# E7H03 - What is a phase-locked loop?

**URL:** <https://forum.openhamprep.com/t/e7h03-what-is-a-phase-locked-loop/1425>\
**Category:** Extra Questions\
**Created:** [January 2, 2026, 7:53pm UTC](https://forum.openhamprep.com/t/e7h03-what-is-a-phase-locked-loop/1425 "2026-01-02T19:53:29Z")\
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**Post date:** [January 2, 2026, 7:53pm UTC](https://forum.openhamprep.com/t/e7h03-what-is-a-phase-locked-loop/1425/1 "2026-01-02T19:53:29Z")

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## Question

What is a phase-locked loop?

## Answer Options

- **A)** An electronic servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator
- **B)** An electronic circuit also known as a monostable multivibrator
- **C)** An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator
- **D)** An electronic circuit consisting of a precision push-pull amplifier with a differential phase input

**Correct Answer: C**

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## Explanation

A Phase-Locked Loop (\text{PLL}) is a fundamental electronic circuit widely used in communications for frequency synthesis, modulation, and demodulation. It is a closed-loop negative feedback system designed to ensure that one oscillator (the Voltage-Controlled Oscillator, \text{VCO}) precisely tracks the frequency and phase of a second, highly stable reference signal.

A \text{PLL} is defined as **an electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator**. The phase detector compares the \text{VCO} frequency (or a divided version) to the reference frequency, generating an error voltage that, after filtering, controls the \text{VCO} to lock it exactly to the reference frequency and phase.

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