The VCO’s frequency can be controlled by the input voltage. An ideal VCO generates a periodic output whose frequency is a linear function of the input voltage. In practice, VCOs are never perfect, hence the output is never perfectly linear. Note that VCOs are bounded, which means that there is a minimum and maximum frequency they are able to generate. A VCO will not generate an output frequency if it is set to operate outside the specified range, or if no reference clock is provided to the input of the PLL. The VCO will rail to the upper or lower frequency limit if the PLL is not locked or if no reference clock is provided.