A low-phase-noise injection-locked differential ring-VCO with half-integral subharmonic locking in 0.18 µm CMOS

Design and implementation of a CMOS differential ring-VCO that locks at half-integral (1.5, 2.5, 3.5, ⋯) as well as integral (1, 2, 3, ⋯) multiples of the injected reference frequency fref are presented. The advantage of half-integral subharmonic locking is that, for a given VCO output frequency step, the output phase noise can be lowered than when using integral subharmonic locking because of the higher (2x) reference frequency. For example, the 1-MHz-offset phase noise at a VCO output frequency of 1.5GHz was −136 dBc/Hz when locked to an integral subharmonic of fref = 0.5 GHz, whereas it was as low as −139 dBc/Hz when locked to a half-integral subharmonic of fref = 1.0 GHz. The ring-VCO was fabricated with a 0.18µm CMOS process. An explanation is given as to why it locks to half-integral subharmonics and how such an oscillator could be designed. Half-integral or, more generally, nonintegral subharmonic locking could make an effective means to reduce the phase noise of high-resolution injection-locked VCOs.