| Research Abstract |
Recent years have seen extensive studies on high-temperature superconductor (HTS) filters with small-size, low loss, and high frequency selectivity. There have been numerous reports and publications on HTS microstrip filters. On the other hand, as filters using coplanar waveguide (CPW) structures require only, one side superconductor film, lower fabrication cost is expected compared with filters using microstrip structures using two-sides superconductor films.
This research project is aimed at developing design and measuring methods of superconductor filters at millimeter-wave frequencies.
In 2002, three measuring methods, including the circular cavity method, the cut-off circular waveguide method, and the four modes in a sapphire rod resonator method, are developed for accurate measurement of the dielectric substrate (MgO) and the frequency dependence of surface resistance of superconductor film (YBa_2Cu_3O_7, i.e., YBCO) at millimeter-waves. The results show that at 30GHz and 70K, the
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loss tangent of the MgO substrate is smaller than 2x10^<-6,> while the surface resistance of YBCO film is about one fifth of that of the copper. Therefore, low loss resonators and filters can be built by using these materials even at millimeter-waves.
Based on these measured results, an H-slot type resonator using CPW structure is proposed, and the simulated results show it owns very high Q-factors at millimeter-waves. Next, a low-loss HTS CPW 4-pole Chebyshev filter is designed using the H-slot type resonators.
In order to realize a high performance filter with low insertion-loss in its passband and sharp attenuation in its stopband, a general circuit model of a filter having one cross coupling path is analyzed, and a new theory is developed for the design of a filter with poles of attenuation in its stopband. By using the derived formulas, the reactance element values in the cross coupling path are determined readily. The attenuation poles can thus be assigned at desired frequencies. Various design examples are provided, together with simulated results, which validate the proposed theory.
The research results have been published with 5 papers in IEICE Transactions on Electronics, 4 technical reports of IEICE, 6 oral presentations at the national conferences of IEICE, 3 papers at international symposiums, and 5 papers at a number of technical workshops. Less
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