In electronic circuit and system design, the word ise is used to refer to any undesired excitation on the system. In other contexts, ise is also used to refer to signals or excitations which exhibit chaotic or random behavior. The source of ise can be either internal or external to the system. For instance, the thermal and shot ise generated within integrated circuit devices are in- ternal ise sources, and the ise 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 ise sources on the performance of an electronic system is called ise analysis or ise simulation. A methodology for the ise 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 ise sources. * Mathematical model or representation for the system that is under the in- fluence of the ise sources.