Electronics and Communication Engineering - Microwave Communication

46. 

Assertion (A): Impatt diode is an avalanche diode.

Reason (R): Avalanche breakdown phenomenon occurs when a p-n junction is reverse biased.

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

Answer: Option B

Explanation:

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.


47. 

A radar has a maximum range of 120 km. The maximum allowable pulse repetition frequency for unambiguous reception is

A. 1250
B. 330
C. 2500
D. 8330

Answer: Option A

Explanation:

No answer description available for this question. Let us discuss.

48. 

In a line with finite attenuation

A. power transmission is least efficient when there are no standing waves on the line
B. power transmission is most efficient when there are no standing waves on line
C. Load power depends on phase constant
D. standing waves will always exist on the line

Answer: Option B

Explanation:

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


49. 

In the given figure a short circuited transmission line resonator If n = 1, 2, 3...

A. For series reasonance βl = np and for parallel resonance
B. For series resonance and for parallel resonance βl = np
C. βl = np for both series and parallel resonance
D. for both series and parallel resonance

Answer: Option A

Explanation:

No answer description available for this question. Let us discuss.

50. 

The electric field in a TWT due to applied signal

A. is directed along the helix axis
B. is directed radially from helix axis
C. is inclined to the helix axis by about 60°
D. is inclined to the helix axis by about 45°

Answer: Option A

Explanation:

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.


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