Introduction to Signal Integrity - Understanding Signal Integrity - page 3

the battery's DC) to pass. As we will see in later chapters, this same behavior also
occurs when signaling over modern circuit board traces.
During debug the sending voltage was incrementally increased to excessively
high values in an unfortunate attempt to improve signaling speed. This destroyed
the cable within a month of the Queen's inaugural message. In fact, the cable never
sent any commercial telegrams and was a financial failure that set off investigations
of financial fraud and technical mismanagement and ruined several careers.
Modern SI engineers and those who manage them can avoid the kinds of sys-
temic problems experienced during the 1858 cable project by learning the ways in
which pulses misbehave when traveling along long lines and learning techniques
for correcting these types of errors.
We begin by defining a pulse, then examine the time and frequency domains,
and describe two of the most important physical constants used in signal integrity
work. We conclude by briefly discussing the effects that many drivers simultane-
ously switching have on power supply noise.

1.3 What Is a Pulse?

A pulse is shown in Figure 1.1. The pulse width in time ( W , usually nanoseconds
or picoseconds) is measured between the two points where the signal crosses half
of the signal amplitude (“the 50% points”), while the pulse period ( T ) is measured
at the pulse base. The rise time ( t r ) is defined as the time required for the pulse to
increase from 10% of its steady state value to 90% of that value. The fall time (not
shown in the figure) is measured between the same voltage levels on the falling edge.
The rise or fall time can be measured in many ways [5, 6], but in this topic we will
stay with the commonly accepted “10/90” definition shown in the figure.
Under some conditions the pulse's rising or falling edge may exceed and then
oscillate around the steady state value. Usually this is undesirable and later chap-
ters describe the transmission line conditions that can cause the overshoot and
ringback conditions shown in the figure.

Overshoot

100%
90%

Ringback

50%

W

10%

t r

T

Figure 1.1 Pulse of width W , period T , and rise time t r overshoots and then rings back below the
steady state value.