Impedance Discontinuity Reduction Between High-Speed Differential Connectors and PCB Interfaces

High-speed serial communication (i.e., Gigabit Ethernet) requires differential transmission and controlled impedances. Impedance control is essential throughout cabling, connector, and circuit board construction.

In controlled impedance, multilayered board differential strip lines are utilized to go from the board interface (e.g. connectors) to the electronic devices. Differential vias are used to go between controlled impedance layers. The differential impedance of these via holes has been extensively studied for high-speed applications. The impedance is dictated by the pin spacing on densely populated semiconductor devices and the dielectric constant of the board material. This usually turns out to be lower than the connecting differential impedance of the strip lines and/or the device.

An impedance discontinuity arises at the interface of a high-speed quadrax and twinax connectors and the attached printed circuit board (PCB). This discontinuity usually is lower impedance since the relative dielectric constant of the board is higher (i.e., polyimide ≈4) than the connector (Teflon ≈2.25). The discontinuity can be observed in transmit or receive eye diagrams, and can reduce the effective link margin of serial data networks.

High-speed serial data network transmission improvements can be made at the connector-to-board interfaces as well as improving differential via hole impedances. The impedance discontinuity was improved by 10 percent by drilling a 20-mil (≈0.5-mm) hole in between the pin of a differential connector spaced 55 mils (≈1.4 mm) apart as it is attached to the PCB.

The effective dielectric constant of the board can be lowered by drilling holes into the board material between the differential lines in a quadrax or twinax connector attachment points. The differential impedance is inversely proportional to the square root of the relative dielectric constant. This increases the differential impedance and thus reduces the above described impedance discontinuity. The differential via hole impedance can also be increased in the same manner. This technique can be extended to multiple smaller drilled holes as well as tapered holes (i.e., big in the middle followed by smaller ones diagonally).

This work was done by Sal Navidi, Rodell Agdinaoay, and Keith Walter of Honeywell Aerospace for Johnson Space Center. For further information, contact the JSC Innovation Partnerships Office at (281) 483-3809.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)}, to Honeywell Aerospace. Inquiries concerning licenses for its commercial development should be addressed to:

   Honeywell Aerospace
   P.O. Box 52199
   Phoenix, AZ 85072

MSC-24855-l