Principle solution for EMC testing using reverberation chamber
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Principle solution for EMC testing using reverberation chamber

Posted Date: 2024-01-20

The traditional semi-anechoic chamber (SAC) or full anechoic chamber (FAC) simulates open field or free space propagation environments respectively. They are recognized by domestic and foreign standards and are widely used in EMC testing. Reverberation Chamber (RVC, also called mixing chamber in the standard) technology adopts a different testing idea from the traditional anechoic chamber. In recent years, the principles and methods of using reverberation chambers to replace traditional anechoic chambers for EMC testing have received a lot of research and considerable development. As the testing requirements for the equipment under test, especially automotive products, become increasingly stringent, reverberation chamber technology is gradually attracting attention.

How the reverberation chamber works

According to the definition in IEC61000-4-21 (GB/T 17626.2.1), a reverberation chamber is a specially designed chamber with a long mixing time to allow the field to spread as much as possible. The reverberation chamber consists of a shielded shell, a mechanical tuner or stirrer, an antenna, a test system and control software.

Figure 1. Typical reverberation chamber configuration (GB/T 17626.21-2014)

Figure 2. Internal components of an actual reverberation chamber

Traditional anechoic chambers need to be laid with absorbing materials to absorb electromagnetic waves as much as possible and avoid multiple reflections and multipath effects to improve field uniformity. In contrast, the reverberation chamber is a metal shielded cavity that has a strong reflection effect on electromagnetic waves and continuously changes the boundary conditions of the electromagnetic field in the space through the rotation of the tuner/stirr, thereby changing the boundary conditions of the electromagnetic field in the space. Generate as many randomly polarized electromagnetic wave signals as possible inside the cavity to produce a continuously changing electromagnetic field distribution, forming a spatially statistically uniform, isotropic, and randomly polarized electromagnetic environment throughout the statistical period. To simulate the actual electromagnetic wave propagation environment.

Figure 3. Schematic diagram of electromagnetic field distribution inside the reverberation chamber.

Compared with traditional anechoic chambers, reverberation chambers have the following characteristics:

►Different workspaces

The traditional anechoic chamber assumes a working plane with uniform field strength in the same polarization direction; the reverberation chamber assumes a three-dimensional working space, in which electromagnetic waves are arbitrarily polarized, and a uniform field is obtained through statistics. Field strength; in comparison, within the same cavity, the reverberation chamber has a larger testing space.

Figure 4. Reverberation room three-dimensional working space vs. traditional darkroom working plane

structureDifferent, different costs

The traditional anechoic chamber is composed of a shielded room, ferrite, absorbing materials, etc.; the reverberation room is composed of a shielded room and a stirrer, which does not require the laying of ferrite and absorbing materials, and the construction cost is low.

► Produce different field strengths

Traditional radio anechoic chambers absorb a large amount of input electromagnetic waves through absorbing materials to avoid radio wave reflection; the reverberation chamber reflects electromagnetic waves multiple times and uses radio waves with high efficiency. Therefore, with relatively small input excitation, the reverberation chamber can produce higher test field strengths.

Working frequency is different

The lowest operating frequency (LUF) of the reverberation chamber is limited by the size of the reverberation chamber, while high frequencies are basically unlimited.

Test times vary

Reverberation chamber testing requires longer testing time than traditional darkroom testing.

Main reference standards for reverberation chamber testing

Key parameters of reverberation chamber

Design parameters: reverberation chamber size; LUF; number of modes; mode density; Q value

Parameters used: working interval; LUF; field uniformity; G value; time constant; ACF/CCF/CLF

Reverberation chamber testing procedures

Reverberation chamber testing simply needs to follow the following steps.

Step One: Field Uniformity Calibration of the Cavity

Verify that the field uniformity of the reverberation chamber meets standard requirements; determine the normalized electric field strength; determine the working interval. This step is performed once the reverberation chamber is built or undergoes major renovation.

Step 2: Confirmation of the maximum loading coefficient of the cavity

Based on the first step, absorbing material is added to the cavity to simulate loading to verify the field uniformity. This step is performed once the reverberation chamber is built or undergoes major renovation.

Step 3: DUT loading preflight

"Quick check" of the cavity performance to confirm that the loading of the cavity is less than the maximum loading when loading is simulated. This step is performed once when testing different DUTs.

Step 4: DUT test

Use the forward power recorded during calibration to perform DUT testing.

Reverberation room test system

A complete reverberation room test system mainly consists of the following parts:

1. Reverberation room system:

Including shell, screen door, stirring paddle, antenna, field strength probe, etc.

2. Similar to the traditional anechoic chamber test system, the test equipment of the reverberation chamber test system mainly includes the following parts:

Signal source, used to generate the required interference signal waveform

Power amplifier, used to amplify the signal generated by the signal source to the desired power

Power meter for monitoring the forward and reverse power of a power amplifier to determine the effective power input into the reverberation chamber

Transmitting antenna for loading interfering signals into the reverberation chamber

Receiving antenna for monitoring field strength during testing

Field strength probe, used to confirm field strength within the working area

3. Test software, used to control the reverberation chamber to complete site verification and automated testing.

Figure 5. Schematic diagram of the structure of the reverberation chamber test system.

Rohde & Schwarz (hereinafter referred to as R&S) TS9982 EMS test system can fully meet the testing needs of reverberation chambers. The system includes various test instruments and test accessories necessary for reverberation chamber testing. It is customized and designed according to international/domestic standards or the customer's own corporate standards. It is controlled by R&S Elektra test software to achieve repeatable and fully automatic testing. Test process.

Reverberation room test automation software

R&S Elektra EMC automated test software fully supports reverberation chamber testing, provides standard or customer-customized test processes, and outputs test reports. Elektra has distinctive features and product advantages:

The software is independently developed by R&S, supports all R&S instruments and equipment required for testing, and mainstream test accessories in the market, and can be expanded as needed;

It supports both reverberation chamber calibration and complete testing process, organically combining calibration and testing into a whole;

All parameters required in the test standard;

Calibrate multiple times and compare test data;

It only needs to update the driver to adapt to different mixing paddles, with strong compatibility;

Continuous evolution, easy to implement new testing requirements through upgrades;

Platform-based software, simple and easy to use.

R&S has been deeply involved in the Chinese market for more than 30 years, delivering more than 100 EMC test systems to Chinese customers every year, and has accumulated rich experience in system integration, delivery and after-sales service. For many years, R&S has always been at the forefront of EMC testing. Today, R&S is working closely with its partners to launch an advanced overall reverberation chamber testing solution in a timely manner. It is committed to providing customers with high-quality reverberation chamber system integration solutions and turnkey services to meet customers' growing new needs for testing and certification. .

Review Editor: Huang Fei


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