Quote:
Originally Posted by G0HZU_JMR
I agree that the classic crystal model is extremely good but it doesn't take into account capacitance to free space. This capacitance might only be a tiny fraction of 1pF but it is there nonetheless.
It doesn't really matter though as long as everyone uses the same classic model and they fudge the C0 capacitance ever so slightly to take into account the tiny effect of the package capacitance to free space. A 2cm long piece of wire still has some capacitance to free space so a crystal will be no different.
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Your concerns are overestimated.
If you look at the typical configuration for parallel resonance you will find that the package capacitance as seen from the crystal's terminals can be considered as an equivalent of two approximately equal capacitances in series, put across the terminals, with their common point connected to the case. As such it will be simply a part of total load capacitance.
What's more important, the values of these capacitances are more constant and they are of better quality than most commonly used types for prescribed load capacitors.
If you now connect the crystal's case to the ground plane as is (or should be) the case in any practical circuit you will end up with a circuit where the capacitance of this ground plane to free space may be some tens or more picofarads (let's call it Ccmfs). But this Ccmfs is, as seen from the crystal's terminals, a capacitance connected to the ground plane and looking into free space, and as such it can be freely ignored, regardless of its absolute value.
Think about a screened crystal oscillator.