Site: Advanced Mobile Telecommunication Technology, Inc. (AMTEL)
500-1 Minamiyama
Komenoki-cho, Nisshin-shi
Aichi-ken, 470-01, Japan
Date Visited: January 30, 1997
WTEC Attendees: R. Ralston (report author), M. Beasley, G. Gamota, H. Morishita, M. Nisenoff, F. Patten, J. Rowell
HOSTS: Mr. Masayuki Aoki, Managing Director
Mr. Yoshiki Ueno, Section Head, 1st Section, R&D Division
Mr. Seitoku Itou, Section Head, 2nd Section, R&D Division
Mr. Mitsunari Okazaki, Section Head, Yokohama Laboratory 1st Section, R&D Division
Dr. Nobuyoshi Sakakibara, Chief Research Engineer, 1st Section, R&D Division
Mr. Hiroshi Kubota, Chief Research Engineer, 2nd Section, R&D Division

BACKGROUND

The panel visited the main site of the Advanced Mobile Telecommunication Technology, Inc. (AMTEL), which is located in Nisshin City, Aichi Prefecture, beyond Nagoya. AMTEL was incorporated in March 1994 through joint contributions from the Japan Key Technology Center (JKTC) and two commercial companies, Denso Corp. and ALPS Electric Co., Ltd. The investment is 70% from the JKTC (a MITI-MPT government-backed entity begun about 10 years ago) and 15% each from the companies. Under the development strategy, the R&D Division is to develop high-performance technology for high-quality mobile telecommunications for six years, at which time it will cease operation. The Planning Division is to continue an additional 20 years with the responsibility for managing the commercialization of the technology. Licenses for intellectual property can be negotiated with any Japanese firm. JKTC launches a few joint ventures of this type each year and currently has 25 such ventures in operation.

The AMTEL headquarters and main site for the R&D Division is in approximately 500 m2 of space leased from Denso, a member of the Toyota Group. The facility is contained within a very spacious building constructed about 1990. This same building houses Denso Research, comprising 300 people. The AMTEL R&D Division has 21 staff members; 5 people are in the Planning Division. Of the 21 technical staff members, 7 were drawn from ALPS and 14 were contributed by Denso. The system concepts are being developed at the Yokohama site of ALPS.

The projected total investment over the 6 years is ¥2,860 million (($24 million). The companies renewed their commitment last year. The budget profile for the next 6 years (in millions of yen) is as follows:

199419951996199719981999
¥525.7522.0711.3386.0402.6309.9

DISCUSSION

Upon arrival shortly after 9:00, Mr. Ueno, who had coordinated the panel's visit to AMTEL, introduced Mr. Aoki and his several associates. Drs. Rowell and Gamota provided overviews of the WTEC mission and this WTEC panel's purpose. Mr. Aoki then provided background information regarding the formation of AMTEL. The AMTEL effort is fully concentrated on HTS planar microwave devices and small, efficient cryocoolers and is targeted at the twin goals of improving the quality of mobile telecommunications and making more effective use of the available spectrum. The panel's hosts felt that both ground and satellite systems will be practical, but expected that ground applications would emerge first. The discussion then dealt with the technical strategy and detailed development issues.

Development Strategy and Resources

The development is being carried out on two parallel thrusts, one involving the microwave planar filters and subsystems, and the other involving compact cryocooler subsystems. The years 1994 through 1996 were dedicated to fundamental component studies in each thrust; the 2-year period beginning in 1997 will develop a subsystem for each, and in the final year, 1999, a total system will be evaluated. The 1st Section has 4 staff at the Yokohama Laboratory considering system issues. HTS films and filters occupy the remaining 8 staff in the 1st Section. In the 2nd Section, 7 staff are involved in cryocooler work and 2 are developing the electrical-thermal interface. The facilities are impressive in their quality and range of instrumentation. In addition to the dedicated AMTEL resources, the AMTEL programs purchase from the larger Denso operation such services as machining and photolithography.

HTS Films

The development includes single-target rf sputter deposition of YBCO on both surfaces of the microwave substrate. Two deposition systems are employed: the one favored for production of most filters has 3 target stations in an on-axis configuration; the other system has 4 target stations in an off-axis geometry. Both MgO and LaAlO3 are used with sizes up to 4 cm2; the substrates are radiatively heated. In the on-axis system, the sputter deposition is done upward. The low-power surface resistance (on both top and bottom substrate surfaces) at 60 K is 0.2 - 0.4 m(. AMTEL researchers have studied the effect of stacking faults in the thin films as a factor in determining surface resistance. While these defects clearly correlate with the higher resistances found in poorer quality films, it is not understood how to further reduce the resistance of the best films (Sakakibara et al. 1996). Patterning of the films is done with ion beam etching, and the films do support good preliminary filter designs.

HTS Microwave Filters

Bent (or hair-pin) microstrip resonator structures are featured in AMTEL filter work. The emphasis is on receiver improvement, which is to be gained both by reduction of in-band noise and by rejection of out-of-band interference. A 9-pole, 2.6 GHz filter had a 34 MHz 3 dB bandwidth with 0.25 dB insertion loss. The 40 dB bandwidth was a relatively broad 68 MHz, with a virtual zero in the low frequency skirt and a slower than ideal roll-down in the high frequency skirt. These distortions are indicative of the typical difficulty of implementing a filter response uncorrupted by crosstalk between nonadjacent resonators, or by feed-through from input to output. The device had an impressive -28 dB return loss, probably due to the MgO substrate. This filter was fabricated on a 40 x 25 mm substrate. Although larger substrates and more poles will be needed for the actual application, there was no explicit description of the design for the ultimate filter response.

Cryocoolers

AMTEL's cooler work is impressive and appears to be completely self-contained. It was acknowledged that, in beginning the effort, it was expected that the cryocooler work would benefit from the substantial technical expertise at Denso gained via the development and manufacture of automobile air conditioning units. In retrospect, the cryocooler area has little in common with the Denso product line, and JKTC constraints do not permit collaborations. Nevertheless, Section 2 appeared to be making substantial headway in the development of prototype pulse tube coolers. A feature of its work is the use of two relief valves rather than the more conventional orifice. This modification has improved efficiency at 90 K from 4.0% of Carnot to 5.3% of Carnot (Hagiwara, Yatuzuka, and Ito 1996, and Hagiwara et al. 1996). The minimum temperature of the prototypes is at or slightly below 65 K. In the tour of facilities the WTEC panel witnessed functioning units and the careful characterization of operating efficiencies under load. Our hosts reported a unit with 0.66 W cooling power at 70 K, working toward a goal of up to 1.2 W at 70 K. Within the telecommunication system, it is expected that a cooler will support 3 filters, hence the need to scale up in cooling power. AMTEL management is considering both pulse tube and Stirling cycles to reach that capacity, and has identified a 5-year MTBF (mean time between failures) as necessary. A compressor with flexure bearings was recently put under life test. Development of low-thermal-leakage coaxial cable (by thinning the outer conductor) was underway, and although work had not yet begun on a long-life vacuum housing, the importance of such a component was recognized. Our hosts acknowledged occasional interactions with R. Radebaugh at NIST (Boulder) and requested information on the small (up to 4 W) cryocoolers available in the United States.

Application Example

Use of HTS to improve a satellite-to-ship communication link was described. The scenario offered as an example was a down-link ship-borne receiver in Tokyo Bay being interfered with from spurious out-of-band intermodulation products radiated by an analog cellular base station transmitting from a shore location. The analog cellular system base stations are licensed to transmit in the 1513-1522 MHz band, and the -50 dBm power in the channel at 1522 MHz is only 8 MHz below the 1530-1545 MHz downlink band. The ship's receiver must maintain -120 dBm sensitivity. AMTEL engineers project that a 10-pole HTS filter with cooled low-noise amplifier (LNA) would maintain the receiver at required sensitivity by rejecting the analog cellular system transmitter spurious signals. For that filter characteristic, an unloaded resonator Q of at least 45,000 is needed. This was compared to the current situation in which the ship receiver uses only a conventional 4-pole dielectrically loaded cavity filter. More poles and sharper frequency skirts at low insertion loss are not possible in that case because of the limited unloaded Q of 13,000. (This value was given for room temperature; possible improvements in the conventional filter through cooling were not mentioned). Intermodulation distortion generated within the example cryocooled system was projected to be no worse than -160 dBm, but that appeared to be based solely on an assessment of the distortion within the LNA (with a third-order intercept point of -27 dBm input), with ideal frequency response simulated for the filter.

In a related example, the effective receiver noise temperature for a satellite ground station was calculated to be substantially improved by the use of an HTS filter in combination with a cold LNA. In this assessment (Ueno et al. 1997), a 1.2 m diameter antenna was looking up at the "cold" sky (effective temperature of 30 K). A reduction in noise temperature from 401 K to 133 K was estimated, and the improvement would permit a three-fold increase in data rate. More of the increase was attributed to the LNA than to the HTS.

The specific system target for the demonstration appeared not to have been identified. Our hosts identified the sharpness of filter response, rather than low insertion loss, as the most important parameter for the wireless systems. Because their R&D cycle has a tight time line, Mr. Aoki was interested in our assessment of market opportunities. The AMTEL staff believed that the new worldwide standard for a code-division multiple-access (CDMA) system near 2 GHz would be a practical target for the HTS technology.

SUMMARY

The R&D at AMTEL is tightly focused on developing commercially viable HTS filters and a cryocooled subsystem for telecommunication base stations. It is a venture founded in March 1994 and jointly funded by the Japan Key Technology Center and two commercial firms, Denso Corp. and ALPS Electric. The R&D phase is planned to run for 6 years, concluding with a demonstration of a subsystem comprised of a bank of HTS filters and a high-efficiency compact cryocooler. The facility was very well instrumented, the staff competent, and the 6-year funding level is planned at ¥2.86 billion (($24 million).

The filters (including film deposition) and cryocoolers are being developed by parallel sections within the 21-staff R&D Division. Now at the mid-point in its development plan, AMTEL has achieved a pulse tube refrigerator with 0.7 W capacity at 70 K, and has demonstrated typical 7-pole microstrip filters near 2 GHz. Over the remaining 3-year period AMTEL researchers intend to scale the refrigerator to 2 W and optimize the filter performance at 10 poles or more. The goals for the final subsystem evaluation are to improve the quality of telecommunication and make more effective use of the available spectrum. The specific system to be improved was not identified.

REFERENCES

Hagiwara, Y., S. Yatuzuka, and S. Ito. 1996. Experimental study on the pulse tube refrigerator with two relief valves. In Digest of 9th International Cryocooler Conference (ICC9), Waterville Valley, NH, 26-27 June 1996.

Hagiwara, Y., S. Yatuzuka, S. Ito, and T. Saito. 1996. Experimental study on the gas flow in a pulse tube refrigerator with two relief valves. Paper presented at the 2nd International Conference on the Use of Non-Artificial Substances, 3-6 September 1996. Published in Science et technique du froid. 3: 163-172 (1996).

Sakakibara, N., K. Saito, M. Fuse, and Y. Ueno. 1996. Crystalline study on surface resistance of YBCO thin films for microstrip bandpass filters. In High temperature superconducting electronics: Fundamentals and applications. Program and extended abstracts of the 1996 Int'l. Workshop on Superconductivity, 24-27 June, 1996, Iwate, Japan.

Ueno, Y., N. Sakakibara, M. Okazaki, and M. Aoki. 1997. High-Tc superconducting filters for receiver front-end of mobile telecommunication base station. Microwave Workshop and Exhibition 10-12 Dec. 1996. In Proceedings of the 1996 IEICE General Conference 1997 (Sogo, Pt. 5): 276-7.


Published: August 1998; WTEC Hyper-Librarian