4-59
Activity dips in the f vs. T and R vs. T when operated with and without load capacitors.  Dip temperatures are a function of CL, which indicates that the dip is caused by a mode (probably flexure) with a large negative temperature coefficient. 
Text Box: Frequency
Frequency
Text Box: Resistance
Resistance
Temperature (0C)
-40
-20
0
20
40
60
80
100
RL2
RL1
R1
fL1
fL2
fR
10 X10-6
Activity Dips
   Anomalies in the f vs. T and R vs. T characteristics, as illustrated above, are called “activity dips”.  The curves labeled fR and R1 are the f vs. T and R vs. T without a load capacitor.  The fL and RL curves are the f vs. T and R vs. T of the same resonator when load capacitors are in series with the resonator.  The load capacitors shift the frequency to higher values; the curves have been vertically displaced for clarity.
   An activity dip can cause intermittent failures.  It affects both the frequency and the resistance (i.e., the Q) of resonators.  When the oscillator gain is insufficient, the resistance increase stops the oscillation.  For example, the clock in one satellite stopped periodically a certain time interval after the satellite entered the earth’s shadow.  As the satellite cooled, the oscillator’s temperature reached the activity dip temperature and the oscillation stopped. Upon further temperature change, the oscillation resumed.
   Even when the resistance increase is not large enough to stop the oscillation, the frequency change can cause intermittent failures, e.g., it can cause a loss of lock in phase locked systems.
   Activity dips are usually caused by interfering modes (e.g., by high overtone flexure modes). Such activity dips are strongly influenced by the crystal's drive level and load reactance.  The activity-dip temperature is a function of CL because the interfering mode’s frequency usually has a large temperature coefficient and a C1 that is different from that of the desired mode.  When the frequency of the interfering mode coincides with the frequency of the main mode, energy is lost from the main mode and an activity dip occurs.
   Activity dip like features can also be caused by processing problems, e.g., loose contacts.  Such dips usually change with temperature cycling, rather than with load capacitance.
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A. Ballato and R. Tilton, "Electronic Activity Dip Measurement," IEEE Trans. on Instrumentation and Measurement, Vol. IM-27, No. 1, pp. 59-65, March 1978; also “Ovenless Activity Dip Tester,” in Proc. 31st Ann. Symp. On Frequency Control, pp. 102-107, 1977.
See also “Overtone Response…” & “Unwanted Modes vs. Temperature” in Chapter 3.

E.P EerNisse, E. Benes, M. Schmid, “The Role of Localized Rotational Imbalance in Drive Level Dependence Phenomena,” Pro. 2002 IEEE Int’l Frequency Control Symp., pp. 2-7, 2002.