Frequency Switching in Dual-Resonance Oscillators

An oscillator with a high-order (>2) resonator has multiple stable modes of oscillations. The stable modes for one such oscillator, having a fourth-order resonator, are found using a nonlinear analysis. By using a proper nonlinear active topology and tank component values, the fourth-order oscillator can generate either of the two distinct frequencies f₁ or f₂. A method is introduced to dynamically switch between the stable modes of oscillations. It is shown that the phase noise of this fourth-order oscillator, when generating only one of its resonant frequencies, is comparable to the phase noise of a second-order oscillator using the same active topology and resonator quality factor. Furthermore, the fourth-order oscillator has better phase noise and/or higher tuning range in VCO implementations compared to the commonly used switched resonator oscillators. The claims have been verified experimentally through an integrated oscillator prototype with f₁ = 2.4 GHz and f₂ = 4.7 GHz fabricated in a standard 0.18 mum CMOS technology. The oscillator draws 1.89 mA current of 1.8 V supply. The 1 MHz offset phase noises of the fourth-order oscillator for f₁ = 2.4 GHz and f₂ = 4.7 GHz are -122.4 dBc/Hz and -123.4 dBc/Hz, respectively.