into space, which will lead to determining whether the system complies with governmental
emission requirements. We will see in later chapters how to account for all these things.
When a conductor must be considered as a distributed series of inductors and capacitors, it
is known as a transmission line. In general, this must be done when the physical size of the
circuit under consideration approaches the wavelength of the highest frequency of interest in
the signal. In the digital realm, since edge rate pretty much determines the maximum
frequency content, one can compare rise and fall times to the size of the circuit instead, as
shown in
Figure 1.2
. On a typical circuit board, a signal travels about half the speed of light
(exact formulas will be in later chapters). Thus a 500 ps edge rate occupies about 3 in. in
length on a circuit trace. Generally, any circuit length at least 1/10th of the edge rate must be
considered as a transmission line.
Figure 1.2: Rise time and circuit length.
One of the most difficult aspects of high-speed design is the fact that there are a large
number codependent variables that affect the outcome of a digital design. Some of the
variables are controllable and some force the designer to live with the random variation. One
of the difficulties in high-speed design is how to handle the many variables, whether they are
controllable or uncontrollable. Often simplifications can be made by neglecting or assuming
values for variables, but this can lead to unknown failures down the road that will be
impossible to "root cause" after the fact. As timing becomes more constrained, the
simplifications of the past are rapidly dwindling in utility to the modern designer. This book
will also show how to incorporate a large number of variables that would otherwise make the
problem intractable. Without a methodology for handling the large amount of variables, a
design ultimately resorts to guesswork no matter how much the designer physically
understands the system. The final step of handling all the variables is often the most difficult
part and the one most readily ignored by a designer. A designer crippled by an inability to
handle large amounts of variables will ultimately resort to proving a few "point solutions"
instead and hope that they plausibly represent all known conditions. While sometimes such
methods are unavoidable, this can be a dangerous guessing game. Of course, a certain
amount of guesswork is always present in a design, but the goal of the system designer
should be to minimize uncertainty.
Summary :
One of the most difficult aspects of high-speed design is the fact that there are a large number codependent variables that affect the outcome of a digital design. Without a methodology for handling the large amount of variables, a design ultimately resorts to guesswork no matter how much the designer physically understands the system. A designer crippled by an inability to handle large amounts of variables will ultimately resort to proving a few "point solutions"
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