Question

A 400 W carrier is amplitude modulated to a depth of 75%.Calculate the total power in AM wave. (i) Explain the types of noise in a communication system. (ii) Compare simplex and duplex mode of communication.

Answer 1

(i) Noise: Noise is any spurious or undesired disturbances that mask the received signal in a communication system.

a) Atmospheric Noise: Atmospheric Noise is also known as static noise which is the natural source of disturbance caused by lightning, discharge in thunderstorm and the natural disturbances occurring in the nature.

b) Industrial Noise: Sources of Industrial noise are auto-mobiles, aircraft, ignition of electric motors and switching gear.

c) Extraterrestrial Noise: Extraterrestrial Noise exist on the basis of their originating source. They are i) Solar Noise ii) Cosmic Noise

Internal Noise are the type of Noise which are generated internally or within the Communication System or in the receiver. They are as follows:

1) Shot Noise : These Noise rises in the active devices due to the random behaviour of Charge particles or carries. In case of electron tube, shot Noise is produces due to the random emission of electron form cathodes.

2) Partition Noise : When a circuit is to divide in between two or more paths then the noise generated is known as Partition noise. The reason for the generation is random fluctuation the division.

3) Low- Frequency Noise : They are also known as FLICKER NOISE. These type of noise are generally observed at a frequency range below few kHz. Power spectral density of these noise increases with the decrease in frequency. That why the name is given Low- Frequency Noise . 

4) High- Frequency Noise : These noises are also known TRANSIT- TIME Noise. They are observed in the semi-conductor devices when the transit time of a charge carrier while crossing a junction is compared with the time period of that signal.

5) Thermal Noise: Thermal Noise are random and often referred as White Noise or Johnson Noise. Thermal noises are generally observed in the resistor or the sensitive resistive components of a complex impedance due to the random and rapid movement of molecules or atoms or electrons. Dark current noise: When there is no optical power incident on the photodetector a small reverse leakage current still flows from the device terminals. This

Dark current contributes to the total system noise and gives random fluctuations about the average particle flow of the photocurrent.

The Dark current noise is given by: where e is the charge on an electron Id is the dark current

Quantum noise: Discrete nature of electrons cause a signal disturbance called Quantum noise or Shot noise.It arises from the statistical nature of the production and collection of photoelectrons.

Sr. No.	Simplex	Duplex
1	It is one way communication	It is a two way communication
2	Information is communicated in only one direction.	Information can transmit as well as receives simultaneously or not simultaneously.
3	ExamplesTV broadcasting, radio broadcasting, telemetry, remote control	ExamplesWalkytalky,telephone,mobile,Radar, FAX,Pager
4