Modulation and Demodulation Interview Questions and Answers:

1. What is meant by communication?

Ans. Communication is the process of establishing connection or link between two points for information exchange.

2. What is communication system?

Ans. The electronic equipment’s used in communication are called communication equipment’s. Different communication equipment’s when assembled together form a communication system.

3. What is modulation?

Ans. The process of superimposing audio signal over the carrier (high frequency) wave is called the modulation.

4. What is amplitude modulation?

Ans. The process of varying amplitude of the high frequency or carrier wave in accordance with the intelligence (code, voice or music) to be transmitted, keeping the frequency and phase of the carrier wave unchanged, is known as the amplitude modulation.

5. What is meant by EM spectrum?

Ans. The EM spectrum is defined as the range of frequencies from approximately 30 Hz to visible light over which the electronic communication takes place.

6. What is difference between frequency and wavelength of EM wave ?

Ans. Frequency is defined as the number of cycles completed per second by the waveform and is expressed in Hz. On the other hand, wavelength (λ) is defined as the distance travelled by an EM wave during time duration of one cycle.

The relation between the two is λ = 3 x 10⁸/f m/s

7. What is modulation index in AM?

Ans. The extent to which the amplitude of the carrier wave is varied by the modulating signal is called the modulation index and is defined as the ratio of change in amplitude of the carrier wave to the amplitude of normal carrier wave.

8. Why the reception in ease of amplitude modulation is usually noisy?

Ans. In case of AM, the reception is usually noisy because radio receiver cannot distinguish between the amplitude variations that represent noise and those contain the desired signal.

9. What is the bandwidth of AM wave?

Ans. The difference between the two extreme frequencies is equal to the bandwidth of the AM wave.

Bandwidth, BW = (ω_c + ω_m) – (ω_c – ω_m) = 2ω_m

10. Why is DSBFC transmission system widely used in broadcasting inspite of the fact that SSBSC or SSB transmission system has numerous advantages over it?

Ans. DSBFC transmission system is widely used in broadcasting because of relative simplicity of its modulating equipment.

11. How can a DSBSC signal be obtained?

Ans. A DSBSC signal is obtained by simply multiplying modulating signal with carrier signal. This is achieved by a product modulator.

12. What is the bandwidth of DSBSC signal ?

Ans. Bandwidth of DSBSC signal = (ω_c + ω_m) – (ω_c – ω_m) = 2ω_m

13. What do you understand by vestigial sideband and where it is used?

Ans. A vestigial sideband modulation system is actually a compromise between DSBSC and SSB modulation systems. In other words, it can be said that it is an optimum choice in which the advantages of DSBSC and SSB modulation systems have been exploited.

In VSB modulation instead of rejecting one sideband completely as in SSB modulation scheme, a gradual cutoff of one sideband is allowed. This gradual cut is compensated by a vestige or portion of the other sideband.

VSB is used for video signal transmission in commercial television broadcasting.

14. What is meant by angle modulation?

Ans. Analog modulation may be defined as the process in which the total phase angle of a carrier wave is varied in accordance with the instantaneous value of the modulating or message signal while keeping the amplitude of the carrier constant.

15. What is meant by frequency modulation?

Ans. Frequency modulation (FM) is that type of angle modulation in which the instantaneous frequency is varied linearly with a message or baseband signal about an unmodulated carrier frequency ω_c. This means that the instantaneous value of the angular frequency will be equal to the carrier frequency ω_c plus a time-varying component proportional to the modulating signal.

16. What is meant by phase modulation?

Ans. Phase modulation (PM) is that type of angle modulation in which the phase angle is varied linearly with a baseband or modulating signal about an unmodulated phase angle (w_ct + Φ_c).

17. What is frequency deviation?

Ans. The change or shift above or below the mean or centre frequency is called the frequency deviation.

18. What is carrier swing?

Ans. The total variation in frequency from minimum to maximum value is called the carrier swing and it is twice the frequency deviation i.e., CS = 2Δf.

19. What is FM index?

Ans. The ratio of frequency deviation to modulation frequency is called the FM index.

20. What is deviation ratio?

Ans. The ratio of maximum permitted frequency deviation to the maximum permitted audio frequency is known as the deviation ratio.

21. What is per cent modulation in FM?

Ans. The per cent modulation when applied to FM, has slightly different meaning than when applied to AM. In FM, it is given as the ratio of actual frequency deviation to maximum permitted frequency deviation and multiplied by 100.

22. In FM radio broadcasting, the modulation is 40%. What is the value of frequency deviation?

Ans. Frequency deviation, f_d = m_ff_m = 0.4f_m

23. In an FM modulation system, the modulation index is doubled. By what percentage does the total transmitted power increase?

Ans. In FM, the amplitude of the carrier wave is not affected during the process of frequency modulation. Hence the total power in the wave remains unchanged irrespective of the degree of modulation. So on doubling the modulation index there will be 0% change in the total transmitted power.

24. What is wideband FM?

Ans. When the value of modulation index m_f is quite large, then in FM a large number of sidebands are produced and hence the BW of FM is sufficiently large. This type of FM system is known as wideband FM.

25. What are the disadvantages of frequency modulation (FM)?

Ans. A much wider channel typically 200 kHz is required in FM against only in 10 kHz in AM broadcast. This is serious drawback of FM. FM transmitting and receiving equipment particularly used for modulation and demodulation tend to be more complex and hence more costly.

26. What is demodulation?

Ans. Demodulation or detection is a process of recovering the original modulating signal (intelligence) from the modulated carrier wave i.e., the demodulation is a process reverse of the process of modulation.

27. What is heterodying action?

Ans. The process of combining or mixing of two signals of different frequencies in order to produce a signal of new frequency is called the heterodying action.