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Issue
91, February 1998
Choosing the Right
Crystal For Your Oscillator
by
Norman Bujanos
Start
•Why Quartz Crystals
• Timing Budget &
Accuracy
• Frequency
Tolerance
• Frequency
Stability
• Aging • Load
Capacitance •
Series and Parallel Resonance •
Frequency
Tolerance and Load Capacitance
• AT vs. BT Cut
• Mode of Operation
•
Package Considerations •
Crystal Placement •
Crystal Clear • References
FREQUENCY STABILITY
Frequency stability is the
second item to add to the timing budget. It is a function
of temperature and is related to the crystal cut type.
The most common crystal cut
types are AT and BT. Their temperature stability curves
are different—a fact that should be considered when
you’re designing a system.
The AT curve is cubic [1],
as depicted in Figure 1. Note that the curve moves between
the +ppm and –ppm areas with temperature.
Figure 1—AT-cut
crystals (solid curve) exhibit a cubic temperature
stability curve. On the other hand, BT-cut crystals
(dashed curve) exhibit a parabolic temperature
stability curve.
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If a system using an AT-cut
crystal is exposed to temperature fluctuations, the temperature
effects tend to average to zero over time. However, an
error is introduced from not operating at 25°C (i.e.,
crystal calibration temperature).
The BT cut, common in low-frequency
crystals, is a parabolic form. Increasing or decreasing
temperatures both cause a decreasing resonant frequency.
Unlike the AT-cut crystal, temperature fluctuation effects
do not average to zero.
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