Tunable differential‐mode filters with wide tuning range and high common‐mode suppression are proposed. Step‐impedance resonators terminated with varactors are proposed and used to construct the tunable filter. Symmetrical parallel coupled lines terminated with varactors are used as a negative coupling bisection, which results in convenient tuning of coupling coefficient and bandwidth control. Capacitors are placed at the four ports to adjust the external quality factor. With these techniques and the choice of a high capacitance ratio varactor, a wide tuning range up to 75% has been achieved. Tunable differential‐mode filters with constant absolute bandwidth (ABW) and constant fractional bandwidth (FBW) are realised, respectively. The constant ABW filter has a −3 dB bandwidth of 95 ± 10 MHz and an insertion loss of 5.7 − 2.5 dB. The constant FBW filter has a −3 dB bandwidth of 9.8 ± 1.2% and an insertion loss of 6.0 − 1.7 dB. In addition, a capacitor is loaded on the symmetry plane to misalign common‐mode resonant frequencies, resulting in high common‐mode noise rejection in the whole frequency range.
In this paper, a novel end-loaded resonator is investigated and applied to design tunable bandpass filter (BPF) and bandstop filter (BSF). The novelty of the resonator lies in that two varactors are added to the two ends of the resonator, and then its resonant frequency can be bi-directionally tuned. As a result, the theoretical frequency tuning range can be significantly extended to approximately double that of the conventional tunable resonator. For demonstration, the proposed resonator is applied to design tunable BPF and BSF. As expected, the tuning ranges are 52.4% and 53.5% for the BSF and BPF, respectively. Good agreements can be observed between the simulated and measured results.
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