A 2.5 GHz CMOS LC VCO with improved phase noise based on the transformer feedback scheme

This paper demonstrates a circuit technique to improve the phase noise of a GHz-range CMOS differential LC VCO. The VCO has two on- chip transformers; primary of each transformer is used as an inductor of the LC tank. A signal current, whose frequency is the same and phase is 90 degrees in advance of the primary coil, is produced in the secondary coil so that the series resistance of the primary coil is cancelled out. As a result, Q-value of the LC tank is increased and the phase noise of a VCO is reduced. The measured phase noise of the 2.5 GHz VCO with a 100 KHz offset was -103 dBc/Hz, and the phase noise improvement in comparison with the conventional VCO at a 100 KHz frequency offset was 10 dB from a 2 V power supply.