A BAW antenna duplexer for the 1900 MHz PCS band

Miniature antenna duplexers are used in modem cellular telephone handsets. We present a new solution utilizing micro-machined, thin Film Bulk Acoustic Resonators (FBAR). A 1900 MHz duplexer with dimensions 5×8×2 mm is achieved. The duplexer, a three port device, achieves low loss from transmitter (Tx) to antenna (Ant) ports with a filter at 1880 MHz and from Ant to receiver (Rx) ports with a filter at 1960 MHz. High isolation from Tx to Rx port is achieved by coupling the two filters with a quarter wavelength line. Filter bandwidth is ~3%. The filters are realized as 21/2 or 31/2 section ladder connected FBARs with the shunt FBARs tuned 3% lower in frequency than the series FBARs to provide a flat pass band. Individual FEAR resonators are free membranes formed of Aluminum Nitride piezoelectric and metal electrode films micro-machined on a silicon substrate. Resonators with electro-acoustic coupling k_t²~3 to 5% and Q~300 are achieved. Filters fabricated in this manner achieve minimum insertion loss ~1 dB, out-of-band rejection of ~20 dB, 60 MHz bandwidth, and an input power capability of 2 Watts. Duplexers are assembled from one each Tx and Rx filter, and a lumped λ/4 line of two inductors and a capacitor. These duplexers have insertion loss <2 dB in the pass bands, and Tx to Rx isolation >45 dB