Deep Dive: E7H03
The correct answer is C: An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator. A phase-locked loop (PLL) is an electronic servo loop that consists of a phase detector (compares phases), a low-pass filter (filters the phase error signal), a voltage-controlled oscillator (VCO) (generates the output frequency), and a stable reference oscillator (provides the reference frequency). The PLL works by comparing the phase of the VCO output with the reference oscillator. The phase detector produces an error signal proportional to the phase difference. This error signal is filtered and used to control the VCO frequency, creating a feedback loop that locks the VCO to the reference. PLLs are widely used in frequency synthesizers, FM demodulators, and clock recovery circuits. They provide stable, accurate frequency generation that can be digitally controlled.
Why Other Answers Are Wrong
Option A: Incorrect. A ratio detector, reactance modulator, and VCO describe an FM demodulator circuit, not a PLL. PLLs use a phase detector, not a ratio detector. Option B: Incorrect. A monostable multivibrator is a one-shot pulse generator, not a PLL. PLLs are continuous feedback loops, not one-shot circuits. Option D: Incorrect. A precision push-pull amplifier with differential phase input doesn't describe a PLL. PLLs are feedback control systems, not just amplifiers.
Exam Tip
PLL = Phase detector + Filter + VCO + Reference. Remember: A phase-locked loop consists of a phase detector, low-pass filter, voltage-controlled oscillator, and stable reference oscillator.
Memory Aid
**P**LL = **P**hase **D**etector, **L**ow-pass, **L**ocked **V**CO (think 'PLL = PDLV')
Real-World Example
Your modern transceiver uses a PLL frequency synthesizer to generate stable frequencies. The PLL compares the VCO output with a crystal reference oscillator. If the VCO drifts, the phase detector generates an error signal that corrects the VCO frequency, keeping it locked to the reference. This provides stable, accurate frequency generation that can be controlled digitally.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E7H
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E7H topic.