Coupling Matrix-Based Design of Fully Reconfigurable Differential/Balanced RF Filters

This letter presents the coupling matrix design of differential/balanced RF filters. They exhibit: i) a highly suppressed common mode that is obtained by multiple transmission zeros and resistively loaded poles and ii) a differential mode that exhibits a quasi-elliptic-type transfer function that can be tuned in frequency and bandwidth (BW) and can be intrinsically switched-off. The filter's reconfiguration properties are attained by only tuning the resonant frequencies of its constituent resonators. Thus, it exhibits less in-band insertion loss than tunable differential architectures in which tuning of couplings is required. For practical validation purposes, a microstrip prototype was manufactured and measured in the 1.4-1.9-GHz range with the following characteristics. Differential mode: center-frequency tuning between 1.36 and 1.9 GHz (1.4:1), BW tuning between 43 and 270 MHz (6.3:1), and an intrinsically switched-off mode with isolation (IS) > 22 dB. Common mode: 70%, 40-dB suppression BW and IS > 60 dB at the center frequency for all tunable states.