Noise Basics and types of Noise
Noise signal is the unwanted electrical signal present in the circuit or system other than the desired useful signal. It can alter frequency or amplitude or phase of the desired signal in a unpredictable and unwanted fashion. It is referred as interference when noise voltage results into improper functioning of the system(or circuit).
Noise can not be completely eliminated or removed; but can be reduced in magnitude so that system functioning does not get affected. When we communicate noise always will be present in the signal due to various factors as explained below. Following section describes different types of noise. Engineers work to explore the possibilities to minimize the effect of these types of noises on the system.
The manmade noise is present usually in urban and industrial areas. Noise generated from automobile vehicles, electrical motors used in aircrafts, leakage produced from high voltage electrical lines, fluorescent lights are examples of noise. These type of noise sources affect the signal in the frequency range from 1 to 600 MHz. The general rule of thumb is that more is the receiver bandwidth more is the noise along with the received signal. The noise can be kept within limit by proper shielding and also by eliminating the coupling channel.
The atmospheric noise is result of spurious radio frequency waves due to lightning and other natural disturbances occurring in the atmosphere. The other sources of this noise are rain, snow and dust storms near the RF antenna. This noise is important from frequence of 10 MHz till 20 MHz. This noise is observed while listening the radio program using short wave radio receiver. Atmospheric noise can be reduced by eliminating sharp points around the antenna.
Extra terrestrial noise
Solar noise, cosmic noise are examples of extraterrestrial noise as explained below.
The sun in our solar system radiates broad range of frequencies at the temperature of about 6000 degree C. These frequencies fall within the band used for various communication purposes by the mankind. This is referred as solar noise.
When the directional antenna is pointed towards the sky to receive the signal, other than the desired signal it also receives random noise from the galaxy. The intensity of this noise varies very widely. This is referred as cosmic noise. The major source of this kind of noise is caused by sun ionized intersetter gas clouds in our galaxy. There are two types of this noise viz. thermal and non thermal. As mentioned it is predominant if antenna is turned towards sky to receive the satellite signal. The cosmic noise exist from the frequency band in the range 15MHz to 100 GHz.
The internal noise is generated due to active as well as passive devices used in the radio receiver and other supporting electronic circuits. This noise is randomly distributed over the entire acquisition range. Hence it is directly related to the bandwidth. shot noise, quantum noise and thermal noise are examples of this type of noise.
shot noise is present in all the active devices including amplifiers.
This is generated due to random variations in the arrival of electrons and holes at the collector terminal of the transistor.
The short noise equation is as mentioned below:
Ish = 2* q* Idc *B;
Where Ish is rms value of shot current
q is electron charge of value 1.56 X 10-19 coulombs.
Idc is diode DC current
B is the bandwidth in Hz.
Thermal noise usually has flat noise density spectrum. It extends well into the infrared at room temperature.
As frequency increases , it reaches to the point where spectrum starts to fall. At this point
another noise referred as quantum noise emerges.
It has power spectral density as defined by equation below.
W(f) = h * f watts/Hz.
Where h is the planck's constant of value 6.63 * 10-34
f is the frequency in Hz
Quantum noise is signifact at optical frequency bands and is negligible at commonly used frequency.
Thermal noise is generated in a resistance or any resistive component of of the impedance. It is caused due to rapid and random motion of the electrons. Temperature of a particle expresses its kinetic energy. At temperature of 0 degree K , kinetic energy of particle becomes zero. Hence noise power produced in the resistor is proportional to absolute temperature and measurement bandwidth.
Thermal noise power, P = K*T*B
Where, B is the bandwidth
K is the boltmans constant equalto 1.38 X 10-23 J/K
T is the temperature in degree Kelvin
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