Near-carrier phase-noise characteristics of narrow band Colpitts oscillators

A theoretical approach to estimate the near-carrier phase-noise level of narrow band transistor Colpitts oscillators is described. Near carrier phase-noise characteristics of reference oscillators in mobile communication systems constantly need to be improved. We proposed a narrow-band Colpitts oscillator for SC-cut resonators in 1996 FCS. This oscillator uses an AT-cut resonator as a band selection filter, and we showed that the near-carrier phase-noise characteristics of the oscillator were improved over 5 dB compared with conventional transistor Colpitts oscillators. An analysis of the close to carrier phase-noise characteristics of the narrow-band Colpitts oscillators is described. The open-loop impedance, that is, a series including a SC-cut resonator and an oscillator active circuit, is calculated using a linear circuit model with a nonlinear transistor model. We show the loaded Q can be improved by introducing the resonator filter in the oscillation circuit. Based on Leeson´s model, the near carrier phase-noise is estimated using calculated Q values. The results correspond well with previous experiments. We also show that the phase-noise can be greatly improved - under -148 dBc/Hz at 10Hz offset - by using a resonator filter with high Q value.