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T-SHAPED I/O FEED BASED DIFFERENTIAL BANDPASS FILTER WITH SYMMETRICAL TRANSMISSION ZEROS AND HIGH COMMON MODE REJECTION RATIO

By R. Gadhafi, D. Cracan, A. A. Mustapha, and M. Sanduleanu

Full Article PDF (478 KB)

Abstract:
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 (SOLR), 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 f0d that extends up to 4.1f0d with a value better than -30 dB. Frequency f0d 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.

Citation:
R. Gadhafi, D. Cracan, A. A. Mustapha, and M. Sanduleanu, "T-Shaped I/O Feed Based Differential Bandpass Filter with Symmetrical Transmission Zeros and High Common Mode Rejection Ratio," Progress In Electromagnetics Research M, Vol. 89, 141-149, 2020.
doi:10.2528/PIERM19111804

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