Ring VCO question - ring length

[knights]

Ring VCO question

I am designing a ring VCO. The delay cell is fully differecntial cell.There are two structure I can select. One is use 4 stage 8phase VCO. Another is use 3 stage 6
phase. For 4 stage ,I have to choose small channel length than 3 stage. So it means if I use 3 stage 6 phase structure, I can get better phase noise.Is that ture?
So whether I can get a conclusion that it is better to choose short ring?
Is that have any other issues?
Thanks in advance.

 

[sxb416]

Re: Ring VCO question

4 stage have good noise but low speed

 

[Sadegh.j]

Re: Ring VCO question

It depends on many things, including the biase currents, make sure you read "Jitter and phase noise in Ring Oscdillators" by Ali Hajimiri, S. Limotyrakis and T. H. Lee, you will find what you are looking for in Equation 34 on page 796.

 

[twlin1]

Re: Ring VCO question

But 4 stages may have poor frequency pushing and may make it worse, even better phase noise.