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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 •
Frequencyt Tolerance and Load Capacitance
• AT vs. BT Cut
• Mode of Operation
•
Package Considerations •
Crystal Placement •
Crystal Clear • References
FREQUENCY TOLERANCE
The frequency tolerance (i.e.,
calibration effect) is the first of the four accuracy-budget
parameters. It is a room-temperature (i.e., 25°C) spec
stating how close the actual crystal frequency is to its
specified frequency. Like most crystal specs, it is in
parts per million.
For example, a 32,768-Hz
crystal may have a frequency tolerance of ±20 ppm. At
25°C, the resonant frequency can be anywhere between 32,768.65536
and 32,767.34464 Hz.
The 32768 crystal is known
as a watch crystal. This system might sound highly accurate,
but when you consider its accuracy impact over a month,
this one parameter alone can cause the watch to be off
by almost 1 min.
Equation 1 shows that a ±20-ppm
frequency tolerance can account for about 52 s per month:
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