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- Question
VSB modulation is preferred in TV because
Options- A. it reduces the bandwidth requirement to half
- B. it avoids phase distortion at low frequencies
- C. it results in better reception
- D. none of the above
- Correct Answer
- it reduces the bandwidth requirement to half
ExplanationVSB (vestigial side band) transmission transmits one side band fully and the other side band partially thus, reducing the bandwidth requirement.
Communication Systems problems
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- 1. The range of a cordless telephone is about
Options- A. 1000 m
- B. 500 m
- C. 100 m
- D. 10 m Discuss
Correct Answer: 100 m
Explanation:
The range depends on quality of instrument and is about 100 m.- 2. Energy content of atmospheric noise
Options- A. does not depend on frequency
- B. decreases as frequency is increased
- C. increases as frequency is increased
- D. either (a) or (c) depending on the temperature Discuss
Correct Answer: decreases as frequency is increased
Explanation:
Atmospheric noise decreases as frequency is increased.- 3. Non-coherently detection is not possible for
Options- A. PSK
- B. ASK
- C. FSK
- D. both (a) and (c) Discuss
Correct Answer: PSK
- 4. Radio broadcasts are generally
Options- A. amplitude modulation
- B. frequency modulation
- C. both amplitude and frequency modulation
- D. neither amplitude nor frequency modulation Discuss
Correct Answer: frequency modulation
Explanation:
Frequency modulation gives noise free reception and is used for radio broadcasts.- 5. Push pull amplifier is
Options- A. voltage amplifier
- B. current amplifier
- C. power amplifier
- D. none of the above Discuss
Correct Answer: power amplifier
Explanation:
Push pull is a power amplifier.- 6. Assertion (A): The phenomenon of differential mobility is called transferred electron effect.
Reason (R): GaAs exhibits transferred electron effect.
Options- A. Both A and R are correct and R is correct explanation of A
- B. Both A and R are correct but R is not correct explanation of A
- C. A is correct but R is wrong
- D. A is wrong but R is correct Discuss
Correct Answer: Both A and R are correct but R is not correct explanation of A
Explanation:
A Gunn diode uses GaAs which has a negative differential mobility, i.e., a decrease in carrier velocity with increase in electric field.This effects is called transferred electron effect. The impedance of a Gunn diode is tens of ohms.
A Gunn diode oscillator has a resonant cavity, an arrangement to couple Gunn diode to cavity, biasing arrangement for Gunn diode and arrangement to couple RF power to load.
Applications of Gunn diode oscillator include continuous wave radar, pulsed radar and microwave receivers.
- 7. A line is excited by a 100 V dc source. If reflection coefficients at both ends are 1 each then
Options- A. there will be no oscillations on line
- B. there will be only 1 or 2 oscillations on line
- C. there will be a finite number of oscillations on line
- D. the oscillations will continue indefinitely Discuss
Correct Answer: the oscillations will continue indefinitely
Explanation:
Because rv = 1 at each end, the line voltage will not reach a steady value and oscillations will continue indefinitely.- 8. Reflex klystron oscillator is essentially a low power device
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
It uses a single cavity resonator for generating microwave oscillations.Its parts are electron gun, resonator, repeller and output coupling.
It operates on the principle of positive feed back.
The repeller electrode is at negative potential and sends the partially bunched electron beam back to resonator cavity.
This positive feedback supports oscillations. Its feature are:
1. Frequency range - 2 to 100 GHz
2. Power output - 10 MW to about 2 W
3. Efficiency - 10 - 20 %
Its applications include radar receivers, local oscillator in microwave devices, oscillator for microwave measurements in laboratories etc.
- 9. A resistive microwave load with ZL = 150 ? is connected to 50 ? coaxial line. SWR is
Options- A. more than 3
- B. less than 3
- C. equal to 3
- D. either (a) or (c) Discuss
Correct Answer: equal to 3
Explanation:
- 10. Assertion (A): A backward wave oscillator has an internal positive feedback.
Reason (R): A positive feedback is necessary for sustained oscillations.
Options- A. Both A and R are correct and R is correct explanation of A
- B. Both A and R are correct but R is not correct explanation of A
- C. A is correct but R is wrong
- D. A is wrong but R is correct Discuss
Correct Answer: Both A and R are correct but R is not correct explanation of A
Explanation:
It is somewhat similar to TWT and can deliver microwave power over a wide frequency band.It has an electron gun and a helix structure. However the interaction between electron beam and RF wave is different than in TWT.
The growing RF wave travels in opposite direction to the electron beam.
The frequency of wave can be changed by changing the voltage which controls the beam velocity.
Moreover the amplitude of oscillations can be decreased continuously to zero by changing the beam current.
It features are:
1. Frequency range - 1 GHz to 1000 GHz.
2. Power output - 10 mV to 150 mW (continuous wave) 250kW (pulsed).
It is used as signal source in transmitters and instruments.
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