Tunable differential-mode bandpass filters with wide tuning range and high common-mode suppression

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.