Abstract

Microwave filters are essential elements in many areas of RF/microwave engineering. The crowded frequency spectrum has put forward higher requirements on channel filters. Not only very high close-to-band rejections are required to prevent interference to or from closely neighboring channels, but also very low insertion loss is required to reduce system noise figure. Some RF/microwave system require in-band group-delay and amplitude flatness to minimize signal degradation even further. Surface resistance of high temperature superconducting (HTS) material is about 1000 times less than the best conventional metals such as copper and silver at microwave frequency. So a significant effort has been put into the research and realization of high performance microwave HTS filters since the discovery of HTS materials. HTS microwave filters with remarkably high performance have been constructed in the past 30 years. For example, by selecting appropriate resonators with high quality factors and employing well-designed coupling structures with week coupling coefficients, some HTS filters can have fractional band width (FBW) blow 0.1% with very high selectivity. The insertion loss of some HTS filters can be as low as 0.05 dB. With above excellent performance, HTS filters can not only eliminate unwanted interference but also minimize noise figure of the microwave system. HTS filters and their front-end subsystems have also been successfully applied to many fields and brought multiple economic returns and social benefit. It is reported that HTS receiver front-end subsystem, which integrate with narrow-band highly selective HTS filters and low noise amplifiers (LNA) in a cryocooler, has been employed in more than 10000 base stations in America. According to the promotion materials, the ultra-low-loss and high sensitivity of the HTS receiver can translate into 10%–15% or more increased base station coverage area and data throughput. In some cases carriers could add 8% or more usable resource blocks and capacity due to the high selectivity of the HTS receiver, valued at hundreds of millions of dollars. Design technology of HTS filters has been developed rapidly at an incredible speed. In recent years, many efforts have concentrated on novel HTS filter design technologies, including those in high power handling capability filters, multiplexer or multiband filters and frequency tunable filters. Some research groups are trying to develop all-HTS integrated microwave front-end receivers, which integrate HTS filter, oscillators, mixers and antenna into a whole chip. The all-HTS integrated front-end receiver could make the system compact, efficient, and be able to exploit the full potential of the superconductive components. In this paper, the research status, trend of development and application progress about HTS filters are reviewed. The best performances of HTS filters ever reached in specifications such as insertion loss, FBW, out of band rejection, skirt slope and in band group delay are firstly reviewed. Then the remarkable progress in the most recent years which stand for the research trends of HTS filter technology is presented in detail. At last the application of HTS filters and their front-end subsystems to mobile communications, space or satellite communication technology, radar detective system, deep space detection and radio astronomy is also reviewed.

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