Coupled resonator filters with differential input and/or differential output

The conventional coupled resonator filters, widely used in mobile communication systems, usually work with unbalanced termination conditions in both input and output electrical ports. However, there are increasing demands from systems engineers to have compact SAW filters which are able to operate with differential input and/or output termination. To meet these demands, two kinds of new innovative structures have been developed. In this paper the basic idea on which the new innovative structures are based are first discussed. This is followed by the presentation of the new structures in detail as experimental devices. The first is a 6-pole transversely coupled resonator filter at 150 MHz with a more than 100 kHz of 3 dB working bandwidth and very low shape factor (the transition bandwidth from -3 dB to -35 dB is less than 40 kHz), built on 34°YX-quartz. It works with both input and output terminals in differential conditions. The other is a longitudinally coupled resonator filter built on 36°YX-LiTaO₃ substrate and working at 860 MHz with a fractional bandwidth of about 1.5%. It operates with unbalanced input but differential output