Aging is the systematic gradual change in frequency over time. Aging
is the result of the deterioration of the frequency determining
parameters due to changes in crystal surface, methods of frequency
adjustment, drive/power-level and hermetic seal. Aging is stated as
deviation in frequency, typically in parts per million (PPM) over
time at a given drive level and/or temperature. EXAMPLE: +/- 3 ppm/year
at 0.5 mW after 30 days
Absorption and desorption are the two main causes of aging.
Absorption refers to the process of depositing material onto the
quartz plate causing the frequency to be reduced from the mass
loading. The material may be a contaminant in the form of
particulate matter trapped inside the holder, plating that is driven
off of the holder during the sealing process, or moisture.
Desorption is the process of a quartz plate throwing off minute
quartz particles over time as the crystal vibrates. As the bits of
quartz are thrown off the quartz plate, mass loading is reduced and
the frequency of the crystal increases.
exhibit alternating increases and decreases in frequency as they are
cycled between hot and cold temperatures typically have high
moisture content inside the package. The evaporation and
precipitation of the water cause alternating absorption and
desorption of the moisture.
The fundamental frequency of the crystal is also an important factor
in aging. Due to the relationship of frequency to thickness of the
quartz plate and the particulate matter involved in absorption and
desorption, higher frequency devices will exhibit more aging than a
lower frequency device.
Manufacturing process, selection of electrode material, holder,
seal-type and internal atmosphere can be optimized to produce
crystals with minimum aging.
vs. HOLDER & SEAL:
Holder/seal selection and internal atmosphere have a direct bearing
on the aging characteristics of the device. Sealing methods that do
no produce contaminates and sealing a crystal in a vacuum offers the
best opportunity to minimize aging. Second to this would be
resistance welded packages with the quartz wafer sealed in a dry
nitrogen atmosphere. There are cost vs. performance trade-offs to be
considered. Typical aging characteristics of holder/seal types are
Solder Seal - Helium: +/- 5 ppm/yr @ 25 deg. C
Resistance Weld - Nitrogen: +/- 3 ppm/yr @ 25 deg. C
Cold Weld - Vacuum: +/- 1 ppm/yr @ 25 deg. C
PRECISION LOW-AGING CRYSTAL HOLDERS:
AGING VS. TEMPERATURE
Aging is affected by temperature. In simplest terms, aging is
accelerated as temperature increases. Aging is typically specified
at room temperature, 25 degrees C. Some applications require
operation at elevated temperatures and the aging rate will be
accelerated in relation to the rate at 25 deg. C. Elevated
temperatures vs. aging rate must be considered and processes
selected to assure meeting aging specifications at temperatures
higher than room temperature.
Moisture vs. Temperature:
Desorption of any moisture that may be trapped in the package is
accelerated an increases in temperature. Conversely, as temperature
is reduced, moisture condenses onto the quartz plate causing mass
loading and a reduction in frequency. A crystal with this anomaly
will exhibit a lower frequency with cooling and a higher frequency
as the moisture is driven off the resonator. To eliminate moisture
content, crystals are processed in a temperature and humidity
Epoxies, Solvents & Contaminants vs. Temperature:
Elevated temperature can also accelerate the out-gassing of epoxies
used to bond the wafer to the internal mounting structure in the
crystal unit. Out-gassed material can precipitate onto the quartz
wafer and in turn reduce the oscillation frequency of the crystal
Crystal manufacturing process includes high-temperature vacuum
bake-out at various stages to drive off moisture, solvents,
contaminants, and drive to completion any out-gassing of epoxies. In
addition to the bake-outs, semi-finished material is stored at
elevated temperatures in a controlled atmosphere and processed
according to a schedule that eliminates lengthy queue between
processes. The goal is to seal the unit as quickly as possible in
order to eliminate the aging mechanisms associated with temperature.
Stress vs. Temperature:
Elevated temperature relieves stresses in the quartz structure as
well as the mounting structure. The relaxation of stresses causes
the frequency to change as a result of as the resonator reaches
equilibrium. A secondary effect of stress relief is a change in
frequency stability through "angle-rotation.
SHORT TERM STABILITY
If aging is the gradual change in frequency over the period of days,
months and years, short term stability is the change of frequency
typically exhibited per day. All of the variable mechanisms that
contribute to longer-term aging contribute to variations in short
term stability. Changes in short term stability are typically in
parts per billion, 10-9 per day.
Large changes in short term stability are called "pops" and "jumps"
. These changes in short term stability also contribute to "retrace"
or hysteresis of the resonator frequency over temperature.
Manufacturing processes are developed and selected to minimize
short-term changes in frequency.
Causes of short term instabilities are:
- Thermal transients
- Activity dips at
- Johnson noise -
thermally induced EMF in a resistive element
- Acoustic losses -
- Fluctuations in
the number of adsorbed molecules
- Changes in
interfaces between quartz, electrodes, mount & bond
measurements can be derived by each of the following methods:
- Two sample (Allan)
- Spectral density
of fractional frequency fluctuation
- Spectral density
of phase fluctuation
- SSB phase noise to
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