Wave transmission lines and networks – Coupling networks – With impedance matching
Reexamination Certificate
1998-12-02
2001-01-30
Lee, Benny (Department: 2877)
Wave transmission lines and networks
Coupling networks
With impedance matching
C333S246000, C174S262000
Reexamination Certificate
active
06181219
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to printed circuit boards and manufacturing methods and particularly to printed circuit boards adapted to carry signals having very high data rates.
As is known in the art, printed circuit boards are used to couple data among electrical components mounted on a surface of such board. This data is coupled through signal conductors disposed at various levels in the board. The board also carries reference potentials, such as ground and component supply voltages. In order to provide isolation between signal conductors in the various levels, reference potential conductive planes are interspersed between the levels of signal conductors.
As is also known in the art, in order to connect to the components, conductive vias are provided which pass from the surface of the board to the interior region thereof for connection to the signal conductors. These vias form what are sometimes called “signal launches.” Further, in many cases it is necessary to electrically connect components on one printed circuit board to components on another printed circuit board. This is typically done with an electric connector assembly. The connector assembly has two parts, each part mounted to a corresponding one of the printed circuit boards. Thus, when it is desired to connect one board to the other board, the connector parts are plugged into one another.
Further, as is known in the art, the data rates of the signals propagating through printed circuit boards are ever increasing. As such data rates increase, it is necessary to improve the efficiency of the electrical coupling of the data passing through one of the printed circuit boards to the other printed circuit board. More particularly, the signal conductor, reference potential plane or layer, and portions of the dielectric of the board therebetween form a transmission line having a predetermined impedance, typically 50 ohms. The effect of the via provides an impedance mismatch to the impedance of the transmission line. One technique used to compensate for this impedance mismatch is to provide compensation elements in the connector. For vias with impedance below the impedance of signal traces in the boards, some connectors are designed to have a higher impedance so that on average the impedance is the same as the interconnect as it is in the board. However, signal reflections which can limit performance of an interconnect depend on changes in impedance.
SUMMARY OF THE INVENTION
In accordance with the invention, a printed circuit board is provided having a conductive via passing from a surface of a dielectric into an interior region of the dielectric. A reference potential layer is disposed in the interior region of the dielectric parallel to the surface. A signal conductor is disposed in the dielectric parallel to the reference potential layer to provide a transmission line having a predetermined impedance. The conductive via is configured to provide an impedance to the transmission line substantially matched to the impedance of the transmission line.
With such an arrangement, impedance matching is provided internal to the printed circuit board thereby enabling connection of such board to another board with reduced reflections caused by the impedance mismatch through the connector launch. We have recognized that matching the average impedance of an interconnect to the impedance of the board can limit performance of an interconnect, particularly for high frequency signals. We have recognized the benefits of vias with impedance that is more similar to the impedance of the printed circuit board.
In accordance with another feature of the invention, a printed circuit board is provided having a conductive via passing from a surface of a dielectric into an interior region of the dielectric. The conductive via has a wider region at a surface region of the dielectric than at an interior region of the dielectric. A reference potential layer is disposed in the interior region of the dielectric. A signal conductor is disposed in the dielectric parallel to the reference potential layer to provide a transmission line having a predetermined impedance. The signal conductor is connected to the conductive via. The conductive via is configured to provide an impedance to the transmission line substantially matched to the impedance of the transmission line. More particularly, the wider, surface portion of the hollow conductive via is selected to accommodate a terminal pin of the connector while the narrower, interior portion of the conductive via is selected for impedance matching.
In accordance with another feature of the invention, a printed circuit board is provided having a conductive via passing from a surface of a dielectric into an interior region of the dielectric. The conductive via has a wider region at a surface region of the dielectric than at an interior region of the dielectric. A plurality of different levels of reference potential layers is disposed in the interior region of the dielectric and parallel to the surface. The plurality of layers have edges terminating at different distances from the conductive via.
In accordance with another feature of the invention, a printed circuit board assembly is provided having a pair of printed circuit boards. Each one of the boards has a conductive via passing from a surface of a dielectric into an interior region of the dielectric. Each one of the printed circuit boards has a reference potential layer and a signal conductor disposed in the dielectric thereof parallel to the reference potential layer thereof to provide a transmission line having a predetermined impedance. The signal conductor of each one of the boards is connected to the conductive via thereof. The conductive via in each one of the boards is configured to provide an impedance to the transmission line thereof substantially matched to the impedance of the transmission line thereof. A first electrical connector is provided having a signal contact connected to the conductive via of one of the boards and a second electrical connector having a signal contact connected to the conductive via of the other one of the boards. The first signal contact of the first electrical conductor is adapted for electrical connection to the second contact of the second electrical connector.
In accordance with another feature of the invention, a pair of printed circuit boards are electrically interconnected through an electrical connector. Each one of the printed circuit boards has a conductive via passing from a surface of a dielectric into an interior region of the dielectric. A reference potential layer is disposed in the interior region of the dielectric parallel to the surface. A signal conductor is disposed in the dielectric parallel to the reference potential layer to provide a transmission line having a predetermined impedance. The conductive via is configured to provide an impedance to the transmission line substantially matched to the impedance of the transmission line. The impedance from one of the boards through the electrical connector to the other one of the boards is substantially constant.
In accordance with another feature of the invention, a method is provided for fabricating a printed circuit board. The method includes determining an impedance of a transmission line in a printed circuit board, such transmission line having a signal conductor separated by a reference potential layer by a portion of a dielectric; and determining a configuration of a conductive via in the dielectric passing from a surface of the dielectric to an interior region of the dielectric, and connected to the signal conductor, to provide the transmission line with a termination impedance substantially matched to the impedance of the transmission line.
In accordance with another feature of the invention, a method for fabricating a printed circuit board is provided. The method includes determining an impedance of a transmission line in a printed circuit board, such transmission line having a signal conductor separated by a referen
Cohen Thomas S.
Gailus Mark W.
Stokoe Philip T.
Jones Stephen E.
Lee Benny
Teradyne, Inc.
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