Tunable duplex filter for adaptive duplexers of advanced LTE handsets

The new generation mobile handsets must support a large number of frequency bands and duplex offsets. Traditionally, an adaptive duplexing technique makes use of a wideband circulator to create initial isolation. A tunable duplex filter is presented in this paper, achieving the required tuning range for LTE Band 1. The demonstrated circuit is implemented in a 250nm Silicon on Sapphire (SOS) process, it provides an isolation of 19dB and 14dB at the transmit and the receive frequencies, respectively. The measured insertion loss from PA to antenna port is better than 3.8dB. The measured third-harmonic suppression is 45.9dB at an input of 24.5dBm. The implemented circuit occupies an area of 3.0mm×2.2mm.