20.2 A 3.09-to-4.04GHz Distributed-Boosting and Harmonic-Impedance-Expanding Multi-Core Oscillator with-138.9dBc/Hz at 1MHz Offset and 195.1dBc/Hz FoM

The rapid development of high-speed communication and radar systems has placed stringent requirements on the spectral purity of RF oscillators. Also, maximizing battery lifetime, efficiency, or power consumption is of great concern for these systems. The oscillator is thus required to have a low phase noise as well as a high figure-of-merit (FoM). Recently, to reduce the noise sensitivity of the LC oscillator, various harmonic shaping techniques [1] –[3] have been reported. By setting the differential-mode (DM) [1], [2] or common-mode (CM) [1], [3] resonances at specific harmonic frequencies, the impulse sensitive function (ISF) is reduced, and a high FoM could be achieved. However, to meet the strict relationship between fundamental frequency and resonances, the frequency tuning plan is complex, and the frequency range is limited. Besides, the minimum phase noise performance is still limited by the reduced power supply of advanced process and the maximum power taken by a single-core resonate tank. To overcome these limitations, the low-phase-noise oscillators have evolved from the single-core to multi-core types [4], [5]. By coupling N oscillators together, it is possible to burn N times higher power and decrease the phase noise by 10log(N), with the FoM unchanged. Nevertheless, the multi-core coupling method needs to be designed carefully to prevent concurrent oscillations [5]. Meanwhile, the conventional multi-core oscillators only achieve an average FoM of 189dB/Hz [4], [5], which is significantly worse than the state-of-the-art single-core oscillators. This paper proposes a distributed-boosting and harmonic-impedance-expanding multi-core oscillator, which can achieve low phase noise and high FoM simultaneously. A distributed-coupling transformer is introduced to obtain the distributed-boosting effect and maintain the coupling relationship of the four oscillator cores. The expanded DM and CM resonances are allocated for harmonic shaping over a wide frequency range. Prototyped in a conventional 40nm CMOS technology, the proposed oscillator operating at 3.09GHz achieves the phase noise as low as -138.9dBc/Hz at 1MHz offset, 195.1dBc/Hz FoM, and 26.6% tuning range.