|
Fig.1 Quartz crystal
structure
Fig.2 Cross section of crystal perpendicular to axis ZZ
Quartz Crystal
A natural crystal of quartz has
a hexagonal habit, as illustrated
in Fig.1.The asymmetrical properties of quartz can be referred
to three sets of major axes in the crystal. The axis ZZ drawn
through the apices of the crystal represents the optical axis
of the crystal. No doubt optical refraction occurs along this
axis. The axes XX joining opposite edges
of a cross-section of the crystal normal to ZZ(Fig.2)are the
electrical or X-axes of the crystal .The axes YY in the same
plane as the X-axes and bisecting the X-axes perpendicularly
are the mechanical or Y-axes of the crystal. Mechanical stresses
along a Y-axis will give rise to electrical stresses along the
corresponding X-axis; electrical stresses along an X-axis will
cause mechanical stresses along the corresponding Y-axis.
The crystals used in crystal
units are normally in the form of plates or elements cut from
synthetic crystal(Fig.3).
Fig.3 Cutting angles
Fig.4 AT-cut frequency-temperature characteristics
The setting tolerance is the maximum
allowable deviation from nominal frequency at a specified temperature(typically25℃).It
is normally specified in parts per million (ppm) or %
of nominal
frequency.
Fig.4 shows the frequency-temperature characteristics for a
thickness-shear mode AT-cut with the angle of cut as a parameter.
Since the AT-cut frequency- temperature characteristic is equivalent
to an equation of the third degree ,it displays excellent frequency
stability over a wide temperature range. |
