A 30-GHz Class-F Quadrature DCO Using Phase Shifts Between Drain-Gate-Source for Low Flicker Phase Noise and I/Q Exactness

In this article, we present a low phase noise (PN) mm-wave quadrature digitally controlled oscillator (DCO) exploiting transformers for class-F operation and harmonic extraction. A third transformer coil is added for the inter-core coupling with the source terminals of the switching transistors. We identify that the inter-core coupling in quadrature oscillators causes an asymmetric flicker noise current, thus degrading flicker PN. As a remedy, we propose a deliberate drain-to-gate phase shift of the switching transistors by means of capacitive loading to fix this asymmetry. The +/- 90(?) I/Q mode ambiguity is also resolved by introducing another phase shift in the source-coupling; it is explained by a simplified analysis with phasor diagrams. Fabricated in the TSMC 28-nm LP CMOS, the prototype achieves PN of -112 dBc/Hz and figure-of-merit (FoM) of -185 dB at 1-MHz offset of 25.7 GHz. The measured flicker PN corner is 140-250 kHz and image rejection ratio (IRR) > 47 dB over the whole 18% tuning range (TR). To the best of the authors' knowledge, it is the best reported PN and IRR for a mm-wave quadrature oscillator.