Basic Technology of Quartz Crystal Resonators
Quartz crystal resonators (often called “crystals”) are widely used in
frequency control applications because of their unequalled combination of high
Q, stability, small size and low cost. Many
different substances have been investigated as possible resonator materials, but
for many years quartz resonators have been preferred in satisfying needs for
precise frequency control. Compared
to other resonators, for example, LC circuits, mechanical resonators such as
tuning forks, and piezoelectric ceramic resonators based or other single-crystal
materials, the quartz resonator has a unique combination of properties.
First, the material properties of single-crystal quartz are extremely stable
with time, temperature, and other environmental changes, as well as highly
repeatable from one specimen to another. The
acoustic loss or internal friction of quartz is very low, leading directly to
one of the key properties of a quartz resonator, its extremely high Q factor.
The intrinsic Q of quartz is about 107 at 1 MHz.
Mounted resonators typically have Q factors ranging from tens of
thousands to several hundred thousand, which is orders of magnitude better than
the best LC circuits.
The second key property of the quartz resonator is its stability with
respect to temperature variation. Depending on the shape and orientation of the crystal blank,
many different modes of vibration can be used and it is possible to control the
frequency-temperature characteristics of the quartz resonator to within close
limits by an appropriate choice. The
most commonly used type of resonator is the AT-cut, where the quartz blank is in
the form of thin plate cut as an angle of about 35°15' to the optic axis of the crystal.
The third essential characteristic of the quartz resonator is related
to the stability of its mechanical properties. Short and long term stabilities manifested in frequency drifts of only a
few parts per million per year are readily available from commercial units.
Precision crystal units manufactured under closely controlled conditions
are second only to atomic clocks in the frequency stability and precision
achieved.
Quartz Plate Orientation
Quartz resonators consist of suitably mounted and metal-deposited plates of
crystalline quartz using bulk acoustic wave (BAW) vibrations. Originally the quartz plates were made from natural quartz, but to day
cultured quartz is used almost exclusively. The plates (also called wafers or blanks) are fabricated at a precise
orientation with respect to the crystallographic axes of the quartz material.
The orientation or "cut" determines frequency-temperature characteristics and other important properties of the resonator.
The above is a schematic diagram of a cultured quartz crystal grown from a Y-oriented seed
crystal for use in fabricating AT-cut resonators. A seed crystal
establishes the initial crystal orientation and encourages growth in the Y
direction at the expense of the Z-axis. Seed crystals are carefully selected to
avoid defects which might propagate as the crystal grows. The position of the
seed crystal is indicated. The lines sloping left from the x-axis mark the
saw cut position for AT plates, the line sloping to the right indicates the
BT-cut. In practice, these angles are very critical and are precisely
determined using Bragg x-ray diffraction.
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