Editing Crystal oscillator (section)

=== Frequency fluctuations ===
Crystals suffer from minor short-term frequency fluctuations as well. The main causes of such noise are e.g. [[thermal noise]] (which limits the noise floor), [[phonon scattering]] (influenced by lattice defects), adsorption/desorption of molecules on the surface of the crystal, noise of the oscillator circuits, mechanical shocks and vibrations, acceleration and orientation changes, temperature fluctuations, and relief of mechanical stresses. The short-term stability is measured by four main parameters: [[Allan variance]] (the most common one specified in oscillator data sheets), phase noise, spectral density of phase deviations, and spectral density of fractional frequency deviations. The effects of acceleration and vibration tend to dominate the other noise sources; surface acoustic wave devices tend to be more sensitive than bulk acoustic wave (BAW) ones, and the stress-compensated cuts are even less sensitive. The relative orientation of the acceleration vector to the crystal dramatically influences the crystal's vibration sensitivity. Mechanical vibration isolation mountings can be used for high-stability crystals.

[[Phase noise]] plays a significant role in [[frequency synthesis]] systems using frequency multiplication; a multiplication of a frequency by N increases the phase noise power by N<sup>2</sup>. A frequency multiplication by 10 times multiplies the magnitude of the phase error by 10 times. This can be disastrous for systems employing [[Phase-locked loop|PLL]] or [[Frequency-shift keying|FSK]] technologies.

[[Magnetic field]]s have little effect on the crystal itself, as quartz is [[diamagnetic]]; [[eddy current]]s or AC voltages can however be induced into the circuits, and magnetic parts of the mounting and housing may be influenced.

After the power-up, the crystals take several seconds to minutes to "warm up" and stabilize their frequency. The oven-controlled OCXOs require usually 3–10 minutes for heating up to reach thermal equilibrium; the oven-less oscillators stabilize in several seconds as the few milliwatts dissipated in the crystal cause a small but noticeable level of internal heating.<ref>[http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=vigwarm Frequency Control|Teaching Resources] {{webarchive|url=https://web.archive.org/web/20100705235246/http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=vigwarm |date=2010-07-05 }}. Ieee-uffc.org. Retrieved on 2010-02-08.</ref>