[SOLVED] What's the role of a inductor in ctrystal circuits?

[nightseas]

This is a reference design of crystal circuit for a CPU.
My question is what's the role of the 470nH inductor and 0.01uF Capacitor in this circuits?
And how can I calculate these parameters?

 

[biff44]

I would guess the inductor stops unwanted modes of oscillation from happening, like if it was a 3rd order crystal the L-C keeps if from oscillating at the 1st order frequency. The 0.01 uF is just a DC block to keep the Xtal IN from shorting out to ground.

22 pF in parallel with 470 nH implies the desired frequency of oscillation is around 50 MHz.

F0=1/(2Π*√(22*10^-12 * 470*10^-9))=49.5 Mhz, and the parallel L-C will look like an open circuit at that frequency. Farther above or below F0, the L-C circuit will look more like a shunt short circuit. So it is probably a 16.5 MHz crystal forced to work at 49.5 Mhz.

 

[nightseas]

I would guess the inductor stops unwanted modes of oscillation from happening, like if it was a 3rd order crystal the L-C keeps if from oscillating at the 1st order frequency. The 0.01 uF is just a DC block to keep the Xtal IN from shorting out to ground.

22 pF in parallel with 470 nH implies the desired frequency of oscillation is around 50 MHz.

F0=1/(2Π*√(22*10^-12 * 470*10^-9))=49.5 Mhz, and the parallel L-C will look like an open circuit at that frequency. Farther above or below F0, the L-C circuit will look more like a shunt short circuit. So it is probably a 16.5 MHz crystal forced to work at 49.5 Mhz.

Thanks, biff44. Now I'm clear about it.