An inverse-class-F CMOS VCO with intrinsic-high-Q 1st- and 2nd-harmonic resonances for 1/f2-to-1/f3 phase-noise suppression achieving 196.2dBc/Hz FOM

Second-harmonic common-mode (CM) resonance has been explored for LC oscillators to improve their phase noise (PN) in the past. Its implementation evolves from an explicit design [1] that relies on an extra tail tank, to a recent implicit design [2], where the resonator itself offers a CM impedance peak at 2x the oscillation frequency (FLO): Explicit design (Fig. 23.5.1-upper): a high-Q tail tank (QTAIL) is desirable to raise its impedance |ZTAIL| at 2FLO and to prevent the loss of LTAIL from penalizing the PN in the 1/f² region [3]. To compare with the theoretical limit (FOMMAX), the FOM in the 1/f² PN region is plotted against LTAIL at different QTAIL. Closing the gap between FOMMAX and FOM imposes an excessive QTAIL of 20 or beyond, which can hardly be achieved and maintained over a wide tuning range.