New sustaining circuit for precision crystal oscillators

Actual state of the art commercial precision 5 MHz crystal oscillators use “linear” sustaining circuits with AGC-control and have, at 1 Hz from the carrier, a phase noise performance of about -120 dBc/Hz, but F.L. Walls [1995] showed, that the fundamental limit is lower, -140 dBc/Hz should be attainable. A new design philosophy for the sustaining circuit is presented which uses a nonlinear approach, is simple and has no AGC, which is one of the causes of not reaching the fundamental limit for the phase noise and which allows the crystal to oscillate as freely as possible during the whole cycle of oscillation without degrading its Q-factor as actual bottoming nonlinear sustaining circuits do. This new sustaining circuit makes it further possible to realize a crystal oscillator which can at the same time approach the fundamental limit of phase noise at 1 Hz and have extremely low aging rate, because it is not overdriving the crystal in order to reach low noise at 1 Hz