UNIT II – ANGLE MODULATION SYSTEMS
PART-A
1. Define frequency modulation. MAY/JUNE 2007Frequency modulation is defined as the process by which the frequency of the carrier wave is varied in accordance with the instantaneous amplitude of the modulating or message signal.
2. Define modulation index of frequency modulation. NOV/DEC 2010
It is defined as the ratio of maximum frequency deviation to the modulating frequency.
β = δf/fm
3. Define phase modulation. NOV/DEC 2004
Phase modulation is defined as the process of changing the phase of the carrier signal in accordance with the instantaneous amplitude of the message signal.
4. Compare WBFM and NBFM. APRIL/MAY 2011
NBFM WBFM
Modulation index less than 1
Modulation index is greater than 1
Frequency deviation 5 KHz Frequency deviation 75 KHz
Bandwidth 2fm Bandwidth 15 times NBFM
Less suppressing of noise Noise is more suppressed
5. Define frequency Deviation. APRIL/MAY 2004
The maximum departure of the instantaneous frequency from the carrier frequency is called frequency deviation.
6. State the Carson’s rule. APRIL/MAY 2011
An approximate rule for the transmission bandwidth of an FM Signal generated by a single tone-Modulating signal of frequency fm is defined as
B =2 ∆f (1+1/ β)
7. What are the disadvantages of FM system? NOV/DEC 2008
1. A much wider channel is required by FM.
2. FM transmitting and receiving equipments tend to be more complex and hence it is expensive
8. What are the applications of phase locked loop? NOV/DEC 2006
Phase-locked loops are widely used in radio, telecommunications, computer and other Electronic applications.
9. Mention advantages of angle modulation over amplitude modulation. NOV/DEC 2010
1. The amplitude of FM is constant. It is independent of depth of modulation.
Hence transmitter power remains constant in FM whereas it varies in AM.
2. Since amplitude of FM is constant, the noise interference is minimum in FM.
3. FM uses UHF and VHF ranges, the noise interference is minimum compared to AM which uses MF and HF ranges.
10. A carrier wave of frequency 100 MHz is frequency modulated by a signal 20sin (200π x 103t).What is bandwidth of FM signal if the frequency sensitivity of the modulation is 25 kHz/v? NOV/DEC 2004
Bandwidth = 2 (δ + fm) = 2 (500 + 100) = 1.2 MHz.
11. Differentiate Phase modulation and frequency modulation. MAY / JUNE 2007
PM FM
Phase of the carrier varies as per amplitude variations of modulating signal Frequency of the carrier varies as per amplitude variations of modulating signal
Modulation index = k Em Modulation index = k Em /
Instantaneous Phase deviation Instantaneous frequency deviation
12. What is diversity reception? NOV/DEC 2008
Diversity reception is used when the signal fades into noise level. These are two types of diversity reception: 1) Space diversity 2) Frequency diversity
Space diversity: It uses two or more receiving antennas separated by nine or more wavelengths. There are separate receivers for each antenna. The receiver with strongest signal is selected.
Frequency diversity: It uses single receiving antenna which works for two or more frequencies. The frequency which has strong signal is selected.
13. What do you understand by FM stereo multiplexing? MAY / JUNE 2009
FM stereo multiplexing is used for stereo transmission. It is basically frequency division multiplexing. It is used for FM radio Broadcasting. The left and right channel signals are used to generate sum and difference signals.FM difference signal and carrier are combined together and sent. Such FM multiplexed signal can be coherently received by stereo as well as mono receiver.
14. Compare the transmission bandwidth required for Narrowband FM
and Wideband FM. DEC 2010
Narrow band FM, BW = 2fm
Wideband FM, BW = 2 [ + fm (max)]
15. Name the methods for detecting FM signals. MAY 2011
Balanced Slope or frequency discrimination
Foster – Seeley or phase discriminator
Ratio detector
PLL demodulator
16 What are the advantages of FM? DEC 2010
• Improved noise immunity
• Low power is required to transmit the signal
• Covers a large area with the same amount of transmitted power.
• Transmitted power remains constant
• All the transmitted power is useful.
17. What are the application areas of FM? MAY 2011
• Radio broadcasting
• Sound broadcasting in TV
• Satellite communication
• Police wireless
• Point to point communication
18. State the condition for wide band FM. MAY 2011
If the modulation index is higher than 10, then it is called wide band FM.
19. Distinguish frequency translation from frequency heterodyning. MAY 2011
Frequency translation Frequency heterodyning
In SSB, the message spectrum is shifted by an amount equal to carrier frequency fc The frequency of the incoming signal is converted to fixed Intermediate Frequency.(IF)
It is used to shift carrier frequencies in AM It is used to get fixed IF in AM detection
It is used at transmitter side It is used at receiver side
20. What are the advantages of ratio detector? NOV / DEC 2013
• It is not affected by amplitude variations on the FM wave.
• Ratio detector can operate with as little as 100mv of inputs.
• Easy to align
• Very good linearity, due to linear phase relationship between primary and secondary
• Amplitude limiting is provided inherently. So additional limiter is not required
• The ratio detector has the advantage of reduced fluctuations in the output voltage compared to Foster-Seeley circuit.
21. What is frequency translation? NOV / DEC 2011
The basic operation involved in single side band modulation is in the form of frequency translation which is some time referred to as frequency changing mixing or heterodyning.
22. How is the Narrow band FM converted into wideband FM? NOV / DEC 2011
FM signal is given by S (t) = Ac cos (2πfct + β sin (2πfmt))
If β is very small compared to one radian, it is called NBFM. If the value of β is very large compared to 1 radian, it is called wideband FM.
23. What is pre-emphasis? Why is it used? APRIL / MAY 2008
• 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 predistortion).
• Pre emphasis is particularly effective in FM systems which are used for transmission of Audio signals
24. What is narrowband FM? NOV / DEC 2007
When the frequency sensitivity Kf is small, then the bandwidth is also narrow which is twice the baseband signal.
25. What do you meant by multitone modulation?
Modulation done for the message signal with more than one frequency component is called Multitone modulation.
26. What are the types of Frequency Modulation?
Based on the modulation index FM can be divided into types. They are Narrow band FM and Wideband FM. If the modulation index is greater than one then it is wide band FM and if the modulation index is less than one then it is Narrowband FM.
27. What is the basic difference between an AM signal and a narrowband FM signal?
In the case of sinusoidal modulation, the basic difference between an AM signal and a narrowband FM signal is that the algebraic sign of the lower side frequency in the narrow band FM is reversed.
28. What are the two methods of producing an FM wave?
Basically there are two methods of producing an FM wave. They are,
i) Direct method
In this method the transmitter originates a wave whose frequency varies as function of the modulating source. It is used for the generation of NBFM
ii) Indirect method
In this method the transmitter originates a wave whose phase is a function of the modulation. Normally it is used for the generation of WBFM where WBFM is generated from NBFM.
29. Give the average power of an FM signal.
The amplitude of the frequency modulated signal is constant .The power of the
FM signal is same as that of the carrier power. P=1/2 Ec2.
30. Define phase deviation.
The maximum phase deviation of the total angle from the carrier angle is called phase deviation.
31. Define the deviation ratio D for non-sinusoidal modulation.
The deviation ratio D is defined as the ratio of the frequency deviation ∆f, which corresponds to the maximum possible amplitude of the modulation signal m(t),to the highest modulation frequency .
D = ∆f / fm
32. What is the use of crystal controlled oscillator?
The crystal-controlled oscillator always produces a constant carrier frequency thereby enhancing frequency stability.
33. How will you generate message from frequency-modulated signals?
First the frequency-modulated signals are converted into corresponding amplitude-modulated signal using frequency dependent circuits. Then the original signal is recovered from this AM signal.
34. What are the types of FM detectors?
Slope detector and phase discriminator.
35. What are the types of phase discriminator?
Foster seely discriminator and ratio detector.
36. What are the disadvantages of balanced slope detector?
1. Amplitude limiting cannot be provided
2. Linearity is not sufficient
3. It is difficult to align because of three different frequencies to which various tuned circuits to be tuned.
4. The tuned circuit is not purely band limited.
37. State the frequency in an FM system is 500 Hz and modulating voltage is 3 V, modulation index is 60. Calculate maximum deviation and bandwidth.
Modulation index m = δ/fm
60 = δ/ 500
δ = 60 x 500 = 30 kHz
Bandwidth BW = 2 δ = 2 x 30 = 60 kHz.
38. Define angle modulation.
In angle modulation, the timing parameters such as phase or frequency of the carrier are modulated according to amplitude of modulating signal.
39. What is modulation index of FM?
Modulation index of FM is defined as the ratio of frequency deviation to modulating frequency.
Mf = frequency deviation / modulating frequency
40. Define percentage modulation.
The percentage modulation is defined as the ratio of the actual frequency deviation produced by the modulating signal to the maximum allowable frequency deviation. Percentage modulation = Actual frequency deviation / Maximum allowable frequency deviation
41. What is the effect of mf on the bandwidth of FM?
When the modulation index increases, the number of sidebands having significant amplitude will also increases. This will increase the bandwidth.
42. What is the use of FM modulation index?
The FM modulation index is used to describe the depth of modulation achieved for a modulating with a given peak amplitude and radian frequency.
43. Why angle modulated waves are more immune to effects of noise?
• Most noise will occur due to unwanted amplitude variations in the modulated wave.
• FM and PM receivers include limiters that remove most of the AM noise from the received signal before the final demodulation process occurs.
• This is process that cannot be used with AM receivers because the information is also contained in amplitude variations, and removing the noise would also remove the information.
44. Define indirect method.
In the indirect method of producing frequency modulation, the modulating signal is first used to produce a narrow band FM signal and frequency multiplication is next used to increase the frequency deviation to the desired level.
45. What do you meant by varactor diode?
Varactor diode is a specially fabricated PN junction diode which is used as a variable capacitor in the reverse biased condition.
46. Define VCO.
The oscillator whose frequency is controlled by a modulating voltage is known as voltage controlled oscillator.
47. What is direct method?
In the direct method, the carrier frequency is directly varied in accordance with the input base-band signal.
48. Mention the applications of varactor diode modulator.
• Automatic frequency control
• Remote tuning
49. Write short notes on advantages of phase discriminator.
• The phase discriminator is much easier to align than the balanced slope detector.
• Only two tuned circuits are necessary and both are tuned to same frequency.
• Linearity is better.
50. What is wideband FM?
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.
51. What are the components in a frequency discriminator?
Frequency discriminator has got two components. Slope detector or diffentiator with a purely imaginary frequency response that varies linearly with frequency. It produces output where the amplitude and frequency vary with message signal. An envelope detector that recover the amplitude variations and produces message signal.
52. What is the function of amplitude limiter in FM system?
The function of amplitude limiter in FM system is used to remove the amplitude variations by clipping the modulated wave at the filter output almost to the zero axis. The resultant wave is rounded off by another BPF that is an integral part of the limiter thereby suppressing the harmonics of the carrier frequency.
53. What do you meant by Multitone modulation?
Modulation done with more tone than one message signal is called multitone modulation.
54. What is instantaneous frequency deviation?
The instantaneous frequency deviation is the instantaneous change in the frequency of the carrier.
55. What is modulation index of PM?
In PM, the modulation index is proportional to the amplitude of the modulating, independent of its frequency.
Mp = Kp Vm where, Kp – Deviation sensitivity
Vm – peak modulating signal amplitude
56. Define deviation sensitivity.
Deviation sensitivity represents the input-output transfer function of the modulators, which give the relationship between what output parameter changes in respect to specified changes in the input signal.
57. What is instantaneous phase deviation?
The instantaneous phase deviation is the instantaneous change in the phase of the carrier at a given instant of time and indicates how much the phase of the carrier is changing with respect to its reference phase.
58. Define instantaneous phase.
The instantaneous phase is the precise phase of the carrier at a given instant of time.
59. Define instantaneous frequency.
The instantaneous frequency is the precise frequency of the carrier at a given instant of time
PART – B
8 MARKS:
1. Differentiate between narrowband and wideband FM. (MAY/JUNE 2007)
2. Discuss the Effects of non linearities in FM systems. (MAY/JUNE 2007)
3. Explain the working of a reactance modulator and derive an expression. (APRIL/MAY2005)
4. Explain the varactor Diode can be used for frequency modulation. (NOV/DEC 2006)
5. Discuss the indirect method of generating wideband FM signal. (NOV/DEC 2008)
6. With a neat Diagram explain Armstrong method generating FM Signal. (NOV/DEC 2008)
7. Discuss the working of ratio detector (NOV/DEC 2008)
8. Discuss the working of balanced slope detector. (MAY/JUNE 2005)
9. Discuss on FM Demodulation using PLL. (MAY/JUNE 2009)
10. Discuss in Details FM stereo multiplexing.
11. Refer Class note – Problems.
16 MARKS:
1. Derive the expression for the frequency spectrum of a FM signal with single tone modulation.(MAY/JUNE 2009)
2. Derive the expression for the frequency modulated signal. Explain what is meant by narrowband and wideband FM using the expressions.(NOV/DEC 2006)
3. Give the method of generation of FM signal. (MAY/JUNE 2009)
4. Explain how FM wave is generated by indirect method. (NOV/DEC 2007)
5. Explain in Generation and Demodulation of FM signal. (NOV/DEC 2004)
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