compromise the earlier rules if the backplane is designed to be an extension of the plug-in boards at high frequencies.

This requires RF bonding the reference planes in the backplane to the corresponding reference planes in the plug-in boards, so that at the highest frequency of concern there appears to be no impedance discontinuity between them. Using shielded connectors helps - their shields should bond their mating halves to each other in 360o (refer to Part 2 of this series), and also bond all along their length to the 0V reference plane on both sides. Whether shielded connectors are used or not, there should be one signal return pin for every one or two (at most) signal or power pins in the connector, and these return pins should be spaced fairly regularly along the entire length of the connector. Most designers would place the return pins according to a regular scheme, but there is some evidence that a randomised allocation can have benefits. Impedance-matched connectors will almost always have a return pin alongside every signal pin, in any case.

It is important to make sure that all the high-speed devices associated with the backplane connector are closer to the middle of the connector, and do not go near to the outer edges of the boards, near the ends of the connector. It is best if the backplane connector extends to occupy the entire length of the card edge, but if it can’t it should still extend well to both sides of the area of the high-speed devices associated with the backplane connector.

5.6Useful references

[1]M. Montrose, EMC and the Printed Circuit Board, design, theory, and layout made simple, IEEE Press, 1999, ISBN 0-7803-4703-X

[2]IEC 61188-1-2 : 1998 Printed Boards and Printed Board Assemblies – Design and use. Part 1-2: Generic Requirements – Controlled Impedance, www.iec.ch.

[3]IPC-2141, Controlled Impedance Circuit Boards and High Speed Logic Design,

Institute for Interconnecting and Packaging Electronics Circuits, April 1996, www.ipc.org.

[4]T Williams, EMC for Product Designers 3rd Edition, Newnes, 1992, ISBN 0-7506-4930-5, www.newnespress.com.

[5]H W Johnson and M Graham, High Speed Digital Design, a Handbook of Black Magic, Prentice Hall, 1993, ISBN 0-13-39-5724-1.

Eur Ing Keith Armstrong C.Eng MIEE MIEEE

Partner, Cherry Clough Consultants, www.cherryclough.com, Associate of EMC-UK Phone: 01457 871 605, Fax: 01457 820 145, Email: keith.armstrong@cherryclough.com