A wide tuning range VCO using capacitive source degeneration

This paper presents a wide tuning range LC voltage controlled oscillator with a new differential negative resistance cell that uses capacitive source degeneration. Traditional differential negative resistance cell use two transistors where the drains are cross-connected to the corresponding gates and the sources are ac grounded. This structure has a fairly large parasitic shunt capacitance. This effective parasitic capacitance combined with the fixed portion of the varactor and the inductor parasitic capacitance limits the tuning range and the maximum size of allowable inductor that is crucial for the large output swing. This paper shows that if the sources of the differential negative resistance cell are capacitively degenerated such that the effective parasitic capacitance is much smaller than that of traditional structures, then the tuning range can be increased or alternately the output swing can be increased for a given tuning range. An appropriate degenerating capacitance and device geometry can be selected to optimize the performance. The tradeoff between negative resistance and effective parasitic capacitance has also been considered. To verify this concept a 5.3GHz VCO with 21.5% tuning range and -106dBc/Hz phase noise at 1MHz offset from 5GHz has been designed using the TSMC 0.25μm CMOS process.