Steepness Improvement of HTS Wideband Bandpass Filter at Low Frequency End

A low order wideband bandpass filter is designed onto the substrate coated with high temperature superconducting (HTS) films on both sides by means of an equivalent model of an interdigital capacitor. In the designed filter, its upper and lower cut-off frequencies can be separately adjusted, and the return loss (RL) in its passband can also be reduced by adjusting the interdigital capacitance. Owing to the low order feature of the designed filter, its edge steepness at the low frequency end is very poor. To improve its edge steepness at the low frequency end, a series resonator is connected with the input end of the filter in parallel to generate a transmission zero. Simulation results prove that the proposed approach can apparently improve the edge steepness of the filter at the low frequency end. All these results demonstrate that the proposed combination design scheme is feasible and suitable to improve the single end edge steepness of the low order bandpass filter.