|
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
AGING
The third item in the timing
budget is aging as it relates to crystal contamination
and drive level. Resonant frequency changes as a function
of time.
It tends to be related to
crystal contamination. That is, particles either drop
off or fall onto the quartz surface. Because this happens
inside the crystal case, there isn’t anything you
can do about it. It’s up to the manufacturer.
However, keeping the drive
level low can reduce aging effects as the crystal is not
knocked around as much. Choose crystals that are hermetically
sealed for best aging characteristics.
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.
|
