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.
Correct Answer: frequency multiplier
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:
Recognize PLL composition and purpose: lock a VCO to a reference signal.Recall that inserting a divide-by-N in the feedback path forces the VCO to run at N * f_ref.Conclude that multiplying frequency is a canonical PLL function.Eliminate unrelated options (power regulation, hysteretic comparators, and open-loop relaxation oscillators).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.