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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
LOAD CAPACITANCE
The fourth timing budget
parameter to consider has to do with load capacitance.
For parallel resonant circuits, there is a load-capacitance
spec.
If the load capacitance of
your circuit doesn’t match the crystal load capacitance,
there is a resonant frequency shift. I’ll discuss
this effect shortly.
One nice characteristic of
quartz crystals is that all of these parameters are independent.
Each one can be adjusted without affecting other parameters.
You can request a crystal
with 5-ppm frequency tolerance, and keep the other parameters
the same or change them. However, you pay a premium for
custom-made crystals.
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