Trifilar-Transformer-Coupled Series-Resonance Oscillator with Enhanced Tuning Range

Despite the potentially superior phase-noise (PN) performance compared to the parallel-tank resonance oscillator, the series-resonance oscillator (SRO) usually suffers from a narrow tuning range (TR) of <10% due to the heavy loading by the large Gm cells as required for oscillation. Typically, enhancing the TR of SRO requires lowering the tank Q-factor at the cost of PN and figure-of-merit (FoM). This paper proposes a quadrature SRO with an enhanced TR using higher-order tanks with a boosted coupling factor by means of trifilar transformers. This maintains the low PN while improving its tuning range (TR) to 22% (i.e., 10.3 to 12.9GHz). Designed in TSMC 28-nm HPC+ CMOS, as per post-layout full-chip simulations, the proposed SRO achieves PN of -124dBc/Hz and -146dBc/Hz at 1 and 10MHz offset, respectively, from the 12.24GHz carrier frequency, while consuming 55mW. This results in an FoM of 188.3–190.1dBc/Hz and an FoMT of 194.7–197.7dBc/Hz across the TR.