Crystal Conundrum

5th element · 7th March 2004, 11:25 AM

When a manufacturer specifies a load capacitance what exactly does this mean.

I know that you connect two capacitors from the crystal legs to ground. But if they state a value of 30pf does that mean use two 15pf or two 30 pf?

The data sheets for the make ive found are useless.

Cheers Matt

5th element · 7th March 2004, 12:57 PM

Someone? Anyone?

GaryB · 7th March 2004, 02:12 PM

It means the effective capacitance across the crystal. For the case you mention where you want a 30pf load, then you need to remember that the two capacitor to ground are effectively in series. To get a 30pf load would require a 60pf cap to ground from each leg.
Of course you also need to consider the capacitance provided by the rest of the circuit and adjust accordingly. More capacitance will pull the frequency slightly higher.
---Gary

7th March 2004, 11:25 AM	#1
5th element diyAudio Member Join Date: Oct 2002 Location: England	Crystal Conundrum When a manufacturer specifies a load capacitance what exactly does this mean. I know that you connect two capacitors from the crystal legs to ground. But if they state a value of 30pf does that mean use two 15pf or two 30 pf? The data sheets for the make ive found are useless. Cheers Matt __________________ What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz! Droggon Attack!

7th March 2004, 12:57 PM	#2
5th element diyAudio Member Join Date: Oct 2002 Location: England	Someone? Anyone? __________________ What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz! Droggon Attack!

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7th March 2004, 02:12 PM	#3
GaryB diyAudio Member Join Date: Sep 2002 Location: Silicon Valley	It means the effective capacitance across the crystal. For the case you mention where you want a 30pf load, then you need to remember that the two capacitor to ground are effectively in series. To get a 30pf load would require a 60pf cap to ground from each leg. Of course you also need to consider the capacitance provided by the rest of the circuit and adjust accordingly. More capacitance will pull the frequency slightly higher. ---Gary