A Transformer-Based Series-Resonance CMOS VCO

The LC parallel tank is widely used in oscillators, and its noise optimization techniques are relatively mature. However, there has always been a bottleneck in phase noise (PN) performance, and the multi-core technique can only improve it partially. This article proposes a brand-new oscillator structure leveraging a transformer-based series tank, which can tolerate larger currents, generate greater oscillation amplitudes, and achieve significant PN suppression. A general active circuit for series-resonance (SR) is analyzed. The proposed series tank provides adjustable low input impedance, is well-suited for high-frequency operation, and suppresses parasitic modes. With the folded transformer, the layout implementation is simplified. An SR voltage-controlled oscillator (VCO) prototype is designed and fabricated using a 65-nm CMOS process. It achieves a measured PN of -132.1 dBc/Hz at a 1-MHz offset and a corresponding figure of merit (FoM) of 187.4 dBc/Hz at a 10.65-GHz carrier frequency. The SR VCO is applied to a phase-locked loop (PLL), which achieves 33.7 fs of jitter with a 100-MHz reference clock, significantly reducing the demands on the reference path.