A VCO is fairly difficult to simulate. When designing a PLL ( phase locked loop) it helps to use a behavioral model of a VCO to begin the design. This is because a behavioral model runs fast and is cheaper in simulation time and engineering time. There are a number of ways to do this. However, a popular way is to use PSPICE ABM modeling to implement the VCO behavioral model. In a recent post Signal Processing Group Inc.’s technical team has released a white paper that describes this. The paper also includes simulation results and the run file used. Interested parties may access this paper from the complementary items link on the SPG website.