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- Question
Which one of the following is not a negative resistance device?
Options- A. Gunn diode
- B. Tunnel diode
- C. Impatt diode
- D. Varactor diode
- Correct Answer
- Varactor diode
ExplanationGunn diode, Tunnel diode and Impatt diodes are negative resistance devices.The limitation of solid state devices at high frequencies include those associated with transit time and junction capacitances.
The devices used are : Transferred electron oscillators (Gunn diode), Avalanche diode oscillators (Impatt diode, Trapatt diode, Masters, Lasers, Tunnel diode, Varactor etc).
Microwave Communication problems
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- 1. The noise figure of multicavity klystron amplifier is very low
Options- A. True
- B. False Discuss
Correct Answer: False
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.
- 2. A coaxial RF cable has a characteristic impedance of 50 ? and C equal to 40 pF/m. The inductance is
Options- A. 1 ? H/m
- B. 10 ? H/m
- C. 0.1 ? H/m
- D. 0.01 ? H/m Discuss
Correct Answer: 0.1 ? H/m
Explanation:
.
- 3. Consider the following statements
- Bunching of electrons occurs in two cavity klystron amplifier.
- Bunching of electrons occurs in multi cavity klystron amplifier.
- Bunching of electrons occurs in reflex cavity klystron amplifier.
Options- A. 1, 2, and only
- B. 1, 2 and 3 only
- C. 1 and 3 only
- D. 2 and 3 only Discuss
Correct Answer: 1, 2 and 3 only
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. Consider the following statements
- Dissipative attenuator has a fixed value of attenuation.
- Reflective attenuator has a fixed value of attenuation.
- Both dissipative and reflective attenuators are available only with fixed attenuation.
- Both dissipative and reflective attenuators are available with either fixed or variable attenuation.
Options- A. 1 only
- B. 1 and 2
- C. 1, 2 and 3
- D. 4 only Discuss
Correct Answer: 4 only
Explanation:
Both dissipative and reflective attenuators are available with either fixed or variable attenuation.- 5. Assertion (A): A half wavelength line can be used as a 1 : 1 transformer.
Reason (R): The input impedance of a half wavelength line is equal to load impedance.
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 and R is correct explanation of A
Explanation:
Since a half wavelength line has an input impedance equal to load impedance, the impedance transformation ratio is 1:1.- 6. If a short line is terminated in characteristic impedance it behaves as an infinite line.
Options- A. True
- B. False Discuss
Correct Answer: True
Explanation:
In a line terminated by Z0, there are no reflections.- 7. A wave Em cos (bx - ?t) is a backward wave.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
It is a forward wave because as t increases, x increases.- 8. Which mode has the lowest cut off frequency in circular wave guides?
Options- A. TE01
- B. TE11
- C. TE20
- D. TE21 Discuss
Correct Answer: TE11
Explanation:
Some applications require dual polarization capability. Circular waveguide has this capability.These analysis uses cylindrical coordinates.
In circular waveguide TE11 mode has the lowest cut off frequency and is the dominant mode.
If D is diameter of waveguide
?c = 1.706 D for TE11 mode
?c = 1.029 D for TE21 mode
?c = 0.82 D for TE01 mode
?c = 1.306 D for TM01 mode.
- 9. The number of TV channels which can be accommodated in a spectrum of 300 MHz is about
Options- A. 5
- B. 10
- C. 50
- D. 200 Discuss
Correct Answer: 50
Explanation:
A TV channel has a bandwidth about 5 MHz. Hence 300 MHz spectrum can accommodate about 50 channels.- 10. In a multicavity klystron amplifier the signal to be amplified develops an ac voltage of signal frequency across the gap in buncher cavity
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.
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