Atmospheric noise or static is caused by lighting discharges in thunderstorms and other natural electrical disturbances occurring in the atmosphere. These electrical impulses are random in nature. Hence the energy is spread over the complete frequency spectrum used for radio communication.
There are numerous types of extraterrestrial noise or space noises depending on their sources.
However, these may be put into following two subgroups.
1. Solar noise
2. Cosmic noise
This is the electrical noise emanating from the sun. Under quite conditions, there is a steady
radiation of noise from the sun. This results because sun is a large body at a very high
temperature and radiates electrical energy in the form of noise over a very wide frequency
spectrum including the spectrum used for radio communication.
Distant stars are also suns and have high temperatures. These stars, therefore, radiate noise in
the same way as sun. The noise received from these distant stars is thermal noise (or black body
noise) and is distributing almost uniformly over the entire sky. We also receive noise from the
center of our own galaxy from other distant galaxies.
Man-Made Noise (Industrial Noise)
Man-made noise or industrial- noise is meant the electrical noise produced by such sources as automobiles and aircraft ignition, electrical motors and switch gears, leakage from high voltage lines, fluorescent lights, and numerous other heavy electrical machines.
Conductors contain a large number of 'free" electrons and "ions" strongly bound by molecular forces. The ions vibrate randomly about their normal positions, however, this vibration being a function of the temperature. Continuous collisions between the electrons and the vibrating ions take place. Thus there is a continuous transfer of energy between the ions and electrons. This is the source of resistance in a conductor. There is a random motion of the electrons which give rise to noise voltage called thermal noise.
The most common type of noise is referred to as shot noise which is produced by the random arrival of 'electrons or holes at the output element of PN junction.
Transit Time Noise
Another kind of noise that occurs in transistors is called transit time noise.
Transit time is the duration of time that it takes for a current carrier such as a hole or current to move from the input to the output.
Flicker noise or modulation noise is the one appearing in transistors operating at low audio frequencies.
Transistor Thermal Noise
Within the transistor, thermal noise is caused by the emitter, base and collector internal resistances.
Partition noise occurs whenever current has to divide between two or more paths, and results from the random fluctuations in the division.