A Wideband Oscillator Exploiting Multiple Resonances in Lithium Niobate MEMS Resonator

This article presents a comprehensive guide to codesign lithium niobate (LiNbO₃) lateral overtone bulk acoustic resonators (LOBARs) and voltage-controlled oscillators (VCOs) using discrete components on a printed circuit board (PCB). The analysis focuses on understanding the oscillator-level tradeoffs between the number of locked tones, frequency stability, tuning range, power consumption, and phase noise. Moreover, this article focuses on understanding the relationship between the abovementioned specifications and the different LOBAR parameters, such as electromechanical coupling (k_t²), quality factor (Q), transducer design, and the resonator size. As a result of this study, the first voltage-controlled MEMS oscillator (VCMO) based on LiNbO₃ LOBAR is demonstrated. Our LOBAR excites over 30 resonant modes in the range of 100-800 MHz with a frequency spacing of 20 MHz. The VCMO consists of an LOBAR in a closed loop with two amplification stages and a varactor-embedded tunable LC tank. By adjusting the bias voltage applied to the varactor, the tank can be tuned to change the closed-loop gain and phase responses of the oscillator so that the Barkhausen conditions are satisfied for a particular resonant mode. The tank is designed to allow the proposed VCMO to lock to any of the ten overtones ranging from 300 to 500 MHz. These ten tones are characterized by average Qs of 2100, k_t² of 1.5%, figure of merit (FOM = Qk_t²) of 31.5 enabling low phase noise, and low-power oscillators crucial for Internet of Things (IoT). Due to the high Qs of the LiNbO₃ LOBAR, the measured VCMO shows a close-in phase noise of -100 dBc/Hz at 1-kHz offset from a 300-MHz carrier and a noise floor of -153 dBc/Hz while consuming 9 mW. With further optimization, this VCMO can lead to direct radio frequency (RF) synthesis for ultralow-power transceivers in multimode IoT nodes.