Network coupled, high performance SAW resonator filters

The dynamic market for compact mobile communication systems has greatly stimulated new SAW design techniques, especially SAW coupled resonator filters. Purely acoustical coupled SAW resonator filter can offer very low loss in the passband but the performance is not good enough for stopband rejection and transition width. Usually, electrical coupling is helpful in this respect and the required performance can be achieved by properly designed coupling networks. In this paper two types of network coupled SAW resonator filter are presented, One is a low loss, small transition width (shape factor of 1.4 from -3 dB to -28 dB) IF filter for DECT system and the other is a single chip duplexer filter for the CT1+ system. Both devices have losses of less than 4 dB over moderate bandwidths and high stopband rejections (⩾45 dB). The duplexer also requires large separation between RX and TX channels without any matching elements. It is well known that the coupling-of-mode theory is a good tool for analysis and design of these kinds of filters. However, since the substrate employed is 36°YX-LiTaO₃, which has the required strong coupling to leaky surface modes, and the velocity of leaky surface mode is very close to that of fast shear bulk wave, there exists strong coupling between two modes within the metal strip periodic structure. This implies modifications to the conventional COM theory in order to accurately model the device operation. In this paper theoretical analysis results based on a modified coupling-of-modes model for both filters are compared with the measured results. Reasonable agreement has been obtained