Oscillators & PLL MCQs

Practice Oscillators & PLL MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Analog ElectronicsOscillators & PLL

A voltage-controlled oscillator (VCO) is an oscillator whose output frequency is primarily controlled by?

AAn applied control voltage
BOnly ambient temperature
CThe load resistance alone
DA fixed diode current with no control input

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Correct Answer: A. An applied control voltage

Explanation: A VCO converts a control voltage into a corresponding change in oscillation frequency and is widely used in modulation and PLL systems.

Reference: Standard analog electronics: voltage-controlled oscillator

Question 2

Analog ElectronicsOscillators & PLL

In a phase-locked loop, the phase detector produces a signal related to?

AThe phase difference between reference and feedback signals
BOnly the output amplitude
CThe transistor collector current only
DThe power-supply voltage

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Correct Answer: A. The phase difference between reference and feedback signals

Explanation: The phase detector compares reference and feedback phases; after loop filtering, its error signal controls the VCO to establish lock.

Reference: Standard analog electronics: phase-locked loop

Question 3

Analog ElectronicsOscillators & PLL

In a phase-locked loop, the loop filter is placed mainly between the phase detector and VCO to?

ASmooth the detector output and shape loop dynamics
BGenerate the reference frequency by itself
CRectify the VCO output for power conversion
DEliminate the need for feedback

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Correct Answer: A. Smooth the detector output and shape loop dynamics

Explanation: The loop filter removes unwanted high-frequency detector components and determines important loop bandwidth and dynamic characteristics.

Reference: Standard analog electronics: PLL loop filter

Question 4

Analog ElectronicsOscillators & PLL

A PLL is said to be locked when the VCO output and reference have?

AA constant phase relationship and the required frequency relationship
BRandomly changing frequencies
CExactly zero amplitudes
DNo feedback connection

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Correct Answer: A. A constant phase relationship and the required frequency relationship

Explanation: In lock, feedback maintains the required frequency relationship and the phase error settles to a constant value appropriate to the loop.

Reference: Standard analog electronics: PLL lock

Question 5

Analog ElectronicsOscillators & PLL

In a simple PLL frequency synthesizer with divide-by-N feedback, a 100 kHz reference and N=50 produce an output frequency of approximately?

A2 kHz
B100 kHz
C5 MHz
D50 MHz

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Correct Answer: C. 5 MHz

Explanation: In lock, the divided output equals the reference, so fout/N=fref and fout=Nfref=50×100 kHz=5 MHz.

Reference: Standard analog electronics: PLL frequency synthesis

Question 6

Analog ElectronicsOscillators & PLL

The capture range of a PLL refers to the range of input frequencies over which an initially unlocked loop can?

AAcquire lock
BMaintain transistor saturation only
CProduce zero output
DOperate without a VCO

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Correct Answer: A. Acquire lock

Explanation: Capture range describes the frequency range from which an unlocked loop can acquire synchronization with the input.

Reference: Standard analog electronics: PLL capture range

Question 7

Analog ElectronicsOscillators & PLL

The lock or hold-in range of a PLL generally describes the range over which an already locked loop can?

ARemain locked while input frequency varies
BStart oscillating with no reference
CSuppress all supply current
DAct as a rectifier

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Correct Answer: A. Remain locked while input frequency varies

Explanation: Once lock is established, the loop can track input-frequency variation over its lock or hold-in range.

Reference: Standard analog electronics: PLL lock range