In a phase-locked-loop circuit, the output of the VCO generates what type of signal?

In a phase-locked loop (PLL) circuit, the output of the Voltage-Controlled Oscillator (VCO) generates an AC signal with a changing frequency. This is fundamental to the operation of a PLL, which synchronizes the output frequency of the VCO to match a reference frequency.

The output signal produced by the VCO varies in frequency depending on the input control voltage, which is derived from the phase comparator within the PLL. When the PLL is locked, the VCO adjusts its frequency to maintain phase coherence with the reference signal, thus producing an AC signal that can change in frequency as needed to keep the system in sync. This dynamic frequency output is essential for applications such as frequency synthesis, demodulation, and signal recovery.

The other options do not accurately describe the output of the VCO in this context. A DC signal, for example, would be constant and not representative of the frequency modulation occurring in a PLL. Similarly, a pulsed signal or fixed frequency signal does not capture the essence of the VCO output's capability to vary based on the control voltage, which inherently results in an AC changing frequency signal.