Digital Circuit Boards: Mach 1GHz

174 Pages·2012·0.957 MB·English

by  Ralph Morrison(auth.)

DIGITAL CIRCUIT BOARDS
DIGITAL CIRCUIT BOARDS
Mach 1 GHz
Ralph Morrison
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LibraryofCongressCataloging-in-PublicationData:
Morrison,Ralph.
Digitalcircuitboards:mach1ghz/RalphMorrison.
p.cm.
Includesbibliographicalreferences.
ISBN978-1-118-23532-4
1. Digitalelectronics. 2. Logicdesign. 3. Integratedcircuits. I. Title.
TK7868.D5M682012
621.382–dc23
2011043534
PrintedintheUnitedStatesofAmerica.
10987654321
Buildings have walls and halls.
People travel in the halls not the walls.
Circuits have traces and spaces.
Energy travels in the spaces not the traces.
Ralph Morrison
A word about the book title
Mach 1 was a barrier in flight for a long time. Aircraft that can go faster than
the speed of sound are more expensive and more difficult to design. There is
a barrier in digital design that occurs at clock rates around 1GHz. One clock
period is one nanosecond, and in this time an electromagnetic wave can travel
about15cminepoxy.Thisisthedimensionofatypicalcircuitboard.Circuits
that can perform near or above 1GHz present a new set of design challenges.
In that sense, there is a barrier to cross. This book discusses the challenges of
designing these faster and faster circuits. Many old ideas must be discarded
and new ones accepted. There are no sonic booms that I know of. I hope the
ride through Mach 1 is a smooth one.
CONTENTS
Preface xi
1
BASICS 1
1.1 Introduction 1
1.2 Why the Field Approach is Important 3
1.3 The Role of Circuit Analysis 4
1.4 Getting Started 5
1.5 Voltage and the Electric Field 6
1.6 Current 7
1.7 Capacitance 8
1.8 Mutual and Self-Capacitance 10
1.9 E Fields Inside Conductors 11
1.10 The D Field 12
1.11 Energy Storage in a Capacitor 12
1.12 The Energy Stored in an Electric Field 13
1.13 The Magnetic Field 13
1.14 Rise Time/Fall Time 15
1.15 Moving Energy into Components 15
1.16 Faraday’s Law 16
1.17 Self- and Mutual Inductance 16
1.18 Poynting’s Vector 17
1.19 Fields at DC 18
Glossary 19
2
TRANSMISSIONLINES 22
2.1 Introduction 22
2.2 Some Common Assumptions 24
2.3 Transmission Line Types 25
2.4 Characteristic Impedance 27
viii CONTENTS
2.5 Wave Velocity 29
2.6 Step Waves on a Properly Terminated Line 30
2.7 The Open Circuited Transmission Line 31
2.8 The Short Circuited Transmission Line 33
2.9 Waves that Transition between Lines with Different Characteristic
Impedances 35
2.10 Nonlinear Terminations 38
2.11 Discharging a Charged Open Transmission Line 38
2.12 Ground/Power Planes 40
2.13 The Ground and Power Planes as a Tapered Transmission Line 41
2.14 Pulling Energy from a Tapered Transmission Line (TTL) 43
2.15 The Energy Flow Through Cascaded (Series) Transmission Lines 45
2.16 An Analysis of Cascaded Transmission Lines 48
2.17 Series (Source) Terminating a Transmission Line 49
2.18 Parallel (Shunt) Terminations 50
2.19 Stubs 52
2.20 Decoupling Capacitor as a Stub 54
2.21 Transmission Line Networks 54
2.22 The Network Program 55
2.23 Measuring Characteristic Impedance 56
Glossary 57
3
RADIATIONANDINTERFERENCECOUPLING 61
3.1 Introduction 61
3.2 The Nature of Fields in Logic Structures 62
3.3 Classical Radiation 62
3.4 Radiation from Step Function Waves 63
3.5 Common Mode and Normal Mode 66
3.6 The Radiation Pattern along a Transmission Line 70
3.7 Notes on Radiation 70
3.8 The Cross Coupling Process (Cross Talk) 71
3.9 Magnetic Component of Cross Coupling 72
3.10 Capacitive Component of Cross Coupling 74
3.11 Cross Coupling Continued 75
3.12 Cross Coupling between Parallel Transmission Lines of Equal
Length 76
3.13 Radiation from Board Edges 78
3.14 Ground Bounce 79
CONTENTS ix
3.15 Susceptibility 80
Glossary 80
4
ENERGYMANAGEMENT 82
4.1 Introduction 82
4.2 The Power Time Constant 84
4.3 Capacitors 86
4.4 The Four-Terminal Capacitor or DTL 87
4.5 Types of DTLs 89
4.6 Circuit Board Resonances 90
4.7 Decoupling Capacitors 90
4.8 The Board Decoupling Problem 92
4.9 The IC Decoupling Problem 93
4.10 Comments on Energy Management 94
4.11 Skin Effect 95
4.12 Dielectric Losses 97
4.13 Split Ground/Power Planes 97
4.14 The Analog/digital Interface Problem 98
4.15 Power Dissipation 99
4.16 Traces through Conducting Planes 100
4.17 Trace Geometries that Reduce Termination Resistor Counts 101
4.18 The Control of Connecting Spaces 101
4.19 Another way to look at Energy Flow in Transmission Lines 103
Glossary 104
5
SIGNALINTEGRITYENGINEERING 106
5.1 Introduction 106
5.2 The Envelope of Permitted Logic Levels 107
5.3 Net Lists 108
5.4 Noise Budgets 108
5.5 Logic Level Variation 109
5.6 Logic and Voltage Drops 110
5.7 Measuring the Performance of a Net 111
5.8 The Decoupling Capacitor 112
5.9 Cross Coupling Problems 114
5.10 Characteristic Impedance and the Error Budget 114
5.11 Resistor Networks 116
5.12 Ferrite Beads 117
x CONTENTS
5.13 Grounding in Facilities: A Brief Review 118
5.14 Grounding as Applied to Electronic Hardware 120
5.15 Internal Grounding of a Digital Circuit Board 123
5.16 Power Line Interference 124
5.17 Electrostatic Discharge 125
Glossary 126
6
CIRCUITBOARDS 130
6.1 Introduction 130
6.2 More about Characteristic Impedance 131
6.3 Microstrip 133
6.4 Centered Stripline 135
6.5 Embedded Microstrip 136
6.6 Asymmetric Stripline 137
6.7 Two-Layer Boards 140
6.8 Four-Layer Circuit Board 143
6.9 Six-Layer Boards 145
Glossary 147
AbbreviationsandAcronyms 149
Bibliography 157
Index 159
PREFACE
The story of Digital Circuit Boards at Mach 1 GHz starts with my friend Daniel
Beeker. Dan is a senior field applications engineer for Freescale Semiconductor. He
was instrumental in getting me interested in circuit board design problems. He was
the one that spurred me into finishing the 5th edition to my book “Grounding and
Shielding,” which was published by John Wiley in 2007. I have rewritten this book
five times since 1967 and when this fifth writing was finished, I really thought I was
through writing books. Obviously, I was mistaken.
Dan sees the problems encountered by his customers. He recommended to his
managementthattheusersmustbeprovidedsomehelpintheformofseminars.They
agreed, and as a result Dan took on a new set of responsibilities. He was tasked to
find speakers and arrange for seminars for Freescale customers. To locate speakers,
Dan turned to his own personal library. The first book he took from the shelf was a
copy of my “Grounding and Shielding.” He then looked into my web site and found
my address. The result was that I was invited to participate in the Freescale Forum in
Orlando and later to give a seminar to his customers in the Detroit area.
The seminar I gave was based on my book and was well received. For those
familiar with my books, I use very simple physics to explain how interference is
generated,howitenterscircuits,andhowthecircuitscanbeprotected.Theprinciples
are the same whether the problem is analog, digital, or rf. I had little trouble bringing
transmissionlinetheoryintomydiscussions.IfoundIhadtocatchuponthelanguage
of circuit boards and how they are built. I had to find out what a BGA was, what
prepreg meant, and what is an interposer board. I had to learn the difference between
a blind and a buried via.
Fortunately, Dan followed through with additional seminars where the speakers
understood the details of circuit board design and could relate more closely with the
details of components and materials. I admire Dan for recognizing that the funda-
mentals must come first. Even though I knew little about the details of board design,
I could show the users how and why layout geometry was critical if they were going
to build successful boards.
Dan wanted me to get closer to the circuit board problems, so he arranged for
me to attend the PCB conference in Santa Clara, given by the UP Media Group.
At the conference I sat in on courses given by experts in the field. I learned a lot
about how circuit boards were designed and built. The talks introduced me to the
designers’ problems. Many of the speakers used a combination of circuit theory and
lore to explain circuit board behavior. This is just the problem I had been dealing
xii PREFACE
with throughout my career. I was in a different field with a new language. I had a lot
to learn. I wanted to understand the digital layout problem based on physics, not on
lore.
I found that digital engineers were working in areas of nanosecond delays and
picosecond rise times. This was an area where I understood the physics but not the
detailsofboardconstruction.Irecognizedthatrealtimedelayswereinvolved,andthis
was not covered by circuit theory. The speakers related their real world experiences
and how they resolved many difficult problems. I remember speakers saying that
energy could be drawn from the ground/power plane faster than from a capacitor.
This got the wheels turning. How fast is fast and how much energy is there? How
fast are capacitors?
Iknewthebasicphysicssothechallengewastolearnthelanguageandmakesense
out of all the material that was being presented. When I got home from the show one
of the first things I examined was how energy is moved from a ground/power plane.
Iassumedacoaxialconnectiontotheconductingplanes.Ifasteploadwasplacedon
this connection, the wave that propagated outward moved in a circular pattern. The
characteristic impedance of the wave depended on the radial distance from the point
ofcoaxialconnection.Irecognizedthattherewerecontinuousreflectionsasthewave
propagated outward and that the energy returned to the source increased with time.
Muchtomysurprise,Ifoundthatthepowercurvedependedonconductorspacingand
notonthedielectricconstant.Withagreaterdielectricconstant,theenergywaspulled
from a smaller area. With a higher dielectric constant and with multiple demands for
energy there would be less cross talk.
This exercise helped me understand the problem of connecting to the
ground/power plane. I recognized that vias were the accepted method of making
connections between layers. Typically, a via geometry has an aspect ratio of unity,
where the characteristic impedance is about 50 ohm. When I assumed that a short
lead length of 50 ohm was used to connect the ground/power plane to a load, I found
out a very important fact. A short section of coaxial line placed between the load and
the ground/power plane increased the rise time significantly. What was happening
was that a large number of wave reflections were required to move energy across
this short connection.
This one fact really caught my attention. It had a far reaching impact on my
thinking. How many places were there, where short sections of transmission lines
were used to connect a load to a source of energy?
The first area I considered was the capacitor. The available books and all the
speakers discussed the natural frequencies of capacitors. The limitations in perfor-
mance were assigned to the series inductance. This was a good explanation for axial
lead capacitors but what about surface mounted components? Because capacitors of
any geometry have a natural frequency when tested with sine waves, the assumption
that resonance is related to a simple series inductance is made. In my view, I was
dealing with step functions and reflections, and these ideas of circuit theory did not
exactly fit. I decided then that I needed to look at digital processes in a consistent
manner. I could not mix step function discussions with sine wave terminology. I had
tocleanupmyunderstandingandexplainthingsinanunambiguousmanner.Resonant