Fundamentals of Signal Integrity
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Signal Integrity Described
By definition, “integrityÓ mean s “complete and unimpaired.”
Likewise, a digital signal with good integrity has clean, fast
transitions; stable and valid logic levels; accurate placement
in time and it would be free of any transients.
Evolving technology makes it increasingly difficult
for system developers to produce and maintain
complete, unimpaired signals in digital systems.
The purpose of this primer is to provide some insight into
signal integrity-related problems in digital systems, and to
describe their causes, characteristics, effects, and solutions.
Digital Technology and the
Information Age
It’s been over twenty years since the personal computer
emerged and almost as long since cellular telephony went
from being a novelty to a consumer necessity. For both, one
trend has remained constant: the demand for more features
and services, and the need for more bandwidth to deliver
them. First-generation PC users were excited about the
power of creating a simple spreadsheet. Now they demand
detailed graphics, high-quality audio, and fast-streaming
video. And, cell phones are hardly a tool anymore for just
conversation.
Our much-smaller world now depends on increasingly more
content and its rapid, reliable delivery. The term “Information
Age” was coined to describe this new interwoven, interde-
pendent, data-based culture.
With the Information Age has come a steady stream of
technology breakthroughs in the fields of semiconductors,
PC bus architectures, network infrastructures, and
digital wireless communications. In PCs— and especially
in servers— processor speeds have escalated into the
multi-GHz range, and memory throughput and internal bus
speeds have risen right along with them.
These dramatically increased rates support computer
applications such as 3D games and computer-aided design
programs. Sophisticated 3D imagery requires a huge amount
of bandwidth at the circuit board level, where the CPU, the
graphics subsystem, and the memory has to move data,
constantly, as the image moves.
Computers are just one facet of the bandwidth-hungry
Information Age. Digital communication equipment designers
(and particularly thos
e developing the electrical and optical
infrastructure elements for both mobile and fixed networks)
are moving toward 40 Gb/s data rates. And digital video
product development teams are designing a new generation
of transmission equipment for high-definition, interactive
video.
Numerous technologies are pushing these data rate
advancements. Serial buses are emerging to break the speed
barriers inherent in older, parallel bus architectures. In some
cases, system clocks are intentionally dithered to reduce
unintended radiated emissions. And smaller, denser circuit
boards using ball grid array ICs and buried vias have become
common as developers look for ways to maximize density
and minimize path lengths
.
Rising Bandwidth Challenges Digital
Design
Tod a y ’s digital bandwidth race requires innovative thinking.
Bus cycle times are now up to a thousand times faster
than they were twenty years ago. Transactions that once
took microseconds are now measured in nanoseconds.
To achieve this improvement, edge speeds today are now
a hundred times faster than before.
Circuit board technology, however, has not kept pace
because of certain physical realities. The propagation time of
inter-chip buses has remained virtually unchanged. Although
geometries have shrunk, circuit boards still need sufficient
space for IC devices, connectors, passive components, and
of course, the bus traces themselve
s. This space equates to
distance, and distance means delay— the enemy of speed.
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