In electronic circuit and system design, the word noise is used to refer
to any undesired excitation on the system. In other contexts, noise is
also used to refer to signals or excitations which exhibit chaotic or
random behavior. The source of noise can be either internal or external
to the system. For instance, the thermal and shot noise generated within
integrated circuit devices are in- ternal noise sources, and the noise
picked up from the environment through electromagnetic interference is
an external one. Electromagnetic interference can also occur between
different components of the same system. In integrated circuits (Ies),
signals in one part of the system can propagate to the other parts of
the same system through electromagnetic coupling, power supply lines and
the Ie substrate. For instance, in a mixed-signal Ie, the switching
activity in the digital parts of the circuit can adversely affect the
performance of the analog section of the circuit by traveling through
the power supply lines and the substrate. Prediction of the effect of
these noise sources on the performance of an electronic system is called
noise analysis or noise simulation. A methodology for the noise analysis
or simulation of an electronic system usually has the following four
components: 2 NOISE IN NONLINEAR ELECTRONIC CIRCUITS - Mathematical
representations or models for the noise sources. - Mathematical model or
representation for the system that is under the in- fluence of the noise
sources.