Reducing phase noise by proper sizing of MOSFETs in LC tuned VCOs

This work identifies the relationships between the sizes of the oscillator´s core MOSFETs and the phase noise in the 1/f region. Three packaged 1 GHz VCOs with the same LC lank circuit, but different gate lengths were designed and fabricated in a standard digital 0.6 μm CMOS technology. The minimum gate length (L_min) of the core MOSFETs does not result in the minimum phase noise. Instead, the minimum phase noise occurs when the gate length is L_opo and L_opt = η·L_min where η is a parameter that depends upon fabrication process and bias current. From measured results, the phase noise can be further decreased by 2 dBc/Hz at 600 kHz offset from 1 GHz center frequency by using the optimal sizes of the core MOSFETs.