Design of Miniaturized Narrow-Band POI-SAW Filter Using Mixed Ladder-DMS Topology

In this brief, a miniaturized narrow-band surface acoustic wave (SAW) filter with low loss, high near-skirt selectivity, and wide stopband is designed, presented, and verified. The proposed 4
$^{\mathrm{ th}}$
-order bandpass filter is composed of a 5-IDT double-mode SAW (DMS) and two synchronous resonators. Firstly, the equivalent in-line extracted-pole circuit is investigated by applying the nodal representations of both the DMS and the synchronous resonators. Then, the bandpass-domain generalized Chebyshev filtering polynomials of the filter is derived by the direct synthesis technique (DST) according to the filtering specifications. And the values of the Eigen frequencies and the frequency invariant (FIR) susceptance are determined by the partial extraction procedure. Based on the synthesized initial dimensions, the performance of the filter can be easily optimized by the coupling-of-modes (COM) model. For verification, a filter prototype is implemented on the piezoelectric on insulator (POI) substrate, with a center frequency of 2493 MHz for Beidou navigation systems, a 3-dB fractional bandwidth of 2.09%, a measured minimum in-band insertion loss of only 0.99 dB, a footprint of only 0.345 mm
$\times0.55$
mm, a small temperature coefficient of temperature (TCF) of−6.2 ppm/°C and a rejection level of better than 40 dB over the lower stopband and 35 dB over the higher stopband.