Noise is an unwanted electrical signal which gets added on a transmitted signal when it is travelling towards the receiver. Electrical noise is defined as any undesired electrical energy. For Example: In audio recording any unwanted electrical signals that fall within the audio frequency band of 0 khz to 15khz will interface with the music and therefore considered as noise.
Noise figure is a figure of merit and used to indicate how much the signal to noise ratio gets degraded as a signal passes through a series of circuits.
Noise can be divided into two general categories:
(i) Correlated Noise : Implies a relationship between the signal and the noise.
(ii) Uncorrelated Noise : It is present all the time whether there is a signal or not.
Uncorrelated can be divided into two general categories: (i) External noise and (ii) Internal noise.