Frequency, Aging, Temperature Stability, and Retrace

FREQUENCY: For a given crystal cut, lower frequency crystals exhibit superior
stability and, for a given frequency, the higher overtone crystals will usually
provide the best stability. A simple rule of thumb is, "the more quartz the
better" down to about 5 MHz below which frequency dividers are usually the best
choice. High frequency oscillators may include phase-locked loops or frequency
multipliers to take advantage of a low frequency crystal's stability. Multiplied
oscillators are preferred above 120 MHz when stability is a key issue.

AGING: New, high quality ovenized quartz crystals typically exhibit small,
positive frequency drift with time unrelated to external influences. A
significant drop in this "aging" rate occurs after the first few weeks of
operation at the operating temperature. Ultimate aging rates below 0.1 PPB per
day are achieved by the highest quality crystals and 1 PPB per day rates are
commonplace. Significant negative aging (dropping frequency) indicates a bad
crystal - probably a leaking package.

A typical aging curve for a new ovenized oscillator.

TEMPERATURE: The primary effect of temperature variations is to change the
oscillator's frequency. Oven oscillators offer the best temperature stability
and largely avoid many of the problems associated with activity dips. Activity
dips are drops in crystal Q which can appear in narrow temperature windows
causing sudden frequency shifts and amplitude variations. Temperature stability
below 0.1 PPB can be achieved but the aging rate often dominates the frequency
error budget after only a few days. The specification should state whether the
stability specification is peak-to-peak over the entire range or whether it is
relative to room temperature. Variation from room temperature is a popular
method of specification since the oscillator is usually tuned at room
temperature. Non-oven XOs and TCXOs may drift slowly to a new frequency after
the ambient temperature changes since the internal thermal time constants can be
fairly long.

RETRACE: When power is removed from an oscillator, then re-applied several
hours later, the frequency will stabilize at a slightly different value. This
"retrace" error is usually specified for a twenty-four hour off-time followed by
a warm-up time sufficient to allow complete thermal equilibrium. Retrace errors
often diminish after warming as though the crystal walks back down its aging
curve when cold and then exponentially approaches the previous drift curve when
activated. Oscillators stored at extremely cold temperatures for extended
periods of time may exhibit a frequency vs. time curve much like the initial
"green" aging curve of a new crystal. In addition to the crystal related effects
described above, mechanical shifts can also occur due to the thermal stresses
from heating and cooling the oven structure. A common retrace error source is
the mechanical device used to adjust the oscillator's frequency. Precision,
multi-turn variable capacitors exhibit good retrace but a good practice is to
turn the screw back slightly after setting to relieve any stress. Most Wenzel
oscillators use special precision potentiometers which exhibit an unusually low
amount of retrace and hysterisis.

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