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

Total Questions: 100

31. Demodulation of SSB signals can be achieved easily by:

Correct Answer: D. Coherent Detector
Solution:

The most efficient way to demodulate SSB (Single Side Band) signals is by using a Coherent Detector. SSB signals do not have a carrier, so an envelope detector would not work as it does for AM signals. A conerent  detector, on the other hand, re-inserts the carrier signal, allowing the original message signal to be recovered through multiplication and filtering. This method ensures accurate phase and frequency matching with the original signal.

32. In case of wideband FM, the modulation index value is:

Correct Answer: A. around zero
Solution:

In Wideband FM (WBFM), the modulation index (m) typically exceeds unity. Modulation index is defined as the ratio of the frequency deviation to the modulating frequency. For WBFM, this deviation is large, and as a result, the modulation index is high.

This leads to multiple sidebands, which give WBFM its wide bandwidth. On the contrary, for narrowband FM (NBFM), the modulation index is typically less than unity, resulting in fewer sidebands and a more confined frequency spectrum.

33. Most of the practical control system require damping factor in the range of:

Correct Answer: D.
Solution:

For most practical control systems, the damping factor (ξ) should lie in the range 0.28 <ξ< 0.7. The damping factor determines how oscillations in a system decay in response to a disturbance or setpoint change.

A system with a damping factor of 1 (or higher) is overdamped and responds slowly. A system with a damping factor of 0 is undamped and will oscillate indefinitely. The range 0.28 to 0.7 ensures that the system responds quickly without excessive oscillation, making it suitable for many practical applications.

34. Consider a sixth order system with characteristic equation

The control system is:

Correct Answer: C. Limitedly stable

35. In two transistor model of an SCR the expression for anode current is given by:

Correct Answer: D.

36. Find the value of inductance in a series inverter circuit having the frequency of 5 kHz and a capacitance 1 µF. If the inverter is operating under resonance condition. The value of inductance is given by:

Correct Answer: A. 40 mH
Solution:For a series resonant circuit, the resonant frequency (fᵣ) is given by:

37. An ammeter of 1 mA possesses a resistance of 100 ohms. This ammeter is to be converted into 1A ammeter. The value of shunt resistance required will be:

Correct Answer: Β. 0.1001 Ω
Solution:

When converting into a galvanometer an ammeter, we use a shunt resistor to bypass the majority of the current, allowing only a small fraction to flow through the galvanometer. The shunt resistance (Rₛ) is given by:

38. Which of the following bridge is used for the measurement of capacitance, dielectric loss of capacitor and loss angle?

Correct Answer: C. Schering Bridge
Solution:

The Schering Bridge is particularly designed for measuring the capacitance and its associated power factor or dissipation factor. It is especially suitable for measuring the capacitance of a cable.

The bridge provides a null indication when the ratio of the products of the impedances in the arms is equal. Dielectric loss, capacitance, and loss angle are all properties that can be measured using a Schering Bridge.

39. For a Lissajous pattern as shown in Figure below, determine the frequency of vertical signal if the frequency of horizontal signal is 12 kHz.

Correct Answer: B. 30 kHz

40. An LVDT is used for measuring the deflection of bellows. The sensitivity of LVDT is 40 V per mm. The bellows is deflected by 0.125 mm by a pressure of 0.8 x 10⁶ N/m² Determine the sensitivity of the LVDT in V per N/m², when the voltage output of LVDT is 3.1 V.

Correct Answer: B. 6.25 x 10⁻⁶ V per N/m²