Explanation: In a frequency synthesizer, the Voltage Controlled Oscillator (VCO) is the active component that generates the output frequency. Its frequency can be varied over a range by an input control voltage. In a Phase-Locked Loop (PLL) synthesizer, the VCO's output is divided down and compared in a phase detector with a stable reference standard. The error signal from the phase detector then adjusts the VCO's control voltage, locking its output to a precise multiple of the reference frequency. The phase detector compares signal phases, the divider scales frequency, and the reference standard provides a stable, fixed frequency; none of these *generate* the variable output frequency themselves. The VCO is the tunable oscillator at the heart of the synthesizer.