4-pole SAW-coupled-resonator filters with solely acoustic proximity coupling

Transverse coupled SAW resonator filters (TCF) are widely used as IF filters in the range between approximately 100 MHz and 500 MHz. They offer a high stopband rejection, steep skirts and insertion losses well below 10 dB. In this paper operation of TCFs with relatively high 3 dB-bandwidths is considered. For the realized 4-pole filters resonator coupling is performed entirely by evanescent acoustic coupling between adjacent resonator tracks as proposed by Coldren (1979). Thereby the detuning of the centre resonators and the resulting deterioration of the passband shape in conventional electrically coupled 2-pole TCF structures is avoided. In electrically coupled TCFs the sensitivity to the inductivity value of a required external coil increases with increasing bandwidth and thus limits wideband operation of these filters. Since in entirely proximity coupled TCFs the coupling strength is determined by photolithography, the sensitivity problem is avoided. Two filters are presented: one with a 3 dB-bandwidth of 292 kHz and the other with 118 kHz. The minimum insertion losses are 2.5 dB and 3.0 dB respectively for filters mounted on printed circuit boards (PCBs) and matched to 50 Ω. A selectivity of 29 dB has been achieved for the narrowband filter driven asymmetrically. The chip length is only 6.8 mm for both devices for centre frequencies of 83 MHz. In the paper the following items are addressed: electrical coupling of SAW-TCFs; the sensitivity on discrete external coupling elements; electrical crosstalk; and the influence of weak resonances of the uncoupled resonators