The electronics industry offers products that continue to amaze us with their advancements in capabilities and speed. The technological improvements, however, do not come without a price. For example, designers of high-frequency PCs, telecommunications equipment, and wireless communications products must find new ways to suppress EMI events.
RF design engineers must constantly be aware of new shielding and suppression techniques to control EMI/RFI. But are there any rules of thumb to help engineers reduce emissions in their designs?
Each product is unique, and only experience can help you find the best solution to EM emissions problems, said W. Drew Peregrim, product manager at AMP. Your best approach is to consider emissions at the circuit level. But that is not always possible. The time between product releases is short, and engineering resources usually are limited, preventing companies from considering emissions until the product is completed. Many designers, therefore, rely on electronic enclosures to solve their emissions problems.
A cure-all generic electronic enclosure solution for high-speed devices is very difficult because the size of the openings must shrink as the frequency increases, said Mr. Peregrim. Additionally, as high-frequency products move from the factory to the home, more importance is placed on aesthetics. Consumer products use plastic enclosures which have more curved surfaces, adding more possibilities for RF emissions. Pliable gaskets that conform to the surface are needed to reduce emissions from these products.
Solutions for reducing emissions vary markedly, depending upon the type of equipment, the I/O type, and the frequencies of operation, said Dr. John Nemec, director of applications at California Micro Devices. For example, the cellular/Personal Communication Services (PCS) products that have a sensitive handset receiver connected through a peripheral port filter must successfully attenuate signals in the 900-MHz to 2.4-GHz range. This is necessary to prevent signals that leak through the peripheral port from impairing the receiver operation.
Unfortunately, it is difficult to protect the handset with filters built from discrete surface-mount components at the required frequencies, continued Dr. Nemec. Specialized filters, like the company’s PAC T low-pass filters, can be used to eliminate noise and spurious disturbances with a filter network that combines resistors, capacitors, diodes, and active elements on a silicon substrate.
To decrease emissions in wireless communications and microprocessor-based equipment, the simplest rules focus on proper board layout and shielding, said Ernest Niemisto, chief engineer at MMC Electronics America. Limit the length of high-frequency signal and clock traces, and keep them away from other signal lines that may be sensitive to interference.
Since many of the newest products are portable, board space is at a minimum, said Mr. Niemisto. Shielding and EMI filters are required to keep down the size of the board and make the best use of limited space.
A high-speed modem is an example of a new design that needs stringent shielding and suppression techniques to control EMI and remain a compact size. Without implementing EMI-control techniques, the present modem circuitry can act as an antenna for picking up noise.
Another new technology expanding the requirements for EMI suppression is PCS, added Mr. Niemisto. EMI filters typically were designed to operate up to several hundred megahertz, but PCS products raise the requirement to the gigahertz range.
Place filters as close as possible to the noise source, rather than shield an enclosure to keep radiation from escaping, said Jeff Bruce, director of engineering at Steward Manufacturing. Applying a ferrite bead to data or power-supply lines close to the active components reduces emissions and suppresses unwanted signals.
When a ferrite is inserted in a circuit, the insertion loss depends on the impedance of the source and the load as well as the ferrite. In many cases, additional ferrites may be applied in series to increase the impedance. Several ferrites also may be added to the circuit during testing to determine the appropriate impedance to suppress the noise signals.
When choosing an EMI suppression component, consider how much attenuation is needed and at what frequency, said Mr. Niemisto. The frequency to be attenuated compared to the frequency to pass will determine which components are best suited for the design.
Capacitors and inductors may suffer from self-resonance, while chip ferrite beads may become lossy at high frequencies, continued Mr. Niemisto. The combination of chip or feed-through capacitors with chip ferrite beads typically offers the best of both worlds.
Technical considerations about a suppression or shielding method are difficult without understanding market trends, said Dr. Nemec. The major issue is cost. Shielding for cabinets and cables, for example, is expensive, so you could investigate a lower-cost filtering solution.
But the first step is to design a solution without a filter or shield. If that does not work, then the next step is filtering. Shielding the design should be the last resort.
Additional factors to consider are the voltage and current rating required by the product and the size limitations, said Mr. Niemisto. The method of attenuation will depend on what already exists in the circuit. Long cable lengths or signal traces may add enough inductance to allow just a capacitor to be an effective filter. The inductance of the circuit-board trace and the length of the traces running next to each other could create crosstalk. In simple applications, matching the characteristics of the EMI filter to the requirements may be enough.
A coordinated design program is needed to meet the combined requirements for emissions and susceptibility, said Ron Brewer of Instrument Specialties. To meet these EMC specifications, three interlinked steps are essential:
Minimize circuit speed and bandwidth.
Reduce PCB coupling loop areas.
Shield all problem circuits.
Keep in mind that higher-speed devices have lower noise margins than low-speed devices, said Mr. Brewer. It is best to use low-speed parallel processing rather than high-speed serial.
High-speed systems should reduce the highest-frequency loop areas to keep emissions to a minimum. For double-sided boards, lay out the clock and sensitive analog loop areas first, said Mr. Brewer. Then provide segregation and isolation between analog and digital circuits.
Shielding can be used as a stand-alone solution for existing circuits, providing the needed attenuation for the difficult-to-control sections of the board. For new designs, shielding can be used in conjunction with, or after, circuit-layout and bandwidth- reduction techniques, said Mr. Brewer.
After you complete the design, test that noise levels are within specification. When you conduct an emissions test and your product fails, you probably will notice that it is well within the limits over most of the frequency range but has a few peaks poking above the regulation limits, said Mr. Peregrim. If not, you may have a serious problem that requires an electronic enclosure and filtered connectors.
To resolve the failures that exceed the limits at a few frequencies, you can:
Increase the shielding effectiveness of the enclosure to meet the level of the worst offending emissions by adding gasketing and decreasing the size of any apertures. This is the easiest, but the most expensive procedure.
Add filtering to reduce the emissions on cables and wires. This is easy to do during emissions testing.
Hunt down the source of emissions and shield it. This requires knowledge of the circuitry.
Experienced RF engineers use the offending emissions frequency to find the source, said Mr. Peregrim. Then they selectively shield them at their source, by adding filters on the circuit board, applying a simple shield over the component, or combining techniques.
Multiple RJ45 Jack Connectors
Available With Filtering
The Multiport Modular Jack Series of filtered RJ45 connectors protects equipment from conducted and radiated EMI. They are interchangeable with existing standard products. Filtering options include ferrite or capacitive film. The series provides from one to eight connector positions. Typical insertion loss is 35 dB at 200 MHz. Applications include LANs, WANs, network cards, broadband transmission equipment, fax/modems, PCs, and copy machines. AMP, (800) 522-6752.
Saves Board Space
The MultiGuard® Series is a four-element transient voltage suppressor array that uses less than 50% of the PCB space required to place eight discrete components in I/O data line circuits. The multilayer construction protects against most voltage transients caused by ESD and inductive switching and provides EMI attenuation. The arrays are available in 5.6, 9, 14, and 18 V with 0.1-J energy ratings. They have a 30-A peak current capability and operate from -55°C to +125°C. AVX, (803) 448-9411.
Custom-Design Shields Provide
>1 GHz E-Field Attenuation
Custom-designed EMI shielding laminates, boxes, and enclosures for medical, telecommunications, computer and network systems are offered with magnetic-field attenuation from 10 kHz and electric-field attenuation to >1 GHz. The designs provide
heat-flow control, integral grounding and mounting points, and insulation. Corrosion is prevented by using materials such as beryllium copper, anodized aluminum, and plated steel. Ben Mer Manufacturing, (800) 458-7597.
Filtering Network Meets
IEEE P1284 Recommendations
The PAC 1284™ is a high-performance resistor-capacitor thin film on silicon network with nine terminating lines per package. It meets IEEE P1284 enhanced parallel-port recommendations for termination and filtering. The network features pull-up series termination, EMI filtering, ESD protection to 2 kV, and a flow-through filter design. Applications include high-speed parallel ports used to communicate with backup drives, ZIP drives, printers, parallel-port SCSI adapters, and scanners. California Micro Devices, (800) 325-4966.
Multilayer Ferrite Chip Beads
Feature 600-W Impedance
The P/N 2744555577 Surface-Mount Wound Bead is available in standard EIA 0603, 0805, 1206, and 1806 sizes. It provides 340-W impedance @ 25 MHz, 600-W impedance @ 100 MHz, and a DC winding resistance of <7.5 mW . The bead, wound 2.5 turns, is offered in a tape and reel version per EIA 481-2 and IEC 286-3 requirements. Applications include board-level switched-mode power supplies and I/O circuitry. Fair-Rite Products, (888) 324-7748.
Straddle Cables and Wires
U-shaped ferrite saddle-bead RFI suppressors fit over cables, wires, and PC board electronics. The magnetic coupling of the suppressors provide high-frequency impedance damping at the source, allowing low-frequencies to pass. They mount with adhesive foam or electronic adhesives. FerriShield, (212) 268-4020.
Laminated Shield Includes
Foils and Cloth on Substrate
The ElectroLaminate Line of shielding and grounding laminates is made of conductive foils and cloth affixed to substrate materials. They can be die cut, scored, and formed. The conductive layer is available in aluminum, copper, tin-alloy copper, and embossed foils of silver-plated nylon, woven, and nonwoven copper cloth. A nonconductive substrate is offered in PVC foam, high- and low-density foams, polypropylene, polyethylene, and board stock such as Mylar®. Conductive and nonconductive pressure-sensitive adhesives are supplied. Instrument Specialties, (717) 424-8510.
Foil Tape Is Conductive
And Corrosion Resistant
The 888-L Tin-Coated Copper Foil Tape is conductive along and through the material. The tape shields EMI and RFI from components, cables, antennas, and motors. The 0.0035″-thick, silver-colored tape has 1 oz of tin-coated foil acrylic and a conductive adhesive polyethylene-coated paper liner. Conductivity through the adhesive is 10 mW . Lamart, (201) 772-6262.
EMI Filter Targeted
For Compact Board Designs
The CNF20 Series is a 0805-package chip EMI filter for compact board designs. Capacitance values range from 22 pF to 2,200 pF, filtering from approximately 1 MHz to >1 GHz. Attenuation at the resonant frequencies ranges from -45 dB to -60 dB. End terminations are nickel. The operating temperature range is -55°C to +125°C. MMC Electronics America, (847) 577-0200.
Miniature Ferrite Core
Is 0.162″ in Height
It is 0.162″ high, has a nickel zinc ferrite core, and features metalized terminals with tin or tin/lead outer surfaces. The device accommodates applications requiring maximum impedance above 200 MHz. MMG North America, (201) 345-8900.
Flexible EMI Gasket Uses
Silicone in Spiral Design
The Flexi-Shield EMI Gasket is a bonded spiral-shaped stainless steel and beryllium-copper material around a silicone tube or cord for EMI shielding, and for rain, wind, and dust sealing. It resists compression set and is easy to handle. The gasket shows no visible wear after 1,000 insertions on a VME front panel. It provides 130-dB shielding effectiveness @ 1 GHz. The groove-mounted gasket flexes to conform to uneven joint surfaces. Cross-sectional diameters for the gasket range from 0.063″ to 0.125″. Spira, (818) 764-8222.
Provides 75-dB Shielding
A form-in-place gasket material made from silver-plated, copper-filled resin cures to create a flexible EMI shield and an environmental seal. The material is applied on any of three planes, and provides shielding effectiveness to 75 dB @ 10 MHz. It adheres to metal or plastic housings, and is applied with a computer-controlled process that dispenses the gasketing onto flanges as narrow as 0.03″. Tecknit, (908) 272-5500.
Copyright 1997 Nelson Publishing Inc.