NTA UGC NET/JRF Exam, February-2023 (Electronic Science)

Total Questions: 100

91. Read the following paragraph carefully and answer these questions.

A pulse radar determines target by round trip time of a pulsed microwave signal. The frequency used by radar transmitter is 10 GHz with transmitted power 2 KW (Pulse power) The Antenna size of(Gt) of 28 dB is used to detect the target (aeroplane) having its cross section area as 12 m². The receiver has its capability as -90 dBm as minimum detectable signal (Pₘᵢₙ). There is an isolation between trans and receive chain as (80-100 dB) determine Radar maximum range.
What is the distance (R) of target if round trip time (t) is known in the radar signal C velocity of signal (EM wave):

Correct Answer: A.
Solution:

For radar, the distance to a target is half the round-trip time multiplied by the speed of the radar signal (which is the speed of light in a vacuum). This is because the signal has to go to the target and come back. So,Given the speed of electromagnetic 2 waves (c) is 3 x 10⁸ m/s, the formula simplifies to R = 150 x 10⁶ x t. The specific distance depends on the round-trip time t provided. Without a specific value for t, it is not possible to calculate the exact distance. However, the relationship between R and t is captured by this formula.

92. Operating frequency of this radar falls in which band? specify it

Correct Answer: С. Х-Вand
Solution:

Radar frequency bands are designated by specific names. The frequency of 10 GHz corresponds to the X-Band, which typically covers the range from 8 GHz to 12 GHz.

This band is widely used for radar applications, especially in missile guidance, weather observations, and airborne radars. It provides a good trade-off between resolution and atmospheric absorption, making it ideal for many radar tasks.

93. If Zʟ is impedance of radar antenna and Zₒ is characteristic impedance of line, then what is reflection coefficient (Γ) of the antenna?

Correct Answer: E. (*)
Solution:

Either question or options is wrong.

94. Express minimum detectable Signal (Pₘᵢₙ) in watts (Pₘᵢₙ in radar receiver):

Correct Answer: A. 10⁻¹² W
Solution:

The minimum detectable signal Pₘᵢₙ of -90 dBm can be converted to watts using the formulaThus the minimum detectable signal in watts is 10⁻¹² W.

95. What is maximum range of this radar (Rₘₐₓ) upto which target can be determined?

Correct Answer: A. 8114 m

96. Read the following paragraph carefully and answer these questions.

In phase control and Integral cycle control the supply voltage is ac and the armature voltage is rectified ac, where as in chopper control the supply is dc and the average armature voltage is proportional to the duty ratio of the chopper. The chopper system is used for dc traction & battery operated vehicle-Choppers using thyristors are more complicated than the phase control system. This is because an auxiliary commutation arrangement is necessaiy for choppers, whereas commutation is natural owing to ac supply in phase control or integral cycle control.
 In a fully controlled three phase bridge without flywheel operation the displacement factor and power factor is:

Correct Answer: B.

97. In a two quadrant single phase SCR Drive armature current becomes continuous when:

Correct Answer: A. β-α is equal to or greater than 180°
Solution:

In a two-quadrant single-phase SCR Drive, the armature current becomes continuous when the conducting angle (or conduction interval) of the SCR is equal to or greater than 180°. The relationship β-α denotes the conduction interval, where a is the firing angle and β is the turn-off angle.

For the armature current to be continuous, β-α should be equal to oror greater than 180°, ensuring that there's no period within the cycle where the current drops to zero.

98. In one quadrant converters such as half controlled bridge circuit or single phase circuit with flywheel diodes for DC motor.

Correct Answer: A. Average output voltage is always positive
Solution:

In one-quadrant converters such as a half-controlled bridge circuit or a single-phase circuit with flywheel diodes for DC motor, the average output voltage is always positive.

This is because the converter operates in only one quadrant of the voltage-current plane, specifically the first quadrant, which signifies positive voltage and positive current. The flywheel diodes maintain the continuity of current and prevent its reversal, ensuring that the voltage remains positive throughout the operation.

99. Filter for chopper should be designed such a way in the DC Drive that the chopper operating frequency should be at least.

Correct Answer: C. 4 times resonant frequency
Solution:

For a chopper's filter in a DC Drive, it should be designed such that the chopper operating frequency is significantly higher than the resonant frequency to avoid resonant conditions.

Operating at a frequency close to or at the resonant frequency can cause large current or voltage spikes and may damage components. By selecting a chopper operating frequency at least 4 times the resonant frequency, we ensure that the system remains stable and far from resonance, minimizing the risk of overcurrents or overvoltages.

100. In a single quadrant chopper how can the direction of dc motor be reversed

Correct Answer: A. by changing the field
Solution:

In a single-quadrant chopper, the direction of the DC motor can be reversed by changing the field polarity. The chopper controls the voltage (and therefore speed) applied to the motor. To reverse the motor's direction, we don't typically modify the chopper frequency or use filters.

Instead, reversing the field polarity changes the direction of the generated torque, and hence, the rotation direction of the motor. It's one of the basic principles of DC motor operation where the direction is governed by the interaction between the armature and field magnetomotive forces.