Question 1

In a klystron amplifier the force on all electrons

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

In a line with finite attenuation

Accepted Answer

If standing waves are not there, whole of the power is absorbed by load.

Question 3

If load impedance ZL >> Z0, then

Accepted Answer

If ZL >> Z0, |rv| and VSWR . Therefore ZL = Z0(VSWR).

Question 4

The electric field in a TWT due to applied signal

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 5

In a klystron amplifier the bunching effect

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 6

Assertion (A): Impatt diode is an avalanche diode. Reason (R): Avalanche breakdown phenomenon occurs when a p-n junction is reverse biased.

Accepted Answer

An Impatt diode has n+ - p - i - p + structure and is used with reverse bias. It exhibits negative resistance and operates on the principle of avalanche breakdown. Impatt diode circuits are classified as broadly tunable circuit, low Q circuit and high Q circuit. The impedance of Impatt diode is a few ohms. The word Impatt stands for Impact Avalanche Transit Time diode. The features of Impatt diode oscillator are : frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and upto 40 W for combination of several diodes), efficiency about 20%. Its applications include police radar systems, low power microwave transmitter etc.

Question 7

The path length between ports 1 and 2 for the two waves in a hybrid ring is

Accepted Answer

The quantities required to be measured in microwave circuits are frequency, power and impedance. Microwave frequency measurement is done by slotted line, resonant cavities and transfer oscillator. Microwave power measurement uses bolometers and microwave power meters. A self balancing bridge working on the principle of power substitution is commonly used. Microwave impedance measurement is done by measurement of reflection coefficient and VSWR. A slotted line and probe is a basic tool for these measurements.' Network analyser enables rapid impedance measurement over a broad frequency range.

Question 8

The reverse voltage applied to a varactor diode

Accepted Answer

The action of varactor diode is due to the capacitance of depletion layer. The thickness of depletion layer depends on the reverse bias which should not cause breakdown.

Question 9

In a TWT the amplitude of resultant wave travelling down the helix remains constant

Accepted Answer

The signal gets strengthened.

Microwave Communication Questions