I have a three stage ring oscillator that oscillate with 1GHz. each delay is a simple inverter. I use a tri state inverter in parallel to the first delay stage in ring structure. The frequency difference between two state of connecting and disconnecting the tri-state inverter is limited to 50MHz. Even the w/L of tri-state inverter is much lower than delay stage in ring structure, but this frequency difference cannot get lower that much. Does anybody knows the reason?