A Passive Phase Noise Cancellation Element
  We introduce a new method for reducing phase noise in oscillators, thereby improving their frequency precision. The noise reduction device consists of a pair of coupled nonlinear resonating elements that are driven parametrically by the output of a conventional oscillator at a frequency close to the sum of the linear mode frequencies. Above the threshold for parametric response, the coupled resonators exhibit self-oscillation at an inherent frequency. We find operating points of the device for which this periodic signal is immune to frequency noise in the driving oscillator, providing a way to clean its phase noise. We present results for the effect of thermal noise to advance a broader understanding of the overall noise sensitivity and the fundamental operating limits. 
Eyal Kenig, M. C. Cross, Ron Lifshitz, R. B. Karabalin, L. G.   Villanueva, M. H. Matheny, M. L. Roukes
cond-mat.mes-hall nlin.PS