The objective of this project is to develop enclosure design methodologies, tools, and guidelines for quantifying shielding performance; and, to identify and quantify coupling paths (and mitigation approaches) at the PCB level that excite cavity modes, which can lead to EMI through perforations in the enclosure. When designing a system, you need to first consider the regulatory and RF environment where the system will operate. Recent work has focused on an investigation of the ways that heatpipes and heatspreaders drive shielded enclosures. Often, over design as well as under design is done because no thought was given to EMC at the beginning of the project.In response to the many EMC problems I have seen in products that are either failing while in the field or that fail their compliance testing while in their final stages of design, I created a three-day EMC by Your Design Seminar (Ref.
My goal was to help digital and analog design engineers become aware of EMC pitfalls, and more importantly, techniques they can incorporate to minimize EMC problems. These specifications will define the maximum emissions allowed from a system and the maximum interference the system must withstand while still functioning properly.
Therefore, a product’s enclosure must reduce both the emissions leaving the product and the interference trying to enter the product.
The amount of reduction of the radio waves passing through the surface is generally referred to as enclosure’s shielding effectiveness (SE).Shielding is one of many ways you can control a system’s emissions and minimize immunity issues. If you plan to use an existing enclosure, then you must test a prototype to confirm it will comply as part of the new design.
If you design a new enclosure, it must meet EMC emissions and immunity requirements in addition to meeting aesthetic, mechanical, and cost constraints.
Designing for both emissions and immunity up front will avoid the possibly of having to redesign the enclosure should the product later be sold in the European Union, which requires immunity testing. Determine shielding effectivenessWhen designing a new enclosure, you need to know the amount of attenuation (reduction of radio waves passing into or out of the enclosure) that is required.


Knowing the regulation and the emissions, you can again determine the amount of shielding needed.
You can estimate the amount of shielding needed by calculating the emissions from a design making a few assumptions. When designing products for the military or for commercial aviation, you need shielding effectiveness (SE) of 30 to 60 dB and perhaps more.Plane waves and electric fieldsPlane waves can be seen as either electric or magnetic since they are related by a constant, the impedance of free space, 377 ?.
Using the 2 ? and the following equation will give a reflected loss of about 33 dB of attenuation which is enough attenuation for most civilian applications, for both emissions and immunity.Figure 1. The good news is that absorption loss increases with frequency and the two complement each other such that across the entire frequency spectrum, any conductive material used as a shield and strong enough to stand by itself will provide enough shielding for almost all applications, including most military enclosures.
The two pennies prevent breaking through any surface film, which would prevent contact with another surface when joining two sections of an enclosure.
When checking for quality on this type of enclosure, be sure to place pennies on opposite sides of a rib, a corner, or a transition.
We ended up putting Mu metal on the wall and floor as well as adding a special enclosure for the monitor. The walls and floor reduced the field to about 50 mG and the special enclosure reduced it to about 6 MG. Mu Metal on a wall shields a monitor from a source of magnetic flux.Making the enclosureFigure 6 shows that even a practical box is far from ideal because it’s built from multiple pieces that may need continuous bonding. Besides these discontinuities, the box has yet to have penetrations for cables entering or leaving the enclosure, an air vent for cooling, and possibly viewing apertures. Figure 8 shows how gasketing comes in many different styles and some even require special consideration be made when constructing the enclosure.


Gaskets can reduce slot size, thus reducing the amount of energy that passes through the enclosure.Conductive tape is often used to cover a seam, especially while testing. Conductive window used for shielding against unwanted signals.Cables are another major source of energy leaving or entering an enclosure. In the bottom photo, by keeping the input and output of the filter separate from each other, the filter can function as it was designed.Figure 10.
For almost all applications, any solid shield thick enough to be made into an enclosure will supply adequate absorption loss, giving a total attenuation sufficient for most applications.ConclusionIn general, shielding effectiveness is determined by seams and joints, not the material. Until the last hole is treated, the best metal box might appear useless.When designing a metal housing “all” metal parts need to be well bonded. Often finger stock or gaskets are needed to bond the metal section of an enclosure together.
After the enclosure is designed, the shield integrity may need to be restored at cooling holes, viewing apertures and component holes. Remember, use two pennies and an Ohm meter to check its quality.Be sure to filter or treat all cables entering or leaving an enclosure. Seminar and Workshop, “EMC by your Design.”• Henry Ott, author of “Electromagnetic Compatibility Engineering,” Wiley 2009 - Used by permission. He has taught at the University of Wisconsin, Oakton Community College, and currently teaches EMC By Your Design Seminars.



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Comments

  1. 12.06.2014 at 21:11:45


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    Author: gerrard_046
  2. 12.06.2014 at 15:26:57


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    Author: Lifeless
  3. 12.06.2014 at 19:48:56


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    Author: ETISH