Distributed effects and coupling in RF integrated circuits
Increasing frequencies and higher levels of integration, including integration of complete systems, results in the increased use of passive components such as transmission lines and inductors. Recent publications of wireless transceivers, for example , have shown 15 or more inductors on the same silicon substrate in an area of the order of 10-20 mm2. Because of the increased frequencies, long interconnects may no longer behave like an ideal wire with zero delay. Instead it may be more appropriate to think of long interconnect as distributed transmission lines. It will take signals a finite amount of time to travel along such transmission lines. The delay from input to output is equivalent to a phase shift of the output signal with respect to the input signal. Such delay can be problematic in achieving signal alignment, for example, in clock distribution systems, or in the design of combining circuits in power amplifiers. However, the same effects can be exploited, for example, in the design of distributed amplifiers. As well, because of the increased frequency and higher density of components, there can be coupling between components, for example, via conductive coupling through the substrate or via electromagnetic (typically inductive) coupling, for example, from parallel current-carrying wires. Such coupling can present difficulties in achieving isolation between components. However, coupling can also be exploited in the design of transformers, noninvasive testing, and injection-locked oscillators.