High-Selectivity Tunable Balanced Bandpass Filter With Constant Absolute Bandwidth

This brief presents a new and effective approach for designing a high-selectivity tunable balanced bandpass filter (BPF) with constant absolute bandwidth (ABW). The filter is composed of two open-loop resonators with a pair of symmetrically loaded varactor diodes to obtain tunable responses. The major design concept relies on choosing the particular position on the resonator for loading the varactor diodes. The desired variation slopes of the external quality factor and coupling coefficient can be easily obtained for realizing the constant ABW of the tunable BPF. Frequency-dependent source-load coupling between the feed lines are introduced for realizing two adaptive transmission zeros (TZs) on both sides of the tunable passband. Thus, during the frequency-tuning process, high selectivity of the proposed filter can be kept. Meanwhile, the common-mode suppression is optimized to an acceptable level in the frequency-agile range of the differential-mode passband by loading the stub and resistor in the series at the center of the resonator. For demonstration, a tunable balanced BPF with the constant ABW and two TZs is designed and implemented. The simulated and measured results are presented, showing good agreement.