Novel VCO Architecture Using Series Above-IC FBAR and Parallel LC Resonance

A quasi-monolithic voltage-tunable film bulk acoustic resonator (FBAR) enhanced oscillator for 2.1 GHz in 0.25-mum SiGe BiCMOS technology is designed, fabricated, and evaluated. The narrow-band FBAR was built above the SiGe circuit through later Si post-processing steps. The oscillator is based on a two-transistor loop structure and uses two resonators, namely a parallel LC tank and an above-IC FBAR in its series-resonant mode. The improvement in phase noise performance is significant compared to a similar reference LC voltage-controlled oscillator (VCO), with the best phase noise being -144.1 dBc/Hz at an offset of 1 MHz and -149.6 dBc/Hz at 3 MHz. The architecture offers advantages in overcoming frequency tuning difficulties usually present when using high-Q resonators. Although the width of the tuning range comes at some cost on phase noise, the measured performance satisfies contemporary wireless standards such as GPS