Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1999-10-12
2001-11-20
Han, Jessica (Department: 2838)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S034000, C428S036100
Reexamination Certificate
active
06320122
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electronic systems. More particularly, the invention relates to an electronic magnetic interference (EMI) surface conducting gasket.
Background of the Invention
The operation of electronic circuitry is often accompanied by the generation of noise currents that when radiated produce EMI emissions. The noise currents and the EMI emissions can cause significant electronic interference with other surrounding electronic circuitry. As such, the sources of these noise currents need to be effectively shielded in order to prevent this disturbance.
Typically, electronic circuitry is placed into an enclosure or housing. In one type of enclosure, there are slots on one wall of the enclosure that are used to house one or more I/O connectors. The I/O connectors are used to connect the electronic circuitry housed inside the enclosure with external peripheral devices. The I/O connector body or housing is a source of noise currents and potential EMI emissions and as such, requires effective shielding.
One EMI shielding technique commonly used for this problem is gaskets made of metal stampings. A stamped metal EMI gasket is a thin metal sheet cut to fit the size and shape of the I/O connectors and includes a number of spring fingers that make contact with a metal enclosure wall. The noise currents generated from the I/O connector housing flow through the fingers and into the enclosure wall without transcending beyond the enclosure.
There are several disadvantages with metal stamp gaskets. First, in order to provide adequate mechanical compliance, the spring fingers are usually longer than they are wide. As such, the spring fingers are electrically inductive and poor conductors for high EMI frequencies. Second, the metal-to-metal contact between the gasket and the enclosure wall degrades any coating or plating on these surfaces thereby corroding the coating and limiting the shielding effectiveness of the gasket. In addition, the metal stamp gaskets are costly to fabricate and require special tooling to manufacture them.
Another EMI shielding technique involves the use of surface conducting gaskets. A surface conducting gasket includes a metallized fabric wrapped over an elastomer core. The conductive surfaces provide many metal contact points. Surface conducting gaskets are beneficial over metal stamp gaskets since they provide constant low impedance grounding and do not suffer from the problems associated with metal-to-metal contact. In addition, the surface conducting gaskets can be die cut using simpler and less costly tools which make them a more economical alternative to metal stamp gaskets.
FIG. 1
illustrates an exemplary surface conducting gasket
100
having a top conductive surface
102
, a bottom conductive surface
103
, and an aperture
104
that is cut to the dimensions of a particular I/O connector. Noise currents generated from the I/O connector housing flow through one of the metallized conductive surfaces
102
,
103
through the metal enclosure wall and return back to the I/O connector housing. However, there are gaps in the conductive surfaces
102
,
103
due to the non-conductive or open edges
106
a
-
106
d
,
108
a
-
108
d
of the gasket
100
and of the aperture
104
. As such, these open edges
106
,
108
provide discontinuities in the conduction paths that are provided by the conductive surfaces
102
,
103
and allow EMI emissions to radiate from the enclosure.
FIG. 2
illustrates another exemplary surface conducting gasket
110
having a top conductive surface
112
, a bottom conductive surface
114
, and a side conductive surface
116
. The metallized fabric is wrapped over one edge of the gasket
110
thereby providing an additional conductive surface. These conducting surfaces
112
,
114
,
116
provide additional return paths for the noise currents and are more effective at shielding EMI emissions when the noise currents align with the conduction paths provided by the conductive gasket surfaces
112
,
114
,
116
.
However, it is difficult to route all orientations of the noise currents to flow in the direction of a particular conduction path. As such, the surface conducting gaskets shown in
FIGS. 1 and 2
, cannot shield all potential noise current and EMI emissions due to the open edges that limit the number of available conduction paths to the enclosure wall. Accordingly, there is a need for an EMI shielding device that can overcome this shortcoming.
SUMMARY OF THE INVENTION
The present invention pertains to an EMI device that provides a low impedance mechanism for conducting noise currents in all orientations back to the source of the noise currents in order to more effectively reduce EMI emissions.
In an embodiment of the present invention, the EMI device is applied to an enclosure that houses electronic circuitry. The electronic circuitry is connected through I/O connectors to external peripheral devices. The enclosure includes slots that enable the passage of the I/O connectors to connect with the electronic circuitry housed in the enclosure. The I/O connectors are a source of noise currents and EMI emissions. As such, the EMI gasket of the present invention is used to more effectively shield the noise currents and the EMI emissions.
The EMI gasket is constructed from an elastomer core that is wrapped with a metallized electrically conductive material. The EMI gasket includes one or more apertures that are used to house one or more I/O connectors. The gasket is die cut and as such includes one or more open edges that are non-conductive. The non-conductive edges are stitched with a conductive material that provides multiple conduction paths between any surface or edge of the gasket and with the enclosure wall. In this manner, the gasket is able to provide a return path back to the source of the noise currents for all orientations of the noise currents.
The EMI gasket of the present invention is beneficial for several reasons. First, it is more effective for shielding noise currents and EMI emissions for all orientations of the noise currents. In addition, it is easy to manufacture and less costly since it does not require specialized tooling.
REFERENCES:
patent: 5569877 (1996-10-01), Yumi
patent: 5712449 (1998-01-01), Miska et al.
patent: 5959244 (1999-09-01), Mayer
http://www.amp-emi.com/panels.html, An Improved Solution to I/O Connector Shielding, APM -EMI Shielding I/O Panels, pp. 1—2, printed off the URL on Oct. 8, 1999.
http://www.chomerics.com, Low-Cost Conductive Elastomer EMI Gaskets, CHOMERICS, pp. 1—2. (no date).
Cherniski Andrew M.
DeBlanc James J.
Dickey David
Han Jessica
Hewlett -Packard Company
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