Research Of HTSC Linear-phase Filters

The microwave filters using HTSC (high temperature superconductivity) thin films that are small in size and mass have the advantages of low resistance and high quality factor, so its performances are much better than the filters using wave-guide, dielectric resonance or conventional planar structures, offer the potential of large reduction in mass and volume of communication systems, and lead to significant cost reduction of communication systems.In this thesis, the characteristics of HTSC materials are presented first. Then some examples announced in some recent papers are given, and the using of HTSC filters is analyzed too. Difficulties such as the design of HTSC microwave passive component , the lithographic processing technology , the encapsulation and the measurement in low temperature and so on have been solved in this thesis. After this, a series of HTSC linear phase filters of different orders, structures and fractional bandwidths are simulated and designed in different ways, using self-equalization or external equalization of group delay. In chapter 2, the couplings of open-loop microstrip resonators are studied and the measured results of a HTSC filter designed with extracted-pole techenology, whose bandwidth is 80 MHz and center frequency is 1750 MHz, has been given. The best measured insertion loss is less than 0.141 dB and the group delay fluctuates about±1.5 ns across the pass band. In chapter 3, a six-pole HTSC linear phase filter is designed and manufactured with prototype structure. The measured filter has a 55 MHz pass band at a center frequency of 1993 MHz. The measured return loss is better than 13.8 dB across the pass band and the best insertion loss is 0.22 dB. The linear phase bandwidth, where the variation of group delay fluctuates about±1 ns, is over 75% of the filter bandwidth. In chapter 4, a specific parallel structure is used to develop a six-pole HTSC linear phase filter. The measured filter has a 40 MHz pass band at a center frequency of 2000 MHz. The measured return loss is better than 17.6 dB across the pass band and the best insertion loss is 0.264 dB. The linear phase bandwidth, where the variation of group delay fluctuates about±1.5 ns, is over 70% of the filter bandwidth. The measured results well satisfy the performance of simulation. In chapter 5, the process of synthesizing and simulating a series of HTSC linear phase filters of different orders with cross-coupled quadruplet structures to realize the self-equalization. The measured 8-order filter has a 15 MHz pass band at a center frequency of 2514 MHz. The best insertion loss is 2.96 dB across the pass band. The linear phase bandwidth, where the variation of group delay fluctuates about±10 ns, is over 70% of the filter bandwidth. In chapter 6, with external equalization technique, an eight-pole HTSC linear phase filter is designed and manufactured, getting access to a single pole group delay equalizer to compensate the variation. And this results in a much flatter group delay in the pass band of filter. The measured filter has a 14 MHz pass band at a center frequency of 2250.86 MHz. The measured return loss is better than 18.97 dB and the best insertion loss is 2.092 dB. The linear phase bandwidth, where the variation of group delay fluctuates about±50 ns, is over 78.57% of the filter bandwidth. In chapter 7, the method and design process of cascaded linear phase filter are discussed. And the simulated data of two classes of 12-order linear phase filter cascaded from different kinds of 6-order linear phase filter with different structures are given respectively.In chapter 3 and chapter 5, some prototype parameters of low-pass linear phase filters are synthesized and given by this thesis itself. Those parameters can play an extremely important role in guiding the following design of various linear phase filters.All the HTSC linear phase filters are manufactured on the double-sided HTSC YBCO (YBa2Cu3O7) films of about 400nm thickness on a LaAlO3 substrate of 0.5 mm. The whole process in developing the filters can be illustrated as follows. First of all, the prototype parameters are cited from other papers or synthesized by this thesis itself with specific optimization methods. Secondly, the lumped parameter models are given, getting access to some formulations to convert low-pass forms to band-pass forms. Then the theoretical responses are achieved by EM simulation solvers. Thirdly, the modes of HTSC resonators and the linear phase filters are built using CAD programs by full-wave field solvers. Subsequently, the simulated responses and theoretical responses are compared. Finally, the HTSC filters are fabricated, installed in metal shielding boxes and measured; the measured results are given and analyzed as well.