Two Tunable Frequency Duplexer Architectures for Cellular Transceivers

In this paper, two architectures for tunable duplexers are presented. The tuning is accomplished through variable capacitance and resistance. The architectures are based on a three element series-parallel resonator, with one pass and one stop frequency. Both architectures rely on filtering as well as cancellation for good Tx to Rx isolation while maintaining low insertion loss. The first architecture, the filtered transformer balanced (FTB) isolator, has single ended transmit and antenna ports and a differential receive port. The second architecture, the cross coupled filtering (CCF) isolator, is fully differential. For a resonator Q of 50, the impedance ratio of the resonator at pass and stop frequencies is 18.5 dB for 3GPP band-I. In the FTB isolator, this results in 1.65-dB Tx and 2.14-dB Rx insertion loss, and 53 dB isolation for a 20-MHz channel bandwidth. In the CCF isolator, this results in 1.9-dB Tx and 1.9-dB Rx insertion loss, and 59-dB isolation for a 20-MHz channel bandwidth. These figures are obtained with a 20% resistive mismatch, showing feasibility of good performance in an environment with changing impedance. As the operating frequencies of cellular systems increase, these structures will become fully integratable due to the reduced sizes of inductors and transformers.