"Divide and conquer" has been the underlying principle used to solve
many engineering and social problems. Over many years engineers have devised systematic
ways to divide a design objective into a collection of smaller projects and tasks
defined at multiple levels of abstraction. This approach has been quite successful
in an environment where a large number of people with different types and levels
of expertise work together to realize a given objective in a limited time. Communication
system design is a perfect example of this process, where the communication system
is initially defined atthe application level, then descried using system level
terms, leading to an architecture using a number of cascaded sub blocks that can
be implemented as integrated circuits.
integrated circuit design process is then divided further by defining the specifications
for circuit building blocks and their interfaces that together form the system.
The circuit designer works with the specifications at a lower level of abstraction
dealing with transistors and passive components whose models have been extracted
from the measurements, device simulations, or analytical calculations based on
the underlying physical principles of semiconductor physics and electrodynamics.
process of breaking down the ultimate objective into smaller, more manageable
projects and tasks has resulted in an increased in the number of experts with
more depth yet in more limited sublevels of abstraction. While this divide-and-conquer
process has been quite successful in streamlining innovation, the overspecialization
and short time specifications associated with today's design cycles sometimes
result in suboptimal designs in the grand scheme of things. Also, in any reasonably
mature field many of the possible innovations leading to useful new solutions
within a given level of abstraction have already been explored. Further advancements
beyond these local optima can be achieved by looking at the problem across multiple
levels of abstraction to find solutions not easily seen when one confines one's
search space to one level ( e.g., transistor-level circuit design).
explains why most of today's research activities occurs at the boundaries between
different levels of abstractions artificially created to render the problem more
tractable. Distributed circuit design is a multilevel approach allowing a more
integral co-design of the building blocks at the circuit and device levels. Unlike
most conventional circuits, it relies on multiple parallel signal paths operating
in harmony to achieve the design objective. This approach offers attractive solutions
to some of the more challenging problems in high speed communication circuit design.
In High-Speed Integrated Communication Circuits
Integration of high-speed
circuits for wireless (e.g., cellular phones) and wired applications (e.g., optical
fiber communications) poses several challenges. High-speed analog integrated circuits
used in wireless and wired communication systems have to achieve tight and usually
contradictory specifications. Some of the most common specifications are the frequency
of operation, power dissipation, dynamic range, and gain. Once in a manufacturing
setting, additional issues, such as cost, reliability, and repeatability, also
come into play. To meet these specifications, the designer usually has to deal
with physical and topological limitations caused by noise, device non-linearity,
small power supply, and energy loss in the components.
of operation is perhaps one of the most important properties of communication
integrated circuits since a higher frequency of operation is one of the more evident
methods of achieving larger bandwidth, and hence higher bit rates in digital communication
systems. A transistor in a given process technology is usually characterized by
its unity-gain frequency shown as fT. This is the frequency at which the current
gain of a transistor drops to unity. While the unity-gain frequency of a transistor
provide a approximate measure to compare transistors in different process technologies,
the circuit built using these transistors scarcely operate close to the fT and
usually operate at frequencies 3-100 times smaller depending on the complexity
of their function.
are two main reasons for this behavior. First, analog building blocks and systems
usually relay on closed loop operation based on negative feedback to perform a
given function independent of these parameter variations. An open loop gain much
higher than one is thus required for the negative feedback to be effective. Even
if no feedback is present and open loop operation is acceptable, a higher gain
usually improves the noise and power efficiency of the circuits.
transistor has to operate at a frequency lower than the fT to provide the desired
gain. Second, passive devices (e.g. capacitors and inductors), necessary in most
high-speed analog circuits, have their on frequency limitation due to parasitic
components that can become the bottleneck of the design. The combination of these
two effects significantly lowers the maximum frequency of reliable operation in
most conventional circuit building blocks and provides a motivation to pursue
alternative approaches to alleviate the bandwidth limitations.
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