Technical column



GTEM chamber is used to measure radiation immunity and radiation disturbance emission of electromagnetic compatibility test products

  The anechoic chamber established as an alternative to the outdoor open field has been widely used because of its perfect performance, but its development to small and medium-sized enterprises is hindered due to the high cost and necessary equipment. The GETM cell introduced here, also known as the Gigahertz (GHz) transverse electromagnetic wave cell, is a new type of electromagnetic compatibility testing equipment developed in the past ten years. The most valuable thing is that the total price of the small chamber itself and its supporting equipment is not too expensive, which can be accepted by most enterprises. Therefore, the GTEM chamber has made great progress in China and has become the first choice for enterprises to carry out radio frequency radiation electromagnetic field immunity tests for equipment that is not too large in size.

  1 Introduction to GTEM Cell

  The GTEM cell is a device designed based on the principle of coaxial and asymmetrical rectangular transmission lines. In order to avoid the reflection and resonance of the internal electromagnetic wave, the GTEM cell is designed as a pointed cone in shape, and its input end adopts an N-type coaxial connector, and then the central conductor is flattened into a fan-shaped plate, called a core plate. A rectangular uniform field area is formed between the core plate and the bottom plate of the small chamber. In order to make the spherical wave (strictly speaking, the spherical wave propagating from the N-type joint to the GTEM cell, but because the designed opening angle is small, the spherical wave is similar to a plane wave) have a good transmission from the input end to the load end Features, the terminal of the core board becomes a non-reflection terminal due to the use of a distributed resistance matching network. The end face of the GTEM cell is also pasted with a wave-absorbing material, which is used to further absorb high-end frequency electromagnetic waves. A test area with uniform field strength is thus created between the core and the bottom plate of the chamber. During the test, the sample is placed in the test area. In order not to affect the uniformity of the field too much due to the placement of the sample, the height of the sample should not exceed 1/3 of the distance between the core plate and the bottom plate.

  2 working principle

  The electric field strength in the GTEM cell is proportional to the input signal voltage V from the N-type connector, and inversely proportional to the vertical distance h between the core plate and the bottom plate: E = V/h In a 50Ω matching system, the voltage between the core plate and the bottom plate is the same as that of the N-type The relationship between the signal input power of the connector satisfies V = (RP) 1/2 = (50P) 1/2, so the field strength E = (50P) 1/2 / h If the difference between the measured value and the theoretical value is considered, The above formula should also be multiplied by a coefficient k, so the actual electric field strength is

  E = k(50P)1/2 / h

  It can be seen from the above formula that if the same power is injected into the GTEM cell, the closer the position of the core plate to the bottom plate (the smaller the h value), the greater the field strength can be obtained; if the field strength in the same phase is generated, the larger the space The input work required at the position (the larger the value of h) is also larger.

  The above conclusions show that for smaller test objects, we can place the test object at a relatively front position in the GTEM cell, so that a sufficiently high electric field intensity can be obtained with a relatively small signal input power. Note that the height of the test object cannot exceed 1/3 of the distance between the core plate and the bottom plate at the selected position.

  3 Application of GTEM cells in EMC testing

  3.1 Electromagnetic field immunity test of radio frequency radiation Advantages of using GTEM cell for electromagnetic field immunity test of radio frequency radiation:

  The electric field intensity generated by GTEM is much greater than that generated by the antenna, so a relatively small RF power amplifier can generate a strong electric field, which greatly reduces the price of the entire test system. This is a very good radio frequency radiation electromagnetic field immunity test solution for equipment that is not too large in size.

  The GTEM cell does not need an antenna for the radio frequency radiation electromagnetic field immunity test, so it can be conveniently used for automatic testing, greatly reducing the testing time and lowering the technical requirements for the testing personnel.

  When the signal source is amplified and injected into one end of the GTEM cell (through the N-type coaxial connector), a strong uniform electromagnetic field can be formed between the core board and the bottom board, and the electric field monitoring probe placed near the DUT monitors this field Strong, and then get the input power value through the computer, directly adjust the signal source in order to achieve the required field strength value. The measurement and control software controls the signal source to scan the frequency of the radiation field with a certain step size. There is also a video monitor (the camera is installed in the GTEM cell, not shown in Figure 4, and the test personnel pass the monitor outside the GTEM cell) to observe the working conditions of the test product under the interference of the radio frequency electromagnetic field.

  Operation method:

       ①Put the sample and field probe in the GTEM chamber;

  ② Connect an external signal source, and establish a uniform electric field in the GTEM cell through a power amplifier;

  ③ Determine the test frequency range, modulation method and modulation depth;

  ④ Adjust the output level of the signal source (note, do not exceed the maximum input level allowed by the power amplifier);

  ⑤ Monitor the field strength of the GTEM cell through the field strength monitor to make it reach the required strength;

  ⑥Repeat steps ③~⑤ to observe and determine the electromagnetic radiation sensitivity of the tested product.

  3.2 Electromagnetic disturbance radiation emission test system of the test object

  In theory, the electromagnetic disturbance radiation emission value test of the test product can also be carried out in the GTEM chamber. At this time, the core board and the bottom plate in the small chamber replace the antenna in the darkroom test to receive the radiation disturbance generated during the work of the test product. The N-type connector of the GTEM cell is connected to the interference receiver, and the radiation emission of the electromagnetic disturbance during the working process of the test product can be tested through the interference receiver. Then use the computer and processing software to determine the test results of the radiation emission of the sample. Note that there is a problem of comparing the test results in the GTEM cell with the test results of the open field or anechoic chamber, find out the rules (build a mathematical model), and make necessary corrections, and this is exactly what the GTEM cell test software wants solved problem. In addition, the position of the test product in the GTEM cell is different, resulting in a difference in the relative distance between the core plate and the bottom plate, which will also be a key factor leading to different test results, and the test personnel must pay full attention.

  The operation method of the electromagnetic disturbance radiation emission test of the sample in the GTEM cell is as follows:

  ①Place the tested product in the GTEM chamber;

  ②External interference receiver, receiving the radiation disturbance level output of the test product;

  ③ Set the scanning frequency range, detection mode and resolution bandwidth according to the test standard requirements;

  ④ The interference receiver tests the radiation disturbance level value of the tested product;

  ⑤Through computer and software for data processing, get the final test results.

  Note that whether the GTEM cell is used for radio frequency radiation electromagnetic field immunity test or for emission test of radiated electromagnetic disturbance generated in the work of the sample itself, there is a polarization problem (testing in an open field and an anechoic chamber, This is achieved by changing the direction in which the antenna is placed). In the GTEM cell, the core board and the bottom board play the role of the antenna, and their positions cannot be changed. Therefore, if you want to change the polarization direction of the electric field, you can only artificially change the placement direction of the test object relative to the core board and the bottom board. come true.

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