Hewlett-Packard Company

Edward C Miller

Guy E Lichtenwalter

A computer system employs electrically lossy transmission systems to dissipate RF energy, and thus reduce RF emissions near the source. An electrically lossy dielectric includes a colloidal dispersion of lossy material, e.g., graphite, in a polymer matrix. A layer of this material can be placed in contact with a conductor or with its surrounding dielectric. A metal layer contacts the other side of the lossy dielectric so that an electrically lossy transmission system is defined. Such transmission systems are defined with respect to signal conductors, shields for coaxial cables, PCB boards, and structural metal, such as heat sinks and vent openings, etc. In some cases, impedance mismatches are used to reflect unwanted energy back through or back and forth through the lossy transmission system. The lossy transmission systems dissipate RF energy so that there is less RF energy to radiate to the computer's exterior. When used in conjunction with known RF shielding techniques, the lossy transmission systems provide for reduced RF emissions, especially for frequencies above 100 MHz.

Using electrically lossy transmission systems to reduce computer RF emissions

Radio frequency emissions within a computer system having a printed circuit assembly in proximity of a metal surface are reduced by the placement of one or more lossy components electrically connected to a metal plane within the printed circuit assembly. A connector electrically connects the lossy components to the metal surface. In one embodiment, the lossy components include two resistors at opposite ends of an edge of the printed circuit assembly that is closest to the metal surface. In an alternate embodiment, a single lossy component is a resistor placed in the middle of an edge of the printed circuit assembly that is closest to the metal surface. In the alternate embodiment, a second connector at a first end of the edge of the printed circuit assembly that is closest to the metal surface directly connects the metal plane to the metal surface, and a third connector at a second end of the edge of the printed circuit assembly that is closest to the metal surface directly connects the metal plane to the metal surface. A spring is used to connect the printed circuit assembly to the metal surface. Ends of the spring are inserted into vias within the printed circuit assembly. Traces on the printed circuit assembly electrically connect the vias to the lossy component. Similarly, the vias may be directly connected to a ground or power plane without an intervening lossy component.

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Los Gatos, CA, United States of America

Edward C Miller