Short Questions and Answers on Angle Modulation (FM and PM)


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

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Ans. Angle 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.

Q.2. What are the types of 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 f is varied linearly with a baseband or modulating signal x(t) about an unmodulated phase angle (ωc t +θ0). This means that in Phase Modulation, the instantaneous value of the phase angle is equal to the phase angle of the unmodulated carrier (ωc t +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 ωc. This means that the instantaneous value of the angular frequency ωi will be equal to the carrier frequency ωc plus 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 derivatives.

Q.6. 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.7. What is carrier signal?

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 × Δw.

Q.8. 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.9. 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.

Q.10. What is the role of modulating frequency fm in 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 fm keeping the amplitude constant, we will get the same frequency deviation at a faster rate.

Q.11. What is sideband FM?

Ans. When the value of modulation index mf is 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.12. What is transmission BW for FM?

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

Q.13. Express the FM signal mathematically.

Ans. FM wave is represented as:

Q.14. 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.15. Ideally how many sidebands are present in the spectrum of FM wave.

Ans. Infinite.

Q.16. What is Carson’s rule.

Ans. Carson’s rule provides a thumb formula to calculate the bandwidth of a single-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 + ωm)

Q.17. Define modulation index for FM. How is it different from that of AM?

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

Q.18. 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.19. 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 :

Q.20. What is the ideal bandwidth of an FM wave?

Ans. Infinite.

Q.21. State Carson’s rule for bandwidth of FM wave.

Ans. B.W. = 2 [δ + fm(max)].

This rule is applicable for the values of modulation index greater than 6.

Q.22. What is the effect of mf on the bandwidth of FM?

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

Q.23. 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.24. 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 mf.

Q.25. Deviation ratio in FM is defined as……….


Q.26. What is the difference between FM and PM?

Ans. The expressions for FM and PM are almost identical. FM and PM cannot 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.27. Define modulation index for PM.

Ans. mp = Φm, Modulation index of PM = Maximum phase change corresponding to the maximum amplitude of the modulating signal.

Q.28. What is narrow band FM? Where is it used?

Ans. It is the FM system with mf lower than 1. The maximum deviation is only 5 kHz. It is used in FM mobile communication, such as police wireless, ambulances etc.

Q.29. What is the effect of change in fm on the deviation δ?

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

Q.30. 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.31. State application of NBFM.

Ans. Police wireless communication.

Q.32. The general name for both FM and PM is……. modulation.

Ans. Angle.

Q.33. The amount of frequency shift in FM is directly proportional to… of the modulating signal.

Ans. amplitude.

Q.34. The number of significant sidebands in FM depends on………

Ans. modulation index.

Q.35. When the modulating signal goes positive in PM, the phase shift becomes…. and carrier frequency…..

Ans. Lagging, decreases.

Q.36. In PM, when modulating signal goes negative, the phase shift becomes……… and carrier frequency…………..

Ans. Leading, increases.

Q.37. We can obtain PM from Frequency modulator by passing the modulating signal through a …….

Ans. Differentiator.

Q.38. FM is produced at the phase modulator output if and only if…………

Ans. The modulating signal is changing its magnitude.

Q.39. If the modulating signal amplitude becomes negative then the carrier frequency….

Ans. Goes below the unmodulated carrier frequency.

Q.40. In PM, the carrier………. is varied in proportion to the amplitude of the modulating signal.

Ans. Phase.

Q.41. The frequency deviation at the output of a phase modulator is maximum at…………….

Ans. the zero crossing of the modulating signal.

Q.42. The bandwidth of an FM wave in terms of number of significant sidebands is….

Ans. BW = 2fm × Number of significant sidebands.

Q.43. In FM, the percent modulation is defined as……………..

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

Q.44. FM produces…. sidebands than AM.

Ans. More.

Q.45. A negative sign on the carrier and sideband amplitudes in the expression for FM means….

Ans. Phase reversal.

Q.46. True or False: The carrier in an FM signal can never drop to zero amplitude.

Ans. False. For some values of mcarrier does drop to zero.

Q.47. The main disadvantage of FM is….

Ans. Its large Bandwidth.

Q.48. The amount of frequency deviation at the output of phase modulator is proportional to ……

Ans. Modulating voltage as well as modulating frequency.

Q.49. 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.50. In PM, the carrier frequency deviation is not proportional to………….

Ans. modulating signal phase.

Q.51. The FM produced by PM is called as……………

Ans. Indirect FM.

Q.52. If the amplitude of modulating signal applied to a phase modulator is constant, then, the output will be…………

Ans. The carrier frequency.

Q.53. The Pre-emphasis is compensated for at the receiver by……….

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

Q.54. The maximum allowed deviation of FM sound signal in TV is 50 kHz. If the actual deviation is 30 kHz, then, the percentage modulation is…

Ans. 60 percent.

Q.55. For FM broadcast application………… is used.

Ans. Wideband FM.

Q.56. FM waves propagate using ……….. propagation.

Ans. Space wave

Q.57. To obtain the same S/N ratio at the same distance from the transmitter, FM transmitter has to transmit…………. power as compared to AM.

Ans. less

Q.58. The process of frequency up conversion is used for………….

Ans. Increasing the carrier frequency.

Q.59. Carrier frequency and frequency deviation can be increased by using a…….. after the modulator.

Ans. Frequency modulator.

Q.60. Why is FM not used for picture transmission?

Ans. Due to the large bandwidth requirement and due to fact that multipath reception of FM signals generates more annoying patterns on the TV receiver screens.

Q.61. Why is FM used for sound transmission in TV?

Ans. In TV, it is necessary that the sound and picture carriers do not interfere with each other. For this, the sound power output should be much lower than the picture power output, without any sacrifice in the range of transmission. This can be achieved by using FM for sound transmission.

Q.62. Frequency modulation transmitters are more efficient because……….

Ans. They can use class C amplifiers.

Q.63. What is wideband FM? Where is it used?

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

Q.64. What is the principle of a variable reactance modulator used for FM generation?

Ans. A voltage variable reactance when placed across a tank circuit will generate FM signal.

Q.65. Varactor diode is used in forward or reverse biased mode?

Ans. Reverse biased mode.

Q.66. State true or false: For highly stable carrier generators, LC oscillators are preferred over crystal oscillators.

Ans. False.

Q.67. Name the method of generation of FM which uses PM?

Ans. Armstrong method.

Q.68. Which devices can be used as variable reactance devices to generate the FM?

Ans. FET, transistor, varactor diode.

Q.69. Increasing the reverse voltage across the varactor diode will cause it capacitance to………

Ans. Decrease.

Q.70. A positive going modulating signal in the frequency modulator using varactor will………. the capacitance and………. the frequency.

Ans. Decrease capacitance, increase frequency.

Q.71. What is the effect of mixing (heterodyning) on mf and δ in FM?

Ans. No change in δ and m. If just changes the center frequency.

Q.72. Another name for varactor diode is……….

Ans. Voltage variable capacitance (VVC).

Q.73. Why indirect method of FM generation is preferred to the direct methods?

Ans. Indirect method gives very high frequency stability. Direct methods cannot do so.

Q.74. The cut off frequency of Pre-emphasis and De-emphasis circuits is……….

Ans. 2122 Hz.

Q.75. What type of FM is used for police wireless?

Ans. Narrow band FM.

Q.76. What is the effect of fm on the noise immunity in FM?

Ans. Noise immunity improves with reduction in fm. At higher values of fm, the noise immunity of FM is poor.

Q.77. Is Pre-emphasis similar to high pass filtering or low pass filtering?

Ans. Pre-emphasis is similar to High pass filtering.

Q.78. How to improve the noise immunity of FM wave?

Ans. By increasing the value of modulation index. This can be achieved by increasing the frequency deviation if fm is constant.

Q.79. What is Pre-emphasis?

Ans. It is the process of artificially boosting the amplitudes of high frequency components in the modulating signal.

Q.80. What is the effect of pre-emphasis?

Ans. Due to boosting of high frequency components, the deviation and modulation index increases with increase in fm. This will improve the noise immunity of FM wave at higher modulating frequency.

Q.81. What is standard value of Pre-emphasis in FM transmission and sound transmission in TV?

Ans. It is 75 µsec.

Q.82. Why is De-emphasis used?

Ans. To bring the artificially boosted high frequency components back to their original amplitudes. It is a low pass filter (LPF).

Q.83. What is the operating principle of a FM receiver?

Ans. FM receivers operate on the principle of superheterodyne.

Q.84. What can you say about the range covered by FM transmission as compared to that covered by AM transmission with same output power?

Ans. The range covered by FM is larger than that of AM.

Q.85. Why phase discriminator is practically preferred?

Ans. Due to easy alignment and better linearity.

Q.86. Why is the linearity of the phase discriminator better?

Ans. Because the operation of this circuit is dependent more on the primary to secondary phase relationship which is very much linear.

Q.87. Why is simple slope detector not used practically?

Ans. Due to its disadvantages such as non-linearity, critical in tuning etc.

Q.88. For broadcasting applications, what type of FM is used?

Ans. Wideband FM.

Q.89. A crystal oscillator whose frequency can be changed by an input voltage is called as……….

Ans. VCO (Voltage Controlled Oscillator).

Q.90. What is the tuning range and IF for an FM receiver?

Ans. The tuning range is 88 MHz to 108 MHz and IF is 10.7 MHz.

Q.91. What is the use of a limiter circuit?

Ans. It is basically a clipping circuit which clips off the undesired amplitude variations (due to noise) in the received FM signal.

Q.92. Where will you include the limiter circuit in an FM receiver?

Ans. Limiter should always be included prior to FM detector.

Q.93. The phenomenon of strong FM signal dominating a weaker signal on a common frequency is called as …………

Ans. Capture effect.

Q.94. What is the operating principle of a slope detector?

Ans. When a variable frequency constant amplitude signal is applied to a tank circuit, we get an output voltage which changes with change in input frequency. Tank circuit is tuned at slightly higher frequency than the unmodulated carrier.

Q.95. What is the operating principle of a balanced slope detector?

Ans. Same as simple slope detector. But, two slope detectors are connected back to back and output voltage is the difference between individual outputs.

Q.96. The receiver circuit that gets rid of FM noise is……..

Ans. Limiter.

Q.97. What are the demerits of a balanced slope detector?

Ans. Difficult to tune as three circuits are tuned at different frequencies, linearity is not good enough and no amplitude limiting possible.

Q.98. Where is the quadrature detector used? Why?

Ans. Quadrature detector can be fabricated in the IC form, hence, they, are widely used in modern receiver circuits.

Q.99. In the Foster-seeley discriminator and ratio detector circuits, an input frequency variation produces a………..which causes the variation in output voltage.

Ans. Phase shift

Q.100. Which discriminator averages pulses in a low pass filter (LPF)?

Ans. Quadrature detector.

Q.101. What is the effect of multiplication on mand δ in FM? Ans. Deviation and mf, both get multiplied by the multiplying factor N.

Ans. Deviation and mf, both get multiplied by the multiplying factor N.