Some knowledge about electromagnetic compatibility laboratory in engineering design

What is electromagnetic compatibility? What is its role? Today we will talk about the electromagnetic compatibility laboratory! Let’s first look at the definition of electromagnetic compatibility laboratory - the electromagnetic compatibility laboratory is for conducting electromagnetic compatibility tests. A special type of laboratory was established. First, let’s explain the basic concept of electromagnetic compatibility: it refers to the ability of electromagnetic systems and subsystems to work normally in their respective electromagnetic environments without reducing their working performance due to electromagnetic interference.

For a given electromagnetic system or subsystem, electromagnetic compatibility has two meanings:

(1) This system or subsystem will not be interfered with by other systems or subsystems and affect its working performance;

(2) This system or subsystem will not cause the above interference to other systems or subsystems. Electromagnetic compatibility work can be divided into two aspects. On the one hand, there are radio management measures, such as using frequency allocation and time division to prevent interference between systems; on the other hand, there are various technical measures, that is, using shielding, filtering and Reasonable wiring, grounding and other measures can suppress interference. In order to standardize the electromagnetic compatibility of electronic or other products, relevant electromagnetic compatibility tests need to be conducted according to relevant international standards or national standards, and the place where electromagnetic compatibility tests are conducted is the electromagnetic compatibility laboratory. A complete system of electromagnetic compatibility laboratories generally includes two types: anechoic chambers and shielded rooms.

A complete system of electromagnetic compatibility laboratories generally includes two types: anechoic chambers and shielded rooms.

A shielded room is just a large iron box that blocks indoor and outdoor radio signals. In layman's terms, it means that the inside can't get out and the outside can't get in, but the electromagnetic waves inside will be reflected and superimposed on the inner wall.

The anechoic room is based on a shielded room, with absorbing materials laid on the inner wall to simulate the effect of an open field. The anechoic room is much more expensive than a shielded room because of the materials posted in the anechoic room. The electromagnetic waves inside will be absorbed when emitted to the inner wall, and there will be basically no mixed wave effect of reflection and superposition. Suitable for testing samples for radiated emission interference. Darkrooms are generally divided into full anechoic chambers and semi-anechoic chambers.

Tests that do not have high requirements on the experimental environment, such as conduction harassment, electrostatic testing, surge testing, lightning strike testing, etc., are all conducted through power lines, so they only need to be conducted in a shielded room; while for space radiation and space harassment, Space-borne harassment or anti-interference has special requirements for space, so it needs to be carried out in a dark room to simulate the space of an open field.

Radiation tests conducted in these three test sites, including full anechoic chamber, semi-anechoic chamber and open field, can generally be considered to be consistent with the propagation laws of electromagnetic waves in free space.

1) Fully anechoic chamber

The fully electromagnetic anechoic chamber reduces the interference of external electromagnetic wave signals on test signals. At the same time, electromagnetic wave absorbing materials can reduce the impact of multipath effects on test results due to reflections from walls and ceilings. It is suitable for emission, sensitivity and immunity experiments. In actual use, if the shielding efficiency of the shielding body can reach 80dB~140dB, then the interference from the external environment can be ignored, and the free space situation can be simulated in a fully anechoic chamber. Compared with the other two test sites, the fully anechoic chamber has the smallest reflections from the floor, ceiling and walls, is least affected by the external environment, and is not affected by the external weather. Its disadvantage is that it is subject to cost constraints and limited testing space.

2) Semi-anechoic chamber

The semi-anechoic chamber is similar to the full anechoic chamber. It is also a six-sided box with a shielded design, and the interior is covered with electromagnetic wave absorbing materials. The difference is that the semi-anechoic chamber uses a conductive floor and is not covered with absorbing materials. A semi-anechoic chamber simulates an ideal open field situation, where the field has an infinitely large conductive ground plane. In a semi-anechoic chamber, since the ground is not covered with absorbing material, a reflection path will be generated, so the signal received by the receiving antenna will be the sum of the direct path and reflection path signals.

3) Open field

An open field is an elliptical or circular test site that is flat, open, with uniform conductivity and no reflectors. An ideal open field has good conductivity and an infinite area. The receiving antenna can receive signals between 30MHz and 1000MHz. The signal will be the sum of the direct path and reflected path signals. However, in practical applications, although good ground conductivity can be obtained, the area of ​​the open field is limited, which may cause a phase difference between the transmitting antenna and the receiving antenna. In emission testing, the open field is used in the same way as the semi-anechoic chamber.

Each test item of electromagnetic compatibility requires a specific test site, among which radiated emission and radiated immunity tests have the most stringent requirements for the site. Because the emission and reception of high-frequency electromagnetic fields of 80 to 1000MHz are completely based on the theory of the superposition of space direct waves and ground reflected waves at the receiving point. If the venue is not ideal, it will inevitably lead to large test errors.

The open test site is an important electromagnetic compatibility test site. However, because the open test site is expensive and far away from the urban area, it is inconvenient to use; or if it is built in the urban area, the background noise level is large and affects the EMC test, so indoor shielded rooms are often used instead. However, the shielded room is a metal closed body with a large number of resonant frequencies. Once the radiation frequency and excitation method of the equipment under test cause the shielded room to resonate, the measurement error can reach 20 to 30dB. Therefore, it is necessary to measure the surrounding walls and top of the shielded room. Installing wave-absorbing materials on the ceiling greatly weakens the reflection, that is, when radio waves propagate, there are only direct waves and ground reflected waves, and its structural dimensions are also based on the requirements of an open test field, so that it can simulate testing in an outdoor open field. This is electromagnetic shielding. Absorbing anechoic chamber, also referred to as EMC anechoic chamber, has become a commonly used EMC testing site. The US FCC, ANCI C63.6-1992, IEC, CISPR and the national military standards GJB152A-97, GJB2926-97 "Accreditation Requirements for Electromagnetic Compatibility Testing Laboratories" and other standards allow the use of electromagnetic shielding semi-anechoic chambers to replace open test sites for EMC testing. .

The EMC darkroom structure usually consists of an RF shielded room, absorbing materials, power supply, antenna, turntable and other parts: the RF shielded room ensures that the test is free from external interference; the absorbing materials ensure the absorption characteristics of the darkroom; the antenna and turntable ensure that the measured object Test according to the status and conditions required by the standard; the power supply system ensures the power used for the test. Auxiliary equipment such as RF shielded doors, ventilation waveguide windows, cameras, lights, power boxes, etc. should be designed to be placed outside the main reflection area as much as possible to avoid any metal parts being exposed in the main reflection area.

The floor of the darkroom is the only reflective surface for electromagnetic waves. The requirements for the floor are: continuous smoothness without unevenness. There should be no gaps exceeding 1/10 of the minimum operating wavelength to maintain the conductive continuity of the floor. The ground wire and power wire in the darkroom should be laid against the wall base and should not cross the room. The wires should also pass through metal pipes and keep the metal pipes well connected with the floor. In order to avoid the impact of radio wave reflection on measurement errors, people and test control equipment should not be within the test site.

Therefore, generally EMC anechoic rooms are composed of a test chamber and a control room. The test antenna and the equipment under test are placed in the test chamber, and the operators and test control instruments are in the control room. If there are high-power amplifier equipment, an amplifier room should be established to place these equipment to avoid interference with the surrounding environment. The darkroom and the control room must adopt independent power supply systems, use power supplies of different phases, and pass through their own filters to prevent interference from the control room from entering the darkroom through the power cord. The above is some knowledge about electromagnetic compatibility, I hope it can help you.