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Aging 
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

Aging Mechanisms

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.

Crystals that 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.

Typical Aging Behaviors

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 as follows:

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:

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 controlled environment.

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 unit.

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:

  • Temperature fluctuations
  • Thermal transients
  • Activity dips at oven set-point
  • Johnson noise - thermally induced EMF in a resistive element
  • Vibration
  • Acoustic losses - Q
  • Fluctuations in the number of adsorbed molecules
  • Changes in interfaces between quartz, electrodes, mount & bond

Short-term stability measurements can be derived by each of the following methods:

  • Two sample (Allan) variance
  • Spectral density of fractional frequency fluctuation
  • Spectral density of phase fluctuation
  • SSB phase noise to carrier ratio

Aging And Short Term Stability

Time Domain Stability

ALAN VARIANCE/DEVIATION:

Allan Deviation

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