Research On HTS Wide-stopband Filters And Multi-band Filters

High-temperature superconducting(HTS)thin films have extremely low microwave surface resistance,which can be exploited to produce band-pass filters(BPFs)with both low insertion and sharp rejection.With the rapid development of modern wireless communication systems,complex electromagnetic space environment presents many new challenges to filter designs.Band-pass filters with high out-of-band rejection and multi-frequency selectivity have aroused increasing research interests.Therefore,researchs on HTS wide-stopband filters and multi-band filters have important significance.This dissertation studies the methods based on quasi-lumped elements to promote the out-of-band rejections of filters with low center frequencies.1.A novel resonator that is constituted by a modified spiral with an embedded interdigital capacitor is proposed.At the fundamental resonant frequency(f0),inductance and capacitance of the resonator are both increased,while at the first spurious frequency(fs),inductance and capacitance of the resonator are both diminished.Thus,a large ratio of fs/f0 is obtained.For demonstration,a compact four-pole HTS filter centered at 568 MHz is designed and fabricated.The filter measurement shows excellent performance with an out-of-band rejection level better than 60.9 d B up to 6.8 f0.2.A three-turn spiral resonator is proposed for designing a wideband band-pass filter.The coupling strength of t he resonator is much increased compared with the normal spiral resonators at the same coupling distances.For demonstration,a 20-pole Chebyshev band-pass filter in L-band with a fractional bandwidth of 22.7% has been successfully designed and fabricated.The measured results without tuning show that it has insertion losses less than 0.29 d B and good return loss large than 13.5 d B.Moreover,the filter also shows a high out-of-band rejection level better than 90 d B up to 2.8 f0.This dissertation also analyzes the approaches to realize high-order multiband filters which has high passband numbers.A new design method which combines two different multi-band sub-filters into a quint-band BPF is proposed.The two sub-filters are integrated with a matching junction with branch lines of different widths to reduce the strong interactions between them.The first two passbands are constructed by four stub-loaded stepped-impedance resonators(SL-SIRs).Moreover,three different types of resonators are combined to form a tri-band response,which corresponds to the last three passbands.Thus,the quint-band filter determines the controllable frequencies and bandwidths.The compact quint-band superconducting filter operating at 2.4,3.5,4.7,5.3,and 5.9 GHz has fraction bandwidths of 11.2 %,6.6 %,4.8 %,7.9 % and 2.9 %,respectively.The measured results without tuning show that it has insertion losses less than 0.25 d B and good return loss large than 12.5 d B.Moreover,the filter also shows a high rejection level between passbands better than 45 d B.