A New Approach to the Modeling of the Dynamics of RF VCO's and Some of Its Practical Implications

A new approach to the modeling of practical radio frequency (RF) voltage controlled oscillators (VCO's) is presented. The usual VCO analysis employs a second-order model. Here we resort to a third-order representation, which is more accurate mathematically and is also practically advantageous. The advantages are due to the fact that practical RF VCO's are based on the Colpitts circuit, which is a third-order system. The model is investigated by 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.