Low-Power Class-C VCO in SiGe BiCMOS With Varactor Embedded in Cross-Feedback Path for Tuning Range Enhancement

The tuning range (TR) of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> voltage-controlled oscillators (VCOs) is primarily limited by the maximum tuning ratio of varactors in a given process, and significantly degraded by parasitic capacitance of the transistors in a cross-feedback path especially for millimeter-wave (mm-wave) VCOs. In this article, a mm-wave class-C VCO is demonstrated in silicon-germanium (SiGe) bipolar complementary metal-oxide–semiconductor (BiCMOS) technology, with varactors embedded in the cross-feedback path to enhance the TR. Equivalent circuit model and derived analytical expressions show that in the presence of large parasitic capacitance at the base of the transistors, introducing varactors in the cross-feedback path significantly improves the overall TR. Therefore, the TR limitation by varactor tuning ratio can be released. Analysis of start-up and phase noise with consideration of base parasitics and practical values of cross-feedback capacitance are given to replenish the conventional class-C analysis and methodology. Based on the analysis, a VCO containing varactors both in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i>-tank and the cross-feedback path is proposed to achieve a continuous TR from 29.6 to 32.4 GHz, low dc power consumption of 10.1 mW, and low-phase noise of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-114.6~\text {dBc/Hz}$</tex-math> </inline-formula> at 10-MHz offset in measurements.