A Low-Phase-Noise Transformer-Feedback VCO With Separated DM and CM Resonance

This letter reports a transformer-feedback voltage-controlled oscillator (VCO) (TF-VCO) with separated differential-mode (DM) and common-mode (CM) resonance for feasibility and low phase noise. With an effort to isolate the DM and CM resonances by the relatively DM resonance-independent single-ended (SE) capacitors connected at NMOS source for CM tuning and a CM cancellation transformer between drain and source, the proposed VCO achieves low resonant frequency interaction between CM and DM resonances. A factor
$ F_{\mathrm{ cm,dm}}$
is defined to describe this interaction. Compared with the other mainstream implicit CM resonance oscillators, the proposed VCO exhibits at least 7.75 times better
$F_{\mathrm{ cm,dm}}$
. Implemented with the 180-nm CMOS process, the 6-GHz VCO occupies a die area of
$0.1~\rm {mm^{2}}$
and consumes 6 mW under 750-mV supply. The measurement result demonstrates a phase noise of −123.7 dBc/Hz at 1-MHz offset with a −191.5 dBc/Hz peak figure-of-merit (FoM) and only undergoes 0.21% frequency shift after calibrated harmonic tuning.