Limiting factor for the performance of high stability quartz crystal oscillators – The residual heat of crystal resonators

The high stability crystal oscillator has been playing the heart of modern electronic communication equipment for decades, because of its excellent performance and reasonable price. It has three most significant indexes, namely, the aging, the phase noise and the frequency temperature stability. We found that when investigating the frequency temperature stability of high stability quartz crystal oscillator, with the increase of the oven control accuracy, oscillator's frequency temperature characteristic is deteriorating instead of improving. This is considered as a result of the impact of the residual heat effect in crystal resonator. In this case, by reducing the drive level properly, the frequency temperature stability increases accordingly, achieving about 1ppb in the temperature range of -20~+70°C, which explains the validity of the residual heat effect. Furthermore, after reducing the drive level appropriately, the abatement of oscillator's aging and the improvement of close-to-carrier phase noise's performance signifies that the residual heat effect of crystal resonators may be the limiting factors for improving the performance of high stability quartz crystal oscillator.