UBC Theses and Dissertations
Transformer-enhanced high-performance voltage-controlled oscillators Lightbody, Samuel
We show that, in comparison to an inductor, an asymmetric transformer can improve the quality factor (Q) of an inductor-capacitor (LC) tank when the tank loss is dominated by the varactor. Near, and at mm-wave frequencies, varactors in complementary metal-oxide-semiconductor (CMOS) processes have significantly lower Q than inductors and transformers. Directly connecting a varactor to the core of an LC oscillator lowers tank Q, and the increased ratio of parasitic capacitance to total tank capacitance limits frequency tuning range (FTR). Instead, magnetically coupling a varactor to the oscillator core using an asymmetric transformer, where the core is connected to the primary and varactor to the secondary, increases tank Q. Furthermore, it permits doubling the varactor bias range and reducing the parasitic capacitance seen at the varactor. Thus, both FTR and Phase Noise (PN) are improved simultaneously. Measurement results for two prototypes in 65nm CMOS are presented. A 25 GHz Voltage-controlled Oscillator (VCO) shows an FTR of 29.8%, a PN of -106.6 dBc/Hz at 1 MHz offset, and an FTR-inclusive Figure of Merit (FoMT ) of -195.04 dBc/Hz. A 60 GHz self-mixing VCO, where the VCO core at 20 GHz is mixed with its common-mode 40 GHz tone, shows an FTR of 18.5%, a PN of -98.9 dBc/Hz at 1 MHz offset, and an FoMT of -193.4 dBc/Hz.
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