EC6402 CT 2marks-16marks, COMMUNICATION THEORY Question Bank, CT Short Answers – ECE 4th SEM Anna University

1.Define noise.
Noise is defined as any unwanted form of energy, which tends to interfere with proper reception and reproduction of wanted signal.
2. Give the classification of noise.
Noise is broadly classified into two types. They are (i)External noise
(ii)Internal noise.
3. What are the types of External noise? External noise can be classified into
1. Atmospheric noise
2. Extraterrestrial noises
3. Man –made noises or industrial noises
4. What are types of internal noise? Internal noise can be classified into 1. Thermal noise
2. Shot noise
3. Transit time noise
4. Miscellaneous internal noise
5. What are the types of extraterrestrial noise and write their origin?
The two type of extraterrestrial noise are solar noise and cosmic noise Solar noise is the electrical noise emanating from the sun.Cosmic noise is the noise received from the center part of our galaxy, other distant galaxies and other virtual point sources.
6. Define transit time of a transistor.
Transit time is defined as the time taken by the electron to travel from emitter to the collector.
7. Define flicker noise.
Flicker noise is the one appearing in transistors operating at low audio frequencies. Flicker noise is proportional to the emitter current and junction temperature and inversely proportional to the frequency.
8. State the reasons for higher noise in mixers.
1. Conversion transconductance of mixers is much lower than the transconductance of amplifiers.
2. If image frequency rejection is inadequate, the noise associated with the image frequency also gets accepted.
9. Define signal to noise ratio.
Signal to noise ratio is the ratio of signal power to the noise power at the same point in a system.
10. Define thermal noise. Give the expression for the thermal noise voltage across a resistor.
The electrons in a conductor possess varying amounts of energy. A small fluctuation in this energy produces small noise voltages in the conductor. These random fluctuations produced by thermal agitation of the electrons is called thermal noise.

11. What is narrowband noise?
The receiver of a communication system usually includes some provision for preprocessing the received signal. The preprocessing may take the form of a narrowband filter whose bandwidth is large enough to pass modulated component of the received signal essentially undistorted but not so large as to admit excessive noise through the receiver. The noise process appearing at the output of such filter is called narrow band noise.
12. Define noise temperature. (In terms of hypothetical temperature)
The available noise power is directly proportional to temperature and it is independent of value of resistance. This power specified in terms of temperature is called as noise temperature. It is denoted by Te .
13. What is shot noise?
When current flows in electronic device, the fluctuations number of electrons or holes generates the noise. It is called shot noise. Shot noise also depends upon operating conditions of the device.
14. What is White Noise?
Many types of noise sources are Gaussian and have flat spectral density over a wide frequency range. Such spectrum has all frequency components in equal portion, and is therefore called white noise. The power spectral density of white noise is independent of the operating frequency.
15. Define noise factor.
Noise factor (F) is defined as the ratio of signal to noise power ratio at the input to signal to noise power ratio at the output