A Wideband Bandpass Filter With Frequency Selectivity Controlled by SAW Resonators

A wideband bandpass filter (BPF) with two cascaded sections, each consisting of a surface acoustic wave (SAW) resonator and one or two transmission lines in parallel, is proposed. The lower and upper rolloffs of the passband are separately controlled by the narrowband characteristics of the SAW resonators to realize high frequency selectivity. The filter principle is analyzed for the two sections using the modified Butterworth-Van Dyke model for SAW resonators. The effects of cascading the two sections using a transmission line are also discussed. To demonstrate the proposed filter topology, two types of resonators are chosen from a limited set of available SAW resonator chips. The filter is designed and simulated based on the data of on-wafer measurements of the chip resonators. It is shown that the wideband behavior of the SAW resonators has impact on the filter. A prototype BPF centered at 2.02 GHz is implemented on PCB combining the discrete chip SAW resonators and microstrip lines. The chips are assembled using 1-mil wire bond technique. The measured filter has a 3-dB fractional bandwidth of 28.8% with high frequency selectivity.