Electronics and Communication Engineering - Microwave Communication

26. 

A reflex klystron oscillator is a

A. low power device
B. high power device
C. high efficiency device
D. both (a) and (b)

Answer: Option A

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.


27. 

Consider the following applications

  1. TV tuning
  2. Active filter
  3. Microwave frequency multiplication
In which of above can a varactor diode be used?

A. 1 2 and 3
B. l and 2 only
C. 1 and 3 only
D. 2 and 3 only

Answer: Option A

Explanation:

Varactor diode is used in all the three applications.


28. 

Consider the following statements

  1. Dissipative attenuator has a fixed value of attenuation.
  2. Reflective attenuator has a fixed value of attenuation.
  3. Both dissipative and reflective attenuators are available only with fixed attenuation.
  4. Both dissipative and reflective attenuators are available with either fixed or variable attenuation.
Which of the above are correct?

A. 1 only
B. 1 and 2
C. 1, 2 and 3
D. 4 only

Answer: Option D

Explanation:

Both dissipative and reflective attenuators are available with either fixed or variable attenuation.


29. 

Impedance inversion may be obtained with

A. a short circuited stub
B. a quarter wave line
C. an open circuited stub
D. a half wave line

Answer: Option B

Explanation:

Impedance inversion occurs when load impedance is not matched with characteristic impedance of line.


30. 

Power gain of a multicavity klystron amplifier is

A. more than 30 dB
B. causes retardation of all electrons
C. causes acceleration of some electrons and retardation of others
D. none of the above

Answer: Option A

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.