In this letter, design methodlogy of a low phase noise multiband film bulk acoustic resonator (FBAR) based voltage controlled oscillator (FBAR-VCO) is presented. It employs a 1.9GHz cross-coupled FBAR-VCO core, and extends oscillation to 0.65GHz, 0.98GHz, 1.96GHz and 3.92GHz by a divider and a multiplier. By analyzing the low frequency instability and proposing the solution based on a capacitor, cross-coupled architecture is employed in 1.9GHz FBAR-VCO core and phase noise degradation is extensively studied for extended frequencies. By considering the effect of transistors' size on the Q-factor and impedance of FBAR, excellent phase noise and high loop gain are obtained. The post-layout simulation shows the proposed multiband FBAR-VCO achieves the lowest phase noise below 150dBc/Hz at 1MHz offset frequency.
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