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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
CRYSTAL PLACEMENT
The old real-estate adage
"Location, location, and location" applies to
crystal placement. Closer to the oscillator is better.
You want to minimize parasitics
introduced by long PCB traces. The board traces add to
the CL value in the crystal model. Remember, a
CL that does not match the CL in the specs
causes a frequency shift (see equation 4).
Also, look around the crystal
area to see if there are any other clock signals or otherwise
frequently changing signals nearby. These signals can
introduce noise into the oscillator. A good (quiet) ground
plane under the oscillator can help eliminate noise problems,
too.
Consider the distance between
the two crystal lead traces as well. This distance adds
to the Co term in the crystal model. Keep the traces
apart by at least the same distance as the crystal width.
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