Electromagnetic compatibility test EMC (Electro Magnetic Compatibility), means that the device or system meets the ability to run in its electromagnetic environment and does not have an unbearable electromagnetic interference in any device in its environment. EMC design and EMC test are complementary. The quality of EMC design is to measure by EMC test. Only in the full process of EMC design and development of the product, EMC compatibility predictions and assessments can be found early to discover electromagnetic interference, and take the necessary inhibitory and protective measures to ensure electromagnetic compatibility of the system. Otherwise, when the product is constitutive or the system is completed, it is not compatible with incompatibility, it is necessary to modify the design or use remedies in the cost of human, material. However, it is often difficult to completely solve problems, and bring a lot of trouble to the use of the system.
Electromagnetic compatible test venues include:
1. Open-wide test site
Theoretical Basis: The transmission and acceptance of the 30-1000 MHz high frequency electromagnetic field is completely equal to each other with spatial direct wave and ground reflection waves.
Standard: ANSI C63.7, CISPR 16
Requirements: flat, empty, uniform electrical conductivity, no reflective elliptical or circular test site
Normalized Site Attenuation NSA: Normalized Site Attenuation NSA: Specifies the technical indicators of test field quality
2. Shielded room
Definition: Specially designed closed chambers that can attenuate RF electromagnetic energy.
Role: For EMC measurements, the shield chamber provides a test environment that meets the requirements.
Shielding performance definition of the shield: The ratio of the electric field strength EO (or magnetic field strength HO) at a point when the shield is shielded and the shielded space is the ratio of the electric field strength E1 (or magnetic field strength H1) of the point when the shield is shielded.
S = EO / E1 = HO / H1
SE = 20LG (EO / E1) (metric using logarithmic unit decibel DB)
SH = 20Lg (HO / H1) (metric using logarithm decibel DB)
3. Electric wave dark room
Anechoic Chamber, also known as the electric wave cancer or the radio wave without reflecting chamber.
The structure is divided into:
1 Electromagnetic shield half-wave dark room Electromagnetic shielded semi-anechoic chamber), simulated open test site
2 microwave electric waves dark room (full electric wave dark room) Microwave Anechoic Chamber
The shield is just a big iron box, which blocks the radio signal in the indoor and outdoor, and the popular saying is that the inside is not going, but the electromagnetic wave inside will be reflected in the inner wall.
On the basis of the shield room, the radio wave is laid in the inner wall, and the effect of an open field is simulated. The dark room is much more expensive than the shield room is expensive. The electromagnetic wave inside is transmitted to the inner wall and is absorbed, and it is basically not a mixed wave effect of reflection overlapping. Suitable for test samples of radiation emission interference. The dark room is generally divided into a full-electric wave dark room and a semi-electric wave dark room.
Tests for experimental environment requirements, such as conducted harassment, electrostatic testing, surge test, lightning strike testing, etc., it is only necessary to do it in the shielded room; for spatial radiation, space harassment Harassment of space propagation or anti-interference has special requirements for space, so it is necessary to perform space in the dark room.
All-electric wave dark room, half-electric wave darkroom, open field, radiation tests in these three test sites can generally be considered to comply with the propagation law of electromagnetic waves in free space.
All-electric wave dark room
The full-electric wave darkroom reduces the interference of the external electromagnetic wave signal on the test signal, and the electromagnetic wave absorbing material can reduce the multipath effect due to the reflection of the wall and the ceiling on the test results, suitable for emission, sensitivity and immunity experiment. In actual use, if the shielding effect of the shield can reach 80 dB to 140dB, the interference of the external environment can be ignored, and the free space can be simulated in the full-electric wave darkroom. Compared with other two test venues, the ground, ceiling and wall reflections are minimal, minimal interference by the external environment, and is not affected by outside weather. Its disadvantage is that the cost restriction is limited.
Semi-electric wave dark room
The semi-electric wave dark chamber is similar to the full-electric wave dark chamber, and is also a six-sided cassette that is masked, and the electromagnetic wave absorbing material is covered inside, and there is a conductive floor, which does not cover the absorbing material. The half-electric wave dark room simulates the ideal open field situation, that is, the site has an infinitely highly good conductive ground plane. In the semi-voltage dark chamber, since the ground is not covered by the absorbing material, the reflection path will be generated so that the signal received by the antenna will be the sum of the direct path and the reflection path signal.
Open a wide range
Open field is flat, empty, very well electrical conductivity, no elliptical or circular test site without any reflectors, the ideal open space has good conductivity, an area unlimited, and receives an antenna between 30MHz to 1000 MHz. The signal will be the sum of the direct path and the reflection path signal. However, in practical applications, although a good ground conductivity can be obtained, the area of the open field is limited, and therefore the phase difference between the transmit antenna and the receiving antenna may result. In the launch test, the use of the open field and the same half-wave dark room.
Each test project of electromagnetic compatibility requires a specific test site, in which radiation emission and radiation immunity testing are most stringent. Since the emission and acceptance of the 80 to 1000 MHz high frequency electromagnetic field is completely based on the theory of spatial direct wave and the ground reflection wave at the reception point. The venue is not ideal, it will inevitably bring a large test error.
Open the test site is an important electromagnetic compatible test site. However, due to the high cost of the warp test site and away from the urban area, the use is inconvenient; or built in the urban area, the background noise level affects the EMC test, the indoor shielding room is often replaced. However, the shielding chamber is a metal blocking body, there is a large number of resonant frequencies, once the radiation frequency and the excitation method of the device causes the shielding chamber to generate resonance, the measurement error can reach 20 to 30 dB, so it is required to be on the shielding chamber. Wall and top The absorbing material is installed, so that the reflection is greatly weakened, that is, only direct wave and ground reflection waves are propagated, and its structural dimensions are also based on the requirements of the open test site, so that the test of the outdoor open field, this is electromagnetic shield The absorbing dark room, referred to as an EMC dark room, which became a more common EMC test site. US FCC, ANCI C63.6-992, IEC, CISPR and national military standard GJB152A-97, GJB2926-97 "Electromagnetic Compatibility Test Lab Recognition Requirements" to allow the use of electromagnetic shield semi-electric waves dark room to replace open test field for EMC test .
The EMC dark room structure is usually composed of RF shielding chamber, absorbing material, power supply, antenna, turntable, etc.: The test is guaranteed by the RF shielding chamber is not subject to external interference; the absorbing material guarantees the absorption characteristics of the dark room; the antenna, the turntable guarantees the measured object Test according to the status and conditions required by standards; power system guarantee the test electrical. RF shielding door, ventilation wave window, camera, lighting, power box, etc., should be as designed outside the main reflection area, avoiding any metal components to the main reflection zone.
The floor of the dark chamber is the unique reflection surface of electromagnetic waves. The requirements for the floor are: continuous and negative convergence. There is no gap that exceeds 1/10 of the minimum working wavelength to maintain the conductive continuity of the floor. The dark room is grounded and the power cord is set to the wall, do not cross the room, the wire should also be wear metal tube, and keep the metal pipe and the floor well. In order to avoid radio wave reflection affecting measurement errors, people and test control equipment should not be in the test site. Therefore, the general EMC dark room is composed of test darkroom and control room, and the inside of the dark room will be tested antenna and the test equipment, the operator and test control instrument are in the control room. If there is a high power amplifier device, it is also necessary to establish a power amplifier room to prevent interference to the surrounding environment. The darkroom and the control room should each use a separate power supply system to use different phases of power, through their respective filters to avoid the interference of the control room to enter the dark chamber through the power cord.
Editor: LQ, Read full article, original title: What is the difference between the shielding room and the radio space in electromagnetic compatibility test?
Article Source: [Micro Signal: FCSDE-SH, WeChat Public Number: Semiconductor PDSA] Welcome to add attention! Please indicate the source of the article.
Our other product:
