Emc Shielding Enclosures

Jiaxing Lantu Chassis Co., Ltd. is located in Haiyan County, Jiaxing City, Zhejiang Province. It is a company engaged in the design, production and professional production of precision metal molds: aluminum shells, aluminum profile shells, communication chassis chassis, 19-inch chassis, non-standard chassis, hardware for The main well-known enterprises. It is widely used in electronics, instruments, meters, industrial control, communications, medicine, aerospace and other fields.

The term EMC shielding refers to a variety of methods used to protect electronic devices from electromagnetic interference (EMI). Electromagnetic interference is unwanted, undesirable noise in an electrical circuit that's linked to an external source. Any device that generates electromagnetic energy can potentially cause interference when it shares an environment with other electronic equipment. EMI can cause a range of performance issues in electronic devices, including loss of signal or inaccurate readings. In industrial and medical settings, electromagnetic interference can result in safety issues when electronic devices fail to function properly.

At high frequencies it is possible for the currents induced on the shield to radiate as well as (or possibly much better than) the original source of the fields. This is possible whenever the maximum dimension of the shield is a significant fraction of a wavelength or larger. At these frequencies, it is generally necessary to enclose the source completely while paying close attention to any apertures, seams or cable penetrations that could allow electromagnetic energy to escape.

An ideal shielded enclosure with infinite conductivity and no breaks would perfectly isolate (electromagnetically) whatever was inside the enclosure from whatever was outside. Even an enclosure with finite, but high, conductivity (e.g. copper, aluminum or steel), would provide essentially perfect isolation in most practical applications if there were no seams, apertures or cable penetrations.

Unfortunately, such an enclosure would not be very practical for electronic devices since it would be impossible to interact electrically with whatever was inside. So the best we can do is to start with a perfect enclosure and carefully evaluate every seam, every aperture and every cable penetration to ensure that no significant interfering signals are allowed to pass from one side to the other.

Apertures are holes in a shielded enclosure such as those required for ventilation, optical displays, plastic components, or mechanical supports. In order for the enclosure to provide shielding, currents must be able to flow on the surface unimpeded. Fortunately, apertures with maximum dimensions that are much smaller than a wavelength provide very little impedance to the flow of currents on a conducting surface. For this reason, if it is necessary to provide a certain amount of open area, it is much better to accomplish this with many small apertures than with a few large apertures.

Generally, the amount of energy escaping an enclosure through small apertures is insignificant compared to the energy escaping through seams, larger openings and wire penetrations. However if the enclosure is well sealed and it is necessary to further reduce the energy escaping through the apertures, then apertures with sufficient depth can be provided to further attenuate the radiated emissions. Making an aperture extend further into the enclosure creates a small waveguide. For apertures with a small cross-section, the frequencies of the sources within the enclosure are likely to be well below the cut-off frequency of the waveguide. Information on designing apertures to be waveguides below cut-off is provided in a later section.

Choosing the right shielded enclosure need not be a difficult procedure if you apply some of the methodical skills you learned in engineering school. For starters, deduce which enclosure material and design provide the appropriate shielding level. Verify attenuation levels required at the frequencies of interest. Since the enclosure is passive, the internal and external environments determine the levels of EMI/RFI emissions and the frequencies at which they need to be attenuated.

Evaluate the design. Understand the construction details and shielding provisions to determine what works. Appraise the limitations of existing enclosure designs and determine if improvements are possible with gaskets or heavier materials such as steel instead of aluminum. Consider the need for access and cable entry to minimize enclosure openings. Determine the impact of cost, weight, shielding deterioration, and environment on the performance and heat dissipation of the enclosure material.

The ideal shielded cabinet is a metal box with no seams or openings because any opening creates a potential EMI/RFI path. This, however, is not a realistic expectation because openings are needed for maintenance, ventilation of equipment, and electrical connections. In addition, no single enclosure works in all situations, and tests performed on different equipment using the same enclosure will not pass automatically.

Meeting the requirements of an EMI shielding standard depends on the operating frequency of the equipment, its susceptibility to EMI/RFI signals, the amount of shielding on the equipment, and its power source. The enclosure design you select will be dictated in part by the level of shielding needed. The shielding effectiveness (SE) of the enclosure is determined by its material, the construction of the frame and panel-joint design. You can verify the SE by performing tests to assess an enclosure's suitability.

SE is measured by establishing the reference intensity of an incident magnetic, electric, or plane-wave electromagnetic field. The test is done in a screen room by transmitting RF energy to a radio receiver and measuring the field intensity at the receiver antenna terminals. The transmitting antenna is placed in the equipment under test, and the field strength is measured. The SE is defined as the ratio of electric or magnetic field strengths before and after the placement of the shield around the equipment under test: