Editing Crystal oscillator (section)

==Crystal oscillator circuits==

The crystal oscillator circuit sustains oscillation by taking a voltage signal from the quartz [[resonator]], amplifying it, and feeding it back to the resonator. The rate of expansion and contraction of the quartz is the [[resonance|resonant]] frequency, and is determined by the cut and size of the crystal. When the energy of the generated output frequencies matches the losses in the circuit, an oscillation can be sustained.

An oscillator crystal has two electrically conductive plates, with a slice or tuning fork of quartz crystal sandwiched between them. During startup, the controlling circuit places the crystal into an [[unstable equilibrium]], and due to the [[positive feedback]] in the system, any tiny fraction of [[noise (electronics)|noise]] is amplified, ramping up the oscillation. The crystal resonator can also be seen as a highly frequency-selective filter in this system: it only passes a very narrow subband of frequencies around the resonant one, attenuating everything else. Eventually, only the resonant frequency is active. As the oscillator amplifies the signals coming out of the crystal, the signals in the crystal's frequency band becomes stronger, eventually dominating the output of the oscillator. The narrow resonance band of the quartz crystal [[electronic filter|filter]]s out the unwanted frequencies.

The output frequency of a quartz oscillator can be either that of the fundamental resonance or of a multiple of that resonance, called a [[harmonic]] frequency.  Harmonics are an exact integer multiple of the fundamental frequency.  But, like many other mechanical resonators, crystals exhibit several modes of oscillation, usually at approximately odd integer multiples of the fundamental frequency.  These are termed "overtone modes", and oscillator circuits can be designed to excite them.  The overtone modes are at frequencies which are approximate, but not exact odd integer multiples of that of the fundamental mode, and overtone frequencies are therefore not exact harmonics of the fundamental.

High frequency crystals are often designed to operate at third, fifth, or seventh overtones. Manufacturers have difficulty producing crystals thin enough to produce fundamental frequencies over 30&nbsp;MHz. To produce higher frequencies, manufacturers make overtone crystals tuned to put the 3rd, 5th, or 7th overtone at the desired frequency, because they are thicker and therefore easier to manufacture than a fundamental crystal that would produce the same frequency—although exciting the desired overtone frequency requires a slightly more complicated oscillator circuit.<ref>[https://web.archive.org/web/20110725032851/http://www.foxonline.com/techdata.htm Quartz Crystal Theory of Operation and Design Notes]. foxonline.com</ref><ref>[http://www.maxim-ic.com/app-notes/index.mvp/id/726 Specifying Quartz Crystals]. Maxim-ic.com (2001-11-19). Retrieved on 2012-06-21.</ref><ref>[http://www.pletronics.com/uploads/datasheets/selection%20crystal.pdf Crystal selection] {{Webarchive|url=https://web.archive.org/web/20130429155236/http://www.pletronics.com/uploads/datasheets/selection%20crystal.pdf |date=2013-04-29  }}. pletronics.com. Retrieved on 2012-06-21.</ref><ref>[http://www.euroquartz.co.uk/crystal_specification.aspx "Crystal Specification"] {{Webarchive|url=https://web.archive.org/web/20130728144600/http://www.euroquartz.co.uk/crystal_specification.aspx |date=2013-07-28 }}. Euroquartz.co.uk. Retrieved on 2012-06-21.</ref><ref>[http://www.beckwithelectronics.com/abracon/quartzan.htm "Quartz Crystal Application Notes"] {{webarchive|url=https://web.archive.org/web/20150623012309/http://www.beckwithelectronics.com/abracon/quartzan.htm |date=2015-06-23 }}. Beckwithelectronics.com. Retrieved on 2012-06-21.</ref>
A fundamental crystal oscillator circuit is simpler and more efficient and has more pullability than a third overtone circuit.
Depending on the manufacturer, the highest available fundamental frequency may be 25&nbsp;MHz to 66&nbsp;MHz.<ref>[http://www.abracon.com/Support/quartz_crystals.pdf "Quartz Crystals Application Notes"]. (PDF) . Retrieved on 2012-06-21.</ref><ref>[https://web.archive.org/web/20101130222332/http://foxonline.com/techfaqs_cry.htm#a9#a9 Frequently Asked Questions about Crystals]. foxonline.com</ref>
[[File:Quartz crystal internal.jpg|thumb|Internals of a quartz crystal.]]
A major reason for the wide use of crystal oscillators is their high [[Q factor]]. A typical ''Q'' value for a quartz oscillator ranges from 10<sup>4</sup> to 10<sup>6</sup>, compared to perhaps 10<sup>2</sup> for an [[LC oscillator]]. The maximum ''Q'' for a high stability quartz oscillator can be estimated as ''Q'' = 1.6 &times; 10<sup>7</sup>/''f'', where ''f'' is the resonant frequency in megahertz.<ref>{{cite web |title=Radio Frequency Spectrum Management and Time and Frequency Standards |url=https://www.sciencedirect.com/science/book/9780750672917 |access-date=24 February 2019}}</ref><ref name="Reference Data for Radio Engineers">{{cite book |title=Reference Data for Radio Engineers |date=2002 |publisher=Elsevier |isbn=978-0-7506-7291-7 |page=Chapter 1 |edition=Ninth }}</ref>

One of the most important traits of quartz crystal oscillators is that they can exhibit very low [[phase noise]].
In many oscillators, any spectral energy at the resonant frequency is amplified by the oscillator, resulting in a collection of tones at different phases.
In a crystal oscillator, the crystal mostly vibrates in one axis, therefore only one phase is dominant.
This property of low [[phase noise]] makes them particularly useful in telecommunications where stable signals are needed, and in scientific equipment where very precise time references are needed.

Environmental changes of temperature, humidity, pressure, and vibration can change the resonant frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and [[crystal oven|OCXO]] which are defined [[#Circuit notations and abbreviations|below]]. These designs, particularly the OCXO, often produce devices with excellent short-term stability. The limitations in short-term stability are due mainly to noise from electronic components in the oscillator circuits. Long-term stability is limited by aging of the crystal.

Due to aging and environmental factors (such as temperature and vibration), it is difficult to keep even the best quartz oscillators within one part in 10<sup>10</sup> of their nominal frequency without constant adjustment. For this reason, [[atomic oscillator]]s are used for applications requiring better long-term stability and accuracy.

===Spurious frequencies===

[[File:clock crystal.jpg|right|thumb|25 MHz crystal exhibiting spurious response]]

For crystals operated at series resonance or pulled away from the main mode by the inclusion of a series inductor or capacitor, significant (and temperature-dependent) spurious responses may be experienced. Though most spurious modes are typically some tens of kilohertz above the wanted series resonance, their temperature coefficient is different from the main mode, and the spurious response may move through the main mode at certain temperatures. Even if the series resistances at the spurious resonances appear higher than the one at the wanted frequency, a rapid change in the main mode series resistance can occur at specific temperatures when the two frequencies are coincidental.
A consequence of these activity dips is that the oscillator may lock at a spurious frequency at specific temperatures. This is generally minimized by ensuring that the maintaining circuit has insufficient gain to activate unwanted modes.

Spurious frequencies are also generated by subjecting the crystal to vibration. This modulates the resonant frequency to a small degree by the frequency of the vibrations. SC-cut (Stress Compensated) crystals are designed to minimize the frequency effect of mounting stress and they are therefore less sensitive to vibration.  Acceleration effects including gravity are also reduced with SC-cut crystals, as is frequency change with time due to long term mounting stress variation.
There are disadvantages with SC-cut shear mode crystals, such as the need for the maintaining oscillator to discriminate against other closely related unwanted modes and increased frequency change due to temperature when subject to a full ambient range. SC-cut crystals are most advantageous where temperature control at their temperature of zero temperature coefficient (turnover) is possible, under these circumstances an overall stability performance from premium units can approach the stability of rubidium frequency standards.