This is the Coaching materials in Modulation Part 1 as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.
Modulation Coaching Materials Part 1
AMPLITUDE MODULATION
1. What is heterodyning?
Ans. Mixing two frequencies across a nonlinear impedance.
2. What waveform is the basis of all complex waveforms?
Ans. The sine wave
3. What is the instantaneous amplitude of a sine wave?
Ans. The value at any given point on the sine wave
4. What term describes how much of a cycle has been completed?
Ans. Phase or phase angle.
5. Define the heterodyne principle.
Ans. Process of combining two signal frequencies in a nonlinear device.
6. What is a nonlinear impedance?
Ans. An impedance in which the resulting current is not proportional to the applied voltage.
7. What is spectrum analysis?
Ans. The display of electromagnetic energy that is arranged according to wavelength or frequency.
8. What two conditions are necessary for heterodyning to take place?
Ans. At least two different frequencies applied to a nonlinear impedance.
9. Name two methods of oscillator keying.
Ans. Plate keying and Cathode keying
10. State the method used to increase the speed of keying in a CW transmitter.
Ans. Machine keying
11. Name a disadvantage of a single-stage CW transmitter.
Ans. Antenna –to-ground capacitance can cause the oscillator frequency to vary
12. What is the purpose of frequency-multiplier stages in a VHF transmitter?
Ans. To raise the low frequency of a stable oscillator to the vhf range.
13. What are the two major sections of a typical AM transmitter?
Ans. Rf and af units
14. When 100 kilohertz and 5 kilohertz are heterodyned, what frequencies are present?
Ans. 100 kilohertz, 5 kilohertz, 95 kilohertz, and 105 kilohertz
15. What determines the bandwidth of an AM transmission?
Ans. The highest modulating frequency
16. What is percent of modulation?
Ans. The depth or degree of modulation
17. With a single modulating tone, what is the amplitude of frequencies at 100-percent modulation?
Ans. One-half the amplitude of the carrier
18. What is the formula for percent of modulation?
Ans. %M = Em/Ec x 100%
19. What is high-level modulation?
Ans. Modulation produced in the plate circuit of the last radio stage of the system.
20. For what class of operation is the final rf power amplifier of a plate-modulator circuit based?
Ans. Class C
21. The modulator is required to be what kind of a circuit stage in a plate modulator?
Ans. Power amplifier
22. How much must the fpa plate current vary to produce 100-percent modulation in a plate modulator?
Ans. Between 0 and nearly two times its unmodulated value
23. The collector-injection modulator is similar to what type of tube modulator?
Ans. Plate modulator
24. When is a control-grid modulator used?
Ans. In cases when the use of a minimum of af modulator power is desired.
25. What type of modulator is the cathode modulator (low- or high-level)?
Ans. Low-level
26. What causes the change in collector current in an emitter-injection modulator?
Ans. Gain is varied by changing the voltage on the emitter
ANGLE AND PULSE MODULATION
27. What are the two types of angle modulation?
Ans. Frequency and phase
28. Name the modulation system in which the frequency alternates between two discrete values in response to the opening and closing of a key?
Ans. Frequency-shift keying
29. What is the primary advantage of an FSK transmission system?
Ans. Resistance to noise interference
30. What characteristic of a carrier wave is varied in frequency modulation?
Ans. Instantaneous frequency
31. How is the degree of modulation expressed in an FM system?
Ans. As the ratio of the frequency deviation to the maximum frequency deviation allowable
32. What two values may be used to determine the bandwidth of an FM wave?
Ans. The number of significant sidebands and the modulation frequency.
33. How does the reactance-tube modulator impress intelligence onto an RF carrier?
Ans. By changing the reactance of an oscillator circuit in consonance with the modulating voltage.
34. What characteristic of a transistor is varied in a semiconductor-reactance modulator?
Ans. Collector-to-emitter capacitance
35. What circuit section is required in the output of a multivibrator modulator to eliminate unwanted output frequencies?
Ans. An LCR filter
36. What characteristic of a varactor is used in an FM modulator?
Ans. Capacitance
37. What type of modulation depends on the carrier-wave phase shift?
Ans. Phase
38. What components may be used to build a basic phase modulator?
Ans. A phase-shift network such a s a variable resistor and capacitor in series
39. Phase-shift keying is similar to what other two types of modulation?
Ans. CW and frequency-shift keying
40. Overmodulating an RF carrier in amplitude modulation produces a waveform which is similar to what modulated waveform?
Ans. Pulse modulation
41. What is prt?
Ans. Pulse-repetition time
42. What is nonpulse time?
Ans. Rest time
43. What is average power in a pulsed system?
Ans. Peak power during a pulse averaged over pulse time plus rest time
44. What action is necessary to impress intelligence on the pulse train in pulse modulation?
Ans. Some characteristic of the pulses has to be varied
45. To insure the accuracy of a transmission, what is the minimum number of times a modulation wave should be sampled in pulse modulation?
Ans. 2.5 times the highest modulation frequency
46. What, if any, noise susceptibility advantage exists for pulse-amplitude modulation over analog-amplitude modulation?
Ans. Both are susceptible to noise and interference
47. What characteristics of a pulse can be changed in pulse-time modulation?
Ans. The time duration of the pulses or the time of occurrence of the pulses
48. Which edges of the pulse can be modulated in pulse-duration modulation?
Ans. Either, or both at the same time
49. What is the main disadvantage of pulse-position modulation?
Ans. It requires synchronization between the transmitter and receiver.
50. If a modulating wave is sampled 10 times per cycle with a 5-element binary code, how many bits of information are required to transmit the signal?
Ans. 50
51. What is the primary advantage of pulse-modulation systems?
Ans. Low susceptibility to noise
DEMODULATION
52. What is the simplest form of CW detector?
Ans. A circuit that can detect the presence or absence of RF energy
53. What principle is used to help distinguish between two CW signals that are close in frequency?
Ans. Heterodyning
54. How does heterodyning distinguish between CW signals?
Ans. By giving a different beat frequency for each signal
55. What simple, one-transistor detector circuit uses the heterodyne principle?
Ans. Regenerative detector
56. What three functions does the transistor in a regenerative detector serve?
Ans. Oscillator, mixer, and detector
57. What does the simplest diode detector use to reproduce the modulating frequency?
Ans. The modulation envelope
58. What is the function of the diode in a series-diode detector?
Ans. Rectifies the RF pulses in the received signal.
59. Which junction of the transistor in the common-emitter detector detects the modulation envelope?
Ans. Emitter-base junction
60. How is the output signal developed in the common-emitter detector?
Ans. By the collector current flow through R4
61. Which junction acts as the detector in a common-base detector?
Ans. Emitter-base junction
62. To what circuit arrangement is a common-base detector equivalent?
Ans. A diode detector followed by a stage of audio amplification.
63. What is the simplest form of FM detector?
Ans. Slope detector
64. What type of tank circuit is used in the Foster-Seeley discriminator?
Ans. A double-tuned tank circuit
65. What is the primary advantage of a ratio detector?
Ans. Suppresses amplitude noise without limiter stages.
66. What circuit functions does the tube in a gated-beam detector serve?
Ans. Limits, detects, and amplifies.
67. What condition must exist on both the limiter and quadrature grids for current to flow in a gated-beam detector?
Ans. Both grids must be positively biased
68. Name two advantages of the gated-beam detector.
Ans. Extreme simplicity, few components, and ease of adjustment.
69. Where is the intelligence contained in a phase-modulated signal?
Ans. In the amount and rate of phase shift of the carrier wave.
70. How is a quadrature detector changed when used for phase demodulation?
Ans. The quadrature grid signal is excited by a reference from the transmitter.
71. In its simplest form, what functions must a radar detector be capable of performing?
Ans. Detecting the presence of RF energy.
72. What characteristic of pulse does a peak detector sample?
Ans. Pulse amplitude or pulse duration
73. What is the time constant of the resistor and capacitor in a peak detector for PAM?
Ans. At least 10 times the interpulse period
MODULATION/RECEIVERS/TRANSMITTERS
74. The power output of a single-sideband transmitter is normally expressed as the _____ power.
Ans. peak envelope *
75. SSB modulation is classified as ______
Ans. AM *
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