When electrical stress is applied to
one axis of a quartz crystal it exhibits the piezo-electric effect: a
mechanical deflection occurs perpendicular to the electric field.
Equally, a crystal will produce an e.m.f. across the electrical axis
if mechanical stress is applied to the mechanical axis.
If the stress is alternating – the
movement of the diaphragm of a crystal microphone is an example –
the e.m.f. produced will be alternating at the frequency of the
movement. If the stress alternates at a frequency close to the
mechanical resonance of the crystal as determined by its dimensions,
then large amplitude vibrations result.
Polycrystalline ceramics possess
similar qualities. Quartz crystals used for radio applications are
slices cut from a large, artificially grown crystal.
The slices are then ground to the
appropriate size to vibrate at a desired frequency. The performance
of an individual slice – the crystal as the end user knows it –
depends upon the angle at which it was cut from the parent crystal.
Each crystal slice will resonate at
several frequencies and if the frequency of the stimulus coincides
with one of them the output, electrical or mechanical, will be very
large.
The vibrations occur in both the
longitudinal and shear modes, and at fundamental and harmonic
frequencies determined by the crystal dimensions.
Figure 7.1A shows a typical natural
quartz crystal. Actual crystals rarely have all of the planes and
facets shown.
There are three opticalaxes (X, Y and
Z) in the crystal used to establish the geometry and locations of
various cuts. The actual crystal segments used in RF circuits are
sliced out of the main crystal. Some slices are taken along the
optical axes, so are called Y-cut, X-cut and Z-cut slabs. Others are
taken from various sections, and are given letter designations such
as BT, BC, FT, AT and so forth.
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