A novel high common-mode (CM) suppression wideband balanced passband filter (BPF) is proposed using the stub centrally loaded slotline resonators (SCLSR) which have two resonant frequencies (odd- and even-modes) in the desired passband. The odd-mode resonant frequency of the slotline SCLSR can be flexibly controlled by the stub, whereas the even-mode one is fixed. Meanwhile, a transmission zero near the odd-mode resonant frequency can be generated due to the main path signal counteraction. First, the wideband single-ended BPF and corresponding balanced BPF are designed based on the slotline SCLSR with the parallel coupled microstrip line input/output (I/O). Ultra wideband high CM suppression that can be achieved for the slotline resonator structure has no resonant mode under CM excitation. Furthermore, by folding the parallel coupled microstrip line I/O, the source-load coupling is effectively decoupled to improve the CM suppression within the passband. The high suppression wideband balanced BPF is fabricated and measured, respectively. Good agreement between simulation and measurement results is obtained.

A novel dual mode bandpass filter (BPF) with improved spurious response is presented in this letter. To obtain low insertion loss, the coupling structure using the dual mode resonator and the feeding scheme using coplanar-waveguide (CPW) are constructed on the two sides of a dielectric substrate. A defected ground structure (DGS) is designed on the ground plane of the CPW to achieve the goal of spurious suppression of the filter. The filter has been investigated numerically and experimentally. Measured results show a good agreement with the simulated analysis.

A high-Tc superconducting filter of the planar structure is proposed for handling higher power signals and for miniaturizing the filter configuration. The filter is designed with a single disk-resonator shared by two degenerate modes to operate as a two-stage bandpass filter. Thereby the proposed filter is expected to possess high power handling capability as a conventional filter with two resonator disks does while the filter configuration is about a half in area compared to the conventional one. The Tchebyscheff type filter with 5.1 GHz center frequency and 2% relative bandwidth was fabricated using a high-Tc superconducting thin film. The passband insertion loss, Lo, was approximately 0.8 dB at 77 K. The low loss performance due to the superconductivity was observed at incident signal levels up to 41.2 dBm (around 15 W) at 20 K, which is limited by the power devices in the measurement setup. In addition, good linearity in the filter responses was confirmed by observing the intermodulation distortion with the two-tone method, which indirectly shows a stable operation with higher power incident signals.