The dense packaging and high-frequency operation so common today in electronic products are a blessing and a curse to equipment manufacturers. Though conserving space and running at several hundred megahertz, these products are susceptible to damaging noise—and cause it as well. Fortunately, shielded enclosures offer protection from the storm of interfering electrical and magnetic waves.
To make an informed choice, know your packaging requirements and the level of shielding effectiveness (SE) you need, said Rob Baxter of Schroff. Examine the shielding techniques used by the manufacturer, especially around doors and entry panels.
To attain the appropriate amount of RF shielding, all external surfaces of the enclosure must be connected conductively, with as few slits as possible, said Mr. Baxter. Know the shielding requirements for electrical, magnetic and electromagnetic fields. The shielding effects of an enclosure should be considered separately for electrical and magnetic fields. Table 1 provides information on SE for various frequency ranges.1
Sometimes, trade-offs are made to attain the stated shielding effectiveness, cautioned Mr. Baxter. For example, improved shielding can be accomplished by reducing the number of access points or cooling and cable I/O openings.
Specifying Shielding
After you decide what frequencies to attenuate, you are ready to specify how much shielding your product needs. Remember, shielding specifications are system and not cabinet requirements, said Ken Gazarek, Vice President of Engineering at Equipto Electronics. If your product was tested without a cabinet, pass on that information to the cabinet manufacturer and also indicate how much shielding you require at different frequencies.
The more you understand your shielding requirements, the more easily the shielding manufacturer can help you meet your attenuation needs. “Problems occur when the system integrator does not know the EMC requirements or when third-party equipment is added to the shielding solution,” said George Correira, Electronics Product Manager at Rittal. “A checklist of factors to be considered (see Table 2) can help you arrive at a suitable EMI/RFI protection solution.”
Today’s RF shielding systems can provide extremely elevated levels of shielding effectiveness at frequencies as high as 100 GHz, said Michael Lahita, President of Shielding Resources Group. To attain the best shielding level, pay close attention to the installation of the enclosure and to components such as doors, electrical filters and honeycomb-vent materials. For high-frequency requirements, observe the RF panel-joint construction. Weaknesses in any of these parts can reduce shielding to the lowest level.
If you use a modular or cell-type system, the cleanliness of the mating surfaces, the screw or bolt spacing and the galvanic compatibility are critical, said Mr. Lahita. To provide proper cooling, add honeycomb vent panels that match the performance of the RF shielding.
Most cabinets are cooled by filtered intake blowers mounted in the bottom front or rear of the rack. Exhaust air is sent out the top or top rear of the rack. Blowers pressurize the rack, keeping dust from entering the cabinet and harming your electronics. Some EMI cabinets use exhaust fans on the top panel and have an intake filtered grill at the bottom front or rear.
Magnetic Shield Adds Weight
You should realize that shielding for the magnetic frequencies adds weight and volume to an equipment design, said Jack White, Chief Engineer at Magnetic Shield. The most cost-effective magnetic shield implementation is done during the product design stage.
The best route to cost-effective magnetic shielding sets a course that will supply the enclosure manufacturer with the type of field (AC or DC), its strength, the desired attenuation and a description of any physical constraints, said Mr. White. Inadequate information causes a shield design to be over-designed and expensive or under-designed and unable to perform as expected.
Is It Cost Effective?
Most shielding manufacturers can design enclosures to meet your frequency and temperature needs at a price to fit your budget. Expect EMI shielded cabinets to range in price from about $1,500 to $15,000. The cost depends on the system specification you want to meet, such as FCC, VDE, MIL-STD-461 or Tempest, and the amount of shielding your product needs at various frequencies, said Mr. Gazarek.
When you shop for prices, you may find they vary as much as 30%, said Mr. Lahita. Make sure you are comparing apples to apples before making a decision. Freight charges often add a significant amount to the bill, so ask if they are included in the final price.
References
1. Rittal Corp., Interference-Free, Rittal RF Shielded Housings and Enclosures, August 1995.
Table 1
Magnetic Field
Electrical Field
The magnetic component is attenuated in relation to the material and its thickness. As the frequency increases, the SE increases.
The electrical component is effectively shielded and the enclosure acts as a Faraday cage.
1 MHz to 100 MHz
Shielding against the magnetic field does not increase any further.
Interference effects develop at the apertures in the enclosure and impair attenuation of the electrical field as the frequency increases.
>100 MHz
The electrical and magnetic components are no longer considered separately.
The electromagnetic waves are attenuated effectively only with electrically conductive
connections to all enclosure surfaces and with a minimum number of slots.
Table 2
|
Question |
Remarks |
|
What are the requirements? |
Determine the permissible interference to the outside and inside environment. |
|
What types of interference are present? |
Check for line-induced and radiation interference |
|
How much interference is allowed? |
Refer to tolerance curves for the H and E fields and account for frequency dependence of the fields. |
|
What aperture accessories are needed? |
Determine if vent grills, glazed doors or cable brushings are needed. |
|
Are special grounding provisions required? |
Yes/No |
|
What additional aspects determine selection of enclosure type? |
Determine if a large or small unit and outdoor siting is needed, and what protective category, paint finish and enclosure material is required. |
|
Is partial screening suitable? |
Yes/No |
|
Can EMI/RFI tests be performed on the unit? |
Yes/No |
|
What are the results from the test? |
Discuss results with enclosure manufacturer |
|
Are modifications required or possible? |
Yes/No |
|
What is the price/performance ratio? Is it acceptable? |
Determine if the requirements are too high or if other provisions can be made. |
Shielded Enclosures
Shielded Cabinets Meet
European EMC Requirements
The FCC/VDE Line of shielded enclosures meets the European Union EMC requirements due to the addition of a metallizing process and EMI gaskets to limit emissions and interference. Built to the specifications of the company’s cold-rolled steel Heavy Duty Line, the FCC/VDE model holds over 3,000 lb of equipment. It also meets MIL-STD S10D for shock and vibration and MIL-STD 901 for shock. The cabinet is available in panels 19″, 24″ and 30″ wide and 17″, 24″, 29″ and 36″ deep with 10 heights in vertical and sloped front consoles. Equipto Electronics, (800) 204-RACK.
Enclosure Doors Sealed
With BeCu Springs
The RFI Doubleprorack has an inner enclosure made of zinc-passivated sheet steel constructed as a hollowed-out metal body to enhance EMI/RFI protection. Doors are sealed with beryllium copper springs to ensure shielding. Shielding meets VG 95373 requirements from 100 MHz to 1 GHz. Knurr USA (805) 526-7733.
Enclosure Supports In-Line
Testing of Cellular Products
The In-Line Shielded Enclosure allows wireless and cellular products to be tested for EMI/RFI without being removed from the conveyor line. It features solid welded metal construction which can be customized. The enclosure’s electrical, logic and pneumatic components interface with the programmable logic control unit. Lindgren RF Enclosures, (708) 307-7200.
Shielded Housing Provides
50-dB to 90-dB Attenuation
The Vario-Rack is an RF-shielded enclosure that provides 50 dB to 90 dB attenuation from 250 kHz to 1 GHz. The frame is made of 1.5-mm-thick sheet steel and has an electrophoretic dip-coat primer and a textured enamel top coat. It is available in widths of 600 mm, heights ranging from 650 mm to 2,100 mm and depths of 650 mm and 850 mm. Rittal, (513) 399-0500.
Aluminum Case Provides
EMI Protection
The Propac case consists of one-piece aluminum die-cast front frame assembled to extruded aluminum side plates. The top covers slide into grooves on the frame and the side plates, providing approximately 35-dB to 90-dB shielding. The unit is suitable for mounting Euroboards as well as individual modules. The case has interchangeable decorative strips and screw channels in side plates that eliminate the need for threaded inserts or nuts. It is available in heights from 2U to 4U and depths from 256 mm to 496 mm. Schroff, (800) 451-8755.
Commercial, Military and
Secure Shielding Available
Level 50, Level 70 and Level 90 enclosures provide a range of shielding for commercial, military and high security, respectively. Level 50, constructed from 16-ga material with tin-plated copper contact surfaces, is suitable for most FCC and VDE test requirements. Level 70 provides 70-dB attenuation from 20 kHz to 200 MHz, supplies ventilating perforations or aluminum and ferrous honeycomb filters, and meets MIL-STD-461 requirements. Level 90 has 12-ga construction throughout and provides 90-dB attenuation from 20 kHz to 200 MHz. Wyle Laboratories Electronic Enclosures, (310) 643-8181.
CRT Shield Eliminates
Magnetic Interference
ImageGuard™ is a five-sided box that eliminates CRT screen interference. It is made from the company’s proprietary CO-NETIC® AA alloy metallic material. The box eliminates screen disturbances caused by electric power substations, high-current equipment, medical instrumentation and arc furnaces. Box sizes are custom made and available in beige, gray, white and matte black. Magnetic Shield, (708) 766-7800.
Copyright 1996 Nelson Publishing Inc.
September 1996