This paper proposes a configuration of the radio frequency surface acoustic wave (SAW) filter with wide tunability for both the passband width and the center frequency. At first, it is shown how the center frequency and the passband width are adjustable with the VC setting. It is also shown that a flat passband and steep transition bands are realizable when multiple stages are cascaded with mirror inversion. Then, influence of Q factors is discussed. Finally, two types of tunable filters are designed, and applicability of this filter topology is demonstrated.

This paper describes the fabrication and preliminary evaluation of a 200 Ω terminated, 1 GHz tunable surface acoustic wave (SAW) filter with electrostatic micro-electro-mechanical-system (MEMS) varactors. The tunable filter was constructed on a 15° Y LiNbO3 substrate to use Love wave with an electromechanical coupling coefficient as high as 25%. The bridge structure of the MEMS varactor was fabricated by Au electroplating on a sacrificial photoresist, which was finally removed by either O2 plasma ashing or O3 water etching. The measured center frequency was 1.07 GHz, and the bandwidth was 171 MHz, i.e., 16% in relative bandwidth. The MEMS varactor, which was connected to a series SAW resonator in parallel, pulled in by an actuation voltage of 150 V DC, and the right passband edge moved to lower frequency side by 15 MHz.

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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 waferlevel transfer-integration of BaSrTiO 3 (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.