A phase locked loop circuit contains a VCO. What does the output of the VCO consist of?

The output of a Voltage-Controlled Oscillator (VCO) in a phase-locked loop (PLL) circuit is an AC variable frequency signal. This is because a VCO generates an output frequency that changes based on the magnitude of the input control voltage. When the control voltage is varied, the frequency of the output signal also varies, making it an AC signal.

In the context of a phase-locked loop, this dynamic frequency output is crucial for synchronizing the frequency of the VCO with a reference signal. The ability to adjust the frequency allows the PLL to maintain a lock on the input signal by continuously tuning the VCO to match the frequency and phase of the incoming signal.

The other options are not suitable because they either describe a constant frequency (which would not be characteristic of a VCO) or incorrectly classify the output as DC when it is inherently an AC signal due to the nature of oscillation. The VCO's function is specifically designed to produce a signal that oscillates at varying frequencies rather than generating a steady DC signal.