The 'aging' of a quartz crystal results in a small change of frequency over time, which may be crucial for engineers when they design their circuits. Two main factors affect the aging in quartz crystals: mass-transfer and stress.
Aging in practice
When looking at aging test results of crystals, the change in frequency is generally greatest during the first year and decays away with time. Nevertheless, if a device is specified at ±5ppm max per year; it does not follow the aging after 5 years will be ±5ppm x 5 years, i.e. ±25ppm. In practice, the example ±5ppm aging device may be only ±1ppm to ±2ppm during the first year of operation and then reduces over subsequent years. The general 'guide rule' is crystal aging of ±10ppm max over 10 years, although the crystal decays usually much less in reality.
The exact aging of a device is difficult to predict over time, as even parts made at the same time and from the same batch of quartz will exhibit slightly different aging characteristics. The production process must be consistent from part to part, from the manufacture of the quartz blank, the electrode size and its placement, to the epoxy used to mount the quartz and its curing thermal profile, all have a slight effect on frequency. Devices can age negatively or positively depending upon the internal causes although parts from one batch tend to follow similar results. In most cases the aging effect is negative in over 90% of parts manufactured.
Accelerated aging
The industry common practice is to use an accelerated aging process to predict long term frequency movement by soaking devices at elevated temperatures and measuring frequency movement at relevant intervals. The general practice is to test crystals using a passive test (i.e. non-powered). The rationale is soaking a crystal at +85°C for 30 days is equivalent to 1 year of aging at normal room temperature. If this test is extended for enough time then the recorded data can be plotted graphically to enable via extrapolation, the prediction of future long -term aging.
Frequency adjustment
Note the aging of quartz effectively changes the frequency tolerance of the crystal, which does not directly influence the stability of the quartz over temperature to any great degree, as this parameter is dictated by the 'cut angle' of the quartz used. For quartz oscillators with a voltage-control function such as VCXOs, TCXOs or OCXOs, the output frequency can be adjusted back to its nominally specified value.
Design
The engineers designing a circuit using either a crystal or oscillator by and large recognize what overall stability figure their equipment must meet over a particular time period. As the tolerance and/or stability of a device decreases then the more important aging becomes. For example, using a TCXO at ±1ppm stability over temperature will require aging to be kept to relatively small values. However, if the total frequency movement allowance of a design is ±200ppm and a device with a rating of ±100ppm is used, then a small amount of aging can effectively be ignored.
Mass-Transfer
Any unwanted contamination inside the device package can transfer material to or from the crystal, causing a change in the mass of the quartz blank, which will alter the frequency of the device. For example, the conductive epoxy used to mount the quartz blank can produce 'out-gassing' which can create oxidizing material within the otherwise inert atmosphere inside the sealed crystal package, hence this production process must be well controlled. Ideally the manufacturing method is as clean as possible to negate any effects and give good aging results.
Stress
Stress can occur within various components of the crystal from the processing of the quartz blank, the curing of the epoxy mounting adhesive, the crystal mounting structure and the type of metal electrode material used in the device. Heating and cooling also causes stress due to different expansion coefficients. Stress in the system usually changes over time as the system relaxes and this can cause a change in frequency.
Dynamic Engineers offer a wide range of devices with superb aging performance such as OCXO3629C_series and TCXO5300Z-UHS-10.0MHz.
For additional information regarding Aging of Quartz Crystal or other RF / microwave topics, please contact your local Dynamic Engineers sales representative or Inquiry@DynamicEngineers.com