Broadly there are two types of noises-external noises and internal noises. External noises include atmospheric noise, solar noise, cosmic noise and industrial noise. Internal noises include thermal agitation noise, shot noise, transit-time noise, flicker and resistance. Atmospheric noise comes into picture when short waves reach a receiver when the receiver is not good enough to receive them. Usually atmospheric noise is caused by lightning discharges in thunderstorms and other natural electric disturbances occurring in the atmosphere. Even though its origin is in the form of amplitude modulated impulses, it spreads over most of the radio frequency spectrum normally used for broadcasting. Solar noise and cosmic noise belongs to the category of extraterrestrial noise. The noise due to solar disturbances is termed as solar noise and is repeated in every 11 years. Cosmic noise is a radio frequency noise originated from distant stars. Industrial noise is caused by fluorescent lamps, automobile and aircraft ignition, electric motors, heavy electric machines and high voltage lines. Thermal agitation noise is also known as Johnson noise which is caused by the different resistive components used in the system. Actually it is due the rapid and random motion of the molecules. Shot noise is generated by random variations in the arrival of electrons or holes at the output electrode of an amplifying device. Transit-time noise is due to the transit-time effect. Flicker is a noise developed in transistors occurs at low audio frequencies. Resistance is another form of noise occurring in transistors.
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