Activity Dip:
The term "activity dip" is used to describe an abrupt increase in the
resistance and a perturbation in the frequency of a crystal unit, followed by an
equally abrupt return to the prior values. Used in such a way, the term
"activity dip" becomes a "catch-all" phrase that describes
an event but does nothing to identify the cause of that event. Such an event
might be caused by a coupled mode, a spurious response, or by a moisture dip.
Determining the Cause of Activity Dips:
If an "activity dip" is observed, identification of the cause is
necessary before that cause can be eliminated. Therefore, the conditions that
exist when the dip is observed must be noted.
If the ambient temperature is held at room temperature (25° C) as the frequency is swept, and if the dip occurs at a frequency higher than the main response, a spurious response is probably the cause. If the dip occurs at a frequency lower than the main response, a coupled mode is probably the cause.
If the ambient temperature is swept and an activity dip is observed at a temperature lower than normal room temperature, the cause is either a coupled mode or an excess of moisture inside the crystal package. A coupled mode may be verified by changing the value of the load capacitance used with the crystal unit. If the temperature point at which the activity dip occurs changes, a coupled mode has been identified. If the temperature at which the activity dip occurs does not change, moisture inside the crystal package is the cause. In this later case, the temperature passing through the dew point of that moisture causes the moisture to condense onto the surfaces of the quartz plate.
If the ambient temperature is swept and an activity dip is observed at a temperature higher than normal room temperature, the cause is almost certainly a coupled mode. As before, a coupled mode may be verified by changing the value of the load capacitance used with the crystal unit.
If the temperature point at which the activity dip occurs changes, a coupled mode has been identified.
Eliminating the Cause of Activity Dips: Elimination of the causes of activity dips is entirely up to the manufacturer of the crystal unit, if those dips occur within the specified operating parameters of that crystal unit. Operating the crystal unit over a wider than specified temperature range or at a higher than specified drive level may well result in activity dips. This is particularly true in the case of crystal units utilizing square or rectangular (strip) resonator plates. In cases where the end user exceeds the specified parameters, the manufacturer should not be held responsible.
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