Conformity Magazine- May 2008 - (Page 55) material is. Looking again at the formula for skin depth, if the frequency of concern is fixed by the electronics to be shielded, the only way we can get more attenuation is by making the material a greater number of skin depths thick. This means either a thicker material, or a thinner skin depth. Making the material physically thicker may not be practical, so we are left with trying to change the skin depth. If frequency is fixed, the only variables left are conductivity and permeability. Conductivity can only be made so high—copper and silver are as good as it gets. Fortunately, low frequency permeability can be quite high in magnetic materials. There are a number materials that offer a high permeability, including steel, permalloy, and specialized compounds such as “mu-metal” (an alloy of 77 % Nickel, 15% iron, with additional copper and molybdenum). These can provide very effective low frequency magnetic shielding without being very thick. Note, however, that the permeability of these materials drops with frequency. At RF frequencies, the relative permeability of these materials drops to near one, so skin depth and absorption performance becomes governed primarily by the materials electrical conductivity. Entries And Exits The last piece in the puzzle concerns the treatment of leads that must enter and leave the shielded enclosure. Sadly, the performance of a well-formed enclosing shield can be completely compromised by inadequate attention to the leads that penetrate it—signal I/O, control, and power connections. It is easy for common mode energy to exit or enter the enclosure if these leads aren’t shielded or filtered. The idea of shielding equipment connections makes intuitive sense if we consider the shield to be a tubular extension of the enclosure. For shielded cabling to work, then, it must be connected to that enclosure. The key point is that it must be well connected to the right place with a connection that is lowimpedance at all frequencies of interest. This usually means a 360 degree conductive connection directly to the enclosing chassis/shield. Drain wires won’t do—too much inductance. Similarly, tying shields to signal ground, or routing them to the chassis via pins and PCB etch won’t work, either. This is the wrong place, and doesn’t form an extension of the enclosure. Cable construction can also be a factor—sometimes the high-impedance connection occurs underneath the cable molding which transitions from the cable shield to the connector. If you aren’t going to contain the energy on the wires inside of a shield, you have to get rid of it. This means low pass filtering (R, C, and occasionally L). RC filtering will bandlimit differential as well as common mode signals, which may impact the decision as to whether to filter or shield. If filtering is done, the filters must be referenced to the case shield at the point of exit. Remember, however, that it is usually common mode energy that we are trying to suppress. With proper attention to layout, magnetic (i.e., transformer) coupling and provide common mode containment. Summing Up Shielding works if you remember the basic principles. Create a continuous enclosure and keep an eye out for mechanical or electrical breaks in its continuity. When power or I/O signals leave the box, think about how you are going to treat the exiting leads. If you pay attention to the details involved, your shields will serve you well. FAST Link www.conformity.com/1770 Low-Profile Gaskets 125LP45, 125LP55 & 125LP60 "Smooth to the Touch" Low closing force Adhesive mounting Bi-directional Gap range - .030" - .130" Durable with 360° No-snag 100db attenuation Also available with Hook-on mounting for additional mechanical security Optional DiamondBack (dB) texture for high frequencies Beryllium Copper Immediate delivery MOUNTING SURFACE Low Profile LowProfile PRESSURE SENSITIVE ADHESIVE .03" MIN. INSERTION Free Samples at www.tech-etch.com/shield ISO 9001:2000 REGISTERED TECH-ETCH, INC., 45 Aldrin Road, Plymouth, MA 02360 mAy 2008 Conformity 55 http://www.conformity.com/1770 http://www.tech-etch.com/shield http://www.tech-etch.com/shield
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