Shielding efficiency test guide
Today, embedded digital processing chips have almost used in all aspects: mobile phones, kitchen appliances, manufacturing equipment, magnetic resonance imaging (MRI) systems - even greeting cards. Adding the Internet of Things, the scope will explode. As digital technology is widely used, Wi-Fi, Bluetooth, cellular and other forms of communication (including higher frequencies) have been greatly expanded.
As a result, in a small, enabled wireless function is still in a luxury sedan, harmful interference can occur. They all need to perform interference testing to ensure that organizations meet regulatory requirements, regardless of them are engineering groups, design companies, equipment manufacturers, telecommunications companies, medical imaging suppliers or other organizations.
The interference test means using a shielded housing. These isolation devices also play an important role in data security and preventing interference against critical measurements and processing equipment.
Just like interference testing requires RF case, the isolation system needs its own test. This document reviews some of the problems and precautions when testing the exterior case. Particularly important is to assess the shielding effect and develop a housing compliance test plan.
Mask housing
The shield is a metal structure of Faraday cage or ground. They can prevent RF energy from entering the housing and leaks out. The type of shielded housing includes an MRI chamber for testing a shell for testing a laboratory (HEMP or Tempest application), a shielded casing or cabinet for wireless communication industries, and a large shielding housing for the high voltage industry. The size of the chassis is different, from the small box to the large region that can accommodate the aircraft.
Small chassis may have a full metal structure. The larger cabinet has more complex requirements. They are sitting above the floor, there is space between the top of the cabinet and the ceiling of the building. Due to the size of the outer casing, the solid metal is too expensive and cumbersome. Instead, they usually use metal network structures. As long as the apertures in the grid are smaller than the frequency wavelength, they can effectively block the signal. Even if some RF energy does penetrate the wall, it should also be severely attenuated, so that any residual amount is negligible.
The mask housing plays a role in two main respects. Larger is compliance testing. Regulators in the United States, the EU and other places have implemented the RF signals through IEEE-299 or EN 50147-1. There is no mandatory restriction, all forms of equipment may transmit any number of electromagnetic radiation and interfere with the normal operation of medical equipment, communication systems, etc.
Regulation compliance requires the measurement of the signal transmitted by the device and compares related criteria. However, the test process faces challenges. Most environments are full of radio frequency energy of radio stations, television, cellular, Wi-Fi, satellite transmission, solar activities, and many other sources. The shielded housing will isolate the test process with the external signal interference, so that the engineer can more accurately measure the device signal level and attenuation to compare with the regulations.
Other general fields that need to be shielded housings are electromagnetic isolation affecting the outside world. The reason may be safety and preventing information loss or isolation with possible unexpected interference.
Shielding performance
The isolation effect of the shielding housing is as good as the ability to prevent the RF field or outward extends beyond its barriers. Engineers, designers and scientists cannot regard them as a matter of course.
No housing is perfect. According to Oxford University 2014, the traditional mathematical model used in Faramis mesh cages has defects, and exaggerated the degree of cancellation in the cage internal RF.
The inevitable aspects of the shell structure will become the root cause of the corrugated field damage. The door enters the cabinet has a hinge and space around its edges, which can be turned on and off to generate weak points in shielding. The copper caps of the edges and hinges provide additional shielding. Over time, especially in a manufacturing plant that may suffer a large amount of vibration, the cover may loosen.
Shielding housing often occurs surface irregular phenomena, such as passing pipes or catheters or fire alarms. Cable interconnects allow cables inside the cage to external cables, may need to be modified to replace other connectors that destroy shielded integrity. Ventilation is another example and is usually problematic in terms of shielding effects.
Test shield housing
Even without changing the compliance criteria, they do need to test the housing regularly - based on standards, usually test every two or two years to prove that they provide the necessary degree of shielding effect. Ideally, it is sufficient to generate 100 dB attenuation on the entire chassis wall. In some manufacturers, industries, or applications, 60 dB to 80 dB can be accepted.
There are other reasons to test the effectiveness of the shield. Modifications to the chassis or its environment may indicate that you need to test. In addition to cable interconnects or earthquake events, there are two examples that may need to be cautious to retest the shielding effect. Electrical products are redesigned, such as increasing the frequency of wireless operations, requiring verification of masking to continue working properly for new design.
The core of the shell test is a test plan. Specific details are varied by the use of the emission standards. Typically, they involve the development of testing processes, which uses basic testing processes to verify the shielding effect of each point in the housing.
The basic program is a person who performs testing two antennas on one side of the housing surface. Typically, each antenna is located 30 cm from the edge of the outer casing, and the tip ends, the tip is aligned with the edge or edge to the edge, and therefore the maximum transmit or receive intensity points to the opposing antenna. One antenna is connected to the transmitter and the other antenna is connected to the receiver. A signal generator connected to the transmitter generates a known signal. The measurement of the signal strength of the receiver allows the tester to calculate attenuation.
Before the measurement signal is attenuated, the person must first calculate the dynamic range to ensure precise measurement. Personnel set the emission and receiving antennas and related equipment at 60 cm away from each other, but there is no thing between the two. Then they measure the maximum signal. Separate measurements only in the case of receiver work, the minimum signal is generated, which is the floor noise of the antenna and the receiver, with additional safety margins (usually 6 dB) to provide error margin. Use the power amplifier and its additional signal noise will be necessary to add another factor that is usually 6 dB.
The dynamic range is the difference between the maximum and minimum signals. The amount of dynamic range must be greater than the amount of signal attenuation required for the housing shield. Otherwise, the attenuation does not seem to be a masking effect of system noise.
The plan lists specific points of the cabinet that requires test - usually walls, floors, ceilings, corners, doors, protrusions, and cable separators - and personnel will test specific frequencies and related dynamic ranges for each point.
Test consideration
Antenna class
Antenna design is different due to its most response frequency. The wider the test frequency range, the wider the selection range of the required antenna. Regulatory standards typically specify an antenna to use. In some cases, there may be optional. For example, within 300 MHz to 1 GHz, the standard may suggest a dipole. Instead, the logs may be more efficient choices for several cycles, because additional gains have extended dynamic range. Any replacement requires the documentation in the test plan and the recalculation of the dynamic range.
equipment
The plan should indicate which devices are needed in addition to antenna, receiver and transmission equipment. This will include an amplifier and a preamplifier when the dynamic range of the test settings is insufficient. The specifications of the cable length are important. The cable will increase attenuation and reduce dynamic range. But the cable must be long enough to reach the top and bottom of the cabinet.
The tripod of the antenna must be non-reflected. Other equipment may include rolling carts, extension lines, and various conventional tools in the room.
Typical errors and omissions
Any chassis test may fail due to the mistakes or errors in the implementation. There are three typical.
Dynamic range
Testing is often taken in the case where there is not enough dynamic range. The result is a measured measurement that cannot be trusted. The solution is to determine the dynamic range, then add an enlargement or different antenna design to increase the gain.
Antenna physics alignment
Antenna is not aligned to cause signal strength to be weak, thereby producing a larger attenuation of the outer casing. Make people away from the antenna and tripod, minimize someone accidentally touching one of the units and moves the possibility of alignment. Another good entertainment principle is to modify the old carpenter's motto, measure twice and cut once. In this case, three measurements should be performed: Please check the distance measurement before measurement attenuation.
Frequency alignment
Make sure all frequencies match correctly.
Maintenance of RF housing requires regular testing to ensure its effectiveness. Develop a reliable test plan and pay attention to the details, it will be able to effectively and accurate RF testing for the equipment.
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