| The microwave surface resistance of high-temperature superconducting materials is much lower than conventional conductors.Filters made from these materials exhibit advantages such as low insertion loss,high sideband steepness,deep out-of-band rejection,and narrow relative bandwidth.Historically,the application of HTS filters has been limited to specialized fields such as satellite communications and electronic warfare due to the peripheral support equipment.However,with the rapid development of quantum computing,there has been a rapid increase in demand for filters with wide stopbands and high suppression.HTS filters,as a representative of high-performance filters,present new opportunities for such applications because they do not require additional complex,bulky refrigeration,vacuum,and control systems in dilution refrigerators.Additionally,the sensitivity and anti-interference capabilities are significantly better than those of conventional LTCC filters.Unfortunately,HTS filters typically suffer from a narrow stopband.To address this problem,our research focuses on extending the stopband of superconducting filters,which mainly includes the following three parts:A design method for ultra-wide stopband lowpass filters(LPFs)is proposed.Initially,we broaden the inherent stopband by dividing the coupled lines of the dualhairpin unit cell,which raises the resonant frequency while maintaining the cutoff frequency.Subsequently,we overlap the virtual grounding of the unit cell’s second harmonic and the substructure resonator’s fundamental mode at the tapping position to suppress them,thereby further extending the stopband up to the third harmonic.Our results demonstrate that the unit cell can be regarded as an independent structure,and the feed lines only affect the return loss without changing the characteristic frequencies of the LPFs.Hence,high-order LPFs can be developed by directly cascading multiple unit cells without considering their cross-couplings.Based on this method,a onesection dual-hairpin LPF is designed,extending the stopband up to 11.3 times the cutoff frequency.Subsequently,we designed a high-order LPF,which is divided into two subfilters,separated by a thin metal plate to prevent box resonances from degrading the stopband.The low-frequency sub-filter provides a cutoff frequency of 500 MHz,and the high-frequency sub-filter extends the upper stopband.The fabricated high-order filter has a cutoff frequency of 483.2 MHz,with a stopband extending to 51.9 times the cutoff frequency(25.07 GHz).The out-of-band rejection is higher than 60 dB,with a roll-off rate of 569 dB/GHz,and an insertion loss of only 0.04 dB.The filter exhibits the advantages of wide stopband,deep rejection,high selectivity,and miniaturization.A design method for ultra-wide stopband bandpass filters is proposed.Initially,an analytical expression for the scattering parameters is derived through the coupling matrix,demonstrating that reducing the external couplings can improve the harmonic suppression.Next,a dual-hairpin resonator is designed,which induces self-cancellation of the electromagnetic field of high-order harmonics,and consequently weakens the external couplings between the resonator and I/O ports.It is proven that designing filters with specific symmetry can suppress the even-order mode of the substructure resonators.An eight-order filter is designed using this approach and stagger-tuning,effectively suppressing six harmonics concurrently.The fabricated filter possesses a center frequency of 365 MHz,a relative bandwidth of 2.74 %,a stopband extending to 17.4 times the cutoff frequency(6.3 GHz),and an out-of-band rejection higher than 60 dB,with an insertion loss of only 0.11 dB.The filter’s advantages include wide stopband,high rejection,miniaturization,and straightforward synthesis.A dual-band lowpass-bandpass filter with flexible passband allocation is presented.First,we introduce a dual-mode unit cell with two independent resonant frequencies whose frequency ratio can be controlled by the connecting line.The lowpass response and bandpass response are respectively realized by introducing feed lines and coupled lines for the unit cell.Furthermore,the coupled lines can introduce a transmission zero in the transition band of the lowpass response,greatly improving the selectivity.The unit cell is then optimized by introducing meander lines and interdigital capacitors,enabling independent control of the cutoff frequency,the center frequency,and the relative bandwidth.The frequency ratio of the two passbands has a wide tunability ranging from 1 to 4,with a maximum bandwidth exceeding 60%.For demonstration,a seven-section lowpass-bandpass filter is designed and fabricated.The measured cutoff frequency of the lowpass response is 0.917 GHz,with a steepness skirt of 1055 dB/Ghz.The center frequency of the passband response is 1.718 GHz,with a relative bandwidth of 13%.Adjusting the connecting lines of the unit cells only affects the cutoff frequency,while maintaining the passband response unchanged.The filter has the advantages of flexible passband allocation,high roll-off rate,miniaturization,and simple synthesis.The dissertation presents several significant innovations.Firstly,we demonstrate that dividing the coupled stubs in a lowpass filter can considerably widen its stopband.We innovatively achieve harmonic suppression by changing the tapping position,which enables an ultra-wide stopband previously only possible with high-order LPFs,now achievable under a one-section LPF.Notably,the fabricated high-order LPF shows the widest stopband reported in literature.Secondly,we achieve multiple harmonic suppression for the first time for the bandpass filter by utilizing self-cancellation of the electromagnetic field.This is made possible by the symmetric layout,which enables simultaneous suppression of six high-order harmonics.The fabricated eight-order filter boasts the widest stopband in literature.Lastly,we introduce an independent control of two passbands in the lowpass-bandpass filter for the first time. |
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