Editing Crystal oscillator (section)

===Resonance modes===
{{anchor|Pull capacitor}}A quartz crystal provides both series and parallel resonance. The series resonance is a few kilohertz lower than the parallel one. Crystals below 30&nbsp;MHz are generally operated between series and parallel resonance, which means that the crystal appears as an [[inductive reactance]] in operation, this inductance forming a parallel resonant circuit with externally connected parallel capacitance. 
[[File:Xtal response.jpg|thumb|220px|right|Frequency response of a 100kHz crystal, showing series and parallel resonance]]
Any small additional capacitance in parallel with the crystal pulls the frequency lower. Moreover, the effective inductive reactance of the crystal can be reduced by adding a capacitor in series with the crystal.  This latter technique can provide a useful method of trimming the oscillatory frequency within a narrow range; in this case inserting a capacitor in series with the crystal raises the frequency of oscillation. For a crystal to operate at its specified frequency, the electronic circuit has to be exactly that specified by the crystal manufacturer.  Note that these points imply a subtlety concerning crystal oscillators in this frequency range: the crystal does not usually oscillate at precisely either of its resonant frequencies.

Crystals above 30&nbsp;MHz (up to >200&nbsp;MHz) are generally operated at series resonance where the impedance appears at its minimum and equal to the series resistance. For these crystals the series resistance is specified (<100&nbsp;Ω) instead of the parallel capacitance. To reach higher frequencies, a crystal can be made to vibrate at one of its [[overtone]] modes, which occur near multiples of the fundamental resonant frequency. Only odd numbered overtones are used. Such a crystal is referred to as a 3rd, 5th, or even 7th overtone crystal. To accomplish this, the oscillator circuit usually includes additional [[LC circuit]]s to select the desired overtone.