hi,
   i think this can help you

A 1.25-GHz 0.35-
m Monolithic CMOS PLL

Based on a Multiphase Ring Oscillator   Lizhong Sun and Tadeusz A. Kwasniewski, Member, IEEE

thanks,
rajasekhar.

Hi Joel,
            Quote:


     Ring-Osc VCO's are inferior in performance to LC VCO's as far as noise is concerned. (Obvously as active implementation is far worse than passive in noise performance).
              But Ring-Osc VCO's are primarily used to reduce area....Si Real estate costs increases with shrinking technologies...
                      Power required in Ring-Osc is variable and in designer's hand...If your Noise specs are not so stringent as in LC VCO's,(typically LC VCO's exhibit Phase Noise of -120 dBc @ 1 MHz Offset), You can work with Ring-Osci's at much lesser Power. The thing is you can trade-off Noise(say only -90 or -100 instead of -120dBc as i said earlier) and reduce power consumption of the VCO in ring osci.
         
              But yes, if you try to make a Ring Osci VCO with comparable Noise Performance as that of an LC Equivalent, Ring Osci might take lesser area but it will consume nearly the same power as LC one.

Multi-Loop Ring Osci -- well a fundoo thing...  but at the same time complex....Normally in a RING OSCI there is one single loop with Odd No. of Inverters. This Loop introduces a phase shift of 180 degrees before gain reaches 0 dB  , basically rendering system unstable and causing it to oscillate.
        The idea is what if you place multiple Loops in Feedback instead of a single one...This idea came to me sometime back..But someone had already worked on it...[ As always Late Well thats usually the case whenever I think I hv thought of something new  :D ]

      I actually thought of this multi-Loop Idea because i wanted better Noise performance from RING OSCI without burning much power n conserving area. I thought Multiple Loops will control Noise Better...But didnt find any substantial improvement in Noise..There is an alteration in speed..You can attain higher speeds at lesser power but didnt see any significant improvement in Noise performance as i had thought of initially.....Anyways, it was worth exploring...  :)

Refer to the paper posted by raja above...It's very exhaustive and a nice read...

regards,
Mayank.