A 3.6 GHz Radio Frequency Circulator based on AlN FBAR Filters

In this study, a 3.6 GHz magnetic-free radio frequency (RF) circulator based on aluminum nitride (AlN) film bulk acoustic wave resonator (FBAR) filters is implemented using spatiotemporal modulation. The circulator is designed at the board level with two identical filters in a sequentially-switched delay element topology, creating nonreciprocity through two external switch modules. The FBAR filters, which exhibit an insertion loss (IL) of 1.3 dB and a fractional bandwidth (FBW) of 61 MHz at 3.64 GHz, are fabricated using a 200 mm commercial AlN-MEMS platform fully compatible with RF CMOS processes. The circulator achieves a moderate IL of 11.6 dB, high isolation (IX) of 30 dB, low intermodulation distortion (IMD) of 24.6 dB, and a high input third-order intercept point (IIP3) of +42 dBm, over a 20-dB isolation bandwidth of 49.9 MHz (1.38%) with a low modulation frequency of 15 MHz.