Product Application Note 14
ISO 9001 CERTIFIED
High-Shock Quartz Crystals and Oscillators
Ruggedized frequency control for demanding applications
Historically, one of the most fragile components
in an electronic system was the quartz crystal
resonator. This is not surprising since the
resonator was composed of a large crystal
mounted by metal clips inside a metal housing,
e.g. the large round-blank AT-cut crystals. This
construction could not survive shocks much
beyond 50-100 g. While these crystals are superb
for large bench top instruments and similar
devices, they are not well-suited for applications
where the device can expect high shocks such as
hand-held devices and munitions. In these cases,
the accelerations can be on the order of thousands
or even tens of thousands of g’s. Clearly, the
classical construction is not adequate for these
applications.
The impetus to change the construction of quartz
crystals and oscillators came from the continuing
drive to miniaturize electronics. A key step in
this miniaturization took place in 1970 when
Statek Corporation pioneered the use of
photolithographic and chemical milling processes
for manufacturing quartz crystals.
These
processes, adapted from those used in the silicon
industry, allow the precise milling of quartz
crystals with dimensions less than 1 mm and
features as precise as a few microns. Another
important step in this miniaturization was the
development of the ceramic package for firmly
mounting the crystal in a rugged housing.
Together, this manufacturing and construction
technique has become the de facto standard for
miniature quartz crystals.
Fortunately, the miniaturization of the quartz
crystal has had the added benefit of greatly
improving their shock and vibration survivability.
Because of its small size, the resonator has low
mass, and so the force on the resonator is low.
512 N MAIN ST, ORANGE, CA 92868-1182
Using strong mounting materials, the resonator is
held firmly in place—the force due to
acceleration is not sufficient to cause the crystal
to dismount. Further, because of its small size
(short blank size or short tuning-fork tines), the
shear forces within the resonator are low and
hence they are able to survive high shocks
without breaking.
Another added benefit of the small size is that the
frequency of the lowest flexure mode of the
resonator can be on the order a few kilohertz or
higher. This has at least two benefits. First, for
shocks that have a characteristic time of about
1 ms or longer, the shock can be treated as a
quasi-static impulse—at any given time the shock
can be approximated as a static acceleration.
Because of this, the buildup in acceleration is
sufficiently slow that it does not excite the flexure
modes of the crystal. Second, since these flexure
modes are high in frequency, they will not be
excited under vibration (which normally does not
extend beyond 2 kHz in typical applications).
This is important in both high-vibration
applications and when manufacturing boards that
are cut out using a router.
With
this
modern
manufacturing
and
construction, the quartz crystal resonator is no
longer the fragile device that it once was. Today,
many manufacturers offer crystals and oscillators
that can survive mechanical shocks of thousands
of g’s.
Even so, common crystals and oscillators are not
appropriate for the most demanding applications,
e.g. munitions and projectile electronics. Here
shock levels can be tens of thousands of g’s. To
meet these requirements, not only must the
resonator be miniature, it must be mounted in
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FAX: 714-997-1256
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Page 2 of 2
• For maximal ruggedness, design for a crystal
frequency in the range of 13 MHz to 50 MHz
(with 16 MHz to 32 MHz being best). Below
13 MHz, the crystals tend to be large. Above
50 MHz, the inverted-mesa designs can be
fragile.
such a way that the shear forces on it are
minimized. For instance, for high-shock AT-cut
crystals, a third-point mount is used where the
non-electrical end of the crystal blank is mounted
to the crystal package. With this, Statek can
manufacture crystals that survive shock levels
beyond 100,000 g. Likewise, with these crystals
and the use of further construction techniques,
oscillators can be manufactured that survive these
same shock levels.
• Be aware that for shock levels beyond a few
thousand g’s, the common crystals and
oscillators may not be appropriate. A crystal
or oscillator specially designed for high-shock
applications may be required, e.g. Statek’s
CX4HG crystals or HGXO oscillators.
When designing a system that must survive high
shocks, it is useful to keep the following
guidelines in mind:
• Smaller crystals/oscillators (found in smaller
packages) tend to be more rugged than larger
crystals/oscillators.
• If it is known that the shock will be applied
along a single direction, a proper choice of
crystal/oscillator orientation can greatly
improve the ruggedness of the system.
• Tuning-fork crystals (typically 10 kHz to
600 kHz) are more rugged than extensionalmode crystals (520 kHz to 2.5 MHz) while
AT-cut crystals (6 MHz and up) tend to be the
most rugged.
• In addition to verifying the requisite
specification on the datasheet, ask the
manufacturer about their history of providing
high-shock devices.
• For tuning-fork and extensional-mode
crystals, crystal size decreases with frequency
and so ruggedness increases with frequency
(for crystals of a given mode).
Quartz crystal resonators and oscillators have
enjoyed decades of success in providing precise
frequency control in electronic systems. And
through their miniaturization, not only do these
devices take up less board space, they are more
rugged. For these reasons, they continue to be
superb choices for precise frequency control.
Rev. A
512 N MAIN ST, ORANGE, CA 92868-1182
714-639-7810
FAX: 714-997-1256
www.statek.com