Crystal Oscillator

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Techman_7

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For a microcontroller, If the external load capacitors of a crystal oscillator are of different values, will the crystal oscillate at the same frequency for which it is designed?
Or will it oscillate at different frequency other than required?

For example, 10pF is the calculated external load capacitors for a 16 MHz crystal and it's working fine.

If this external cap value is changed to

Case 1: 20pF both and
Case 2: one cap is 10pF and another cap is 20pF.


What will happen in both the cases?
 

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There are G equations in attached that describe effects of the external C's.
 

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  • Crystal Oscillators.pdf
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C1=C2=2*CL for CL = rated f load such as 1/20+1/20 = 1/10
Reducing one cap reduces the net series C and raises f by a few PPM if needed to offset tolerance error.
It also raises the AC voltage across the side with the smaller C.


1740499895664.png
 

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Problem is that C1 and C0 are rarely specified in crystal datasheets. Here's a typical load capacitance pulling curve for a crystal with 7 pF nominal CL:

Screenshot_20250226_075636_Dropbox.jpg
 

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