|
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 CLEAR?
Hopefully, terms like BT
cuts, third-overtone mode, and parallel resonance no longer
send chills down your back. This brief overview of all
the crystal specs should give you a bit more confidence
when it comes to dealing with them.
As I mentioned, for accuracy,
you need to consider four important specs. I discuss them
in more detail in the "Crystal Specs" sidebar.
The sum of the errors contributed
from each spec is the total system timing error. Since
system cost is probably an issue, do not overspecify the
crystal. Try to determine how much timing error your system
can tolerate. Then, select the appropriate crystal using
the information given here [3,4].
The other crystal parameters
deal mostly with how the crystal is being used. For high-frequency
applications, you’ll almost certainly need an overtone
crystal. For very low-frequency applications, it will
be fundamental but a BT cut.
Basically, you want to design
the best system you can at a particular price. Invest
the time in planning for a good system.
|
