This model shows a voltage-controlled oscillator with feedback control to regulate the output voltage. The oscillation frequency is controlled by the reverse bias voltage applied to the to the varactor diode. The model shows how behavioral and component-level modeling can be mixed when designing a circuit. Provided that the variable trans-conductance device has a fast dynamic response relative to the oscillation frequency, the implementation details are not important. Similarly, the PI controller is in abstracted form, and could either be implemented with op-amps, or in software.
The plots below shows the output voltage of the voltage-controlled oscillator (vco) circuit. Both the true output and the averaged output are shown. The PI controller ensures that oscillations are centered about the reference value of 0.2 V.