Question 1

In a three cavity klystron amplifier, the oscillations are excited in

Accepted Answer

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.

Question 2

Which of the following lines is non-radiating?

Accepted Answer

Since fields are confirmed within a coaxial cable it is non-radiating.

Question 3

As the frequency is increased, the charging MVAR in a cable

Accepted Answer

Charging current and MVAR are proportional to frequency.

Question 4

A line has Z0 = 300 ∠ 0° Ω. If ZL = 150 ∠ 0° Ω, reflection coefficient is

Accepted Answer

Reflection coefficient

Question 5

A 10 km long line has a characteristic impedance of 400 ohms. If line length is 100 km, the characteristic impedance is

Accepted Answer

and is independent of line length.

Question 6

Microwave resonators are used in

Accepted Answer

They are used in all the devices.

Question 7

In a klystron amplifier the input cavity is called

Accepted Answer

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.

Question 8

Assertion (A): TWT uses a focussing mechanism to prevent the electron beam from spreading. Reason (R): In a TWT the electron beam has to travel a much longer distance than in klystron.

Accepted Answer

In a klystron the resonant structure limits the bandwidth. A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave. The velocity of wave propagation along the helix structure is less than velocity of light. The beam and wave travel along the structure at the same speed. Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave. Therefore the signal gets strengthened and amplified output is delivered at the other end of tube. The main features of TWT are : 1. Frequency range - 0.5 GHz to 90 GHz 2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz 3. Efficiency - about 5 to 20% 4. Noise - about 5 dB for low power TWT 25 dB for high power TWT TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc.

Question 9

For a 50 Ω resistor for 3 GHz application, the stray capacitance should be less than

Accepted Answer

XC should be at least ten times greater than R. Therefore for 50 Ω resistor at 3 GHz, C should be less than 0.1 pF.

Question 10

Which of the following devices uses a slow wave structure?

Accepted Answer

In a klystron the resonant structure limits the bandwidth. A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave. The velocity of wave propagation along the helix structure is less than velocity of light. The beam and wave travel along the structure at the same speed. Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave. Therefore the signal gets strengthened and amplified output is delivered at the other end of tube. The main features of TWT are : 1. Frequency range - 0.5 GHz to 90 GHz 2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz 3. Efficiency - about 5 to 20% 4. Noise - about 5 dB for low power TWT 25 dB for high power TWT TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc.

Question 11

In high frequency circuits impedance matching can be done by open circuited stubs short circuited stubs transformer Which of the above correct?

Accepted Answer

All the three can be used for impedance matching in high frequency circuits.

Question 12

In a reflex klystron oscillator, repeller electrode is at

Accepted Answer

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.

Question 13

Assertion (A): A backward wave oscillator can be used as a sweep generator. Reason (R): The frequency of oscillation of a backward wave oscillator can be changed by varying the voltage which controls the beam velocity.

Accepted Answer

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.

Question 14

Assertion (A): PIN diode is commonly used for microwave control. Reason (R): A PIN diode uses heavily doped p and n materials.

Accepted Answer

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.

Question 15

Assertion (A): The velocity factor of a line is the ratio of wave velocity on the line to speed of light. Reason (R): If the conductors of a line are immersed in a non magnetic insulating liquid, the wave velocity increases.

Accepted Answer

R is wrong because wave velocity decreases.

Question 16

The magnitudes of OC and SC input impedances of a transmission line an 100 Ω and 25 Ω. The characteristic impedance is

Accepted Answer

Question 17

Assertion (A): A quarter wave transformer is used to match a resistive load to a transmission line. Reason (R): A quarter wave transformer is a transmission line of quarter wave length.

Accepted Answer

A line of length is called quarter wavelength line. Such a line is used for impedance matching. If A quarter wave line can match a source impedance Zin with load impedance ZL by selecting a proper value of Z0 so as to satisfy equation. Such a line is also called transformer.

Question 18

In a loss line RL < Z0, then

Accepted Answer

Since RL < Z0, rv is negative.

Question 19

The correct sequence of parts in klystron amplifier are

Accepted Answer

Cathode is the first part and anode is the last.

Question 20

Which of the following terminations makes the input impedance of a line equal to characteristic impedance Z0?

Accepted Answer

If ZL is load impedance and Z0 is characteristic impedance. Input impedance =. If ZL = Z0 the input impedance = Z0.

Microwave Communication Questions