Site: Saitama University
Department of Electrical and Electronic Systems
255 Shimo-Okubo, Urawa
Saitama 338, Japan
Date Visited: January 27, 1997
WTEC Attendees: R. Ralston (report author), M. Beasley, G. Gamota, H. Morishita, M. Nisenoff, F. Patten, J. Rowell
Hosts: Prof. Yoshio Kobayashi, Faculty of Engineering
Prof. Susumu Takada, Faculty of Engineering
Dr. Yasuhiro Nagai, Supervisor, Applied Frequency Technology Research Group
	Information Hardware Systems Laboratory
	NTT Integrated Information & Energy Systems Laboratories
	Musashino-Shi, Tokyo 180, Japan

BACKGROUND

Saitama University is a national university located in Urawa City, in the center of Saitama Prefecture, about 1.5 hours travel time west of central Tokyo. The university has 8,000 students, of whom 2,500 are in engineering, and of those, approximately 400 are studying electrical engineering.

Professor Kobayashi has been at Saitama since 1965 and has long been active in the field of dielectric-resonator-based filters. He is a member of several professional engineering groups, and he sits on the Superconductor Applications Committee of the IEEE Microwave Theory and Techniques Society.

Professor Takada joined the university in 1996, moving there after leading the superconductive electronics activity at the MITI's Electrotechnical Laboratory (ETL) for several years. His last post at ETL was as general research counselor. He continues his interest in developing LTS digital circuits, but he has not yet obtained funding to establish the required facilities. In addition to his interest in building LTS circuits, he may start an HTS film deposition effort.

Prof. Kobayashi and Prof. Takada each teach one graduate-level course in superconductivity, on the respective topics of HTS and LTS devices. They find that the undergraduate background of physics students prepares them better for these courses than the background of engineering students.

Professor Kobayashi's group is comprised of one technician (for fabrication of 3-D cavity structures), and 15 students (7 undergraduates and 5 first-year and 3 second-year master's candidates), none of whom are PhD students. The master's students can frequently get a national scholarship, but no support is provided directly by him (or the university). He is able to attract students interested in superconductivity, although typically 80-90% of the graduates go into communication fields with Toshiba, NEC, Murata, Matsushita, or NTT and do not continue in superconductivity. Saitama is the only university in Japan known to the WTEC panel that has a professor experienced in microwave filter design who is applying that experience to superconductive implementations. (There is, in addition, a Prof. K. Sawaya of Tohoku University who has been consulted by AMTEL in its characterization of films using a dielectric resonator.) Direct annual support from companies to Prof. Kobayashi is approximately ¥5 million. A grant of ¥500,000 is made by ISTEC.

DISCUSSION

As the WTEC panelists arrived, we were greeted by Professor Takada and escorted to Professor Kobayashi to exchange business cards with our hosts. The technical topics are described below. During the general discussion period, Drs. Gamota and Rowell described the WTEC mission and purpose of this WTEC panel. Near the close of the session the panel was graciously treated to lunch by our hosts.

Microwave Filters

The panel then spent one hour in Prof. Kobayashi's testing laboratory, where, with posters and sample devices, he described his work on dielectric-resonator cavity filters. His lab is a compact facility, reasonably well outfitted with computers and microwave instrumentation (of U.S. manufacture) for the design and characterization of resonators and filters.

Prof. Kobayashi is placing emphasis on high-power filters for satellite communications, probably because his skill is centered in 3-D dielectrically loaded cavities, which because of larger volume can tolerate more circulating power than 2-D planar configurations. He correctly identifies his group's focus on power handling as unique within Japan. Despite his ambition of providing filters for such an application in 2-to-3 years, he has not yet addressed cooler and cryopackaging issues. The relatively weak applications pull was consistent with the difficulty in judging, from within the university, the level of corporate enthusiasm at NEC for insertion into satellites. He is also interested in terrestrial wireless applications.

It was made clear by the presentation that Prof. Kobayashi has substantial links to Murata Mfg. Co., Ltd., a supplier of dielectric pucks for cavity filters, and to NTT; both companies have previously licensed his conventional filter designs. One of these was a dual-mode patch type (approximately 15 x 10 x 3 cm) which diplexed the cellular bands of 1487-1491 MHz and 1439-1443 MHz.

The basic HTS filter structure has a cylindrical bulk-formed YBCO cavity wall. These are supplied by Dr. Shuji Yoshizawa of the Central Research Laboratory of Dowa Mining Co., Ltd., and by Dr. Tetsuo Oka of the IMRA Material R&D Co., Ltd. Currently, the samples have a surface resistance at 10 GHz and 77 K, similar to that of cold Cu. Prof. Kobayashi anticipates eventual improvements in the YBCO. Murata has provided dielectric pucks of Ba (Sn, Mg, Ta) O3 where the doping provides a temperature stable dielectric???r ª 24.2, tan ? ª 2 x 10-5). A 4-pole, 0.3% filter at 12 GHz can be screw tuned at room temperature and stay essentially on frequency when cooled to 77 K. A 2-pole, 0.3%, 12 GHz filter was operated at 77 K and tested up to 50 dBm. This device (Kobayashi et al. 1995) had less than 0.2 dB insertion loss up to 37 dBm (5 W), and a projected third-order intercept of 100 dBm. An issue identified for high power handling is the proper cooling of the coaxial dielectric rods. The filter insertion loss rises rapidly above 10 W input, attributed to heating of the rods. Although a comparison was made between a low-power resonator Q of 80 K at 77 K achieved here versus a Q of only 20 K by Com Dev of Canada, the comparison was incomplete because of the differences in cavity volume.

A variety of HTS and conventional cavity filters were on display, with frequencies at 1.5, 5, 12, and 27 GHz. The group puts substantial effort into the characterization of various dielectrics (NdGaO3, C- and R-face sapphire, MgO, etc.) using a closed cycle cryocooler (Sumitomo Heavy Industries, Ltd.). Many of the designs are optimized to give equal loss contributions from the (super)conductor and dielectric.

NTT HTS APPLICATION ACTIVITIES

Subsequent to the tour of the microwave area, Dr. Nagai of NTT joined us. He presented a review of the NTT activities and described the reasons for a cessation of the HTS development there.

The profile of the NTT program was given as follows:

1986 HTS development began in NTT with 7 researchers. Their mission was communication subsystems. A first target, buried facilities (cables), was abandoned midway and a second target, cellular base stations, was pursued.

1987-1994 Device development was ongoing over a broad area, which included antennas, filters, transmission lines (for optical modulators), equalizers, and mixers.

1986-1988 Device researchers interacted with NTT system engineers in the mobile, transmission, and optical device sections. Although interested, these system engineers were conservative. They were very concerned about both material growth complexities and refrigeration. As a result, the device researchers were unable to convince NTT headquarters that the HTS technology would be practical. Management concluded that employing 100 researchers (many of them only involved part time with superconductivity) was excessive.

1987-1993 NTT ramped down the HTS device efforts, including the microwave work, but did continue materials research (films by Dr. Suzuki at Ibaraki and Dr. Miyazawa at Atzugi) and a program on new "quantum" structures (a transistor by Dr. Miyazawa).

present NTT DoCoMo, a separate (spinoff) company and pace-setting wireless service provider in Japan, is testing HTS filters supplied by U.S. (not Japanese) firms. Dr. Toshio Nojima, Group Leader, Research Section, R&D Department, NTT Mobile Communications Network, Inc., was identified as the principal investigator for this effort. The parent NTT firm is very busy with R&D in multimedia and will not return to the HTS microwave effort. The three more basic research activities are expected to continue at NTT.

LTS DIGITAL

Professor Takada reviewed the earlier collaborative ETL effort with industry to develop a hybrid computing system and gave an overview of the current efforts. He believes that LTS is well matched to switching applications (rather than computation) and perceives that the FED funding in novel devices is also generally pointed at switching as well. When questioned about the possibility of an LTS foundry for research circuits, he indicated that NEC (Tsukuba) was certainly capable, but the typical approach in Japan is for each big company to do everything internally, and thus it has been difficult to have such cooperation. The restructuring of companies during the economic downturn will, he thought, induce more national collaboration. The universities are also being effected by the slowdown, and although government funding for university R&D is increasing, it will likely focus on basic work at the top tier universities. Hence, collaboration between most universities and industries will be increasingly important.

SUMMARY

The one academic group in Japan focused on passive microwave applications of HTS is lead by Professor Kobayashi at Saitama University. In keeping with its expertise in dielectric-resonator-based cavity filters, the group has made and characterized a variety of structures using bulk YBCO cylinders and custom-doped dielectrics, with all materials supplied by industry. The numbers of poles are modest and the filter characteristics can be improved if the surface resistance of the bulk YBCO can be reduced below that of cold copper. Masters students find industrial positions in the microwave area, but not involving superconductivity. Prof. Takada, recently arrived from ETL, is planning to continue his efforts in LTS digital circuits. As would be expected of the university environment, the applications pull is only weakly felt on superconductive electronics.

REFERENCES

Kobayashi, Y., M. Sasaki, T. Senju, Y. Kasuga, K. Haginuma. 1995. Intermodulation characteristics of high-power bandpass filter using dielectric rod resonators loaded in a high-Tc superconducting cylinder. In Proceedings of the 1995 IEEE MTT-S International Microwave Symposium (held 16-20 May 1995 in Orlando, FL) 2: 733-6.

Kobayashi, Y., T. Senju, and M. Sasaki. 1994. High-power characteristics of a bandpass filter using dielectric-loaded high-Tc superconducting cavities. In 1994 Asia Pacific Microwave Conference Digest.

Kobayashi et al. 1995. IEICE Technical Report 95 (179): 51-6.


Published: August 1998; WTEC Hyper-Librarian