An analysis and improvement of balanced-type SAW filters

The degradation mechanism of balance performance for balanced-type SAW filters was analyzed. The authors have confirmed that the parasitic coupling between input and output IDTs makes the balance characteristics degrade. The measurement results show good agreement with simulations by assuming the parasitic capacitances. The balance characteristics can be improved by decreasing the parasitic capacitances. Further, the much better performance can be obtained by connecting inductors between input and output IDTs. Parallel resonance circuit with additional inductor and the parasitic capacitance can solve the problem successfully. The experimental balanced-type SAW filter shows an excellent performance. These values satisfy the system requirements.