A capacitance boosted full-octave LC VCO based 0.7 to 24 GHz fractional-N synthesizer

This paper describes a technique in which a series inductance is used with a varactor to increase the varactor maximum to minimum capacitance ratio, thus enabling increased tuning range in an LC VCO. This technique is demonstrated through the implementation of an LC VCO tunable over a range greater than an octave, specifically 10.5 to 24 GHz (78%). The VCO is implemented within a fractional-N synthesizer in 32nm SOI CMOS to achieve continuous tuning from a fixed reference and achieves -128 to -119 dBc/Hz phase noise at 10MHz offset from the carrier across the tuning range. Output divider circuits are used to obtain all frequencies down to 660MHz.