Phase-locked loop (PLL) — typical applications: A PLL can be used as a(n) ___________. Select the most appropriate function for which PLLs are widely employed in electronics.

Introduction / Context:
Phase-locked loops (PLLs) are feedback control systems that lock the phase and frequency of a voltage-controlled oscillator (VCO) to a reference. They appear in communication systems, clock generation, and motor control. This question asks you to identify a standard, textbook application for a PLL from among several distractors that belong to different circuit families.

Given Data / Assumptions:

• PLL contains a phase detector, loop filter, and VCO.
• Once locked, the VCO frequency is a rational multiple of the reference when frequency division is included in the loop.
• Idealized block behavior; implementation details vary across ICs.

Concept / Approach:
PLLs can multiply frequency by using digital divider ratios in the feedback path. For example, if the feedback includes a divide-by-N counter, the VCO will lock at N times the reference frequency. This principle is the basis of clock synthesizers in microprocessors and RF local oscillators. In contrast, a series voltage regulator is a power-management circuit, a relaxation oscillator is a timing circuit not controlled by phase lock, and a Schmitt trigger is a comparator with hysteresis—none rely on phase locking to a reference.

Step-by-Step Solution:

Verification / Alternative check:
Clock synthesizer ICs and RF PLL chips routinely specify multiplication factors and phase noise characteristics. Laboratory PLL experiments demonstrate clean fractional-N or integer-N frequency multiplication while maintaining phase coherence with the reference.

Why Other Options Are Wrong:

• Series voltage regulator: regulates DC voltage; does not lock phase/frequency.
• Relaxation oscillator: free-running timing; not locked to a reference.
• Schmitt trigger: digital conditioning with hysteresis; no VCO or phase detector.

Common Pitfalls:
Assuming “oscillator” always means free-running; forgetting that a PLL’s advantage is frequency/phase control relative to a reference via feedback.

Final Answer:
Frequency multiplier.