The recent development of high-temperature superconducting (HTS) filters for the receiver front-end of mobile telecommunication base stations and small cryocoolers is described in this paper. The merits of the HTS filters are small size, low noise and sharp skirt. One of these filters combined with a cryogenic low-noise amplifier contributes greater noise reduction to the receiver and improves the Carrier/Noise power ratio (C/N) of the transmission. The HTS filters, having high quality factors, are also effective for reducing out-of-band interference. On the other hand, cryogenic cooling is an essential and important technology for HTS filters. A reliable, low-cost, small cryocooler is required. In the USA, HTS filter systems have been field-tested using prototype equipment with small cryocoolers.
We prepared a Bi-based superconductor using a variety of Pb-oxides and found that the samples prepared at temperatures above 830°C using Pb-oxides contained the 2223-phase; especially, the samples prepared at temperatures above 855°C using Pb2O3 showed a zero-resistance temperature above 90K. The XRD results showed that the volume fraction of the 2223-phase superconductor was increased by doping with Pb2O3. The chemical bonding nature of the samples which prepared using Pb2O3 was different from those of the samples prepared using other Pb-oxides. The results may indicate that Pb2O3 has a peculiar effect on the formation of a 2223-phase.