A phase locked loop system has a voltage-controlled variable-load ring oscillator (VLCO) that operates in a frequency band determined by a selected load on each stage of the ring oscillator. Each stage of the VLCO has multiple load selection transistors, each coupled to a load capacitor. Apparatus is provided for driving the load selection transistors according to a load configuration; and apparatus is provided for determining an operating load configuration such that a period of a divided reference signal approximately matches a period of a divided VLCO signal with the VLCO control voltage input clamped to a reference voltage. Once the load configuration is set, the loop is allowed to lock. In a particular embodiment, devices are provided for slowly tweaking the VLCO load to help keep the VLCO operating near an optimum control voltage despite drift of circuit parameters with temperature or time.