Tunable balanced bandpass filter with constant absolute bandwidth and high common mode suppression

Abstract

A reconfigurable balanced bandpass filter with constant absolute bandwidth (ABW) and high common-mode (CM) suppression by using the interdigital coupling compensation circuit structure is presented here. The odd- and even-mode equivalent circuits of the presented balanced bandpass filter are presented and analysed. The differential mode and CM signals are misaligned by the half-wavelength resonators, which are coupled by interdigital coupling circuit to improve the coupling coefficient and decrease the circuit dimension. Changing the voltage of the four varactors, which are placed at two ends of the half-wavelength resonators with interdigital coupling circuits, can tune the coupling coefficient and resonant frequency at the same time. A tunable balanced bandpass filter with ABW is designed and realised. The measured results indicate that the differential-mode tuning centre frequency range extends from 0.97 to 1.72 GHz, the 3 dB ABW is 80 ± 5 MHz, and the CM suppression is >30 dB in the differential-mode operating passband. Experimental results are in good agreement with the theoretical and simulated ones.