Fundamental limits on the frequency instabilities of quartz crystal oscillators

The frequency instabilities of precision bulk acoustic wave (BAW) quartz crystal oscillators, and the contributions from the instabilities of resonators, sustaining circuits, and ovens, are reviewed. The fundamental limits on the achievable frequency stabilities, and the degree to which the fundamental limits have been approached to date are examined. Included are the instabilities as a function of time, temperature, acceleration, ionizing radiation, electromagnetic fields, humidity, atmospheric pressure, power supply and load impedance. Most of the fundamental limits are zero or negligibly small, a few are finite. The authors speculate about the progress which may be achievable in the future with respect to approaching the fundamental limits. Suggestions are provided about the paths that may lead to significant stability improvements