Electronically-Controllable Bandpass Planar Filter with Ultra-Large Bandwidth-Tuning Ratio and Enhanced in-Band Amplitude Flatness

A class of electronically-reconfigurable quasi-elliptic-type compact bandpass filter (BPF) for application in the intermediate-frequency (IF) stages of modern satellite-communications receivers is presented. By means of dynamic transmission-zero (TZ), external-quality-factor, and pole control, it allows to obtain an ultra-large bandwidth-tuning ratio with enhanced inband amplitude-flatness characteristics in its adaptive transfer function. This is achieved while keeping the BPF inter-resonator couplings static, which results in lower in-band insertion-loss levels. The coupling-matrix foundations and theoretical synthesis examples of the engineered tunable BPF concept are presented. Furthermore, for experimental demonstration, a I-GHz three-pole/two-TZ varactor-tuned microstrip BPF prototype is developed and measured. It exhibits a 1-dB-bandwidth-tuning ratio of 13.9:1 that goes from ultra-wideband to narrow-band states with minimum in-band insertion loss in the range 1.2-6.4 dB.