A Comparative Study of 8-Phase Feedforward-Coupling Ring VCOs

This brief studies comparatively the different 8-phase feedforward-coupling (FC) ring voltage-controlled oscillators in terms of their oscillation modes, oscillation frequency, and phase noise. Among the 3 feasible topologies, the one with sub-feedback loops containing 2 direct-path inverters + 1 feedforward-path inverter (i.e., 2D + 1F) can achieve a better phase noise than the common 4D + 1F topology at the same frequency and power budget. It is also shown that the bimodal oscillation problem of the 2D + 1F topology can be eliminated by inserting an auxiliary FC path. Prototyped in 65-nm CMOS, the proposed 2D + 1F topology at 3.1 GHz shows a phase noise of -95.1 dBc/Hz at a 1-MHz offset, corresponding to a 159.9-dBc/Hz figure-of-merit that is 4.2 dB higher than the common 4D + 1F topology.