Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2001-12-14
2003-04-01
Gibson, Randy W. (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S816000, C361S800000, C361S799000, C361S753000, C174S034000
Reexamination Certificate
active
06542384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reducing electromagnetic radiation emanating from computer enclosures, and particularly, from enclosures for relatively inexpensive, mass-produced computer systems employing printed circuit board (“PCB”) technology.
2. Description of Related Art
As a by-product of normal operation, electronic equipment frequently emits undesirable electromagnetic radiation, often referred to as electromagnetic interference or “EMI.” At the same time, equipment specifications require a maximum acceptable level of EMI outside of an electronics enclosure, to comply with government regulations and other performance requirements. For computer systems, EMI requirements are generally complied with by enclosing the computer system in an enclosure made of metal or other conductive material. Openings in the enclosure may be covered with a metallic grill or mesh screen, and the enclosure as a whole constitutes a “Faraday cage.” To maintain a perfect Faraday cage over a wide bandwidth, no apertures above a specified size may exist in the shield. The higher the frequency of the EMI, the smaller the shield apertures should be.
In practice, computer enclosures contain some apertures that are not covered by grills or screen, because of penetrations for connectors, openings for insertions of items such as floppy disks, and assembly joints, among other things. Despite openings such as these, EMI requirements for most computer systems have been easily complied with in the past using relatively simple, low-cost enclosures. But the advent of modern computer systems has made compliance with EMI requirements more difficult, particularly for lower-end computer systems that are sold in an extremely cost-competitive market.
That is, it is generally more difficult to meet EMI requirements for modern systems without undesirable cost increases in the enclosures or other system components. Various changes in the industry underlie these new difficulties. For one thing, the increasingly high frequency of commonly available microprocessors, especially above about 500 megahertz, means that computer enclosures must be better sealed against transmission of RF radiation. The higher-frequency microprocessors emit EMI at higher frequencies, which, in turn, can emanate from an enclosure through smaller openings than EMI at lower frequencies. And as frequencies and edge rates in high-speed digital designs continue to increase, EMI as a result of radiation through slots, apertures, and seams in shielding enclosures is becoming increasingly problematic. There is sufficient energy at low-order clock harmonics to cause EMI problems above a few hundred MHz as a result of exciting cavity modes of the enclosure, and efficiently driving even small length slots and apertures that are unavoidable in a practical design.
Consolidation among manufacturers, and the drive towards cost reduction in the computer industry in general, also plays a role. To build a computer system at a competitive cost, a computer manufacturer typically will include certain components that are only available as stock items from a limited number of manufacturers. Peripheral DVD drives are an example of a typical stock item made by relatively few manufacturers. The computer manufacturer often has no direct control over the particular configuration of these stock items. Consequently, a particular stock component that is otherwise desirable may provide a pathway for EMI, particularly at high frequencies, to radiate from a computer enclosure. This pathway may be blocked by customization of the component, but customization of a stock item can add substantially to the system cost.
In particular, certain peripheral components, such as CD and/or DVD drives, essentially include a rectangular-tubular metallic enclosure, much like a rectangular cross-section waveguide, that is not shielded from transmission of EMI at one or both ends, and are generally mounted inside computer system enclosures with one unshielded end passing through the computer enclosure. For example, in disk drives such as CD, DVD, and floppy disk drives, the disk door on the exterior of the computer enclosure is frequently made of a nonconductive plastic material that does not block transmission of EMI. At the same time, EMI may enter the opposite end of the peripheral enclosure via a cable connection or opening. Hence, the peripheral enclosure can form an efficient waveguide for transmission of EMI at certain frequencies to the exterior of the computer enclosure. For example, certain modern CD/DVD peripherals provide an efficient waveguide for transmission of EMI at frequencies in the range of about 800-1000 MHz. Other components and compartments within computer enclosures may also act as waveguides at these and other frequencies, depending on the details of the component and its relationship to other components of the computer system.
One approach for eliminating the waveguide effect of peripherals such as disk drives is to construct the opening door of a metallic or conductive material. This approach is likely to add to the cost of the component, and is not effective when the peripheral door is open. Another approach is to employ multiple ground points for the peripheral component, but this approach may increase assembly cost, and multiple ground points are subject to being disrupted during repair or replacement of the peripheral component.
A computer enclosure may contain other EMI openings which, for one reason or another, are difficult to block in a reliable, relatively permanent, and low-cost manner. It is desired, therefore, to provide an alternative method and apparatus for preventing EMI transmission through such openings in computer enclosures, including but not limited to openings created by disk drive peripheral components, that overcomes the limitations of the prior art.
SUMMARY OF THE INVENTION
The present invention provides a shielded riser card assembly for reducing electromagnetic radiation from a computer enclosure, that overcomes the limitations of the prior art. The riser card assembly comprises a four-layer riser card having a connector adjacent its lower edge, such as a connector for an NLX system board. A cable connection socket is on the riser card a first distance away from the connector, and a plurality of traces on a surface of the riser card run between the cable connection socket and the connector. A sheet of conductive material covers the plurality of traces and is spaced a second distance apart from the surface of the riser card. At least one fastener connected is to the sheet and attached to the riser card. The fastener conductively connects the sheet of conductive material to a ground layer (i.e., a signal return layer) of the riser card. A plurality of non-conductive spacers are disposed between and in contact with both of the sheet and the riser card, for maintaining the sheet a predetermined distance away from the riser card. The sheet and spacers may be attached together to form a separate auxiliary shield assembly when detached from the riser card.
The riser card and auxiliary -shield assembly may be adapted for application to a computer enclosure enclosing at least one source of electromagnetic radiation of a computer system, where the enclosure includes an unshielded opening in the computer enclosure through which an undesirable amount of electromagnetic radiation from the source is capable of passing to an exterior of the computer enclosure.
Generally, the riser card and auxiliary shield assembly is for computer systems having a system board mounted to a frame of the computer enclosure, and a riser card connected to the system board within the computer enclosure. The riser card is a PCB having at least one ground layer and a signal layer, and extends transversely from the system board. The riser card divides an interior space of the computer enclosure into two compartments, a source compartment containing the source of electromagnetic radiation, and an unshielded compartment having th
Brovald Russel K.
Luckenbihl Randall C.
Radu Sergiu
Gibson Randy W.
O'Melveny & Myers LLP
Sun Microsystems Inc.
Vu Phuong T.
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