Abstract: Methods for designing and developing models for simulating the behavior of clock signals and in particular those generated by phase-locked loop (PLL) circuits are provided. The clock period of a phase-locked loop circuit's variable frequency oscillator signal may be modeled by combining the inverse of the oscillator frequency rounded up to the simulation time scale with the inverse rounded down to the simulation time scale. The variable frequency oscillator signal may further be synchronized with a reference clock signal at a rate determined by the relationship between the reference clock signal and the variable frequency oscillator signal. A parameter may indicate a target range for the deviation between the two signals and a runtime monitor may be used together with the parameter setting to decide whether synchronization is required and make the appropriate adjustments.