Sorry, maybe i should modify my title to "how to shift the tuning characteristic of my VCO"
The DC bias at the anode of the varactor is about 1.7V, so while i apply control voltage at cathode, it has to be ranging from 1V~3V to get the linear characteristic, if I can lower the DC voltage at the anode from 1.7V to about 0.8V, the tuning characteristic will "shift" to about 0.1V~2.2V for linear tuning.
However, because of the use of nMOS cross-coupled topology, the DC level at the varactor's anode is so high, is there anyway to solve this problem? I've tried complementary topology, but the phase noise performance is too bad....