Detailed explanation of EMC radiated emission test and conducted emission test

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Detailed explanation of EMC radiated emission test and conducted emission test [Copy link]

Why electronic products need to undergo EMC electromagnetic compatibility testing and how to do EMC testing are important and indispensable links for hardware and PCB engineers in electronic design from the beginning to the end.

It is understood that IT, radio and telecommunication products in almost all countries around the world need to undergo electromagnetic compatibility (EMC) testing before becoming qualified electronic products and circulating in the market. The EMC testing process is designed to verify whether the electromagnetic interference transmitted by the product is within an acceptable range and to ensure that your product and its functions are not negatively affected by electromagnetic interference.

Since electromagnetic compatibility (EMC) is so important, what do we need to do when doing EMC project testing on our products?

A complete product project EMC test includes the following steps:

EMC Radiated Emissions Tests: Tests that measure the electromagnetic interference (EMI)

EMC immunity testing: Testing that measures a product's susceptibility to electromagnetic interference from other sources

Electrostatic discharge (ESD) testing: The discharge of static electricity when two objects come into contact. Examples of ESD include the static electricity sometimes felt after drying clothes in a clothes dryer or lightning.

Conducted immunity testing: injects current into the input and output cables of the system under test

Conducted Emissions Testing: RF energy transmitted from electrical equipment back into the electrical system through the power lines

RF Compatibility Testing: RF is the frequency of electromagnetic radiation of energy useful for communication. The testing we provide focuses on the ability of the antenna-connected RF receiver/transmitter subsystem operating within the system to operate properly without performance degradation due to antenna-to-antenna coupling.

Today we will explain in detail the EMC radiation emission test and conducted emission test:

EMC external emission test

Definition: The external emission mainly tests the radiation interference intensity of the equipment to the outside world during normal operation. The equipment under test shall be arranged according to the typical installation requirements at the client end, and the wires shall comply with the length and model specified in the specification. Except for the equipment specified in 36.201.1a, other equipment and systems shall be grouped, classified and tested according to GB4824 (Limits and Measurement Methods of Interference Characteristics of Industrial, Scientific and Medical (ISM) Radio Frequency Equipment).

Range: External transmission includes frequencies from 9kHz to 18GHz

(1) 9KHz-30MHz frequency band
In this frequency band, the magnetic field H is measured. When the EUT is small, it is placed in a large magnetic loop antenna (LLA) to measure the induced current of the disturbance magnetic field. When the EUT is large, the remote antenna method is used to measure the magnetic field strength of the disturbance at a specified distance using a single small loop.

(2) 30MHz-18GHz frequency band
The equipment and site required for measuring the electric field E: below 1GHz in this frequency band, i.e. 30MHz-1GHz, are as follows: the site is selected in an open area or a semi-anechoic chamber, simulating semi-free space, with a 0.8m high wooden table, a 360-degree rotating turntable, a measurement distance of 3, 10m and the corresponding limit, an adjustable antenna height of 1-4m, and antenna vertical polarization and horizontal polarization tests. The broadband antenna, coaxial cable, and interference meter form a 50Ω matching transmission system. The impedance of the antenna, the impedance of the coaxial cable, and the input impedance of the interference meter should be equal. Impedance mismatch will cause reflection, form standing waves, and affect the accuracy of the readings. The characteristics of the interference meter are judged by using quasi-peak or peak measurements.

Above 1GHz, i.e. 1GHz-18GHz, the equipment and site required are roughly as follows: in a fully anechoic chamber, simulating free space, the receiving antenna is set at the same height as the EUT, the turntable still needs to rotate 360 degrees, the test distance is 3m, a small-aperture directional antenna is used, both horizontal and vertical states must be measured, and the lowest end of the antenna should be more than 25cm from the ground when placed vertically to avoid affecting the performance of the antenna. The spectrum analyzer is set to the maximum hold mode and the logarithmic dB display mode. The measurement results are indicated by the peak or average value of the electric field strength (not quasi-peak value). A 1MHz resolution bandwidth and video bandwidth are used for peak measurement, and a 1MHz resolution bandwidth is still used for average measurement, but the video bandwidth should be greatly reduced to 10Hz, which is equivalent to adding a low-pass filter.

Radiated Emissions - Test Procedure:

Place the antenna and receiver 3m away from the outer wall of the EUT (equipment under test), and adjust the center of the antenna to 2m above the ground; turn off the EUT first and test the environmental noise level; turn on the EUT, adjust the wiring method (including the height of the antenna, the polarization direction of the antenna, and the angle of the turntable), and select the maximum radiation point; execute different functions of the EUT to find the maximum radiation mode; rotate the antenna to test the horizontal and vertical radiated interference respectively; move the antenna with the EUT as the center and select as many test points as possible for measurement; select the highest radiation level as the measurement result; the test can be carried out in an open field and a semi-anechoic chamber; turn off the equipment.

EMC conducted emission test

Conducted Emission test, referred to as CE, is also called conducted disturbance, which refers to an electromagnetic phenomenon in which the voltage or current inside the system is transmitted through the signal cable, power line or ground line and becomes a source of interference for other systems or equipment. Conducted emission test is also usually called disturbance voltage test, and any product with power line will be involved, including many DC power supply products. In addition, signal/control lines also have conducted emission requirements in many standards, which are usually expressed by the limit of disturbance voltage or disturbance current (the two have a mutual conversion relationship).

Conducted Emissions - Test Configuration, Procedure:

The EUT should be placed on a test bench 80~90cm above the ground. The surface of the test bench should be a conductive or non-conductive plane with a metal grounding plate. Generally, the conductive or non-conductive test bench is selected based on the actual environment and location of the EUT. For example, portable equipment can be placed on a non-conductive test bench, and equipment installed in the cabin needs to be tested on a metal conductive test bench. The power line under test is connected to the power grid through the power impedance stabilization network. The cable of the device under test can be placed according to the requirements of the standard and different lengths can be selected.

The standard test configuration for conducted emissions testing of desktop equipment is shown below.

① Extra long cables should be bundled and folded into 30~40cm bundles;

②Connect the terminal resistor to the end of the unused I/O signal cable for terminal matching. It is particularly important not to leave it hanging.

③The projection distance between EUT and LISN on the ground is 80cm;

④The distance between all cables and GND is more than 10cm;

⑤The end of the hanging cable should be at least 40cm away from GND to ensure that there will not be excessive spatial coupling.

The standard test configuration for conducted emissions testing of vertical equipment is shown below.

① Extra long cables should be bundled and folded into 30~40cm bundles;

②Connect the terminal resistor to the end of the unused I/O signal cable for terminal matching. It is particularly important not to leave it hanging.

③EUT and GND should be insulated;

④ The distance between all cables and GND is more than 10cm (this has a very big impact because the common-mode current on the cable will change greatly).

The purpose of the conducted emission test is to evaluate and regulate the level of conducted electromagnetic disturbance emitted by electronic products through power lines, signal lines, control lines or ground lines according to unified standards (limits).

When the noise frequency of electronic equipment is less than 30MHz, it mainly interferes with the audio frequency band. For such low-frequency electromagnetic waves, the cable of the electronic equipment is not even a wavelength of a wave (the wavelength of 30MHz is 10m), and its radiation efficiency into the air is very low. In this way, if the noise voltage induced on the cable can be measured, the degree of electromagnetic noise interference in this frequency band can be measured. This type of noise is conducted noise, such as intermittent noise, the click of electric irons, washing machines, and rice cookers. The interference noise measured here is only for the continuous disturbance voltage with a frequency below 30MHz. As shown in Figure 1.1 below, it is the test configuration for conducting disturbance test at the input port, LISN is the linear impedance stabilization network, EUT is the equipment under test, LOAD is the load of the output port, and Figure 1.2 below shows the test configuration for conducting disturbance test at the output port.

Figure 1.1 Schematic diagram of conducted disturbance test at input port

Figure 1.2 Schematic diagram of conducted disturbance test at output port

Source: Electromagnetic Compatibility Home