T-Shaped I/O Feed Based Differential Bandpass Filter With Symmetrical Transmission Zeros and High Common Mode Rejection Ratio

Abstract

A T-shaped feed based differential microstrip bandpass filter (BPF) with high commonmode (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 f d 0 that extends up to 4.1f d 0 with a value better than −30 dB. Frequency f d 0 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.

Narrowband LTCC Filter With Length-Reduced End-Coupled Resonators

“…Performances comparisons with other narrowband implementations are listed in Table 2. The implemented BPF has the smallest average passband insertion loss, greatest size reduction, and narrowest FBW among BPFs in [1][2][3][4][5][6][7][8][9][10][11][12]. Although BPFs in [13,14] achieved a smaller size and narrower [13], and extra via transition is needed with a higher passband insertion loss [14].…”

“…Narrowband bandpass filters (BPFs) have been widely adopted and increasingly required for military phased array radar transceivers. Narrowband BPFs can be constructed with combined structures [1], split ring resonator [2,3], resistor-loaded resonator [4], cross-coupling structure [5], substrate integrated waveguide [6,7], open-loop resonator [8], right-left hand resonator [9], and end-coupled resonators [10][11][12]. Traditional planar end-coupled BPF is constructed with cascaded half-wavelength resonators, and the interconnection between adjacent resonators is via end-coupling effect.…”

“…Secondly, the coupling gap distance between adjacent resonators is limited less than 50 µm according to printed circuit board (PCB) fabrication process, which is not enough for filter design. BPFs in [8][9][10] solved the above coupling gap problem by placing resonators on different layers, but helped little for size reduction as the length of resonator was not reduced. BPFs in [13,14] achieved 80% size reduction using a stacked structure in a low temperature co-fired ceramic (LTCC) substrate.…”