Aging and Drift
Based on the terms and definitions recommended by the CCIR
(Consultative Committee on International Radio)
, aging and drift can be defined in the following way.
Aging is the systematic change in frequency with time due to internal
changes in the oscillator, that is, the frequency change with time when factors
external to the oscillator (environment, power supply, etc.) are kept constant.
Drift is defined as the systematic change in frequency with time of an
oscillator, that is, drift is due to a combination of factors, i.e., it due to
aging plus changes in the environment and other factors external to the
oscillator. Aging is what one
specifies and what one measures during oscillator evaluation.
Drift is what one observes in an application. For example, the drift of an oscillator in a spacecraft is
due to (the algebraic sum of) aging and frequency changes due to radiation,
temperature changes in the spacecraft, and power supply changes.
There are many interrelated factors involved in aging.
Some of the most common are: internal contamination, crystal surface
changes, wire fatigue, small irreversible changes in the crystal lattice, out
gassing of the materials, various thermal effects, mounting stresses, and
over-driving the crystal. Typical
aging figures for metal enclosure crystal units operating in the frequency range
of 10 to 20 MHz are 1.0 – 5.0 ppm / 1st year; while the values for
glass enclosure crystals are 0.1 – 1.0 ppm / 1st year.
AT-cut vs. SC-cut
Of many crystal-cut types available, most applications use either AT-cut or
SC-cut quartz resonators, and in many instances both cuts are candidates.
It is worthwhile to compare them on the basis of eight characteristics:
Turnover Temperature:
For OCXO applications the crystal may be operated at either of its two
turnover temperatures. Here the
SC-cut is superior to the AT-cut in two important ways.
First, the SC-cut’s f - T curve is much flatter near turnover
temperature than the AT-cut’s determined by the three temperature
coefficients. Second, the true
SC-cut is much less sensitive to temperature gradients, allowing much faster
warm-up. These qualities make it
the preferred choice for precision oscillator applications.
Static f - T Characteristic:
Both AT- and SC-cut resonators have a static frequency-temperature
characteristic that is well described by a third-order polynomial in
temperature. For the AT-cut, the
inflection temperature [at which the curvature changes sign] is within a few
degrees of room temperature. In
other words, the f - T curve is approximately anti-symmetric about a temperature
close to 25°C. This makes the
AT-cut suitable for non-temperature-controlled applications, such as simple
oscillators and TCXOs. By contrast,
the inflection temperature for the SC-cut resonator is 92°C.
Most SC-cuts are used in OCXOs, where the higher inflection temperature
results in an important advantage, that is, the f-T curve is quite flat in the
vicinity of the oven set point, which is adjusted to the resonator upper or
lower turnover temperature. Consequently,
temperature control is less critical than that an AT-cut crystal was used.
Other advantages of the SC-cut from its f – T characteristics are its
relative freedom from the activity dips.
Dynamic f-T Characteristic (Thermal Transient Effects):
When a step change is made to the ambient temperature, the frequency of an
SC-cut resonator changes smoothly, without overshoot or ringing, in a manner
corresponding to a critically- damped system.
The AT-cut dynamic f - T characteristics, on the other hand, has very
pronounced overshoot and rubber band effect.
An important reason for choosing an SC-cut for an OCXO is its greatly
improved dynamics, which is, in fact, the hallmark of properly designed SC-cut
based OCXOs.
Aging:
If we compare equal frequency and overtone, the SC-cut is slightly better
than the AT-cut, on account of its slightly greater thickness.
This difference, however, is usually insignificant. Because the greater
current-handling capability of the SC renders its frequency less sensitive to
current changes, in oscillators the SC-cut may exhibit better aging.
Current Handling:
This refers to the maximum current at which a resonator can be operated
without a significant (reversible) change in frequency.
Generally, this is markedly higher for SC-cut than for AT-cut resonators.
The consequences for oscillator applications are an improved phase noise
floor by operating a higher current, and reduced sensitivity to drive level
change, which may affect aging of the oscillator frequency.
