The VCO reveals the main difference between voltage and phase domain models. In a voltage domain model, the VCO model in steady state outputs an oscillatory voltage. In a phase domain model, the VCO model in steady state outputs a ramp representing the phase of the VCO. In some phase domain models, the VCO model in steady state outputs a constant representing VCO frequency.

In a voltage domain model, the dividers are counters. In a phase domain model, they are gain blocks.

In a voltage domain model, the PFD operates on the edges of the reference and divider waveforms. In phase domain model, the PFD is an error amplifier and sometimes includes an integrator.