# Electronics and Communication Engineering - Microwave Communication

Exercise : Microwave Communication - Section 3
26.
A reflex klystron oscillator is a
low power device
high power device
high efficiency device
both (a) and (b)
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?
1 2 and 3
l and 2 only
1 and 3 only
2 and 3 only
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?
1 only
1 and 2
1, 2 and 3
4 only
Explanation:

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

29.
Impedance inversion may be obtained with
a short circuited stub
a quarter wave line
an open circuited stub
a half wave line
Explanation:

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

30.
Power gain of a multicavity klystron amplifier is
more than 30 dB
causes retardation of all electrons
causes acceleration of some electrons and retardation of others
none of the above
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)