Discussion
Why is a varactor sometimes included in an oscillator's tank?
Answer: B
A varactor's capacitance varies with the reverse bias applied, so a control voltage tunes the frequency - which is exactly what a phase-locked loop needs to steer its oscillator. It does nothing for amplitude or harmonics, and it lowers the quality factor rather than improving it, since it is a lossier component than a good fixed capacitor. Every voltage-controlled oscillator trades some quality factor for tunability, and how much is the central compromise in its design.
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