4-50
In (a), the oscillator was kept on continuously while the oven was cycled
off and on.  In (b), the oven was kept on continuously while the oscillator
was cycled off and on.
OVEN
OFF
(a)
14 days
14 days
OSCILLATOR
OFF
OSCILLATOR ON
(b)
OVEN ON
Df
f
15
10
5
0
15
5
0
10
Text Box: X 10-9
X 10-9
OCXO Retrace
   The lack of repeatability of the f vs. T characteristics of oven controlled crystal oscillators (OCXOs), called "retrace," is illustrated above. Retrace is defined as the nonrepeatability of the f vs. T characteristic, usually at the oven temperature of an OCXO, upon on-off cycling under specified conditions.  Retrace is a function of the storage temperature during the off period.
   The OCXO retrace example above shows that upon restarting the oscillator after a 14 day off-period, the frequency was about 7x10-9 lower than what it was just before turn-off, and that the aging rate had increased significantly upon the restart.  About a month elapsed before the pre-turn-off aging rate was reached again. (Figure shows Df/f in parts in 109 vs. time in days.)
   Retrace limits the accuracies achievable with OCXOs in applications where the OCXO is on-off cycled. Typical OCXO retrace specifications, after a 24 hour off period at about 25°C, range from 2 x 10-8 to 1 x 10-9.  Low-temperature storage during the off period, and extending the off period, often make the retrace worse.
   The causes of hysteresis and retrace are not well understood.  The experimental evidence to date is inconclusive.  The mechanisms that can cause these effects include strain changes in the resonator’s mounting structure, changes in the quartz, oscillator circuitry changes, contamination redistribution in the crystal enclosure, and apparent hysteresis or retrace due to thermal gradients.


W. L. Smith, and W. J. Spencer, "Quartz Crystal Controlled Oscillators," Final Report, U.S. Army Contract DA36-039 SC-85373, Report No. 25335-H, 15 March 1963, AD-419717.

R. A. Sykes, W. L. Smith, and W. J. Spencer, "Studies on High Precision Resonators," Proc. 17th Annual Symposium on Frequency Control, pp. 4-27, 1963, AD423381.  Proc. copies available from NTIS.

J. A. Kusters and J. R. Vig, "Thermal Hysteresis in Quartz Resonators - A Review," Proc. 44th Annual Symposium on Frequency Control, pp. 165-175, 1990, IEEE Catalog No. 90CH2818-3. This paper is also available at <http://www.ieee.org/uffc/fc>