  This letter presents the experimental demonstration of a film bulk acoustic
resonator (FBAR) circulator at 2.5 GHz. The circulator is based on
spatio-temporal 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 mm2) consists of three identical FBARs connected in wye configuration.
The FBAR0s quality factor (Q) of 1250 and piezoelectric coupling coefficient kt
2 of 3% relaxes the modulation requirements, achieving non-reciprocity with
small modulationto- RF frequency ratio bettter than 1:800 (3 MHz:2.5 GHz).
