S-Parameters for Signal Integrity

Описание

Preface
This book is the culmination of about ten years of writing that began while I was developing software for signal integrity analysis in oscilloscopes, which my company,
Teledyne LeCroy, ended up branding as Eye Doctor. During the development of this software, I found many recurring patterns in the solutions of s-parameter systems that allowed
me to do a lot of things with a slight change of the recipe. And in the development of the
SPARQ and the WavePulser, two time-domain reflectometry instruments used to measure
s-parameters, I learned a lot about s-parameters and calibrations.
Why s-parameters and signal integrity? Around the turn of the millennium, the field of
signal integrity changed dramatically. I’m not even sure the field had an actual name at that
time. As a check, I looked at two books written by a foremost authority on signal integrity
and one of my favorite authors, Howard Johnson. His first book [1] published in 1993 has no
index entry for signal integrity, while his second book [2], published in 2003, has extensive
entries. As signal integrity became more about signal propagation (in the title of Dr.
Johnson’s second book), the field tended increasingly towards electromagnetic properties
and today heavily overlaps with the field of microwaves and radio frequency (RF); with
this has come an increasing use of scattering parameters, or s-parameters, and the vector
network analyzer (VNA), which is the primary instrument for measuring them.
The fields of s-parameters and microwaves and RF are primarily about the frequency
domain, while the business of signal integrity is the time domain. Despite the wide use of
frequency-domain electromagnetic analysis, it’s in the end mostly about whether a bit that
is transmitted as a one or zero gets to the receiver and is interpreted properly.
S-parameters can be a confusing topic in many ways, including how they are measured,
what they represent, how they connect to circuit theory, how they are manipulated either
to integrate or remove them from a measurement, and especially how they interact with
waveforms in the time domain. This book is mostly about these things.
My feeling is that it is always good to understand better what is going on, even if one
is one is relying on software simulation tools or measurement instruments. Both of these
require their own level of understanding and expertise. Any experienced engineer has been
burned by the simulator or measurement instrument, either by doing something incorrectly,
or because of the limitations of these tools, which is why we constantly try to correlate the
two.
My expertise is mostly in measurement instruments, which are in themselves often quite
intricate and expensive, and have an industry of their own. Software and simulation, meanwhile, have grown into a giant industry, and the advances in the technology, especially
electromagnetic field solvers, have greatly increased the ability of signal integrity engineers.
In some ways, however, they have weakened our minds, especially when overly relied on,
and certainly have emptied our wallets. My original goal in the writing of this book was to
teach methods that can be employed to solve relatively simple problems. These simple probxiii

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