AM-FM conversion by the active devices in MOS LC-VCOs and its effect on the optimal amplitude

Abstract

Large oscillation amplitudes in differential MOS LC-VCOs change the operation region of the MOS negative-resistance devices and thus modulate their parasitic capacitances. As a result, the effective tank capacitance and the oscillation frequency change with respect to the oscillation amplitude for a fixed tuning voltage. The resulting frequency-versus-amplitude curve has a maximum for differential amplitude approximately equal to the threshold voltage of the MOS devices which corresponds to the optimum amplitude that minimizes AM-to-FM conversion. The presented analysis is confirmed with simulation and measurement results for a 2 GHz differential NMOS VCO fabricated in a 0.25 mum BiCMOS process. The optimum amplitude for lowest AM-to-FM conversion also yields the lowest phase noise for this oscillator