How does a quartz crystal resonator work: the basics

  • Sep 27,2022
  • 386 VIEWS

For the operation of a quartz crystal it is found that the piezo-electric effect converts the electrical impulses to mechanical stress which is subject to the very high Q mechanical resonances of the crystal, and this is in turn linked back into the electrical circuit.

The quartz crystal can vibrate in several different ways, and this means that it has several resonances - each on a different frequency.

Fortunately the way in which the quartz crystal blank is cut from the original crystal itself can very significantly reduce this. In fact the angle of the faces relative to the original crystal axes determines many of its properties from the way it vibrates to its activity, Q, and its temperature co-efficient.

When looking at how a quartz crystal resonator works, it can be seen that there are three main ways in which a crystal can vibrate:

Longitudinal mode:   In the longitudinal mode the crystal extends and shortens. This causes the centre to become thinner as the crystal extends and thicker as it shortens.

Low frequency face shear mode: For crystals that work in this mode, the whole rectangular shape of the crystal vibrates as shown below.

High frequency shear:   The high frequency shear mode is used for most quartz crystal resonators working in the region 1 MHz upwards.

The way in which the crystal blank is cut from the bulk crystal affects the way in which it vibrates. In particular the angle it is cut relative to the axes of the bulk quartz crystal has a major effect. There are several standard ‘cuts’ which are given names, and these have known and useful performance parameters. A cut known as the AT cut is used for most crystals found in traditional radio and electronics circuits and this typically vibrates using the high frequency shear mode.

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