Design of a split type filter for wide band cellular system

This paper presents a design technique in order to obtain wide band filters for band 41 with passband of 194 MHz (7.48 % in relative bandwidth) and band 28 with 45 MHz (5.77 %) based on Y-cut and X-propagation lithium tantalate (YX-LiTaO
₃
) substrate. In case of using common leaky surface acoustic wave (SAW) on YX-LiTaO
₃
, conventional ladder filter design and/or double mode SAW (DMS) filter design method cannot be applied for such wide band filters because of its small electro-mechanical coupling factor (K
²
). To cover the shortage of K
²
, a new design technique, we called split type filter, is proposed. Split type filter is basically composed by 2 sub filters, which are interconnected in parallel each other. The 2 sub filters are designed to have a different center frequency in each and a sub passband that is almost half of system bandwidth. In this paper, the detail of the design method for split type filter is discussed, and the filter performances with low insertion loss and sharp steepness for band 41 and band 28 are demonstrated.