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Transmission Lines in Digital Systems for EMC Practitioners

Transmission Lines in Digital Systems for EMC Practitioners

Clayton R. Paul

ISBN: 978-1-118-14399-5 November 2011 288 Pages


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This is a brief but comprehensive book covering the set of EMC skills that EMC practitioners today require in order to be successful in high-speed, digital electronics.  The basic skills in the book are new and weren’t studied in most curricula some ten years ago.  The rapidly changing digital technology has created this demand for a discussion of new analysis skills particularly for the analysis of transmission lines where the conductors that interconnect the electronic modules have become “electrically large,” longer than a tenth of a wavelength, which are increasingly becoming important.  Crosstalk between the lines is also rapidly becoming a significant problem in getting modern electronic systems to work satisfactorily.  Hence this text concentrates on the modeling of “electrically large” connection conductors where previously-used Kirchhoff’s voltage and current laws and lumped-circuit modeling have become obsolete because of the increasing speeds of modern digital systems.  This has caused an increased emphasis on Signal Integrity.

Until as recently as some ten years ago, digital system clock speeds and data rates were in the hundreds of megahertz (MHz) range.  Prior to that time, the “lands” on printed circuit boards (PCBs) that interconnect the electronic modules had little or no impact on the proper functioning of those electronic circuits.  Today, the clock and data speeds have moved into the low gigahertz (GHz) range.

Preface xi

1 Transmission Lines: Physical Dimensions vs. Electric Dimensions 1

1.1 Waves, Time Delay, Phase Shift, Wavelength, and Electrical Dimensions, 4

1.2 Spectral (Frequency) Content of Digital Waveforms and Their Bandwidths, 10

1.3 The Basic Transmission-Line Problem, 22

2 Time-Domain Analysis of Two-Conductor Lines 31

2.1 The Transverse Electromagnetic Mode of Propagation and the Transmission-Line Equations, 32

2.2 The Per-Unit-Length Parameters, 37

2.2.1 Wire-Type Lines, 37

2.2.2 Lines of Rectangular Cross Section, 47

2.3 The General Solutions for the Line Voltage and Current, 50

2.4 Wave Tracing and Reflection Coefficients, 54

2.5 A Simple Alternative to Wave Tracing in the Solution of Transmission Lines, 60

2.6 The SPICE (PSPICE) Exact Transmission-Line Model, 70

2.7 Lumped-Circuit Approximate Models of the Line, 75

2.8 Effects of Reactive Terminations on Terminal Waveforms, 84

2.8.1 Effect of Capacitive Terminations, 85

2.8.2 Effect of Inductive Terminations, 87

2.9 Matching Schemes for Signal Integrity, 89

2.10 Effect of Line Discontinuities, 96

2.11 Driving Multiple Lines, 101

3 Frequency-Domain Analysis of Two-Conductor Lines 103

3.1 The Transmission-Line Equations for Sinusoidal Steady-State (Phasor) Excitation of the Line, 104

3.2 The General Solution for the Line Voltages and Currents, 105

3.3 The Voltage Reflection Coefficient and Input Impedance of the Line, 106

3.4 The Solution for the Terminal Voltages and Currents, 108

3.5 The SPICE Solution, 111

3.6 Voltage and Current as a Function of Position on the Line, 112

3.7 Matching and VSWR, 115

3.8 Power Flow on the Line, 117

3.9 Alternative Forms of the Results, 120

3.10 Construction of Microwave Circuit Components Using Transmission Lines, 120

4 Crosstalk in Three-Conductor Lines 125

4.1 The Multiconductor Transmission-Line Equations, 125

4.2 The MTL Per-Unit-Length Parameters of Inductance and Capacitance, 131

4.2.1 Wide-Separation Approximations for Wires, 135

4.2.2 Numerical Methods, 145

5 The Approximate Inductive–Capacitive Crosstalk Model 155

5.1 The Inductive–Capacitive Coupling Approximate Model, 159

5.2 Separation of the Crosstalk into Inductive and Capacitive Coupling Components, 166

5.3 Common-Impedance Coupling, 172

5.4 Effect of Shielded Wires in Reducing Crosstalk, 173

5.4.1 Experimental Results, 182

5.5 Effect of Shield Pigtails, 183

5.5.1 Experimental Results, 187

5.6 Effect of Multiple Shields, 188

5.6.1 Experimental Results, 188

5.7 Effect of Twisted Pairs of Wires in Reducing Crosstalk, 197

5.7.1 Experimental Results, 203

5.8 The Shielded Twisted-Pair Wire: The Best of Both Worlds, 209

6 The Exact Crosstalk Prediction Model 211

6.1 Decoupling the Transmission-Line Equations with Mode Transformations, 212

6.2 The SPICE Subcircuit Model, 215

6.3 Lumped-Circuit Approximate Models of the Line, 231

6.4 A Practical Crosstalk Problem, 237

Appendix A Brief Tutorial on Using PSPICE 245

Index 267