A Wide Stopband Wideband HTS Filter Using Stepped-Impedance Resonators With an Interdigital Capacitor Structure

A stepped-impedance resonator with an interdigital capacitor (IDC) structure has been proposed to design a wideband filter with a wide and deep stopband. With the increase of the IDC part, there is always a maximum ratio of a spurious resonance frequency to a fundamental resonance frequency whether we change the length or number of interdigital fingers. We increase the width of a microstrip line loaded with the IDC structure to reduce the fundamental resonance frequency and increase the spurious resonance frequency. The harmonic ratio of the proposed resonator is raised to 5.6. Moreover, we bend the narrow microstrip line into a meander line to realize the strong coupling for a wideband filter. A ten-pole high-temperature superconducting filter centered at 2.128 GHz with a 22.7% bandwidth is fabricated for demonstration. The filter is fabricated on a 0.5-mm-thick MgO substrate with a circuit size of 26 x 10 mm. The measured maximum insertion loss in passband is 0.11 dB and the return loss is better than 17.3 dB. The measured out-of-band rejection is better than 80 dB up to 8.728 GHz, which is about 4.1 times of the center frequency.