6H-6 High-Suppression SAW Filters Based on Double-Resonance Structure

A novel type of surface acoustic wave bandpass filter is presented. The operation of the device is based on the double-resonance filter structure in which relatively long interdigital transducers (IDTs) are coupled acoustically. The filter consists of 3 main IDTs on a leaky-wave 42deg-LiTaO₃ substrate, separated by short IDT sections with a reduced period (distributed gaps). Electrical input signal is connected to one of the side IDTs and electrical output to the other. The middle IDT is either left open (floating) or grounded. If the middle IDT is floating, acoustic energy is transferred from input to output by the resonances created in the gaps, corresponding to two 2-IDT filters cascaded. When the middle IDT is grounded, it effectively blocks the acoustic wave propagation from input to output. Therefore, the filter can be switched off by grounding the middle IDT. The device features a 3-dB relative bandwidth of -2% at 1575 MHz, with relatively low insertion loss (IL) in the passband, steep transitions and high rejection levels. Experimental results show minimum IL of 3.1 dB in the filter passband and suppressions on the order of -50 dB (after removal of electromagnetic feedthrough), with a relative -3-dB bandwidth of 27 MHz (1.7% of center frequency)