A new approach to the modeling of the dynamics of RF VCO´s and some of its practical implications

An approach to the modeling of practical radio frequency (RF) voltage controlled oscillators (VCOs) is presented. The usual VCO analysis employs a second-order model. The authors resort to a third-order representation that is more accurate mathematically. The representation is practically advantageous because practical RF VCOs are based on the Colpitts circuit, which is a third-order system. The model is investigated using an approximate linear approach, which is then improved through nonlinear analysis based on the harmonic balance method. The analyses reveal the existence of a relatively unknown mode of a shifting bias superimposed on the oscillation, which may cause interference at the system output. Furthermore, the analytical treatments bring forth a recommendation for a useful procedure of changing the oscillator parameters in attempting a frequency change. This leads to a major improvement of the circuit performance, when employed as a VCO. The analytical and simulation findings are supported favorably by experimenting with a practical circuit