Distributed Integrated Circuits
"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.
The 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.
This 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).
This 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.
Issues 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.
Frequency 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.
There 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. Therefore the 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.
