Clock speeds in electronics are rising, especially in PCs where a 500-MHz processor is within sight. These faster computers are great for zipping through your complex data bases and graphic-intensive programs, but they also present challenges to manufacturers needing to meet stringent EMC requirements at these higher speeds. What can you do to minimize this potential hazard?
The radiated emissions and susceptibility characteristics are interrelated, and a coordinated program is required to meet design requirements, said Ron Brewer, vice president of EMC technical services at Instrument Specialties. You need to consider circuit speed, bandwidth, PCB coupling, and shielding.
Proper termination and board layout are the first orders of business, said Dr. John Nemec, director of applications at California Micro Devices. Unterminated lines and those with impedance discontinuities have signal reflections and generate common- mode frequencies that are prone to radiate or couple into other lines.
Consider filtered I/O connectors for the new high-speed clock rates, said Charles Blichasz, director of new business development at AMP. Distributed-element, Pi, capacitive, and inductive filter-type connectors are typical configurations.
The 3-dB point of the filtered connector must be coordinated with the rate of the data stream associated with the I/O connector, said Mr. Blichasz. For low data rates with slow rise times, higher capacitive values of the I/O connector can be tolerated.
As data rates increase with correspondingly faster rise times, the amount of capacitance that can be tolerated on the I/O line becomes less and the filter needs to be specified more accurately. It may be necessary to increase the 3-dB point to a higher frequency, such as 50 MHz or 100 MHz, to assure that the data stream is not adversely affected by the filter. A low-capacitance filter in the 300-pF to 50-pF range will assure signal integrity at these speeds.
The automotive industry uses cast-metal enclosures for many of the electronic control units installed in cars, and PC manufacturers may have to do the same when they cannot decrease the length of the signal path, said Ernest Niemisto, chief engineer at MMC Electronics America. Metal cans or enclosures that cover the EMI-producing sections solve most noise problems.
If signal lines must be run in and out of a shielded enclosure, filters may be needed because they can remove the noise before the signal enters the enclosure, added Mr. Niemisto. Threaded metal filters usually are best to use in a metal chassis or enclosure. Less robust solutions use feed-through EMI filters soldered inside a lightweight enclosure or onto the circuit board.
Source suppression for today’s electronics is very difficult to implement effectively, said Joe Butler, market manager at Chomerics. At higher frequencies, more circuit traces radiate; and as package density increases, the separation and layout options are reduced. Additionally, the process of finding the best source suppression typically is trial and error, which may add to the cost and the design timetable.
The likelihood of needing a gasket to fill gaps in an enclosure is more important as frequencies increase, said Christian Brull, product manager at Schlegel Systems. For example, 40-dB attenuation is needed for a 0.118″ opening at 500 MHz.
Metal-to-metal connections that previously did not require any gaskets now may leak at the second or third harmonic, continued Mr. Brull. One method would be to weld the contacts. Another solution uses a thin gasket with a conductive fabric over an open-cell urethane foam that is soft enough to follow and fill the unevenness of the joint.
Often a gasket can pay for itself by eliminating the need for additional fasteners or welds, added Bonnie Kunkel, general manager of Spira Manufacturing. Some companies may balk at the idea of using a gasket because of the upfront cost, but these same companies would pay an engineering technician to spend 10 minutes screwing and unscrewing the fasteners each time maintenance is needed.
Applying a gasket around the entire opening of an enclosure may not be necessary. In some cases, you can obtain a significant reduction in emissions with just a 1/2″-segment of an adhesive-backed conductive gasket.
Shielding is a noninvasive suppression technique that works for radiated emissions and susceptibility signals, said Instrument Specialties’ Mr. Brewer. Because shielding is not inserted into the circuit, it does not affect high-frequency operation. For existing designs, shielding can be used as a stand-alone solution to an EMC problem. In new designs, use it in conjunction with, or after, circuit-layout and bandwidth-reduction measures have been implemented.
Shielding frequently eliminates the need for EMI filters, said Mr. Brewer. But if filters are required for conducted emissions, the shielding provides a low-inductance ground sink and isolates filter inputs and outputs to prevent RF energy bypass. Shielding also can provide an isolated ground reference to reduce internal circuit coupling.
Reliance on a shielded enclosure can reduce design time, but also can lead to cost overruns unless effective shielding strategies are established early, said Joe Butler of Chomerics. This is especially true for high-frequency products.
Chomerics recommends that you:
Design plastic housings so they can be plated or coated.
Pay attention to the material used in gaskets. Often, installation costs more than the gasket, especially for hand-held products and equipment with extensive internal shielding.
Specify less-expensive commercial-grade shielding rather than military-grade when it offers acceptable performance.
Use several short lengths of compressible, self-stick gaskets to ground enclosure parts.
Test products in an anechoic or semi-anechoic chamber or an open-field test site because it is the best method for obtaining accurate EMC results.
Available Tests
The key for selecting the best gasket is its conductivity, said Spira’s Ms. Kunkel. EMC gaskets can be graded using the transfer-impedance test method. The MIL-G-83528 method for determining shielding effectiveness (SE) may be popular, but can be unreliable. Tests based on this standard have measured SE as high as 93 dB at 2 GHz on a piece of newspaper. Obviously, newspaper should not have any shielding capabilities.
The data from the military standard is not always flawed, said Ms. Kunkel. But the test results should be confirmed with another method, such as transfer impedance.
Of course, test equipment is available to check the quality of EMI shielding and filtering on a PCB. For example, the PCB can be laid on a scanner and a probe moved over the board to provide a spectral-analysis view. The graph of the emitted noise is shown on a screen.
The tests and equipment that you need vary according to your product and whether it is for use in the United States or abroad.
According to AMP’s Mr. Blichasz, some tests are:
FCC Part 68—for hazardous voltage and current.
CISPR 22 and EN 55022—for Information Technology Equipment.
IEC 801-3—radiated RF.
IEC 1000-4-3—RF electromagnetic field.
EN 61000-4-3—for transmitters and walkie-talkie equipment.
IEC 801-2 and IEC 1000-4-2—ESD.
EN 61000-4-2—static from humans.
IEC 801-4 and IEC 1000-4-4—electrical fast transients.
EN 61000-4-4—for relays and motors.
IEC 801-6—conducted continuous wave.
IEC 1000-4-6—conducted induced RF energy.
EN 61000-4-6—power and signal lines near transmitters.
For CE Marking, radiated emissions tests must be performed according to EN 55022 in an open-field test site, said Schlegel’s Mr. Brull. The standard specifies limits at 10 m for Class A and 30 m for Class B products. Limits are 30 dB µV/m at 30 to 200 MHz and 37 dB/m at 230 to 1,000 MHz.
Reducing Emissions
Limiting emissions is the goal of any EMC engineer, but some techniques work better than others. For example, reducing interference at the source is important. To do this, use multilayer boards with a ground plane, said Mr. Brull. This allows you to separate high-frequency and low-frequency circuits.
For most electronics equipment manufacturers, cost and size determine design rules for suppression and shielding products, said MMC’s Mr. Niemisto. Metal shielding can add weight as well as cost. Spray-on conductive paint can offer some shielding, and some metal-impregnated plastic and paper-based shielding also are effective.
To eliminate EMI problems, begin with the circuit board to determine the source of the noise and implement any possible corrective changes, such as increasing the ground plane or rerouting the signal, said Mr. Niemisto. EMI suppression components can prevent noise problems as long as there is an appropriate ground plane. However, shielding should be used if the circuit still radiates noise.
If you use high-speed interconnects, consider using filtering and fiber-optic cables, said Instrument Specialties’ Mr. Brewer. For high-speed circuits, isolate analog and digital sections. Power-supply suppression also should be considered to prevent internally generated noise.
Future Requirements
The EMC suppression-component industry is using materials that work efficiently at high frequencies, reducing the need for additional components to eliminate EMI, said MMC’s Mr. Niemisto. For example, improved dielectrics have reduced the size of capacitive filters. The mixing of dielectric with ferrite material has created a filter with the low-frequency capability of a ferrite and the high-frequency functional advantages of ceramic. This reduces board space and improves cost-effectiveness.
As more work is done at home with high-speed communications links to the office, the associated EMC requirements need to be met. The FCC and EU regulations for home-based vs commercial products are more stringent, driving the need for more effective shielding and suppression materials, said Mr. Brewer of Instrument Specialties.
There are very few products today that are not microprocessor based, said AMP’s Mr. Blichasz. These products create new markets for shielding and suppression and require innovative techniques to meet stringent cost requirements. For example, the more expensive filtering used for the medical instruments in hospitals will not meet the needs for portable instruments used in the home health-care market.
Expect more double-duty gaskets which act as environmental as well as EMI shields, said Schlegel’s Mr. Brull. They eliminate the need for two gaskets in enclosures used outdoors.
Smaller gaskets that are easy to include in the assembly process are very popular with some manufacturers, added Mr. Brull. They want gaskets that are easy to install and do not require gloves to protect a worker’s hands.
The existing market for computers is becoming saturated, forcing manufacturers to lower prices, said Instrument Specialties’ Mr. Brewer. This cost mandate trickles down to shielding-component manufacturers. So here’s the best piece of advice from the shielding-component makers: Eliminate much of your unnecessary shielding costs by working with a shielding supplier during the design stage of your product.
Shielding/Suppression Products
Filtered Connector Used for
High-Speed Transmission
The Quiet Line Media-Filtered Modular Jack combines a filter and connector into an integrated assembly to save board space. It has a Category 5 performance rating for high-speed data transmission. It accommodates LAN hubs and LAN-compatible peripherals including printers and workstations. AMP, (800) 522-6752.
Filtering Network Satisfies
IEEE P1284 Recommendations
The PAC 1284™ is a high-performance resistor-capacitor thin film 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, and a flow-through filter design. Applications include high-speed parallel ports used to communicate with backup drives, ZIP drives, printers, parallel-port SCSI adapter, and scanners. California Micro Devices, (800) 325-4966.
Gasket Meets Mechanical
Requirements for Enclosures
The SOFT-SHIELD® 5000 Series Gasket meets the shielding and mechanical performance requirements for commercial electronic enclosures. It is made with conductive cloth over soft urethane foam and provides >90 dB shielding from 30 MHz to 1 GHz. At 10 GHz, it provides >75 dB. The gasket deflects 40% and requires <1 lb/in. of closure force. Gasket profiles include rectangular, square, and D shapes. Chomerics, (781) 935-4850.Metallized Gasket Provides
Shielding to 85 dB at 1 GHz
Ultra-Soft Knit consists of a metallized nylon yarn over a flame-retardant, low- density polyester foam core. The material has a closure force of 8 lb/ft at 30% deflection. The gasket features a closely knit stitch that provides 85 dB shielding from 10 kHz to 1 GHz. It is available in silver- and tin-plated nylon yarn to meet galvanic compatibility requirements. Compression set is 10% at 50% deflection. Instrument Specialties, (717) 424-8510.
Filter Attenuates Frequencies
From 40 MHz to 1 GHz
The LFA Series EMI Filter provides -30 dB attenuation from 40 MHz to 1 GHz. It uses a combination of dielectric and ferrite materials. Cutoff frequencies are 10 MHz, 22 MHz, 47 MHz, and 100 MHz. Typical applications include telecommunications, computers, and audio-visual equipment such as digital TV and DVD. MMC Electronics America, (847) 577-0200.
Environmental Shielding Gasket
Resists Paint, Cleaning Agents
The Environmental Perimeter Shield (EPS™) gasket has a UV-stabilized outer cladding that resists paint, cleaning agents, and moisture. The inner shielding layer is offered in a nickel-copper or silver material. The core is a high-resiliency polyether polyurethane foam that has low closure force and low compression set properties. Shielding effectiveness is 80 dB for the nickel-copper material and 60 dB for the silver material from 30 MHz to 1 GHz. Schlegel Systems, (800) 204-0863.
Ferrite Polymer Absorbs
Interference to 500 MHz
Ferrite Polymer Composite FPC C351 film is used for EMC applications and absorbs interference at frequencies to 500 MHz. It is suitable for high-temperature applications to 200°C and is UL 94-VO listed. The material thicknesses range from 0.2 mm to 0.4 mm and have copper coatings from 35 to 100 µm. The homogenous mixture of ferrite powder and plastic can be processed into thin and flexible film for various EMC shielding applications. Siemens Matsushita Components, (011) 49 89 636-28083.
Flexible Gasket Uses
Silicon in Spiral Design
The Flexi-Shield EMI Gasket bonds a spiral-shaped stainless steel and beryllium-copper material around a silicon tube or cord for EMI shielding and sealing against rain, wind, and dust. It resists compression set and is easy to handle. The material shows no visible wear after 1,000 insertions. It provides 130-dB shielding effectiveness at 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 Manufacturing, (818) 764-8222.
Surface-Mount Filter Has
Capacitance to 5,000 pF
The 4700 Series of surface-mount EMI filters offers a capacitance range from 100 pF to 8,200 pF and insertion loss to 70 dB at 1 GHz. The filters measure 0.315″ × 0.083″. The cross-sectional body allows for easy installation onto PCBs via common soldering processes. The insulation resistance, when measured at 25° C with 100 VDC and a charging current limited to 50 mA, is a minimum of 10,000 MW . Tusonix, (520) 744-0400.
Copyright 1998 Nelson Publishing Inc.
June 1998