A study of the thin film bulk acoustic resonator filters in several ten GHz band

The first K to Ka band filters using thin film bulk acoustic resonators (FBARs) have been developed. The filters are ladder type filters configured with air-gap type FBARs and designed using an equivalent circuit based on a Butterworth-Van-Dyke (BVD) model. Center frequency, fractional bandwidth, minimum insertion loss and suppression in out-band were 23.8 GHz, 3.4%, -3.8 dB and -13 dB in the K band filter, and were 29.2 GHz, 3.4%, -3.8 dB and -11 dB in the Ka band filter, respectively. 24.7 GHz in resonant frequency, 285 in resonant Q, 291 in anti-resonant Q and 6.0% in effective electro-mechanical coupling coefficient k_eff ² were obtained in the K band FBAR. Additionally, it was derived and verified experimentally that the loss is proportional to the product of the resonant frequency and the capacitance ratio in the FBAR. This relation allowed the limitation of the FBAR filter to be discussed for higher frequencies.