T-shaped I/O feed based differential bandpass filter with symmetrical transmission zeros and high common mode rejection ratio

A T-shaped feed based differential microstrip bandpass filter (BPF) with high common-mode (CM) rejection ratio is presented. The filter comprises two magnetically coupled conventional square open-loop resonators (SOLRs), with capacitive coupled T-shaped input-output (I/O). The choice of the T-shaped I/O coupling feed enables a higher common-mode suppression of −57 dB at f0^dthat extends up to 4.1f0^{dwith a value better than−30 dB. Frequencyf}0^d is the cutoff frequency of the differential-mode (DM) passband. Moreover, this feed can symmetrically position two transmission zeros (TZs) at the upper and lower stopbands. This yields a highly selective and compact filter. Additionally, a T-shaped feed only excites the odd mode of the filter resulting in a wide stopband with high out of band rejection. The upper and stopband rejection of the filter is better than −50 dB. To demonstrate the design, DM and CM lumped models of the filter are proposed and studied. The filter operates at 1.263 GHz with a fractional bandwidth (FBW) of 3.9%. The design is validated experimentally by characterizing DM, CM, common-mode to differential-mode (CD), and differential-mode to common mode (DC). Moreover, the group delay (GD) response of the filter is measured, and a significantly flat response is observed with a maximum delay variation of only 0.88 ns in the 3 dB bandwidth.