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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
TIMING BUDGET & ACCURACY
When selecting a crystal,
carefully consider how accurate your system has to be.
Crystal selection and oscillator design must be weighed
equally.
These factors work together,
influencing the system operating frequency and cost. Typically,
the more accurate a system is, the more expensive it is
to build.
If you’re building a
system with an RTC, your target accuracy should be ±2
min. per month. PLL reference clocks, however, can tolerate
less accuracy.
Four crystal parameters play
a key role in system accuracy. The contribution from each
must be added to obtain the total system accuracy.
Each parameter can be adjusted
without influencing the others. This parameter independence
makes customizing crystals attractive.
However, customization results
in increased system cost. Look carefully at your system
requirements. Try to use readily available, off-the-shelf
crystals.
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