Discussion
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- Question
Assertion (A): A line of length
and short circuited at far end has an input impedance of infinity.Reason (R): A line of length
and short circuited at far end behaves as a parallel resonant circuit.
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
- Correct Answer
- Both A and R are correct and R is correct explanation of A
ExplanationA parallel tuned circuit has an infinite impedance if R is zero.
Microwave Communication problems
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- 1. A duplexer is used to
Options- A. couple two antennas to a transmitter
- B. isolate the antenna from the local oscillator
- C. prevent interference between two antennas connected to receiver
- D. use an antenna for reception or transmission without interference Discuss
Correct Answer: use an antenna for reception or transmission without interference
Explanation:
Same antenna is used for transmission and reception.- 2. In a vacuum tube, the transit time of electron between cathode and anode is important at
Options- A. low frequencies
- B. high frequencies
- C. both (a) and (b)
- D. frequencies which are neither very low nor very high Discuss
Correct Answer: high frequencies
Explanation:
At high frequencies transit time is large as compared to the period of microwave signal.- 3. The bunching action which occurs in multicavity klystron amplifier can be represented by Applegate diagram
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
A Klystron is a vacuum tube used for generation/amplification of microwaves.An electron beam is produced by oxide coated indirectly heated cathode and is focussed and accelerated by focussing electrode.
This beam is transmitted through a glass tube. The input cavity where the beam enters the glass tube is called buncher.
As electrons move ahead they see an accelerating field for half cycle and retarding field for the other half cycle.
Therefore, some electrons are accelerated and some are retarded. This process is called velocity modulation.
The velocity modulation causes bunching of electrons. This bunching effect converts velocity modulation into density modulation of beam.
The input is fed at buncher cavity and output is taken at catcher cavity.
In a two cavity klystron only buncher and catcher cavity are used. In multi cavity klystron one or more intermediate cavities are also used.
The features of a multicavity klystron are :
1. Frequency range - 0.25 GHz to 100 GHz
2. Power output - 10 kW to several hundred kW
3. Power gain - 60 dB (nominal value)
4. Efficiency - about 40%.
A multicavity klystron is used in UHF TV transmitters, Radar transmitter and satellite communication.
- 4. When a line short circuited at far end, the minimum voltage occurs at
Options- A. far end
- B. source end
- C. midway between source and far end
- D. none of the above Discuss
Correct Answer: far end
Explanation:
When a line is short-circuited at far end, the voltage at far end is zero.- 5. A branched duplexer requires
Options- A. TR tubes
- B. ATR tube
- C. both TR and ATR tube
- D. none of the above Discuss
Correct Answer: both TR and ATR tube
Explanation:
A branched duplexer funds use in narrow bandwidth applications.TR tube stands for transmit-receive tube and ATR tube stands for antitransmit receive tube.
Both these tubes are used in branched duplexer.
- 6. A quarter wave line short circuited at load end behaves as
Options- A. an inductance
- B. an inductance and capacitance in series
- C. a capacitance
- D. an inductance and capacitance in parallel Discuss
Correct Answer: an inductance and capacitance in parallel
Explanation:
A quarter wave line short-circuited at far end behaves as a parallel tuned circuit.- 7. In the given figure the reflected voltage wave after first reflection is
Options- A. 18 V
- B. -18 V
- C. 30 V
- D. -30 V Discuss
Correct Answer: -18 V
Explanation:
.
- 8. If other parameter are constant, transmission loss at 5 GHz as compared to that at 0.5 GHz is
Options- A. 20 dB more
- B. 20 dB less
- C. 700 dB more
- D. 100 dB less Discuss
Correct Answer: 20 dB more
Explanation:
As frequency increases, transmission loss increases slightly.- 9. In Reflex Klystron oscillator the focussing electrode is at a high potential
Options- A. True
- B. False Discuss
Correct Answer: False
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.
- 10. Assertion (A): PIN diode can be used as attenuator and limiter.
Reason (R): PIN diode has a thin intrinsic layer.
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 PIN diode has an intrinsic (i) layer between p and n layers. When reverse bias is applied depletion layers are formed at p-i and i-n junctions.The effective/width of depletion layer increases by the width of i layer. It can be used as a voltage controlled attenuator.
At high frequencies the rectification effect ceases and impedance of diode is effectively that of i layer.
This impedance varies with the applied bias. It is used in high frequency switching circuits, limiters, modulators etc.
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