### Communication Theory–Modulation Systems (Unit 4)–Two Marks with answers

Anna University

147402: Communication Theory

SEM / YEAR: IV/ II

QUESTION BANK

UNIT-IV

PERFORMANCE OF CW MODULATION SYSTEMS

PART A

1. What do you understand by Capture Effect in FM?

Ans:

When the interference signal and FM input are of equal strength, the receiver fluctuates back and froth between them .This phenomenon is known as the capture effect.

2. What is sensitivity and selectivity of receivers?

Ans:

Selectivity:

Selectivity of a receiver is defined as its ability to select the desired signals among the various signals.

Sensitivity:

It is defined as a measure of its ability to receive weak signals

3. What is the purpose of pre emphasis and de emphasis in FM?

Ans:

The psd of noise at the output of FM receiver usually increases rapidly at high frequencies but the psd of message signal falls off at higher frequencies. This means the message signal does not utilise the frequency band in efficient manner. Such more efficient use of frequency band and improved noise performance can be obtained with the help of pre emphasis and de-emphasis.

4. Define SNR.

Ans:

It is defined as the ratio of signal power to the noise power.

5. How to achieve threshold reduction in FM receiver?

Ans:

When the carrier to noise ratio reduces tocertain value, tyhe message information is lost.

The performance of the envelope detector deteriorates rapidly and it has no proportion with carrier to noise ratio.

6. This is called thershold effect.

Ans:

As the input noise power is increased the carrier to noise ratio is decreased the receiver breaks and as the carrier to noise ratio is reduced further crackling sound is heard and the output SNR cannot be predicted by the equation. This phenomenon is known as threshold effect.

7. What is meant by FOM 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/Ni)

8. What is the SNR for AM with small noise case?

Ans:

(So/No)/(Si/Ni) = (Ka2P)/ (1+ Ka2 P)

9. What is threshold effect with respect to noise?

Ans:

When the carrier to noise ratio reduces to certain value, type message information is lost.

The performance of the envelope detector deteriorates rapidly and it has no proportion with carrier to noise ratio. This is called thershold effect.

10. What is FM threshold effect?

Ans:

As the input noise power is increased the carrier to noise ratio is decreased the receiver breaks and as the carrier to noise ratio is reduced further crackling sound is heard and the output SNR cannot be predicted by the equation. This phenomenon is known as threshold effect.

11. Define Pre-emphasis and De-emphasis.

Ans:

Pre-emphasis

The premodulation filtering in the transistor, to raise the power spectral density of the

base band signal in its upper-frequency range is called pre-emphasis (or pre distortion) Pre emphasis is particularly effective in FM systems which are used for transmission of audio signals.

De-emphasis

The filtering at the receiver to undo the signal pre-emphasis and to suppress noise is called de-emphasis.