CLASS: II YEAR / IV SEMESTER ECE
SUBJECT CODE AND NAME: EC 2252 – COMMUNICATION THEORY
UNIT I – AMPLITUDE MODULATION SYSTEMS
PART-A
1. Define modulation? DEC-2010
Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal.
2. What are the types of analog modulation? DEC-2010
Amplitude modulation.
Angle Modulation
1. Frequency modulation
2. Phase modulation
3. Define depth of modulation (or) Modulation Index. APRIL/MAY 2005
It is defined as the ratio between message amplitude to that of carrier amplitude.
m = Em / Ec.
4. What are the degrees of modulation? APRIL/MAY 2004
• Under modulation. m<1
• Critical modulation m=1
• Over modulation m>1
5. What is the need for modulation? MAY-2011
Needs for modulation:
Ease of transmission
Multiplexing
Reduced noise
Narrow bandwidth
Frequency assignment
Reduce the equipments limitations
6. What are the advantages of VSB-AM? APRIL/MAY 2011
• It has bandwidth greater than SSB but less than DSB system
• Power transmission greater than DSB but less than SSB system
• No low frequency component lost. Hence it avoids phase distortion
7. Define demodulation. DEC-2010
Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation.
8. A transmitter supplies 10 Kw to the antenna when modulated. Determine the total power radiated when modulated to 50%. NOV/DEC 2005
m=0.5; Pc=10 kW
Pt=Pc (1+m2/2)
Pt =11.25 kW
9. Give the methods of generating SSB-SC-AM. And mention some applications of SSB-SC. NOV/DEC 2008
The two methods of generating the SSB-SC waves are
1. Frequency discrimination or Filter method.
2. Phase discrimination method.
Applications:
1. Police Wireless communication
2. SSB telegraph system,
3. Point to point radio telephone communication
4. VHF and UHF communication systems.
10. Define Periodic Signals and any two important spectral properties of periodic power signals. MAY -2011
A signal is said to be periodic if it repeats after certain period.
X (t) =X (t+T).
Property:
1. Linearity
2. Time shift
3. Frequency Shift.
11. What do you mean by image frequency? APRIL/MAY 2004
The local oscillator (fo), input signal frequency (f) and If some other frequency, f = fsi + 2fi appears at the input of mixer, then it produces fi at the output of the mixer fsi image frequency.
12. Define VSB transmission. Mention its application. MAY-2011
Definition:
One of the sideband is partially Suppressed and portion of the other sideband is transmitted. This Portion compensates the suppression of the sideband. It is called vestigial sideband transmission.
Application:
VSB is mainly used in TV transmission.
13. Define amplitude modulation. NOV/DEC-2005
Amplitude modulation is the process by which amplitude of the carrier signal is Varied in accordance with the instantaneous value (amplitude) of the modulating signal, but frequency and phase remains constant.
14. Define transmission efficiency of AM signal. MAY/JUNE 2006
It can be defined as the ratio of power in sideband to total power.
% ή = (Power in sideband / Total power) x 100
15. What are the advantages of SSB-SC modulation over AM? MAY/JUNE 2007
• Since only one sideband is transmitted, required transmitter and channel bandwidth is only fm.
• Power requirement is reduced
• Effect of noise at the receiver circuits is reduced because of narrow bandwidth.
16. What are the advantages of superheterodyne receiver? MAY/JUNE 2006
• The selectivity of this receiver is better since IF amplifiers are narrowband, and they operate only at IF.
• The design of IF amplifiers is relatively simple since they operate only at IF.
17. What are the causes of linear distortion? MAY 2010
• Non ideal characteristics of channel in terms of magnitude, phase or both.
• Signal is not strictly bandlimited
18. State the applications of FDM. DEC 2010
• Multiple channels can be accommodated in the same available bandwidth.
• A guard band between the channels avoids overlapping of channel spectrums.
• The transmission band is best utilized with the help of FDM.
19. What is meant by detection? MAY 2011
Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation.
20. What are the advantages of VSB? MAY 2011
• The frequencies near fc are not suppressed
• Bandwidth is reduced compared to DSB
21. What are the types of AM modulators?
There are two types of AM modulators. They are
• Linear modulators
• Non-linear modulators
Linear modulators are classified as follows
• Transistor modulator
There are three types of transistor modulator.
• Collector modulator
• Emitter modulator
• Base modulator
• Switching modulators
Non-linear modulators are classified as follows
• Square law modulator
• Product modulator
• Balanced modulator
22. Give the classification of modulation.
There are two types of modulation. They are
• Analog modulation
• Digital modulation
Analog modulation is classified as follows
• Continuous wave modulation
• Pulse modulation
Continuous wave modulation is classified as follows
Amplitude modulation
• Double side band suppressed carrier
• Single side band suppressed carrier
• Vestigial side band suppressed carrier
Angle modulation
• Frequency modulation
• Phase modulation
Pulse modulation is classified as follows
Pulse amplitude modulation
Pulse position modulation
Pulse duration modulation
Pulse code modulation
Digital modulation is classified as follows
Amplitude shift keying
Phase shift keying
Frequency shift keying
23. What is single tone and multi tone modulation?
• If modulation is performed for a message signal with more than one frequency component then the modulation is called multi tone modulation.
• If modulation is performed for a message signal with one frequency component then the modulation is called single tone modulation.
24. The antenna current of an AM transmitter is 8A when only carrier is sent. It increases to 8.93A when the carrier is modulated by a single sine wave. Find the percentage modulation.
Solution:
Given: Ic =8A It=8.93A m=0.8
Formula: It=Ic (1+m2/2) ½
8.93=8(1+m2/2) ½ m=0.701 It=8 (1+0.82/2) ½ It=9.1A
25. What are the types of AM detectors?
1. Nonlinear detectors
2. Linear detectors
26. What are the types of linear detectors?
1. Synchronous or coherent detector
2. Envelope or non coherent detector
27. Compare AM with DSB-SC and SSB-SC.
AM signal (DSBFC) DSB-SC SSB-SC
Bandwidth = 2fm Bandwidth = 2fm Bandwidth = fm
Contains USB, LSB, carrier Contains USB,LSB Contains LSB or USB
More power is required for transmission Power required is less than that of AM. Power required is less than AM &DSB-SC
28. Compare linear and non-linear modulators.
Linear modulators Non-linear modulators
Heavy filtering is not required Heavy filtering is required
These modulators are used in high level modulation These modulators are used in low level modulation
The carrier voltage is very much greater than modulating signal voltage. The modulating signal voltage is very much greater than the carrier signal voltage.
29. How will you generating DSBSC-AM?
There are two ways of generating DSBSC-AM such as
1. Balanced modulator 2. Ring modulators
30. What are advantages of ring modulator?
1. Its output is stable.
2. It requires no external power source to activate the diodes.
3. Virtually no maintenance.
4. Long life.
31. Draw the block diagram of coherent detector.
32. Define multiplexing.
Multiplexing is defined as the process of transmitting several message signals simultaneously over a single channel.
33. Define sensitivity.
It is defined as a measure of its ability to receive weak signals.
34. Define selectivity.
Selectivity of a receiver is defined as its ability to select the desired signals among the various signals.
35. Define stability.
It is the ability of the receiver to deliver a constant amount of output for a given a given period of time.
36. Define super heterodyne principle.
It can be defined as the process of operation of modulated waves to obtain similarly modulated waves of different frequency. This process uses a locally generated carrier wave, which determines the change of frequency.
37. What are the drawbacks of emitter modulator?
1. The amplifier is operated in class A mode, thus the efficiency is low.
2. The output power is very small. Thus it is not suitable for generating high level modulation.
38. Define FDM & frequency translation.
Frequencytranslation:
The single sideband modulation basically performs frequency translation. it is also called frequency changing, mixing or heterodyning. The single sideband modulation the message spectrum is shifted. By an amount equal to the carrier frequency fc.
FDM:
For voice transmission, the frequency range of 300 to 3400 Hz is used. When more number of such voice channels is to be transmitted then each channel is given a fixed frequency slot and then transmitted. This is called Frequency Division Multiplexing.
39. What is the power saving in DSB-SC-AM and SSB-SC AM?
Power saving in DSB-Sc-AM is 66.7%.
Power saving in SSB-Sc-AM is 83.3%.
40. Differentiate between DSB-SC and SSB-SC:
DSB-SC
1. Suppression of carrier results in economy of power.
2. It is commonly used in carrier current telephony system, in which one sideband is filtered out to reduce the width of the channel required for transmission.
3. It offers secrecy.
4. It increases the efficiency because the carrier is suppressed.
SSB-SC
1. Bandwidth of SSB is half that of DSB-SC AM. Thus twice the number of channels can be accommodated at a given frequency spectrum.
2. No carrier is transmitted, hence possibility of interference with other channels are avoided.
3. It eliminates the possibility of fading. Fading occurs due to multipath propagation of electromagnetic waves.
41. What is AM envelope?
The shape of the modulated wave (AM) is called AM envelope which contains all the frequencies and is used to transfer the information through the system.
42. Define sidebands.
• Whenever a carrier is modulated by an information signal, new signals at different frequencies are generated as part of the process. These frequencies are called side frequencies or side bands.
• Assuming a carrier frequency of Fc and a modulating frequency of Fm, the upper sideband FUSB and lower sideband FLSB are computed as follows:
FUSB= Fc – Fm
FLSB = Fc + Fm
43. What is the bandwidth of AM?
The bandwidth of AM signal is twice of the maximum frequency of the modulating signal. BW = 2 fm
44. Which is called as percent modulation?
When modulator index is expressed in percentage, it is called percent modulation. Percent modulation gives the percentage change in the amplitude of the output wave when the carrier is acted on by a modulating signal.
% modulation = (Vm ∕ Vc ) * 100 or ma x 100
45. Write the expression for modulation index in terms of Vmax and Vmin?
Modulation index (ma) = Vmax – Vmin / Vmax + Vmin
Where, Vmax = Vc+ Vm Vmin = Vc-Vm
46. What is the total power of AM modulated wave?
Total power Pt = [carrier power] + [power is LSB] + [power in USB]
47. What is under modulation?
ma < 1 when Vm < Vc
Here the envelope of the amplitude modulating signals does not reach the zero amplitude axes. Hence the message signal is fully preserved in the envelope of the AM wave. An envelope detector can recover the message signal without any distortion.
48. Define critical modulation.
ma = 1 when Vm = Vc
Here the envelope of the amplitude modulating signals just reaches the zero amplitude axes. The message signal remains preserved. An envelope detector can recover the message signal without any distortion.
49. Define over modulation.
ma > 1 when Vm > Vc
Here both positive and negative extensions of the modulating signals are cancelled or clipped out. The envelopes of message signal are not same. Due to this envelope detector provides distorted message signal.
50. Write the modulation index expression for multiple modulating frequencies.
When two or more modulating signals are modulated by a single carrier. Then the modulation index is given by ma =[ (m12)+ (m22)+……]1/2
Where, ma = total resultant modulation index
m1, m2,…. = modulation indices due to individual modulating components.
51. Write the expression for carrier power of an AM wave.
The average power dissipated in a load by an unmodulated carrier is equal to the rms carrier voltage squared divided by the load resistance.
Pc = Vc2 / 2R Where, Pc – Carrier power
Vc- Peak carrier voltage
R- Load resistance
52. Mention the relationship between carrier power and total power.
Pt = Pc ( 1+ ( ma2/2))
Where Pt- Total power of modulated wave, Pc- Carrier power and ma= modulation index
53. Describe the relationship between the carrier and sideband powers in an AM DSBFC wave?
PUSB= PLSB= (ma2/4) x 100
Where, ma= modulation index and carrier power Pc= Vc2/2R
54. What is meant by diagonal clipping?
If the rate fall of modulation envelope during negative half cycle of the modulating voltage is faster than rate of voltage fall across RC combination, the output fails to follow the modulation envelope resulting in distorted output. This type of distortion is called as diagonal clipping.
55. Define envelope detector.
A detector circuit whose output follows the envelope of the modulating signal which is used to reproduce the modulating or message signal is called as envelope detector.
56. Write the bandwidth of VSB.
The transmission bandwidth of VSB modulation is BT = fm + fv Where, fm is the message bandwidth and fc is the width of the vestigial sideband.
PART – B
8 MARKS:
1. Explain the generation of SSB using Balanced Modulator. (NOV/DEC 2005)
(Or)
How is SSB Generated? Explain. (APRIL/MAY2004)
2. Draw the circuit diagram of a ring modulator and explain its operation. (NOV/DEC 2006)
3. Give the method for generation of DSBSC and SSB Waves. (MAY/JUNE 2009)
4. What is frequency translation? Explain. (NOV/DEC 2007,2008)
5. Define Sensitivity, Selectivity and fidelity and image frequency of a receiver system. (MAY/JUNE 2007)
6. Discuss the operation of Costas loop in Detail. (MAY/JUNE 2009)
7. Draw the block diagram for FDM and Explain. (MAY/JUNE 2009,2006)
8. Comparison Various Amplitude modulation Systems. (MAY/JUNE 2006)
9. Show that the AM signal can be recovered irrespective of the value of percentage modulation Using synchronous detection? (MAY/JUNE 2006)
16 MARKS:
1. Derive an expression for an AM wave and the frequency spectrum of an AM signal. (APRIL/MAY2004)
2. Explain in detail the method of producing amplitude modulated signal (or) Explain & Give the Method for generation and demodulation circuits of AM wave. (NOV/DEC 2004, 2003, APRIL/MAY2004)
3. Draw the Block Diagram of AM transmitter which uses high level modulation and low level Modulation explains its function.
(NOV/DEC 2004,2006, 2009,MAY/JUNE 2007, 2006, 2008)
4. Draw the block diagram of super heterodyne receiver and explain its operation. Explain the function of Each Block.
5. Draw the block diagram for the generation & demodulation of a VSB signal & explain the principle of operation. (MAY/JUNE 2007)
6. Explain the modulation and demodulation of AM signals. (NOV/DEC 2004)
7. Explain the DSBSC, SSB Modulation and detection Technique. (NOV/DEC 2007)
8. Refer Class note – Problems.
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