Most assuredly, your new product must pass certain EMC emissions and immunity tests before being presented to the global marketplace. As a result, at some time during the design process you need to address the many elements that contribute to total EMC compliance. While we recognize it is only part of the solution, a shielded enclosure can play a major role in improving the performance of your product.
Before PCB layout begins you must apply the rules of good EMC design. Guidelines regarding power and signal distribution and ground planes are available in textbooks and technical publications. Also, manufacturers of magnetic, ferrite, and shielding materials can advise about on-board EMI suppression techniques.
I/Os and interconnections between PCBs must be low impedance and RF filtered. Use only fully shielded cables. Manufacturers of RF connectors and cables can provide some helpful advice. For multiple cards in a card cage, it also may be advantageous to provide additional shields between the cards.
The chasis is an important component of any product. Utilize good design practices so it is not a contributor of EMI problems. To pass conducted EMC emissions and immunity tests, use EMI filters on the main incoming power lines to the chassis.
Now EMC testing takes center stage. In doing your own preliminary EMC testing or working with a test lab, develop a sense of what the major sources of EMC problems are. Maybe an improved grounding scheme or the use of aluminum foil around a cable, connector, or box will localize the trouble.
Design a Shielded Enclosure
If wrapping foil around the box alters the EMC test results favorably, you may need a shielded enclosure. The ultimate shielded enclosure is a lot like the aluminum foil wrapping because it provides a continuous conductive surface enclosing a unit with no openings.
When designing an enclosure, keep openings few and small and use EMI gaskets where possible. If you can slide a business card into an opening, then the opening is too large. Bond connectors to the chassis. You are creating a sealed enclosure so calculate the cooling requirements to see if you need a fan or a more powerful fan than originally planned. And to achieve the best airflow and shielding characteristics, install honeycomb filters in vents and fan openings.
If there is a window or a display, consider deposited transparent thin-film conductive coatings. For seams, use EMI gaskets, welding, soldering, or conductive caulk or bolt them together. Where discontinuous chassis parts touch to form a seam, make them galvanically compatible to avoid corrosion. Eliminate paint or other insulating material between them. Use protective coatings that are conductive and prevent corrosion.
The construction of larger cabinets is more critical than for smaller enclosures. The seams are longer, and there is more area through which RF leakage or penetration can occur. These cabinets should be more rigid, more true in basic construction. Corners and joints should be seamed using overlap or butt methods.
The frame needs to be sturdy enough to mount multiple racks and support a great deal of weight. There must be provisions for cables, ventilation, casters, doors, and various options. You may need a viewing window or a special shelf for keyboards or monitors.
Test for Best Results
A shielded enclosure helps you pass both radiated EMC emissions and immunity tests. It attenuates RF signals originating outside the chassis, which improves the product’s immunity to radiated disturbances. RF signals that emanate within the chassis also are attenuated, which reduces radiated emissions to the outside world.
To determine the enclosure-shielding performance you need, consider the range of frequencies over which your product must be tested for radiated EMC emissions and immunity. A common question is, “What is the highest clock rate or frequency generated in the system?”. That’s because you may encounter emissions up to the tenth harmonic. A 300-MHz computer, for example, may need to be tested up to 3 GHz.
Subject your product, without the shielded enclosure, to the appropriate radiated EMC emissions and immunity testing. Get a plot of the actual emissions and immunity levels, in decibels, over the entire frequency range. From this, subtract the applicable specification limit curve to determine how much level reduction in decibels you need at each frequency to pass the test. That is basically the shielding performance specification of the enclosure, to which you should add some margin.
Measure the Shielding Effectiveness
The shielding effectiveness of an enclosure is not constant, but varies with frequency. Over the broad frequency range in typical EMC tests, most shielded enclosures with openings and seams can attenuate your measured levels by 40 to 60 dB. If you need greater attenuation than that to pass the tests, place more emphasis on the enclosure as the only line of defense. You also may want to retrace previous steps to realize other improvements.
Measure the shielding effectiveness of an enclosure by setting up transmit and receive antennas at fixed distances and establishing a reference level at the receiver. Then, place one antenna inside the enclosure and measure the reduced level at the receiver. This amounts to “inserting” a wall between the antennas and is similar to the insertion-loss measurement in transmission lines.
With the receiving antenna inside the enclosure, you’re measuring the attenuation of external signals or the improvement in immunity. That’s how the shielding effectiveness of enclosures is specified in MIL-STD-285 or NSA65-5. When the antennas are reversed, reciprocity does not hold.
A shielded enclosure will attenuate an external RF signal better because of losses caused by absorption and reflection. The main attenuation of an internal signal is by absorption only, as reflected energy tends to stay within the chassis and isn’t really lost.
To get a good overall picture, take both sets of measurements. As with transmission lines, reflections are caused by an impedance mismatch. The impedance of an RF signal in free space is 377 W , but the impedance of a conductive enclosure may be only 1 or 2 W .
A chain is only as strong as its weakest link. The correct selection and use of EMI gaskets and honeycomb filters prevent RF leakage through seams and openings and help maintain the high shielding effectiveness rating of an enclosure.
Most people are not experts in all areas of EMC. But you can get the job done by adhering to sound methodology and following a process of isolating and eliminating problems. Competent applications assistance is available, so take advantage of it whenever possible.
References
1. Violette, M., “The 10 Basic Steps to Successful EMC Design, Part 1: Steps 1 to 5,” EE-Evaluation Engineering, December 1997, pp. 68-74.
2. Violette, M., “The 10 Basic Steps to Successful EMC Design, Part 2: Steps 6 to 10,” EE-Evaluation Engineering, January 1998, pp. 70-80.
3. Brewer, R. and Fenical, G., “Shielding: The Hole Problem,” EE-Evaluation Engineering, July 1998, pp. S4-S10.
NOTE: These articles can be accessed on EE’s TestSite at www.evaluationengineering.com. Select EE Article Archives and use the key word search.
Copyright 1999 Nelson
February 1999