The Printed Circuit Designer's Guide To... Signal Integrity by Example

PEER REVIEWERS This book has been reviewed for technical accuracy by the following experts from the PCB industry. Happy Holden Consulting Technical Editor, I-Connect007 Happy Holden is the retired director of electronics and innovations for Gentex Corporation. Happy is the former chief technical officer for the world’s largest PCB fabricator, Hon Hai Precision Industries (Foxconn). Prior to Foxconn, Holden was the senior PCB technologist for Mentor Graphics and advanced technology manager at Nan Ya/ Westwood Associates and Merix. Happy previously worked at Hewlett-Packard for over 28 years as director of PCB R&D and manufacturing engineering manager. He has been involved in advanced PCB technologies for over 47 years. Eric Bogatin Eric Bogatin is currently the dean of the Teledyne LeCroy Signal Integrity Academy. Additionally, he is an adjunct professor at the University of Colorado- Boulder in the ECEE department, where he teaches a graduate class in signal integrity and is also the editor of Signal Integrity Journal. Bogatin received his BS in physics from MIT, and MS and PhD in physics from the University of Arizona in Tucson. Eric has held senior engineering and management positions at Bell Labs, Raychem, Sun Microsystems, Ansoft and Interconnect Devices. Bogatin has written six technical books in the field, and presented classes and lectures on signal integrity worldwide. MEET THE AUTHOR Fadi Deek Signal/Power Integrity Specialist Corporate Application Engineer In 2005, Fadi received his B.S. degree in computer and communications from the American University of Science and Technology (AUST) in Beirut, Lebanon. That same year, he joined Fidus Systems as a design engineer. He designed circuit boards at Fidus for three years. In 2010, Deek received his M.S. in electrical engineering from the University of Arkansas in Fayetteville. He soon joined Mentor Graphics as a corporate marketing engineer. In 2013, Deek became a corporate application engineer supporting the HyperLynx® tool suite. In parallel, he is also pursuing his Ph.D. at the University of Colorado in Boulder under the supervision of Dr. Eric Bogatin. The Printed Circuit Designer’s Guide to...™ Signal Integrity by Example Fadi Deek Mentor Graphics Corp., a Siemens Business © 2017 BR Publishing, Inc. All rights reserved. BR Publishing, Inc. dba: I-Connect007 PO Box 1908 Rohnert Park, CA 94927 U.S.A. ISBN: 978-0-9982885-2-9 Visit I-007eBooks.com for more books in this series. The Printed Circuit Designer’s Guide to...™ Signal Integrity by Example CONTENTS Introduction .......................................... 1 Chapter 1 Impedance ............................................ 4 Chapter 2 Reflections and Terminations .......................18 Chapter 3 Crosstalk ............................................. 28 Chapter 4 Differential Pairs .................................... 40 Summary and Conclusion ........................... 52 Glossary .............................................. 53 References ........................................... 55 INTRODUCTION In our current high-speed regime, interconnects are no longer transparent. Interconnects screw up the pristine performance of the signals coming off the chips. If you do not consider these problems and design them out of your product from the beginning, there’s a good chance your product will not work. This is what signal integrity is really about: how the electromagnetic fields of the signals interact with the boundary conditions of the dielectrics and conductors. Or, in the circuit's view, how the voltages and currents of the signals interact with and are distorted by the transmission lines and discontinuities of the interconnects. The general process we use to eliminate signal integrity problems is to first be aware of the problems we might encounter, and then follow the best design principles to design them out. Since every product is really customized, with its own set of tradeoffs between performance, cost, risk and schedule, ultimately, we have to optimize each design individually. This is most effectively done by applying analysis techniques such as rules of thumb, approximations and numerical simulations. We use these tools to explore design space as “virtual prototypes” trying different approaches and evaluating the “bang for the buck” to make engineering tradeoffs. 1 But we can’t follow this process blindly and just run simulation after simulation. The most effective engineers are those who have a firm grasp of the essential principles of signal integrity. The farther up the learning curve, the more effectively we can apply the best design principles and explore tradeoffs. That’s what this first book in the series is all about: exploring the essential principles by example. The examples are based on an essential principles signal integrity boot camp developed by Mentor, a Siemens business. In this book, we introduce examples of five out of the six different problems that can arise in leading-edge products and some of the design solutions. More information about the essential principles, their fundamental basis, and how we apply them, can be found in a book by Eric Bogatin, Signal and Power Integrity—Simplified, published in 2010 by Prentice Hall. It’s important to keep in mind that when using simulations to explore design space, we must always practice safe simulation. This means never performing a measurement or simulation without first anticipating what you expect to see. If it is not as you expect, there is always a reason that is worth exploring. In each example, we’ll apply the essential principles to illustrate the problem, the root cause, the solutions, and what we expect to see. Then we will use simulations to build virtual prototypes and explore design space to illustrate the problems and solutions. The four examples in this book include: 1. Designing controlled impedance transmission lines 2. Engineering proper terminations to minimize reflection noise 3. Reducing crosstalk 4. Optimizing differential pair design and termination We hope the information in this book helps you to better manage signal integrity issues in your next PCB design. 2 3 CHAPTER 1 Impedance Characteristic Impedance vs. Instantaneous Impedance A transmission line, or a trace on a printed circuit board with its associated return path, is electrically defined by two properties: its characteristic impedance, or Z , and its time delay, TD. As a signal propagates0 down the signal and return path, it will continuously encounter an instantaneous impedance. This means the signal will apply a voltage and drive a current through each infinitesimal section of the transmission line as shown in Figure 1-1. The impedance the signal sees is the instantaneous impedance. In a uniform transmission line, the instantaneous impedance is the same each step along the transmission line. That single impedance value is the characteristic impedance of the transmission line. This means that a non-uniform transmission line does not have just one impedance that characterizes it. v 10.0 nH C1 Signal path R2 Vin i 100.0 pF 0.0 ohms Return path ∆x Figure 1-1: Signal traversing a transmission line. So, what elements of the transmission line can affect its impedance? To answer the question, a stripline configuration will be used. For the analysis covered in this section, an advanced high-speed analysis tool was used to model several types of transmission lines, wires, cables 4 and connectors. One of the options available is a stripline modeler shown in Figure 1-2. Figure 1-2: Stripline modeler. How does each term affect theZ and the TD? The two dielectric height parameters will affect the capacitance per length of the trace by the following rough approximation,0 where is the material permittivity, is the width of the conductor and is the C = εw/H ε height or the separation between each conductor. By decreasing any of w H the heights H1 or H2, the capacitance will increase. The connection between the characteristic impedance and the capacitance per length is Z . Any increase in the capacitance per length will decrease Z . 0=√(L/C) Also, the width of the0 trace, if increased, will decrease Z since it will increase the capacitance per length. 0 Another factor that will affect the capacitance per length is the dielectric constant. From the capacitance approximation, any increase in ε will increase the capacitance per length. 5 The length of the conductor will not affectZ since, as mentioned before, the instantaneous impedance is constant in a uniform transmission line. 0 Other parameters will have some effect on the impedance, but it will be minimal. As an experiment, Table 1-1 shows the impact on characteristic impedance from a 10% change in each parameter. The initial nominal values of the parameters produced a 49.6-ohm impedance. To capture the new impedance, only one parameter was changed at a time and then reset to its initial value. Only H1 was varied, since the impact of H2 would be identical. We would expect that the things that affect capacitance per length the most would have the biggest impact. An item like dissipation factor should have no impact on the characteristic impedance at all. As can be seen from Table 1-1, the dielectric constant and line width had the biggest impact whereas the loss tangent had no impact at all. Parameter Initial value for Z=49.6 10% increase New Z % change in Z T 1.35 1.485 49 1.2 W 5 5.5 47.6 4 H1 7 7.5 50.7 2.2 Er 4.3 4.73 47.2 4.8 Lt 0.02 0.022 49.6 0 Table 1-1: Comparison of parameter effect onZ due to 10% increase. 0 When it comes to the time delay, TD, the characteristic impedance will have no impact. Only the length of the transmission line and the dielectric constant are expected to have an effect. The velocity of the signal can be calculated using where c is the speed of light in air. This formula shows that the velocity and the dielectric constant are inversely v=c/√(ε_r) proportional.

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