New devices feature ruggedized frequency control for demanding applications
BY GREGORY A. BURNETT and
FARZIN JAHED
Statek
Orange, CA
http://www.statek.com
The classical quartz crystal oscillator is historically one of the most fragile components in an electronic system. This is not surprising since the quartz crystal resonator within the oscillator was composed of a large crystal such as a large round-blank AT-cut crystal mounted by metal clips inside of a metal housing. This construction could not survive shocks much beyond 50 to 100 g. While these crystal oscillators are superb for large benchtop instruments and similar devices, they are not well suited for applications where the device can expect high shocks such as handheld 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.
Quartz crystal resonators and oscillators continue to be superb choices for precise frequency control.
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 with the development of the photolithographic and chemical milling processes for manufacturing quartz crystals. These processes, adopted from those used in the silicon industry, allow the precise milling of quartz crystals with dimensions under 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.
Miniaturization vs. benefits
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. 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 can 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 build-up 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, such as 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 also be mounted in 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 nonelectrical end of the crystal blank is mounted to the crystal package. With this, crystals can be made that survive shock levels up to 100,000 g. Likewise, with these crystals and the use of further construction techniques, oscillators can be manufactured that survive these same shock levels.
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.
In general, tuning-fork crystals (typically 10 kHz to 600 kHz) are more rugged than extensional-mode crystals (520 kHz to 2.5 MHz), while AT-cut crystals (8 MHz and up) tend to be the most rugged.
For tuning-fork and extensional-mode crystals, crystal size decreases with frequency and so ruggedness increases with frequency (for crystals of a given mode).
For AT crystals, crystals in the range of 13 to 50 MHz are the most rugged (with 16 to 32 MHz being best).
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.
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.
In addition to verifying the requisite specification on the datasheet, don’t be afraid to ask manufacturers about their history of providing high-shock devices. ■
For more on quartz crystal oscillators, visit http://electronicproducts-com-develop.go-vip.net/passives.asp
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