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 (F
_LO
): Explicit design (Fig. 23.5.1-upper): a high-Q tail tank (Q
_TAIL
) is desirable to raise its impedance |Z
_TAIL
| at 2FLO and to prevent the loss of L
_TAIL
from penalizing the PN in the 1/f
²
region [3]. To compare with the theoretical limit (FOM
_MAX
), the FOM in the 1/f
²
PN region is plotted against L
_TAIL
at different QTAIL. Closing the gap between FOM
_MAX
and FOM imposes an excessive Q
_TAIL
of 20 or beyond, which can hardly be achieved and maintained over a wide tuning range.