What is a phase-locked loop?
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.
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 Application
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.
FCC Part 97.3Key Concepts
Why Other Options 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.
题目解析
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.
考试技巧
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.
记忆口诀
**P**LL = **P**hase **D**etector, **L**ow-pass, **L**ocked **V**CO (think 'PLL = PDLV')
实际应用示例
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.
错误选项分析
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.
知识点
Phase-locked loop, PLL, Phase detector, VCO, Reference oscillator
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.