A voltage controlled oscillator (VCO) is controlled by a separate phase locked loop (PLL). The PLL includes a first variable delay circuit of m stages which receives a reference frequency signal and produces a delayed signal which is compared in a phase comparator with the reference frequency signal....http://www.google.com/patents/US4912433?utm_source=gb-gplus-sharePatent US4912433 - VCO controlled by separate phase locked loop
