Noise
Noise is random, undesirable electrical noise that enters the communication system via communication medium and interferes with the transmitted message due to proper reception & reproduction original signal at receiver is not possible.
Noise is basically classified into to categories namely:
a) Internal Noise
b) External noise
a) Internal Noise: Noise which get generator within the receiver or communication system.
Internal noise are further classified as:
1.Shot noise
Thermal Noise
Transit time Noise
- Partition Noise
Flicker Noise
b) External Noise: Noise whose source are external to that of communication system.
External noise are further classified as:
1)Atmospheric noise
2) Extraterrestrial noise
3) Man-made noise
Internal Noise
1. Shot noise: It is produced by ‘particle-like’ behavior of electrons and photons when an external excitation is available to produce current. Shot noise is avoidable only if we reduce all current to zero.
2. Thermal Noise/ Johnson Noise: The thermal noise is due to thermal agitation of electrons in a conductor. It is distributed across the entire spectrum and that is why it is also known as white noise (as the frequency encompass over a broad range of frequencies). Average thermal noise power is given by;
Pn = kTB watts
3.Transit-time noise: Transit-time noise is a similar phenomenon to shot noise in that it affects systems more as they get smaller due to the quantized nature of electricity. Transit-time noise results when a signal frequency's period is the same as the time an electron takes to travel from sender to receiver. The noise results from the statistical variations in actual electron flow.
4. Partition noise:
Partition noise is generated when the current get divided between two or more path.
It is generated due to the random fluctuation in the division. Therefore the partition noise in a transistor will be higher than that in diode. The device like gallium arsenide FET draw almost zero gate bias current, hence keeping the partition noise to its minimum value.
5. Flicker Noise:
The flicker noise will appear at frequencies below a few kilohertz. It is sometimes called as "1/f" noise.
In semiconductor device, the flicker noise is generated due to the fluctuationin the carrier density.
The mean square value of flicker noise voltage is proportional to the sqaure of direct current flowing through the device.
b) External Noise
1)Atmospheric noise
2) Extraterrestrial noise
3) Man-made noise
1) Atmospheric noise:
This type of noise gets produced within the Earth's atmosphere. The common source of this type is lighting. This type of noise is in the form of impulse or spikes which covers a wide frequency typically upto 30MHz.The sputtering, cracking etc heard from the loud speakers of radio is due to atmospheric noise. This noise becomes significant above 30MHz.
2) Extraterrestrial noise
This type of noise originate from the sources which exist outside the earth's atmosphere. Hence this noise is also called as deep space noise.
The noise originating from the sun & the outer space is known as Extraterrestial noise.
The extraterrestial noise can be sub-divided into two groups:a) Solar Noise b) Cosmic noise.
3) Manmade noise
The manmade noise is genrated due to the make & break process in a current carrying circuit.
The example are the electrical motor, welding machine, ignition system of automobile, thyristorised high current circuit, switching gears etc.
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