A dual-tank LC VCO topology approaching towards the maximum thermodynamically-achievable oscillator FoM

There exists a fundamental limit in improving the phase noise performance of LC-tank oscillators. Impediments to reach this limit are first discussed, and then a clipping LC VCO topology based on dual tank is presented to mitigate them. This topology can approach within 3 dB of the maximum thermodynamically achievable figure-of-merit (FoM) limit. Compared to conventional class-B/C/D/F oscillators, it is capable of reducing both close-in and far-out phase noise. As a proof of concept, a prototype 4.17–4.95 GHz VCO in a 0.13-μm CMOS process achieves a phase noise of −97 and −143 dBc/Hz at 30 kHz and 3 MHz offset, respectively.