A capacitive transformer-based class-C oscillator

This paper presents a novel class-C VCO by exploiting a capacitive transformer-based (CT-based) resonator. Assuming the same Q-factor, the CT-based resonator has a lower equivalent resistance than the conventional LC resonator, which results in reduction of the phase noise. Moreover, using the second node of resonator for biasing core transistors’ gates enhances the loop gain and provides a robust start-up for biasing MOS gates of the active pair. Furthermore, compared to the conventional oscillator, a larger tail capacitance can be used in the proposed class-C oscillator, which results in greater phase noise improvement. Additionally, the oscillation amplitude calculated accurately in this paper demonstrates that using the CT-based resonator negligibly affects the maximum oscillation amplitude in the CMOS class-C oscillator in the deep-submicron processes. The proposed oscillator is implemented in standard 90-nm CMOS technology and operates at 3.5–3.7 GHz. Its phase noise at 1 MHz and 3 MHz offset frequencies are −129 dBc/Hz and −138.4 dBc/Hz in 3.6 GHz, respectively. The proposed structure occupies 0.49 mm2 core area and consumes 9.2 mW power with the FoM 190.3 dBc/Hz at 3 MHz offset frequency.