Tunable Balanced Bandpass Filter With Constant Bandwidth and High Common-Mode Suppression

This paper presents tunable balanced bandpass filters with constant fractional bandwidth (FBW) and constant absolute bandwidth (ABW), respectively. The equivalent differential- and common-mode circuits of the proposed filters are analyzed. The differential-mode bandwidths can be maintained constant when the frequency is tuned by selecting proper coupling regions and controlling the external quality factor Qe. To suppress the common-mode response, different elements are loaded in the middle of the resonators to misalign common-mode resonant frequencies. The common-mode response can be suppressed without degrading the differential-mode performance. For demonstration, two tunable balanced filters with constant FBW and ABW are implemented, respectively. Comparisons of the calculated, simulated, and measured results are presented to verify the theoretical predications.