**Anna University**

**147402: Communication Theory**

**SEM / YEAR: IV/ II**

**Q****UESTION BANK**** **

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UNIT-III

**NOISE THEORY **

**PART A **

1. Define Shot noise.

Ans:

The noise caused by random fluctuations in the motion of charge carriers in a conductor.

2. What is White noise?

Ans:

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.

3. Calculate noise figure and equivalent noise temperature for a receiver connected to an antenna whose resistance is 100Ω and equivalent noise resistance is 50Ω.

Ans:

F = (1 + (Req / Ra)) = ( 1 + ( 50 / 100) ) = 1.5.

4. Define Noise equivalent Bandwidth.

Ans:

The noise equivalent bandwidth of the filter is defined as the bandwidth of an ideal filter at which the noise power passed by real filter and ideal filter is same.

5. Define Thermal noise. Give the expression for the thermal noise voltage across a register.

Ans:

Thermal noise is the name given to the electrical noise arising from the random motion of electrons in a conductor.

6. What is meant by figure of merit of a receiver?

Ans:

The ratio of output signal to noise ratio to channel signal to noise ratio is called figure of merit. Figure of Merit = (SNR)o/(SNR)c = (So/No)/(Si/nNi)

7. Define Noise Figure.

Ans:

It is defined as the ratio of input signal to noise ratio to the output signal to noise ratio.

F = Input (s/n) / output (s/n).

8. Define Noise Temperature.

Ans:

The available noise power is directly proportional to temperature and it is independent of value of resistance. this power specified in terms of temperatures is called as noise temperature.

9. What is partition Noise?

Ans:

In an electron tube having one or more positive grids, this noise is caused by irratic partition of the cathode current among the positive electrodes. In a transistor, the partition noise is created from the random fluctuation in the division of current between the collector and base.

10. Define a random variable?

Ans:

Random variable is defined as a rule or mapping from the original sample space to a numerical sample space subjected to certain constraints. Random variable is also defined as a function where domain is the set of outcomes ωεs and whose range is R, is the real line.

11. What is a random process?

Ans:

A Random process X (s,t) is a function that maps each element of a samples space into a time function called sample function. Random process is a collection of time functions.

12. What is a stationary random process?

Ans:

When the statistical properties of a process do not change with time, it is called Stationary process.

13. A receiver connected to an antenna of resistance of 50Ω has an equivalent noise resistance of 30Ω. Find the receiver noise figure.

Ans:

F = (1 + (Req / Ra)) = ( 1 + ( 30 / 50) ) = 1.6.