Design And Development Of Very Narrow Bandwidth High Temperature Superconducting Filter In VHF Band

With the continuous and rapid development of information technology,a large number of radio devices have been developed,which makes the limited spectrum resources more and more tight,and the mutual interference between various communication devices makes the frequency utilization significantly reduced.A large number of wireless communication systems exist in the VHF band,which leads to the serious situation of mutual interference between signals.In order to avoid the mutual interference of adjacent channels and to improve the spectrum utilization,the performance of filters needs to be improved.In this thesis,three ultra-narrow bandwidth filters based on high-temperature superconducting thin-film materials are designed,which improve the frequency selectivity of the signals and enable the filter performance to be improved.The ultra-narrow bandwidth can ensure that the interference signal of adjacent channels can be effectively filtered out as well as reduce the noise of the system despite the target signal reception.The design solution can effectively solve the problems of complex signals and serious interference between devices in the VHF band,but the low frequency and narrow bandwidth are also two design difficulties.The filter circuit is based on the Chebyshev low-pass prototype filter design method,which is characterized by the existence of regular ripple undulation in the passband range,and this feature is also the key reason for the enhanced steepness of the filter band edge and better frequency selectivity.In addition,its theory is very mature and widely used.The combination of comprehensive computer optimization techniques and co-simulation techniques can improve the efficiency of superconducting filter circuit design work.The design of the resonator is based on a suitable substrate material and a spin-in-spin-out(SISO)spiral structure,which allows the filter circuit to be arranged in a limited substrate area.The uniform impedance resonator and stepped impedance resonator structures are compared and analyzed in terms of the wide frequency domain response curve of a single resonator,the wide frequency domain response curve of the filter and the coupling curve of adjacent resonators,and finally the uniform impedance resonator is used.In this thesis,four commonly used substrate materials,magnesium oxide,lanthanum aluminate,sapphire,and strontium tantalum lanthanum aluminate,are introduced,and suitable substrate materials are selected for the filter design work according to the specific design requirements of the filter.The algorithm of extracting the coupling matrix and the electromagnetic simulation software simulation greatly improved the design accuracy.The high-temperature superconducting films are prepared by multi-source thermal co-evaporation technique to improve the uniformity of the films.The fabrication of microstrip circuits using semiconductor planar fine machining process effectively improves the achievement of product design specifications.In this thesis,three high-temperature superconducting filters with high band edge steepness and high external quality factor are designed using the surface zero resistance property of superconducting materials:a fourth-order and fifth-order high-temperature superconducting filter with a center frequency of 119.2 MHz based on magnesium oxide(Mg O)and a sixth-order ultra-narrow bandwidth high-temperature superconducting filter with a center frequency of 127.5 MHz based on lanthanum aluminate(La Al O₃).The design idea is to first define the design specifications,and then to develop a high-temperature superconducting filter(HTS filter).The design idea is to first clarify the design specifications,then use the co-simulation technique to give the equivalent circuit of the filter,the theoretical curve,the length of the resonator,the coupling spacing,the port position and the simulation results to design the high-temperature superconducting filter,and finally to test the RF performance of the superconducting filter to get the actual test results.The simulation results show that the band edge steepness,insertion loss,return loss,and out-of-band rejection of the three filters meet the design requirements.The actual casting,assembly,and testing of the fourth-order superconducting filter were carried out,and the actual test obtained a filter relative bandwidth of 0.8‰and a band edge steepness of 241.0 d B/MHz or more,which achieved more satisfactory results overall.