Materials for filtering and frequency control in the next generations of mobile communication systems

Abstract
A very large development of the mobile radiocommunications is expected to occur in the 15 next years. In the introduction, the most important among the present and future systems are listed and their main characteristics are briefly indicated. The case of the Universal Mobile Telecommunication System (UMTS) presently studied in the European community to enter service in the beginning of the 2000's is particularly discussed since, it is now, the most ambitious project in this field. The equipments of the future numerical radiocommunication systems will include a large number of piezo-electric devices for filtering and frequency generation. New characteristics (higher frequencies, larger bandwidth, very reduced dimensions, etc...) will be required for these devices so that new developments will be necessary to find solutions to these requirements. It is believed, that the corresponding evolutions will be very significant of the main trends in piezo-electricity for the next ten years. This paper review the UHF and VHF filtering technologies and the frequency generation techniques the most likely to be used in the future radiomobile systems. For each kind of devices (filters for either UHF or an intermediate frequency and frequency sources), we discuss the most probable requirements, the present state of the different technologies, the new advances required to develop highly miniaturized devices for the mobile equipments or high performances sub-assembly for the base stations. In most cases, it appears that the availability of new piezo-electric materials and the obtainment of better characteristics for the existing materials will be essential factors in the achievement of the required characteristics for these devices. The possibilities of integrating piezo-electric resonators or filters with the micro-electronics contributing to the same or to related functions is also discussed together with the interest of developing new thin film piezo-electric materials permitting this integration.