Tetsuya Kimura

Bandwidth-tunable SAW filter based on wafer-level transfer-integration of BaSrTiO 3 film for wireless LAN system using TV white space

Cognitive radio technology on TV white spaces has been promoted worldwide to solve the spectrum shortage problem due to the explosive increase in personal communication systems such as smartphones. One of the most difficult challenges to utilize TV white space for personal use is the miniaturization of frequency and bandwidth tunable filters to select vacant TV channels. We have developed a one-chip bandwidth tunable filter by wafer-level transfer-integration of BaSrTiO3 (BST) film varactors and surface acoustic wave (SAW) resonators on a lithium tantalate wafer. The 3 dB bandwidth of the bandwidth tunable filter is tuned between 3.25 MHz and 6.25 MHz by applying 7V to the varactors, while the center frequency was constant at 1.004 GHz, as designed. These filters were installed in our developed prototype cognitive radio systems based on the IEEE802.11af draft and successfully demonstrated wireless LAN communication on TV frequency band.

Bandwidth-tunable filter consisting of SAW resonators and BaSrTiO 3 varactors directly fabricated on a LiTaO 3 wafer

A prototyped bandwidth-tunable filter consisting of SAW resonators and BaSrTiO3 (BST) varactors is discussed in this paper. In general, the BST varactors require high deposition temperature (>600°C) in order to obtain good varactor performances. Thus special cared wafer integration process of BST varactors onto a 42°YX-LiTaO3 wafer was newly developed to prevent performance deterioration in SAW resonators. The bandwidth-tunable filter composed of ten pieces of both surface acoustic wave (SAW) resonators and BST varactors on the 42°YX-LiTaO3 substrate was successfully fabricated using the integration process. The measured center frequency of the filter is 1GHz, and the 3dB bandwidth is electrically tuned from 4.5 to 9.6MHz by adjusting a DC supply voltage from 0 to 10V.

Improvement of insertion loss of band pass tunable filter using SAW resonators and GaAs diode variable capacitors

Tunable filters with a wide band tunable range are required for mobile phones with multi-bands and cognitive radio systems to simplify their circuits. Previously, the authors prototyped a band pass tunable filter, which continuously changes frequency from 1734 to 1844 MHz (6.1%) using surface acoustic wave (SAW) resonators and Si diode variable capacitors (varicap). However, the insertion loss was large compared with calculated one because the Q factor of the Si diode varicap was as low as 1 at 1.8 GHz. In this study, the authors improved the insertion loss of the tunable filter using GaAs varicaps. Compared with the Si varicap, the GaAs varicap has a higher Q factor of about 41 at 1.8 GHz. The authors fabricated a tunable filter composed of two SAW resonators and the GaAs varicaps instead of the Si varicaps on a printed circuit board (PCB). As a result, the insertion loss was drastically improved by 12 dB at the maximum, and the frequency characteristic approached the calculated one. The tunable range was also improved from 6.1% to 6.8%.