Design of high selectivity low-loss ladder filters for balanced loads

Ladder resonator filters (LDRF) provide the lowest insertion loss IL=0.6-1.2 dB as compared with other types of LSAW filters, such as longitudinally coupled resonator filters. A drawback of ordinary LDRF is their limited selectivity (or minimal attenuation in the stop band) about UR=40-45 dB. In addition some problems arise when matching LDRF to balanced or differential symmetric loads. The present paper describes basic symmetric Π-type and T-type sections as well as various structures of ladder filters that allow symmetric loads. An extra advantage of symmetric filters is their potentially higher selectivity thanks to low level of parasitic electromagnetic signal. The results of our study have been illustrated by a number of 600-1500 MHz filters exhibiting high selectivity UR=55-70 dB and small insertion loss IL = 1.2-4.0 dB.