The methods of maintaining low frequency stability in FBAR based cross-coupled VCO design

In this letter, we propose the methods to mitigate low frequency instability in a film bulk acoustic resonator (FBAR) based cross-coupled voltage-controlled oscillator (FBAR-VCO). The first method employs a capacitor and while the other one employs a series inductor-capacitor. Using the post-layout simulation, a comparative study of the performances such as phase noise and figure of merit (FOM) of the designed FBAR-VCO is carried out. It is found that both the methods inhibit the low frequency instability successfully and the additional advantage of the second method is 2 dB improvement of FOM. Compared to other published FBAR-VCOs, this work has very low phase noise, below -150 dBc/Hz at 1MHz offset frequency, and much better FOM below -210 dB.