Editing Crystal oscillator (section)

== Aging ==
Crystals undergo slow gradual change of frequency with time, known as aging. There are many mechanisms involved. The mounting and contacts may undergo relief of the built-in stresses. Molecules of contamination either from the residual atmosphere, [[outgassing|outgassed]] from the crystal, electrodes or packaging materials, or introduced during sealing the housing can be adsorbed on the crystal surface, changing its mass; this effect is exploited in [[quartz crystal microbalance]]s. The composition of the crystal can be gradually altered by outgassing, diffusion of atoms of impurities or migrating from the electrodes, or the lattice can be damaged by radiation. Slow chemical reactions may occur on or in the crystal, or on the inner surfaces of the enclosure. Electrode material, e.g. chromium or aluminium, can react with the crystal, creating layers of metal oxide and silicon; these interface layers can undergo changes in time. The pressure in the enclosure can change due to varying atmospheric pressure, temperature, leaks, or outgassing of the materials inside. Factors outside of the crystal itself are e.g. aging of the oscillator circuitry (and e.g. change of capacitances), and drift of parameters of the crystal oven. External atmosphere composition can also influence the aging; [[hydrogen]] can diffuse through nickel housing. Helium can cause similar issues when it diffuses through glass enclosures of [[rubidium standard]]s.<ref>[http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=agingmec Frequency Control|Teaching Resources] {{webarchive|url=https://web.archive.org/web/20100706001506/http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=agingmec |date=2010-07-06 }}. Ieee-uffc.org. Retrieved on 2010-02-08.</ref>

Gold is a favored electrode material for low-aging resonators; its adhesion to quartz is strong enough to maintain contact even at strong mechanical shocks, but weak enough to not support significant strain gradients (unlike chromium, aluminium, and nickel). Gold also does not commonly form oxides; it adsorbs organic contaminants from the air, but these are easy to remove. However, gold alone can undergo delamination; a layer of chromium is therefore sometimes used for improved binding strength. Silver and aluminium are often used as electrodes; however both form oxide layers with time that increases the crystal mass and lowers frequency. Silver can be passivated by exposition to [[iodine]] vapors, forming a layer of [[silver iodide]]. Aluminium oxidizes readily but slowly, until about 5&nbsp;nm thickness is reached; increased temperature during artificial aging does not significantly increase the oxide forming speed; a thick oxide layer can be formed during manufacture by [[anodizing]].<ref name="google1">{{cite book|author=Jerry C. Whitaker|title=The electronics handbook|url=https://books.google.com/books?id=08wHm9EqX20C&pg=PA198|access-date=26 April 2011|date=23 December 1996|publisher=CRC Press|isbn=978-0-8493-8345-8|pages=198–}}</ref> Exposition of silver-plated crystal to iodine vapors can also be used in amateur conditions for lowering the crystal frequency slightly; the frequency can also be increased by scratching off parts of the electrodes, but that carries risk of damage to the crystal and loss of Q.

A DC voltage bias between the electrodes can accelerate the initial aging, probably by induced diffusion of impurities through the crystal. Placing a capacitor in series with the crystal and a several-megaohm resistor in parallel can minimize such voltages.

Aging decreases logarithmically with time, the largest changes occurring shortly after manufacture. Artificially aging a crystal by prolonged storage at 85 to 125&nbsp;°C can increase its long-term stability.

=== Mechanical damage ===
Crystals are sensitive to [[shock (mechanics)|shock]]. The mechanical stress causes a short-term change in the oscillator frequency due to the stress-sensitivity of the crystal, and can introduce a permanent change of frequency due to shock-induced changes of mounting and internal stresses (if the elastic limits of the mechanical parts are exceeded), desorption of contamination from the crystal surfaces, or change in parameters of the oscillator circuit. High magnitudes of shocks may tear the crystals off their mountings (especially in the case of large low-frequency crystals suspended on thin wires), or cause cracking of the crystal. Crystals free of surface imperfections are highly shock-resistant; [[chemical polishing]] can produce crystals able to survive tens of thousands of [[G-force|g]].<ref>[http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=vigaccel Frequency Control|Teaching Resources] {{webarchive|url=https://web.archive.org/web/20100706000011/http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=vigaccel |date=2010-07-06 }}. Ieee-uffc.org. Retrieved on 2010-02-08.</ref>

Crystals have no inherent failure mechanisms; some have operated in devices for decades. Failures may be, however, introduced by faults in bonding, leaky enclosures, corrosion, frequency shift by aging, breaking the crystal by too high mechanical shock, or radiation-induced damage when non-[[swept quartz]] is used.<ref>[http://www.am1.us/Papers/U11625%20VIG-TUTORIAL.PDF Quartz crystal resonators and oscillators for frequency control and timing applications]: a tutorial by John R. Vig, U.S. Army Communications-Electronics Command</ref> Crystals can be also damaged by overdriving.

=== 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>

===Drive level===
The crystals have to be driven at the appropriate drive level. Low-frequency crystals, especially flexural-mode ones, may fracture at too high drive levels. The drive level is specified as the amount of power dissipated in the crystal. The appropriate drive levels are about 5 μW for flexural modes up to 100&nbsp;kHz, 1 μW for fundamental modes at 1–4&nbsp;MHz, 0.5 μW for fundamental modes 4–20&nbsp;MHz and 0.5 μW for overtone modes at 20–200&nbsp;MHz.<ref name="actcrystals1">[http://www.actcrystals.com/techinfo/crystalterminology_txt.html Crystal Terminology] {{Webarchive|url=https://web.archive.org/web/20050126132513/http://www.actcrystals.com/techinfo/crystalterminology_txt.html |date=2005-01-26 }}. Actcrystals.com. Retrieved on 2010-02-08.</ref> Too low drive level may cause problems with starting the oscillator. Low drive levels are better for higher stability and lower power consumption of the oscillator. Higher drive levels, in turn, reduce the impact of noise by increasing the [[signal-to-noise ratio]].<ref>[http://www.axtal.com/data/publ/ukw1979_e.pdf Design of crystal oscillator circuits], a course by B. Neubig</ref>

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