An FBAR Circulator

This letter presents the experimental demonstration of a film bulk acoustic resonator (FBAR) circulator at 2.5 GHz. The circulator is based on spatiotemporal modulation of the series resonant frequency of FBARs using varactors and exhibits a large isolation of 76 dB at 2.5 GHz. The FBAR chip (0.25 mm
²
) consists of three identical FBARs connected in wye configuration. FBAR's quality factor (Q) of 1250 and the piezoelectric coupling coefficient k
_t
²
of 3% relax the modulation requirements, achieving nonreciprocity with small modulation to RF ratio better than 1:800 (3 MHz: 2.5 GHz).