to restore it; The loss of function or performance that cannot be restored due to hardware or software damage or data loss of the device. (Depending on the specific requirements of the manufacturer, client, or purchaser)

        Surge immunity test

        Surge immunity test, also known as lightning strike test. Simulate the phenomenon of lightning strikes in nature through experiments. It is the energy generated by the short circuit and arc fault of the equipment combination to the ground system coupled with external lightning, or directly hitting the external circuit. The injected large current flows through the grounding resistor or external circuit impedance, mainly simulating the interference of the above energy on electronic products through connecting cables (power lines, etc.). In practical use, it may cause the product to crash, reset, malfunction, or burn out.

        The national standard used for surge (impulse) immunity test is GB/T17626.5-2019 "Electromagnetic Compatibility Testing and Measurement Techniques - Surge (Impulse) Immunity Test". The standard describes two different waveform generators, one is a combination wave generator (voltage waveform: 1.2/50 µ s, current waveform: 8/20 µ s), and the other is a combination wave generator (voltage waveform: 10/700 µ s, current waveform: 5/320 µ s).

        1. The waveform used for power line experiments (1.2/50 µ s and 8/20 µ s), open circuit voltage wave and short-circuit current wave internal resistance: differential mode (line to line) 2 ohms, common mode (line to ground) 12 ohms. The waveforms of 1.2/50 µ s and 8/20 µ s are shown in the figure.

        (1.2/50 µ s) Open circuit voltage waveform

Wavefront time: Tf=1.67 * T=1.2 * (1 ± 30%) µ s

Duration: Td=Tw=50x (1 ± 20%) µ s

Note: 1.67 is the reciprocal of the difference between the threshold values of 0.89 and 0.3

(8/20 µ s) Short circuit current waveform

Wavefront time: Tf=1.25 * Tr=8 * (1 ± 20%) µ s

Duration: Td=1.18xTw=20 * (1 ± 20%) µ s

Note 1:1.25 is the reciprocal of the difference between the threshold values of 0.9 and 0.1

Note 2: 1.18 is an empirical value

2. (10/700 µ s) - (5/320 µ s) waveform, used for synchronous interconnect cables directly connected to outdoor communication networks

(10/700 µ s) Open circuit voltage waveform

Wavefront time: Tf=1.65 * T=10 * (1 ± 30%) µ s

Duration: Td=Tw=700 * (1 ± 20%) µ s

(5/320 µ s) Short circuit current waveform

Wavefront time: Tf=1.25 * Tr=5 * (1 ± 20%) µ s

Duration: Td=Tw=700 * (1 ± 20%) µ s

Source: WeChat Official Account of Shenzhen Qianhong Electrical Testing Co., Ltd