Experimental results for a longitudinally coupled 5-IDT resonator filter with distributed gaps

Longitudinally coupled resonator filters (CRFs) provide unbalanced-balanced operation with wide bandwidth, low loss and high suppression levels. One-track filters with five transducers show good performance at 2-GHz frequencies. At high frequencies, however, the resistive losses arising from the relatively wide aperture of the CRFs can degrade the performance. Reducing the insertion loss in the 1.8-2.2 GHz frequency range remains a challenge. A 5-IDT filter has 4 gaps between IDTs where the periodicity of the grating is broken, resulting in additional loss. We show that replacing these gaps with short transducer sections reduces the insertion loss of the device. We present devices on 42°-LiTaO₃ at 1842 MHz with wide bandwidth of 4.5% and 40 dB of suppression, with a minimum insertion loss less than 1 dB in the best devices, and a maximum insertion loss of 1.2 dB in the passband. We also discuss the layout of the contact pads and the connections and its effect on the device performance and balance characteristics.