Quadrature VCO ( QVCO)

Hello,

I am looking for some sources that can teach me about QVCO. I have a good understanding of a VCO and have designed one in the past (Current starved).
I have searched the web and found some papers that talk about QVCO design but they assume that the reader already knows at the very least the basics of the QVCO.
Can someone point me in the right direction.
Any help greatly appreciated.

In IEEE ,there are lot of papers about QVCO.In addition to, Grebennikov explains some techniques in his book.