The parasitical cap of current source in VCO

[hebu]

I read some papers which discuss the LC tank VCO before, long time ago, I remeber the parasitical cap in the drain node of tail current source will affect
the VCO operation in speed and noise. But I forgot how it affect and what's
the exact phenomenon. Any body know this? and tell me more about this.
Thanks,

 

[sunking]

the drain node can be a voltage controlled cap.
So at difference voltage the cap will be changed.