Variable Negative Gm Technique for RF LC VCO with Very Large Tuning Range

When the tuning range of the RF LC VCO gets very large, the VCO start up condition, VCO oscillation swing and phase noise performance will vary obviously. These issues result from the fact that the equivalent parallel resistance of the LC tank varies a lot due to very large tuning range. In this paper, we propose a variable negative Gm technique to solve these problems. From simulation results, it is found that by using this technique the VCO start up condition at different frequency bands can be independently met. The variation of the VCO oscillation swing can be effectively reduced. The phase noise fluctuations between different bands can be decreased close to the intrinsic level.