Hardware Reference
In-Depth Information
it is also quite rapid, which means tremendous forces can be generated. For example, the surface
acceleration of a 50MHz crystal can exceed five million times the force of gravity.
Crystal resonators are made from slabs of quartz sawed from raw quartz crystal stock. The raw stock
slabs are cut into squares, rounded, and ground into flat discs called
blanks
. The thinner the disc, the
higher the resonant frequency; however, there are limits as to how thin the discs can be made before
they break. The upper limit for fundamental mode resonators is approximately 50MHz. At that
frequency, the discs are paper thin and are generally too fragile to withstand further grinding. Still,
higher-frequency crystals can be achieved by using harmonics of the fundamental frequency, resulting
in crystals of up to 200MHz or more. Even higher frequencies can be achieved by using frequency
synthesizer circuits, which use a base crystal-generated frequency fed to a circuit that then generates
multiples of frequency that can extend well into the gigahertz or terahertz range. In fact, crystal-based
frequency synthesizer circuits generate the high operating speeds of modern PCs.
The crystal packages, as well as the shape of the actual quartz crystals inside the packages, can vary.
The packages are usually a metal can that is either cylindrical or oblong in shape, but they can also
have other shapes or be constructed of plastic or other materials (see
Figure 3.45
).
Figure 3.45. Crystal packages of varying shapes.
The sliver of quartz inside the package is normally disc shaped, but it is shaped like a tuning fork in
some examples.
Figure 3.46
shows a cylindrical crystal package with the cover removed, exposing
the tuning fork-shaped sliver of quartz inside.
Figure 3.46. Crystal interior showing the quartz tuning fork.
Most crystals use a disc-shaped sliver of quartz as a resonator. The disc is contained in a
