Contents

Introduction

1 Degradation Of Rise Time In Interconnects

1.1. Propagation issues in interconnects

1.2. Behavior of components at high frequencies

1.3. Effect on transmission of signals on interconnects

1.4. Measurement of rise time

1.5. Conclusion

2 Electromagnetic Modeling of Interconnects

2.1. Global modeling of signal integrity

2.2. RC interconnect model

2.3. Capacitive and inductive modeling

2.4. LC line modeling

2.5. Application to electronic packages and MCM

2.6. Conclusion

3 Controlled Impedance Interconnects

3.1. Why control impedance?

3.2. Influence of rise time on signal degradation

3.3. Model of a controlled impedance interconnect

3.4. Interconnects onPCBs

3.5. Impedance control for a microstrip configuration

3.6. Analysis of propagation in interconnects

3.7. Effect on data bus configuration

3.8. Application to clock distribution

3.9. Conclusion

4 Propagation on Transmission Lines

4.1. Transmission line model

4.2. Propagation modes related to substrate

4.3. Equation of propagation on transmission lines

4.4. Conclusion

5 The S-Parameters Testing Technique

5.1. Definition of measured parameters

5.2. The S-parameters principle

5.3. Measurement of S parameters

5.4. Measurement of characteristic line impedance

5.5. Measurement of line capacitance

5.6. Components on PCB and de-embedding techniques

5.7. Characterization of dielectric materials for interconnects

5.8. Conclusion

6 Time-Domain Reflectometry Analysis

6.1. Principle of TDR

6.2. Reflection and transmission of voltage

6.3. Measurement of characteristic impedance

6.4. Reflection on reactive loads

6.5. Extraction of equivalent schemas

6.6. Discontinuities in cascade

6.7. Conclusion

7 Interference and Cross-Talk in Interconnects

7.1. Coupling and interferences due to substrate

7.2. Theory of coupling between lines

7.3. Application to high-speed cables, buses and connectors

7.4. Conclusion

Bibliography

Index

First published 2014 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd
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John Wiley & Sons, Inc.
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©ISTE Ltd 2014
The rights of Fabien Ndagijimana to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2014935740

British Library Cataloguing-in-Publication Data
A CIP record for this book is available from the British Library
ISSN 2051-2481 (Print)
ISSN 2051-249X (Online)
ISBN 978-1-84821-550-4

Introduction

Electromagnetic compatibility considers the problems of equipment cohabitation and develops methods to control, predict and resolve the effects of various disturbances:

– emission of disruptions occurring on power and linking cables;

– radiated emissions near other equipment;

– immunity to disturbances propagating on power and linking cables;

– immunity to radiated disturbances;

– immunity to electrostatic discharges.

Signal integrity handles disturbances in the equipment itself: signal distortions during its propagation on networks, from cable to integrated circuit via connectors, printed circuits boards (PCBs) and electronic packages.

Figure I.1. Phenomena to account for in interconnects for signal integrity

Audio-video applications currently require higher and higher transmission speed and signal processing circuits, which leads to an ever-larger spectral occupation and thus to higher frequencies to be transmitted. At the same time, the presence of multiple parasite phenomena limits the bandwidth of the transmission channel:

– inductive or capacitive connectors and links;

– mismatch phenomena in transmission lines;

– cross-talk;

– noise related to the switching of power supplies and other circuits.

Thus, the transmission of the signal across a limited bandwidth transmission channel composed of packages, connectors and lines is manifested by distortions that can cause transmission errors, low signal speed and, thus, an overall reduction in system performance.

To control the signal integrity, in Table I.1 we have summarized the principal phenomena to be taken into account depending on the network levels.

Table I.1. Main phenomena in interconnects for signal integrity (***: high level of importance **: mean level of importance *: low level of importance)

Specific analysis and measurement tools are used to understand and optimize circuits in order to take into account the points summarized above:

– the technique of temporal reflectometry;

– the S parameters concept.

These different points are addressed in this book, based on practical observations to begin with and then introducing basic principles that will enable the readers to understand the concepts and grasp the most complex signal integrity problems.

Chapter 1 is dedicated to the reduction of the rise time of digital signals linked to transmission in a limited bandwidth channel and the effect on the parameters of the digital signal. The influence of speed and rise time of the signal on its spectral occupation is discussed and illustrated through examples drawn from high-speed applications.

Chapter 2 addresses the modeling of interconnects. The Input/Output Buffer Information Specification (IBIS) model used for the comprehensive modeling of active circuits and interconnects is introduced in order to define the modeling requirements at different levels of a system. The choice of models, composed of combinations of resistances, inductances and capacitances according to the type of interconnects, is presented. Taking into account losses in conductors and substrates is a critical point in interconnects circuit models. The determination of these parameters enables us to predict the performances of circuits. This chapter discusses the specific case of packages, which involves inductance and capacity matrices to account for the nearby presence of leads.

Chapter 3 introduces controlled impedance, mismatching and the various reflections associated with them. The concept of transmission lines and matching essential for data buses is discussed, and illustrated through typical cases of printed buses or lines. This chapter also includes the introduction of methods of time domain and frequency domain analysis that are often used in signal integrity.

Chapter 4 discusses the concept of transmission lines, which are necessary to handle controlled impedance interconnects. Propagation in lines and the associated parameters, such as the input impedance of a transmission line, are introduced so as to anticipate the measurement techniques discussed in Chapters 5 and 6.

Chapter 5 introduces the concept of S parameters, which is used to determine circuit and system performances in the frequency domain. S parameters enable us to analyze the spectral behavior of interconnects and approve their performances for high-speed signals of which the spectrum extends to radiofrequencies. In particular, the characterization techniques used to design controlled impedance lines are discussed.

Chapter 6 addresses techniques of time-domain measurement via time-domain reflectometry (TDR). This method enables us both to characterize network performance and to locate defects, discontinuities and sources of mismatch in high-speed circuits.

Chapter 7 introduces the phenomenon of coupling at the source of interference and cross-talk in packages and data buses. Essential parameters for cable and data bus performance are discussed and their impact is explained.

This book is based on the professional training conducted by the author as part of ongoing studies at the University of Grenoble. It can serve as basic training for technicians or engineers wishing to approach the design of high-speed circuits or mixed digital and radiofrequency systems with serenity.

1

Degradation of Rise Time in Interconnects

Rise time is one of the main parameters of a digital signal: the higher the speed, the more rise time must be controlled. Interconnects generally behave like low-pass circuits, and propagation in limited bandwidth circuits leads to an increasing rise time, thus limiting the performance of transmission systems. This chapter discusses the degradation of rise time resulting from propagation in interconnects. We will focus on the relationship between the bandwidth of signals in the base band and the modifications in parameters of these signals, in particular rise time, when they propagate in a limited bandwidth channel.

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