What is the main criterion to choose number of stages while designing ring oscillator??
Say my VCO operating range is 400M to 800M. How can i choose the no of stages??
u know from the basic equation of the Ring oscillator that the F depends on the N * the delay of the single inveter
where N is the number of stages , so u should check the propagation delay of the inverter and then use it to get the number of stages required to get this frequency
if u want a ring VCO ,, then u should see how will u vary the propagation delay of the inverter
another way to make a broad band VCO is to change both the propagation delay and the stages
Actually it depends on your design parameter, let say u need low power VCO, so u can used least number of cell, but the drawback is the gain. Actually for ring oscillator there is Barkhausen Criterion for get reliable oscillator.
In terms of power and speed, less stage is better.But usually VCO gain of less-satge design is larger compared to more stage design. In terms of VCO phase noise, we don't want VCO gain too large. Another consideration is more-stage VCO is more sensitive to power-supply noise than fewer-stage. So here are some trade-off depends on your PLL architecture and application.