An S-parameter transmission model approach to VCO analysis

O in RF and microwave frequencies tend to be a mysterious circuit block in that their behaviors are not fully explained. In particular, designing a wide-tuning voltage controlled oscillator (VCO) still involves a lot of guesses and many printed circuit board (PCB) revisions, even with the help of today’s highly sophisticated design tools. One problem in the design process lies in the absence of an analysis method that gives full insight into oscillation performance represented in an easy-tounderstand format. Traditionally, all oscillators are classified as either a negative resistance oscillator (a.k.a. reflection oscillator) or a feedback oscillator. The criterion is whether there is an “obvious” feedback circuit. In RF and microwave frequencies, it becomes difficult to construct a feedback circuit without introducing excess phase shift. Therefore, almost all oscillators in the frequency range are classified as a negative resistance oscillator. Figure 1 describes the negative resistance oscillator and its analysis. In the negative resistance model, oscillators are represented as a combination of an active element and a passive element. The active element generates negative resistance and drives the passive element, which is usually a resonator. When the negative resistance beats all losses in the resonator and the active element, oscillation builds up. The negative resistance analysis is so common, yet there are questions as to the validity of this analysis. Also, some oscillation parameters, like loaded-Q, cannot be derived from this analysis. Loaded-Q is one of the most important fundamental parameters because it dictates spectral purity (phase noise). Missing loaded-Q information when analyzing an oscillator prevents the designers from having a full understanding of their own design.