question about the crystal currrent

in Vittoz's paper, there is comment about the current of crystal: because of the high Q, the current is sinusoidal for any of voltage v. I don't understand of this. Can anyone explain to it? thanks.

because of high Q, only single frequency can be passed.

"because of high Q, only single frequency can be passed. "

why? can explain it in detail? thanks

The question is fuzzy, but I'm sure that you'll understand better if you find out the (fictual) replacement circuit of a crystal oscillator. Check out the following link.

http://www.maxim-ic.com/appnotes.cfm/an_pk/2127

The crystal acts as a narrow bandpass filter. This means any harmonics, etc of the crystal current will be suppressed.

The crystal current shall never exceed the specification of the crystal, usually 1-2 mW, to avoid noise and crystal degradation.