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

Total Questions: 100

51. Read the following statements regarding decoder:

(a) It is similar to demultiplexer.
(b) It does not have a data input.
(c) It is having data input lines.
(d) It converts n bit binary input to 2" output lines.
(e) It converts digital signals to analog signals.
Choose the correct answer from the options given below:

Correct Answer: C. (a), (b) & (d) only
Solution:

A decoder is a combinational circuit that converts binary information from n input lines to a maximum of 2ⁿ unique output lines.
(a) Decoders are similar to demultiplexers. In fact, they can be viewed as a special type of demultiplexer where there is no data input and only address lines.
(b) This is correct. Decoders don't have a "data input" per se. Instead, they take an n-bit input and direct it to one of the 2ⁿ outputs.
(d) This is also correct. An n-bit decoder will have 2ⁿ output lines. For example, a 2-bit decoder will have 4 output lines.
(e) This is incorrect. Decoders work entirely in the digital domain, so they do not convert digital to analog signals.

52. In 8051 micro controller:

(a) INTO and INT1 pins are normally low
(b) Timerl Interrupt Flag is auto cleared after occurrence
(c) Serial interrupt is auto cleared after occurrence
(d) Timer0 Vector location is 000B H
Choose the correct answer from the options given below:

Correct Answer: C. (b) and (d) only
Solution:

The 8051 microcontroller is a widely used microcontroller in embedded systems and has specific behaviours with respect to its interrupts and pins.
(b) This is true. The Timer1 Interrupt Flag gets auto-cleared after it's been triggered.
(d) Timer0 Vector location is indeed 000BН in the 8051 microcontroller.

53. In 8051

(a) INTO and INT1 can be made edge triggered by programming TCON
(b) On reset serial communication interrupts has highest priority
(c) Upon reset all interrupts are enabled
(d) Only one interrupt is set aside for serial communication.
Choose the correct answer from the options given below:

Correct Answer: D. (a) and (d) only
Solution:

The 8051 microcontroller has a variety of interrupt options, and their behaviour can be modified using specific registers.
(a) This is correct. By programming the TCON (Timer Control) register, the external interrupts INTO and INT1 can be configured as either edge-triggered or level-triggered.
(d) The 8051 microcontroller has a dedicated interrupt for serial communication, which is correct.

54. In 8086

(a) The stack memory is a FIFO (First In First Out)
(b) The JMP instruction cannot be prefixed with segment override prefix
(c) SAHF and POPF instructions affect the flag register
(d) DAS instruction cannot be used for 8 bit operations.
Choose the correct answer from the options given below:

Correct Answer: B. (b) and (c) only
Solution:

The 8086 microprocessor, a 16-bit microprocessor by Intel, has specific operations and instructions that dictate its behaviour.
(b) JMP (Jump) instruction in the 8086 microprocessor can indeed have a segment override prefix if the jump is to a different segment.
(c) Both SAHF (Store AH into Flags) and POPF (Pop Flags) instructions do affect the flag register. SAHF copies the lower byte of the AH register into the lower byte of the flag register, and POPF pops the top of stack into the flag register.

55. Varactor diodes have the following properties:

(a) Junction capacitance varies with bias voltage
(b) Junction capacitance varies with time
(c) Non linearity of varactor diodes makes them suitable for frequency multipliers
(d) Linearity of varactor diode makes them suitable for frequency multipliers. Choose the correct answer from the options given below:

Correct Answer: C. (a) and (c) only
Solution:

Varactor diodes, also known as varicap diodes, are used primarily in electronic tuning applications.
(a) This is true. The junction capacitance of a varactor diode changes with the applied bias voltage. This property allows them to be used in tuning circuits.
(c) The non-linearity of the varactor diode's capacitance with respect to voltage makes them suitable for applications like frequency multipliers. This non-linear characteristic is what allows them to interact with a high-frequency signal in a way that generates harmonics.

56. The properties of Dirac Delta are

Choose the correct answer from the options given below:

Correct Answer: A. (a) and (c) only
Solution:

The Dirac Delta function is a unique mathematical entity used extensively in the realm of continuous signals and systems.
(a) This is accurate. At t = tₒ, the value of the Dirac Delta function spikes to infinity.
(c) For any time t that isn't equal to t, the value of the function is zero. This ensures that the function integrates to one over all time.
The Dirac Delta function, although an abstract concept, proves to be invaluable when representing impulses in continuous-time systems.

57. The RF section of Radio Receiver has:

(a) Better coupling of receiver to Antenna
(b) Less Image-frequency rejection
(c) Greater Gain
(d) Allowing Re-radiation of local oscillator through the antenna of receiver. Choose the correct answer from the options given below:

Correct Answer: C. (a) & (c) only
Solution:

The RF (Radio Frequency) section of a radio receiver is crucial in the early stages of signal processing.

(a) One primary goal of the RF section is to have efficient coupling to the antenna, which ensures that the receiver captures as much of the transmitted signal as possible.

(c) Another objective is to amplify weak signals received from far-off stations, so having a significant gain in this section is necessary.

The RF section sets the stage for subsequent processing stages, ensuring that the received signal is of adequate strength and quality.

58. In amplitude modulation:

(a) Amplitude of carrier is varied by modulating signal
(b) Modulation index is between 0 and 1
(c) Bandwidth is infinite
(d) Bandwidth is twice of minimum modulating frequency.
Choose the correct answer from the options given below:

Correct Answer: B. (a) and (b) only
Solution:

Amplitude Modulation (AM) is one of the oldest methods for transmitting voice and music over radio waves.

(a) True. In AM, the amplitude of a carrier wave is modified in accordance with the instantaneous value of the modulating signal.

(b) The modulation index (or depth of modulation) in AM can range between 0 (no modulation) and 1 (100% modulation). This index essentially describes the extent to which the carrier is modulated by-the input signal.

AM has been a foundation of broadcasting for over a century and is simple, yet effective.

59. Waveguides below cutoff have attenuation for:

(a) Reflections from obstacles, discontinuities and misaligned waveguide sections (b) Losses due to currents flowing in waveguide walls
(c) Losses in dielectric filling the waveguide
(d) Gain in dielectric filling the waveguide.
Choose the correct answer from the options given below:

Correct Answer: B. (a), (b) & (c) only
Solution:

Waveguides are structures that guide electromagnetic waves from one point to another. Their operation is contingent on numerous parameters, including their dimensions and the frequency of the signal.

(a) True. If the waveguide is below its cutoff frequency, it will not propagate the signal efficiently. Reflections can occur from obstacles, discontinuities, or misaligned sections of the waveguide.

(b) Currents that flow on the walls of the waveguide can cause resistive losses, especially if the waveguide is below its cutoff frequency.

(c) If the waveguide has a dielectric fill, there can be losses due to the dielectric material absorbing some of the energy.

Waveguides find uses in a variety of applications, especially at microwave frequencies where traditional transmission lines become inefficient.

60. An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. It has

(a) one cycle surge current = 2121.32 A
(b) one cycle surge current = 1121.32 A
(c) I²t rating = 45000 Amp²
(d) I²t rating = 1257358 Amp²
Choose the correct answer from the options given below:

Correct Answer: B. (a) and (c) only