crystal load capacitor sizing

"Jamie Morken"

Probably.

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None that I know of. It's not that clear cut.

Probably insignificant. What is the crystal's purpose? If it's clocking a microcontroller, I wouldn't worry too much about it since you are probably only talking about a few Hz difference. If it's in a model rocket, you may want to consider a ceramic resonator instead of a crystal for durability reasons. One thing you don't want is too little capacitance or you'll have start-up/stability problems. Waking up from SLEEP mode can be a killer too. It's easy to start an oscillator with power application, but when a micro is sleeping it can be difficult to cause enough imbalance to start the oscillator going. In some cases you might need to have one cap at a different value to help.

As far as the clock crystal goes, what kind of long term accuracy are you needing? If you are worried about individual pF of load capacitance, you are only talking about perhaps a 1 or 2 seconds/week change in accuracy. Temperature changes will probably have a larger effect. If you need extreme accuracy, then use a trimmer cap.

For an AT cut it'll probably be in the very low tens of ppm. In the total error budget (initial frequency offset, temp variations, etc.) this isn't entirely in the noise. But that's a good thing: it's why a trimmer cap gives you enough control to set the frequency exactly.

All of what you said about parallel load caps and strays is pretty much on target, BUT in the real world the capacitor values are usually subject to the constraint that they ensure rapid startup of the oscillator and that the oscillator run stably and with a "good" amplitude (not too high to avoid damage to the crystal or accidentally run it in an overtone mode, but high enough that the microprocessor actually clocks). If it so happens that this is somewhere near the crystal's load capacitance, that's great.

Actually ensuring reliable crystal oscillator startup and operation over a wide temp/Vcc/process variation range is not generally an easy problem.

Tim.

If you characterize the crystal(s) to determine the motional parameters then you can calculate the effects.

If you had to get reliable startup characteristics as well as accuracy you could probably change the distribution of the two capacitances -- but it'll be work.