Interview Questions and Answers on Angle Modulation

Q.1. What do you mean by angle modulation ?

Ans. Angle modulation is 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.

Q.2. What are the types angle modulation ?

Ans. There are two types of angle modulation as under :

(i) Phase Modulation (PM)

(ii) Frequency Modulation (FM)

Q.3. Define phase modulation ?

Ans. Phase modulation (PM) is that type of angle modulation in which the phase angle Φ is varied linearly with a baseband or modulating signal x(t) about an unmodulated phase angle (ωct + θ0). This means that in Phase modulation , the instantaneous value of the phase angle equal to the phase angle of the unmodulated carrier (ωct + θ0) plus a time-varying component which is proportional to modulating signal x(t).

Q.4. Define frequency modulation ?

Ans. Frequency modulation is that type of angle modulation in which the instantaneous frequency ωi is varied linearly with a message or baseband signal x(t) about an unmodulated carrier frequency ω. This means that the instantaneous value of the angular frequency ωwill be equal to the carrier frequency ωplus a time-varying component proportional to the baseband signal x(t) .

Q.5. What is frequency deviation ?

Ans. The instantaneous frequency of FM signal varies with time around the carrier frequency ωc. This means that the instantaneous frequency of FM signal varies according to the modulating signal .The maximum change in instantaneous frequency from the average frequency ωc  is called frequency deviation .

Q.6. What is carrier swing ?

Ans. The total variation in frequency from the lowest to the highest point is called carrier swing. Obviously

the carrier swing = 2 × frequency deviation = 2 × Δω

Q.7. What is percent modulation an for FM ?

Ans. The term ” percent modulation ” as it is used in reference to FM refers to the ratio of actual frequency deviation to the maximum allowable frequency deviation. Thus 100% modulation corresponds to 75 kHz for the commercial FM broadcast band and 25 kHz for television .

percent modulation M = (Δ factual /Δ fmax)×100

Q.8. What is sideband FM ?

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

Q.9. What is transmission BW for FM ?

Ans. For n sidebands the bandwidth of FM wave is given by

BW = 2 n fHz

Q.10. What is Carson’s rule ?

Ans. Carson’s rule provides a thumb formula to calculate the bandwidth of a signal-tone wideband FM . According to this rule, the FM bandwidth is given as twice the sum of the frequency deviation and the highest modulating frequency. However, it must be remembered that this rule is just an approximation. Mathematically, BW = 2 (Δω + ω)

Q.11. What are disadvantages of FM order ?

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

Q.12. Define the term deviation in context with the frequency modulation.

Ans. Deviation “δ” is the amount of change in carrier frequency from its unmodulated value.

Q.13. How to vary the deviation?

Ans. Deviation is proportional to the instantaneous value of the modulating voltage, so by changing the amplitude of modulating signal, it is possible to change the deviation.

Q.14. What is the role of modulating frequency fmin frequency modulation ?

Ans. The rate at which the carrier frequency varies from its center value is equal to the modulating frequency. So with increase in fkeeping the amplitude constant, we will get the same frequency deviation at a faster rate.

Q.15. Define modulation index for FM. How is it different from that of AM ?

Ans. Modulation index for FM is m= δ/f. It is ratio of frequencies. In AM the modulation index is the ratio of amplitudes. Secondly the value of m, FM can be higher than 1, where as the value of m for AM can be at the most equal to 1 .

Ans. Angle

Ans. Amplitude

Q.18. If the modulating signal amplitude becomes negative then the carrier frequency _______ .

Ans. Goes below the unmodulated carrier frequency.

Ans. Phase

Q.20. When the modulating signal goes positive in PM the phase shift becomes ______ and carrier frequency _____ .

Ans. Lagging , decreases

Q.22. We can obtain PM from frequency modulator by passing the modulating signal through a  ________ .

Ans. Differentiator

Q.23. FM is produced at the phase modulator output if and only if _______ .

Ans. The modulating signal is changing its magnitude.

Q.24. The frequency deviation at the output of a phase modulator is maximum at ________ .

Ans. The zero crossing of the modulating signal.

Q.25. The bandwidth of an FM wave in terms of number of significant sidebands is ______ .

Ans. BW = 2  fm  × Number of significant sidebands

Q.26. The number of significant sidebands in FM depends upon _______ .

Ans. Modulation index .

Q.27. Deviation ratio in FM is defined as ________ .

Ans. Deviation ratio = Maximum allowable deviation / Maximum modulating frequency

Q.28. In FM the percent modulation is defined as  _______ .

Ans. Ratio of actual carrier deviation to maximum carrier deviation .

Ans. More

Q.30. A negative sign on the carrier and sideband amplitudes in the expression for FM means ______ .

Ans. Phase reversal

Q.31. The carrier in an FM signal can never drop to zero amplitude. True or False.

Ans. False. For some values of  mcarrier drops to zero.

Q.32. The amount of frequency deviation at the output of phase modulator is proportional to ______ .

Ans. Modulating voltage as well as modulating signal .

Q.33. For an FM signal, the maximum deviation occurs at ______ point of the modulating signal .

Ans. At the positive and negative peaks of modulating signal .

Q.34. Why FM is called as a type of angle modulation ?

Ans. Because along with frequency, the phase angle of the carrier also changes with instantaneous amplitude of modulating signal .

Q.35. Express the FM signal .

Ans. FM wave is represented as :  EFM = A sin [ ωct + mf  sin   ωmt ]

Ans. Infinite.

Ans. Infinite .

Q.38. How to calculate the amplitudes of various sidebands in the spectrum of FM ?

Ans. The amplitude of sidebands depend on the J coefficients and the J coefficients in turn depend on the modulation index  m.

Q.39. Justify the total transmitted power in FM always remains constant .

Ans. This is because, the peak amplitude A of the FM wave always remain constant, The transmitted power is given by :

is always constant .

Q.40. State Carson’s rule for bandwidth of FM wave .

Ans. BW = 2 [ δ + f m(max)] . This rule is applicable for the values of modulation index greater than 6 .

Q.41. What is the effect of mf  on the bandwidth of FM ?

Ans. With increase in m, the number of sidebands having significant amplitude will also increases. This will increase the bandwidth .

Q.42. Why is FM called a constant bandwidth system?

Ans. Because the bandwidth of FM changes marginally even when the modulating frequency is changed to a great extent .

Q.43. What is the difference between FM and PM ?

Ans. The expression for FM and PM are almost identical. FM and PM can not be distinguished at a constant modulating frequency. However, with change in modulating frequency fm, the modulation index of FM will change and that of the PM won’t.

Q.44. Define modulation index for PM .

Ans. m= Φ, modulation index of PM = maximum phase change corresponding to the maximum amplitude of the modulating signal.

Q.45. What is the effect of change in fm on the deviation δ ?

Ans. No effect . δ is independent of the values of f.

Q.46.Why is FM more immune to effects of noise ?

Ans. Because the noise can distort the amplitude of an FM wave. As the information is contained in the frequency deviation changes, hence amplitude distortion due to noise does not affect the information being carried.

Q.47. In PM, the carrier frequency deviation is not proportional to _______ .

Ans. Modulating signal phase.

Q.48. The FM produced by PM is called _______ .

Ans. Indirect FM .

Q.49. If the amplitude of modulating signal applied to a phase modulator is constant then the output will be _____ .

Ans. The carrier frequency .

Q.50. The main disadvantage of FM is _______ .

Ans. Its large bandwidth .

Q.51. The pre-emphasis is compensated at the receiver by _______ .

Ans. A low pass filter called de-emphasis circuit .

Ans. 60 percent.

Q.53. What is wideband FM ? Where is it used?

Ans. It is the FM system having modulation index higher than 1. The maximum permissible deviation in wideband FM is 75 kHz. It is used in entertainment broadcasting ( Vividh Bharti or sound transmission in TV ).

Q.54. What is narrow band FM ? Where is it used?

Ans. It is the FM system with mlower than 1 . The maximum deviation is only 5 kHz. It is used in FM mobile communication. Such as police wireless, ambulance etc.

Ans. True .

Q.56. State application of NBFM.

Ans. Police wireless communication