An unbalanced/balanced MCF for cellular phone

Abstract

In a monolithic crystal filter (MCF) which has an unbalanced-input and a balanced-output, the attenuation on the lower side has poor repeatability caused by the variation of capacitance between each electrode. We examined an unbalanced/balanced MCF that has good and repeatable attenuation characteristics. We obtained more than 90 dB at fo-900 kHz for a 2-pole MCF (center frequency is 109.65 MHz, 3rd overtone, terminating impedance is 800 ohm//-0.3 pF). We also examined an unbalanced/balanced 3-pole MCF which has good characteristics. This is because the capacitance between each electrode is close to zero due to the shield effect of the center electrodes.