Crystal unit structure
A quartz crystal unit includes a small
disc or strip of quartz that is processed to an exact size and
thickness dependent on the customer specified resonating frequency. The
quartz is plated with conducting electrodes and mounted in a
hermetically sealed protective enclosure (see Figure 1).
The electrodes connect to leads
that pass through the
base assembly via glass-to-metal seals.
Crystal units are often encapsulated
together with other circuitry to realize a fully functional module,
e.g. an oscilllator or a complex filter. Figure 2 illustrates a simple
crystal oscillator. Because of the nature of the crystal unit, correct
handling is
very important.
Mechanical shock
Crystal components are manufactured
to withstand a
certain level of mechanical shock.
These levels are outlined
within the environmental specifications for each individual
component type throughout out specification sheets.
Excessive levels of shock can cause a
change to the electrical characteristics, which will most likely
manifest itself as a
change of frequency. Severe mistreatment, such as dropping
onto a hard surface, may well result in actual breakage.
Handling leads
Excessive bending of leads can cause
damage to the glass-to-metal seal, which can result in loss of
hermeticity of the enclosure. Enclosures are filled
with a dry inert gas and loss of
hermiticity will result in a rapid deterioration of the product due to
atmospheric contamination. Care should therefore be taken
when handling a crystal not to pull or bend the
leads. If the component needs to be
moved in a way that involves bending, the lead should be bent slightly
away from the glass seal to avoid cracking it. The recommended minimum
radius of curvature is product dependent, e.g. 2 mm for HC49 crystals
and 1 mm for UM1s.
Tape-and-reel product
Before using crystal components on
automated placement machines, tests should be undertaken to test the
level of shock that the crystal devices will be subjected to during the
placement process. If necessary the shock level should be reduced.
Temperature
If
crystals are subjected to extreme temperatures
outside storage temperature limits, the electrical performance can be
affected, resulting in eventual failure.
Electrostatic discharge (ESD)
Only at extreme voltages
can static electricity be seen, heard or even felt, but even the lowest
voltages can damage electronic circuits. The damage caused to
oscillators as a result of EDS may not immediately be evident but can
be delayed, causing the oscillator circuitry to degrade, which in turn
can cause failure of the oscillator in the field. Although quartz is
not necessarily susceptible to ESD damage, the associated electronic
circuitry contained within an
oscillator is and should
be considered as an electrostatic
discharge sensitive (ESDS) device. ESDS devices should only be handled
in an ESD protected area (EPA), where typically wrist straps
are used. Any transportation should be undertaken using the appropriate
protective packaging.
All packaging should be marked with a
warning notice, and protective measures and packaging should conform to
BS EN 100015. For a more detailed breakdown of the precautions
that should be taken, please contact our Customer Support Department.
General information
The approximate thickness of the
quartz disc or strip is given by the following equation:
Related links:
Ordering
information
OCXOs
TCXOs
VCXOs
Crystal
oscillators
Semtech
approved components
Zarlink
approved components
Quartz
crystals
Rubidium
clocks
Programmable
oscillators
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