Noise is a limiting factor is many receivers and other systems. Essentially, it is unwanted energy that sets the sensitivity of a receiver. Noise masks signals of interest at lower levels, preventing the receiver from detecting them. It is almost unavoidable and exists in all electronic systems, from audio and baseband electronic devices through the highest terahertz frequencies and optical electronic systems. While it cannot be eliminated, it can be controlled and managed, enabling receivers to operate effectively with low-level signals.
All components at normal operating temperatures generate some amount of noise when power is applied, due to the random motion of the electrons that account for the flow of current. This is one form of noise, known as thermal noise, associated with the heat that is a byproduct of the applied power. This heat is generated at all temperatures above absolute zero (−273ºC) since, in theory, charge carriers do not move at a temperature of absolute zero.